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Forest soils in the Anthropocene
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Forest soils in the Anthropocene
2 Dan Binkley*
School of Forestry, Northern Arizona University, Flagstaff, AZ, United States * Corresponding author
ABSTRACT The formation of forest soils continues across centuries and millennia. Many of the key characteristics of soils develop very slowly. The slow, long-term development of soils is coupled with much faster processes that substantially change soils over a period of years and decades. A century ago, soil scientists looked for explanations of soil development using concepts limited by simple assumptions and expectations. Quantification became more important as the 20th century advanced, with concepts retreating into the background. Case studies illustrate both these long-term and more-rapid processes and patterns of change. Detecting, monitoring, understanding and predicting change will be a key focus for forest soil science and management. In the Anthropocene, the breadth of human influences will be the major soil-forming factor in forest soils.
A world without soils Imagine a world without soil. Soil, as we know it, evolves from the interaction of life, chemistry and physics. For the majority of Earth’s history, there was no soil because there was no (or very little) biotic presence across the terrestrial face of our planet. The Earth had an abiotic skin, and the ecstatic nature of soil (Logan, 2007) has been possible for only 10% of Earth’s existence. Prior to the expansion of life into terrestrial geography, the skin of the Earth would have been substantially modified from its original geology. Primary minerals are generally unstable at the temperature, pressure, and oxygen concentrations at the Earth’s surface. Mineral weathering would have proceeded under the influence of oxidation and carbonic acid hydrolysis, both of which were facilitated by atmospheric legacies of the biogeochemistry of marine ecosystems. The hydrological cycle removed some of the products of geochemical reactions, leaving behind residual materials and secondary minerals. The texture of the regolith depended on the chemistry and physics of in situ reactions along with losses and deposition of transported material. The three-dimensional structure of the regolith would have been simply related to texture, with some additional features developing for sodic materials and shrink-swell clays. Long-term development of the characteristics of the regolith would have been the balance between processes increasing entropy (oxidation, mineral weathering, downhill flow of water) and geologic and hydrologic processes that provided continuing opportunities for entropic processes (geologic uplift, replenishment of atmospheric chemistry by marine ecosystems, and hydrology driven by sun and wind).
Global Change and Forest Soils. https://doi.org/10.1016/B978-0-444-63998-1.00002-1 Copyright Ó 2019 Elsevier B.V. All rights reserved.
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CHAPTER 2 Forest soils in the Anthropocene
The world of opportunities expanded when the one-way entropic stories of regolith formation were expanded into new dimensions by the influence of terrestrial life. Life brought three major potentials for turning mere regolith into soils. First, the coupling of plant life and decomposer communities created an engine that took carbon dioxide from the atmosphere and injected it into the soils at concentrations ten to one hundred times greater than the concentration in the atmosphere. The high partial pressure of carbon dioxide in soils pushed the carbonic acid equilibrium, dramatically increasing rates of mineral weathering. Second, undecomposed organic molecules had the potential to change patterns of ion transport within and beyond the soil. Much of the classic horizonation that lies at the heart of soil taxonomy derives from the influence of organic chemistry on the dissolution and precipitation of ions (particularly aluminum and iron). The third potential was perhaps the most important: the three-dimensional alteration of soils by plants and their biotic communities. Plant roots couple soils to the atmosphere through their water conducting systems, and with the movement of water comes the movement of nutrients and other ions. The death of plant tissues places these elements near the surface of the soil, essentially reversing the one-way downward flow driven by entropy. The persistence of organic matter in the soil is also a key aspect of the structure of soils, from the gluing together of particles into microaggregates, to macrochannels of decaying roots, to the mixing activities of soil animals. The full ensemble of chemistry, physics and life creates soils, with the potential for increasing the entropy in the universe by harnessing high-energy photons to split water and create stores of energy bound in organic molecules. These molecules embody the potential gradients that develop and operate in soils, with entropy increasing as energy-rich organic molecules oxidize to low-energy states of water and carbon dioxide. But why does this overall train of energy not run out as racing reactions in the soil increase entropy? Why do large, concentrated pools of plant-required nutrients remain in soils rather than dissipate loosely into the environment? Accumulation of these pools of organic matter and nutrientenriched upper soil layers might not be surprising if activity were slow and time spans were short. However, rates of turnover of pools are actually quite rapid relative to the time course over which soils develop. Could it be that soils somehow developed in concert with plant life such that interactions developed in mutualistic ways, as the evolution of an integrated, complex adaptive system? This idea was explored in a special issue of Geoderma, with the overall conclusion that the logic of selection at the scale of soil/plant systems did not appear tenable (van Breemen, 1993). Instead of simple high-entropy possibilities, we find ourselves contemplating truly complex, dynamic, and yet not totally unstable systems of soils interacting with plants that drive further changes in soils. The dynamics over timescales of seconds and days can be as large as those across seasons and years. Yet the long-term major changes in soils often result from very slow accumulations of very small net changes, such as the gradual reduction in soil hydraulic conductivity that can result from clay translocation or iron oxide deposition, which further reduces soil aeration, slows biogeochemical cycles, and lowers plant growth. This is the context humans encounter when engaging with soils to support growth of plants: a fascinating suite of interacting processes (inorganic and organic) across scales of time and space in ways that lead to very non-static outcomes. Forest soils began to evolve on Earth with appearance of trees in just the most recent 10% of Earth’s history (Xu et al., 2007). The evolution of forest soils across the Earth would have changed substantially with the evolution of trees and other life. Major influences of humans on forest soils developed only in the past few millennia, resulting from changing fire patterns (and subsequent tree species success), livestock grazing (altering tree species composition), harvesting, and converting forest soils into
Case study 1: one step on Hans Jenny’s ecological staircase
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agricultural soils. At the beginning of the Anthropocene most of Earth’s forested soils have a weak or strong legacy of human influences, and our influences will only increase. How will the forest soils of the future be shaped? The chapters of this book develop a wide range of insights. This chapter introduces five cases studies to illustrate some major themes of the changing forest soils of our planet: • • • • •
how very small changes lead to major differences in soil evolution; how soil change is not simple, because of confounding interacting factors; how conversion to agriculture and back to forest changed a particular soil; how intensive silviculture influenced soils across a large region; and how multiple challenges limit our ability to detect important soil changes at the scales that really matter.
Case study 1: one step on Hans Jenny’s ecological staircase Geologic uplift is an important process in rejuvenating soils, bringing fresh low-entropy materials into contact with oxygen and life. Hans Jenny’s favorite example of uplift and soil development was an “ecological staircase” on the Pacific Coast of northern California (Jenny et al., 1969; Jenny, 1989). Gradual uplift provided ecosystems and soils that developed on the same parent material, but with varying distance along the dimension of time. One of the steps in the staircase has soils that have been developing for half a million years or so. The portions of the step that are close to the edge of the terrace sustain productive forests of redwood and Douglas-fir, on fertile, well-drained soils (Fig. 2.1). Farther from the edge of the terrace are stunted forests of Bishop pine, Bolander pine, and Mendocino cypress, eking an existence from poorly drained, infertile soils. Each square meter of the terrace has received something like 500 million liters of rainwater over the eons. The water drained away near the edges of the terrace, but greater distances away from the terrace edges led to somewhat more poorly drained conditions leading to a series of biogeochemical developments. The geochemical and organic weathering of minerals generated iron and aluminum oxides, and a large portion of these oxides was transported away from the well-drained soil. The poorly drained backsides of the terrace experienced an accumulation of precipitated oxides which formed iron pans not far below the surface. The iron pans restrain biological activity by sealing off the nutrient supply in the deeper soil from access by the roots, and further ponding of water (especially in winter) leads to poor oxygen supply for roots and soil biota. This ancient step on the ecological staircase supports two very different sorts of soils, not because of differences in parent material, climate, or the length of time for soil development. The differences stem from the spatial topographic effect of how well water could drain in relation to distance from the terrace edge. These differences were substantial across the soil profile, and even deeper into what is referred to as the Critical Zone: ecological processes in the upper soil alter biogeochemistry deep below the soil surface in ways that can feedback and alter vegetation and the upper soil (Richter and Billings, 2015). Biology always plays a major role when soils differ across topographic (and other) gradients. Factors that alter the species composition and productivity of plant communities in turn have soils reshaped by those vegetation differences. Hans Jenny once thought that soil development could
FIG. 2.1 Location: the difference a few meters can make. The ecological staircase along the coast of northern California (Jenny et al., 1969) includes this “step” that has been developing soil for about half a million years since uplift raised the terrace above the ocean. Well-drained conditions near the edge of the terrace (left) allowed the development of deeper soils with higher nutrient supplies for trees (redwood and Douglas-fir). Poorer drainage away from the terrace edge (right) restricted removal of weathered iron and aluminum, leading to a feedback effect of increasing limitation on drainage as a cemented iron pan formed. Current soil condition in the poorly drained portions of the terrace support stunted vegetation of Bishop pine, Bolander pine, and Mendocino cypress. Figure from Binkley (2015). Photos: pygmy vegetation from Sarah Bisbing, others from Ron Amundson.
Case study 2: rainfall and time in Hawaii
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be quantified by the independent, non-interacting influence of factors (Jenny, 1941, 1961), but the circularity of cause and effect always challenges single-factor stories of the development of soils over long periods of time over landscapes.
Case study 2: rainfall and time in Hawaii Under warm tropical conditions in Hawaii, the leaching losses of calcium (Ca) from the original lava parent material might be expected to increase with increasing rainfall. This was indeed a pattern found after 170,000 years of soil development (Fig. 2.2): half or more of the original Ca remained for sites with <500 mm yr1 rainfall, and sites with more than 1500 mm yr1 lost almost all the original Ca (Chadwick et al., 2003). This simple pattern might seem to fit Jenny’s hope that soil-forming factors operate independently, but the actual story of soil development on these lava flows was not so simple. Organic matter accumulation increased with increasing rainfall, up to about 1500e2000 mm yr1 and then declined. Soils receiving about 3000 mm yr1 of rain had about the same amount of organic matter as those receiving only 500 mm yr1. Non-crystalline clay-sized particles increased asymptotically with rainfall, coupled with a decline in poorly crystallized clay-sized particles. Cation exchange capacity increased almost linearly with rainfall, but exchangeable base cations showed a threshold with a major drop off above 1200 mm yr1. The picture of soil development becomes even more complicated when considering changes over time as well as in relation to rainfall (Stewart et al., 2001). The rate of mineral weathering was initially high when the lava began weathering, especially with high rainfall (Fig. 2.2). Depletion of weatherable minerals from wet locations in the first 100,000 years of soil development resulted in subsequent weathering rates that were higher for drier portions of the rainfall gradient. The curves in Fig. 2.2 probably do not represent the actual quantitative relationships that happened over 170,000 years, because the climate was probably drier for much of that time.
FIG. 2.2 170,000 years of rainfall in Hawaii removed calcium that was originally present in a lava, with higher rainfall leading to greater losses (left). The pattern of Ca removal over time would depend on the twofold effect of rain; higher rainfall leads to higher weathering and leaching, but after some time the depletion of the original material substantially reduces weathering rates (right). The effect of rainfall was not linear for Ca depletion or for Ca weathering, and the dynamics were not uniform across time. (Left) From data in Chadwick et al. (2003). (Right) From data of Stewart et al. (2001).
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CHAPTER 2 Forest soils in the Anthropocene
Quantitative studies of soil development can provide good representations of how soils change over time in response to aspects of climate, vegetation, and other soil forming factors. These quantitative estimates may have rather limited scope because so many factors vary greatly with variable interactions. Parent material is a term that covers hundreds to thousands (or more) possibilities. The characteristics of one lava flow are not the same as another, especially as magma bodies age. One limestone is never quite like another, and there are so many kinds of granitic rock that it may not be clear where the class of granitic igneous rocks stops and other classes begin. The influence of rainfall is not consistent across temperature gradients, and the patterns probably differ based on major vegetation types. Broad scale patterns may be interesting and somewhat useful to think about, but the value for any single location, environment and time may be quite small. The characteristics of soils shift across space and through time, and we can explain some of the variation in quantitative terms, but the wonderful diversity is only partially captured in either simple or complex models.
Case study 3: can trees heal soils? The Calhoun forest experience Major characteristics of soil form over very long periods of time, such as the half-a-million years of development of the terrace soils in the ecological staircase. How dynamic are forest soils over a shorter time period, such as the career of a forest scientist? Unfortunately we have little insight into the rates of changes of forest soils at a time scale of one to a few decades. This is the timescale for the next three case studies in this chapter. The small number of case studies currently available (Lawrence et al., 2013) may not be able to provide the robust insights we need, but they do suffice to give us a key overall insight: forest soils commonly change in major ways at times scales of decades. A working group focused on examining rates of change in forest soils will provide much greater insights in coming decades (see Richter et al., 2011; Lawrence et al., 2016). One of the best characterized case studies comes from the Calhoun Experimental Forest in the Sumter National Forest in eastern South Carolina, USA (Richter et al., 1999, 2014; Fig. 2.3). The historical forests that developed on this site over recent millennia were composed of various hardwood species (oaks, hickories, and others), with some conifers (including shortleaf pine and loblolly pine). European colonization of the landscapes led to forest clearing and two centuries of intensive soil exploitation for cotton, tobacco, corn, wheat, and livestock. The soil suffered massive erosion, and even subsistence-level agriculture was untenable or only marginally successful by the mid-20th century: “Nowhere in the country have hydrological processes in the soil been altered by past land use to a greater extent than in the South Carolina Piedmont” (Lou Metz, quoted in Richter et al., 2014). The USDA Forest Service established the Calhoun Experimental Forest to develop knowledge that would support restoration of soils and forests. Could the degraded soils of the southeastern US provide the water and nutrients required by a planted forest, and could the trees restore soil fertility? Seedlings of loblolly pine were planted, and soil samples were collected and analyzed. The scientists had the great foresight to archive samples that allow the changes in the soil to be traced across decades. The pine forest grew very well, attesting the trees’ ability to cope with challenging conditions. The repeated soil sampling and archiving provided a suite of insights, only some of which might have been predicted: •
The upper mineral soil did accumulate organic matter and carbon (C), but these gains were matched by losses from the deeper soil;
Case study 3: can trees heal soils? The Calhoun forest experience
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FIG. 2.3 Soils degraded by two centuries of agricultural land use (left photo of pine seedlings planted in the Calhoun Experimental Forest) can support reforestation. Five decades of forest growth (such as in the forest on the right) substantially change the soils, but trees could not miraculously reverse all aspects of soil degradation. (Left) From archives of the USDA Forest Service; Richter et al. (2014).
• • •
• •
• •
•
Within each mineral soil horizon, relatively small changes in soil C pools over time did not indicate a lack of activity, but rather a close balance between high rates of C input and C loss; The overall change in soil carbon (C) storage in the soil came from the development of a massive O horizon; Reforestation led to strong soil acidification of the mineral soil, largely from the net movement of soil cations into trees and the O horizon. Iron and aluminum increasingly dominated the exchange complex of the mineral soil, and relatively strong-acid organic compounds dominated in the O horizon; The accumulation of calcium (Ca) in the trees and O horizon was offset by depletion of exchangeable Ca in the mineral soil; In contrast to Ca, potassium (K) accumulation in the trees and O horizon totaled about 20 times the exchangeable K in the soil, indicating that weathering replenished this pool as rapidly as it was used; One of the most important elements driving soil change was iron (Fe), with oxidation, reduction, and solution movement reshaping the soil; The mass balance of nitrogen (N) provided one of the best tests of whether unexpected sources or losses of N might alter ecosystems, and no evidence was found for unexpected fluxes; and finally, Sampling only portions of the forest (such as the trees) and soils (such as the 0e30 cm depth mineral soil) would have missed much of the biogeochemical changes that occurred in 5 decades.
How much value do these insights provide for understanding forest soil change in other places and in other conditions? The statistical power would be near 0, as the replication of plots within a single site provides no degrees of freedom to assess a population of inference that is larger than this single site. The non-statistical value, in contrast, is extremely high. The observed changes were related to processes that explained the overall changes. For example, the depletion of soil exchangeable Ca
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CHAPTER 2 Forest soils in the Anthropocene
clearly indicated that depletion should be expected when mineral phases are not present to restock the supplies through weathering. Where weatherable elements are present, rates of weathering can keep up with accumulation rates in trees (as for K in this case). We can strongly infer that sustaining long-term fertility of the soils, even for low-demanding pine trees, will depend on how the nutrient capital of the site is conserved. The changes in soil C storage have high value in underscoring that O horizons are often the most important horizon in forest soils. Further, relatively small changes in C storage do not reflect robust, stable pools of soil C, but the close balance between high rates of input and loss. High flux rates indicate we should not be surprised if soil C changed rapidly or slowly, as the net size of the pools of stored C could be changed by relatively small changes in these fluxes.
Case study 4: soil changes from intensive silviculture across Brazil, with a side trip to Hawaii Even the most fascinating insights from a well-developed case study cannot tell us about rates of change that occur in other forests. This sort of knowledge requires sampling of a larger population of sites across landscapes or regions. Perhaps the “hardest working” forest soils in the world are found in Brazil, where intensive silviculture produces crops of 30þ m tall trees in just 6 years (Fig. 2.4). The quantity of wood that might be harvested from the Calhoun forest pine plantations once every 50 years would be less than the wood removed every 6 years in Eucalyptus plantations in Brazil. Few forest soils could supply the nutrients for this rate of growth over multiple generations, and intensive fertilization is required. If fertilization restocks the soil nutrient supplies, can the fast-growing trees sustain soil organic matter and C? Cook et al. (2014) tracked changes in total soil C in the 0e30 cm depth of over 300 sites across a 1200-km latitudinal gradient in Brazil for 20 years (3 rotations). The overall rate of change averaged 0.2 Mg ha1 yr1, out of a total C stock of 29 Mg ha1. The trend was not uniform across rotations or regions. The most tropical region (Bahia) showed no net change. In the two subtropical regions, one showed no change, and the other a more substantial loss (0.9 Mg ha1 yr1). Most of the loss appeared to occur in the first rotation, with no significant change in later rotations. Across all sites and periods, the best predictor of change in soil C was the amount of rain during the dry season. An increase of 100 mm yr1 in dry-season rain was associated with a 0.5 Mg ha1 yr1 increase in soil C. The documented pattern leaves many open questions about the processes that led to the overall average outcome of small changes in soil C in response to intensive silviculture. Did the pattern result from small inputs of C to the soil balanced by small outputs, or were the fluxes offsetting but large? Would the effect have been the same if intensive fertilization was not included in the silviculture? Would the effect have been different for another tree species? How much of the C story was determined by the trees themselves or by the soil biotic community? The network study was not designed to answer questions about processes behind the patterns, but again some insights are possible from a single site where large budgets supported detailed investigations. A plantation of Eucalyptus trees was established in Hawaii on soils that were formerly cropped with sugarcane. The carbon isotope ratios of cane-derived organic matter differ from organic matter derived from trees, allowing the change in soil C to be partitioned between loss of old soil C (labeled naturally by the cane organic matter) and the gain of new C from trees. The low spatial variability in
Case study 4: soil changes from intensive silviculture across Brazil
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FIG. 2.4 Intensive silviculture of Eucalyptus plantations in Brazil (upper) yield an average of 250 m3 ha1 of stemwood in 6-year rotations, across very different soil types (sandy Entisol lower left, clayey Ultisol middle). Permanently marked locations (lower right) in over 300 stands were used by Cook et al. (2014) to measure changes in soil C over the course of 3 rotations.
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CHAPTER 2 Forest soils in the Anthropocene
the ash-derived soil allowed very precise measurement of changes in soil C over a single 8-year rotation (Binkley et al., 2004). The initial soil content of C was very high, with 13.8 kg m2 in the O horizon plus 0e45 cm depth mineral soil. No net change occurred over 8 years, because a small rate of loss of old soil cane-derived C (0.144 kg m2 yr1) was matched by a gain of new tree-derived soil C (0.136 kg m2 yr1). Very heavy additions of inorganic nitrogen (N) fertilizer had precisely no effect on these fluxes. However, a nearby plantation on the same soil type showed that soil C changes differed very much with choice of tree species planted. Soils beneath N-fixing Falcataria trees accumulated 0.9 kg C m2 yr1 compared to Eucalyptus plots (Kaye et al., 2000). About 0.5 kg C m2 yr1 accumulated from C recently added by the N-fixing trees, and an additional 0.4 kg m2 yr1 remained from old cane-derived C. Why did the N-fixing trees increase soil C when inorganic N fertilization did not? The microbial communities were very different beneath the two tree species, with higher biomass of fungi and lower counts of bacteria under Eucalyptus. The tree species were also associated with very different soil animal communities; earthworm densities were several-fold higher under the N-fixing Falcataria (Zou, 1993). The Hawaii experiments demonstrate how a pattern can be explained by investigating processes, but then the insights from those processes become patterns themselves in want of further process-based investigation. The statistical inferences from the Eucalyptus study with 300 sites were far stronger than in the pine reforestation study or the comparison of Eucalyptus and N-fixing Falcataria at single sites. The in-depth investigations that were possible at the single sites were not feasible (at least budgetarily) across the 300 Eucalyptus sites. Forest soil science clearly benefits from experimental designs at both ends of the spectrum, from single sites to networks across a thousand km. But would it be possible to develop an experimental design that would have some of the power of both single-site intensity and many-site representativeness?
Case study 5: detecting soil change across Sweden The pine and spruce forests of Sweden (Fig. 2.5) take an average of about a century to accumulate the mass of wood found in a 6-year-old Eucalyptus stand in Brazil or a 50-year-old pine stand in South Carolina. The forest soils of Sweden are all very young and many are shallow, forming after the melting of glaciers about 10,000 years ago. Swedish soils tend to be quite stony, posing major challenges for quantitative sampling of soil characteristics. The Swedish Survey of Forest Soils ambitiously samples soils at the permanent plots used for forest inventory across the country (MarkInfo, 2018). About 2000 forest sites are resampled on a decadal basis, with some records extending 5 decades. A wide range of information is collected at each site, including vegetation, O horizon, and mineral soils. Much of the data is available online, including maps of soil characteristics and some changes over time. The work has been quite valuable, including providing an empirical basis for testing models that predict storage and dynamics of forest soil C (Ortiz et al., 2013). An example of the information derived from this work shows that C concentrations in B horizons generally decline from the southwest to the northeast (Fig. 2.5). The pH of forest horizons tends to increase across the same geographic gradient as base saturation of the exchange complex increases. This level of mapping can show associations among variables, but would not indicate if the pattern of pH derived from the influence of carbon or base saturation, from the influence of C accumulation on base saturation (through higher exchange capacity), or from other factors.
Case study 5: detecting soil change across Sweden
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FIG. 2.5 Forest soils Sweden with high concentrations of C in the B horizon (left) tend to have lower pH (middle) and lower base saturation (right). National Forest Soils Inventory datasbase, MarkInfo (2018), http://www-markinfo.slu.se/, reprinted from Binkley and Ho¨gberg (2016).
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CHAPTER 2 Forest soils in the Anthropocene
Repeated sampling allows investigation into rates of soil change, and issues of quality assurance in sampling and analysis are fundamentally important. An analysis of change in the pH of O horizons over 3 decades indicated substantial acidification of forest soils across the country (Wilander and Lundin, 1999), and most of the changes appeared to warrant statistical confidence (Fig. 2.6). A thorough assessment of the patterns of change over time can look at possible confounding factors in a resampling program. In this case, the decline in pH was associated with an increase in base saturation, which would not be expected if acidification was driven by acidic deposition (Binkley and Ho¨gberg, 2016). What could drive down pH while increasing base saturation? This combination of changes would indicate that the “acid strength” of the soil surfaces increased, and indeed the C concentration of the O horizon samples increased over time. An apparent strong change in the acid strength of organic
FIG. 2.6 The pH of O horizons appeared to decrease in about half of the forests in Sweden over three decades, with the large declines in southern, west central, and north eastern Sweden (significance of change: *, p < 0.05; **, p < 0.01; ***, p < 0.001, with red [gray in print version] indicating declines and blue [dark gray in print version] indicating increase). MarkInfo (2018), National Forest Soils Inventory datasbase, http://www-markinfo.slu.se/, reprinted from Binkley and Ho¨gberg (2016).
Moving into the future of forest soils
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compounds in the O horizon may have resulted from an inconsistent identification of the boundary between the O and A horizons across the decades. The apparent acidification of the O horizon over time may not indicate that the chemistry of the O horizon itself actually changed; the apparent pattern may be only an artifact of the challenge of keeping sampling techniques consistent across decades.
Moving into the future of forest soils A look through the history of soil science shows that conceptualizations were at the heart of the science’s development. The “science” developed in the late 1800s and early 1900s when actual information on real soils was limited. In the absence of strong evidence, concepts were developed to explain how soils developed, and why different parts of the Earth’s surface developed such different soils. Simple assumptions are often appealing, but are not always helpful. Early concepts about soil development were sometimes expressed in ways that gave them the aroma of science, but without the substance. The soil scientist Vasily Dokuchaev (1899, 1951) appears to be the first person to represent soil development in the form of an equation: S [ f ðcl; o; pÞto
where S ¼ soil, cl ¼ climate, o ¼ organisms, p ¼ geologic substrate, and to is a measure of relative age. Three decades later, an ecologist (Shaw, 1930) wrote about “Potent Factors of Soil Formation” in the journal Ecology, and he created his own equation (without reference to Dokuchaev): S [ MðC D VÞT D D
where S ¼ soil, M ¼ parent material, C ¼ climate, V ¼ vegetation, T ¼ time and D ¼ deposition or erosion. Shaw may not have expected that the equation would actually be quantifiably tested. Wilbert Weir’s classic text (1936) highlighted soil formation and the “conjoint” action of soil forming agents, but he did not seem to expect that expression of soil development in the form an equation would have actual value. Hans Jenny’s 1941 book on soil formation had the strongest confidence in an equation-format for soil development. Jenny proposed a modification of Dokuchaev’s equation, which he felt could be quantified because the soil forming factors did not interact with each other. This non-interacting assumption probably reflected his understanding of the state of mathematical approaches, where differential equations would have limited the opportunity for examining interactions even if they were important. Two decades after Jenny’s book, Marlin Cline’s classic paper on The Changing Model of the Soil (1961) concluded “I would cite the increasingly quantitative character of our model as the accomplishment of greatest impact over the past 25 years.” Cline was not referring to hopeful equations that covered the full suite of soil development, but less ambitious, quantifiably measurable features. Five decades later Richter and Yaalon (2011) updated Cline’s essay, and they focused very heavily on quantifiable aspects of soils. Their single issue of a conceptual nature was that we can no longer conceive of soils as “natural bodies,” but as complex systems that are now under increasing human influence. We have essentially reached a point where applications and advances in forest soil science can be based on quantification across time and space, without expectations that nebulous concepts will be a key sources of insight.
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CHAPTER 2 Forest soils in the Anthropocene
How will forest soils change into the future? The answers to this question will develop in the context of the soil processes and patterns described in this chapter, and a wealth of details follow in later chapters. Three broad issues are likely to be important for a majority of the world’s forest soils: changing climate, changing species, and changing rates of nutrient inputs and outputs. Future climates will be different from those that shaped the soils we inherited. We know that chemical reactions generally speed up when temperature increases, so it might seem that warming climates will increase rates of decomposition and decrease stocks of carbon in soils. Forests are not so simple, of course. Increasing temperatures will change growth rates as well as decomposition rates, and most of the C that accumulates in soils is stabilized in a variety of ways that may or may not respond to temperature (Binkley and Fisher, 2020). Perhaps current patterns of soil C across geographic gradients in temperature could give a simple, general expectation for warmer future soils. Global-scale tallies of the C content of tropical, temperate, and boreal soils clearly show that warmer sites store more soil C (Jobba´gy and Jackson, 2000). At the scale of a single country, the C content of Swedish forest soils shows a similar trend, declining by half from the warmer southern portions of the country to the colder northern landscapes (Berggren Kleja et al., 2010). This pattern of increasing soil C with increasing temperature does not seem to be universal, however. A temperature gradient in Hawaii (Giardina et al., 2014) and a comparison of hardwood and pine forests across the eastern United States found no relationship between soil C and temperature (Fissore et al., 2009). Across the contiguous United States, forest soils show lower C with increasing temperature (Domke et al., 2017), contrary to the global pattern. The forest soils across Brazil do show a significant decline in carbon concentration of the 0e20 cm mineral soil (Fig. 2.7), with the temperature effect appearing to become weaker with higher temperatures. However, this correlation between temperature and soil C across Brazil should not be interpreted as a causal relationship, especially as temperature simply seems to covary with factors (rainfall and clay content) that account for a much higher proportion of the variation across the 1400 locations. Perhaps the most defensible conclusion about future stocks of C in forest soils is that the evidence is too variable to support inferences of a simple, overall pattern with any change in temperature, either across sites or within sites. A second major driver of the development of future forest soils comes from the changing species composition in response to human activities, both intended and unintended. Tree species differ substantially in their influences on forest soils (Binkley and Fisher, 2020), and managed forests typically aim for large areas dominated by one species (or in some cases, a few species). The intentional choice of species in managed forests will lead to soils that reflect the dominant species rather than the suite of species that historically shaped soils. Two unintentional changes in species composition are the extirpation of native species and invasion of exotic species. Introductions of exotic insects and diseases have dramatically reduced some of the dominant species of forests in the eastern United States, including losses of American chestnut, American Elm, eastern hemlock, and various species of ash. Forest soils undoubtedly change when dominant species are removed, but these changes are complex and not well understood. Invasive species have great potential to change forest soils, especially when the species increase fire (such as exotic grass in Hawaii, Litton et al., 2006), reduce fire (exotic cattle grazing in the American Southwest, Covington and Moore, 1994), or substantially increase inputs of nitrogen (such as invasive Falcataria moluccana in Hawaii, Hughes and Denslow, 2005).
How will forest soils change into the future?
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FIG. 2.7 Across the vast country of Brazil, forests experience mean annual temperatures from about 16 to 28 C. The temperature gradient is associated with a very gradual decline in the C concentration of the 0e20 cm soil depth (upper graph) across 1400 sampling locations. A temperature increase from 20 to 25 C is associated with a decline from 1.6% C to 1.5% C. However, the apparent temperature relationship “effect” is only a result of covariation with factors that account for a great proportion of the variation among the sites, such as the combined influence of annual rainfall and clay concentration (lower graph). Any conclusion about geographic patterns in temperature driving changes in soil C would be spurious. Map and data provided by Clayton Alcalde Alvarez, Suzano Pulp and Paper.
The third major factor shaping future forest soils is the addition and removal of nutrients. The biogeochemistry of forests entails relatively small inputs and outputs of nutrients, and the growth of most forests is limited by the supply of one or more nutrient elements (Binkley and Fisher, 2020). Intensive forest management often includes fertilization; a typical application of phosphorus fertilizer represents the equivalent of a century or millennium of normal input of P. The burning of fossil fuels has an unintended consequence of increasing the atmospheric deposition of N to forests by several-fold across very large portions of the Earth. Forest harvest entails some amount of removal of an ecosystem’s nutrient capital, in excess of historical background rates of loss. These issues of nutrient gains and losses will be important in shaping the future soils of most forests, but the various processes and rates of nutrient changes in forests are too complex and variable among forests for any simple generalization (local details will always be important). At a broader scale, the quantitative domain of forest soil science will change into the future. It will be enriched by ever increasing data on soil conditions, both across landscapes and over time. It will
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CHAPTER 2 Forest soils in the Anthropocene
give us more powerful representations of average patterns (how soil conditions tend to be shaped), as well as the details needed for understanding which drivers might lead a particular soil to be above or below the average expectation. However, we will never have all the knowledge needed to understand each part of a landscape in detail. Soil management will always find value in acknowledging some irreducible kernel of uncertainty, and responding with some humility. The future forest soil science will have explicit consideration of how humans are shaping the soils as we accelerate into the Anthropocene (Richter et al., 2015).
Acknowledgments The chapter was substantially improved by insights provided by Matt Busse, Chris Johnson, Dale Johnson, Dave Morris, and Dan Richter.
References Berggren Kleja, D., Olsson, M., Syensson, M., Jansson, P.-E., 2010. Soil C dynamics in Swedish forest soils e gradients from south to north. In: Jandl, R., Olsson, M. (Eds.), Cost Action 639, Greenhouse Gas Budget of Soils under Changing Climate and Land Use (BurnOut). Federal Research and Training Centre for Forests, Natural Hazards and Landscape (BFW), Vienna, pp. 81e84. Binkley, D., 2015. Commentary: ecosystems in four dimensions. New Phytologist 206, 883e885. Binkley, D., Fisher, R.F., 2020. Ecology and Management of Forest Soils, fifth ed. Wiley, New York. Binkley, D., Ho¨gberg, P., 2016. Revisiting the influence of nitrogen deposition on Swedish forests. Forest Ecology and Management 368, 222e239. Binkley, D., Kaye, J., Barry, M., Ryan, M.G., 2004. First rotation changes in soil carbon and nitrogen in a Eucalyptus plantation in Hawaii. Soil Science Society of America Journal 68, 1713e1719. Chadwick, O.A., Gavenda, R.T., Kelly, E.F., Ziegler, K., Olsond, C.G., Elliott, W.C., Hendricks, D.M., 2003. The impact of climate on the biogeochemical functioning of volcanic soils. Chemical Geology 202, 195e223. Cline, M.G., 1961. The changing model of soil. Soil Science Society of America Proceedings 25, 442e446. Cook, R.L., Stape, J.L., Binkley, D., 2014. Soil carbon dynamics following reforestation of tropical pastures. Soil Science Society of America Journal 78, 290e296. Covington, W.W., Moore, M.M., 1994. Southwestern ponderosa forest structure: changes since Euro-American settlement. Journal of Forestry 92, 39e47. Dokuchaev, V.V., 1899. Report to the Transcaucasian Statistical Committee on Land Evaluation in General and Especially for the Transcaucasia. Horizontal and Vertical Soil Zones. Tipogr. kantselyarii Glavnonachal’stvuyushchego grazhdanskoi chast’yu na Kavkaze, Tiflis [in Russian]. Dokuchaev, V.V., 1951. Writings, vol. 6. Akademia Nauk, Moscow, p. 381. Domke, G.M., Perry, C.H., Walters, B.F., Nave, L.E., Woodall, C.W., Swanston, C.W., 2017. Toward inventory-based estimates of soil organic carbon in forests of the United States. Ecological Applications 27, 1223e1235. Fissore, C., Giardina, C.P., Swanston, C.W., King, G.M., Kolka, R.K., 2009. Variable temperature sensitivity of soil organic carbon in North American forests. Global Change Biology 15, 2295e2310. Giardina, G.P., Litton, C.M., Crow, S.E., Asner, G.P., 2014. Warming-related increases in soil CO2 efflux are explained by increased below-ground carbon flux. Nature Climate Change 4, 822e827. Hughes, R.F., Denslow, J.S., 2005. Invasion by a N2-fixing tree alters function and structure in wet lowland forests of Hawaii. Ecological Applications 15, 1615e1628.
References
25
Jenny, H., 1941. Factors of Soil Formation: A System of Quantitative Pedology. McGraw-Hill, New York. Jenny, H., 1961. Derivation of state factor equations of soils and ecosystems. Soil Science Society of America Proceedings 25, 385e388. Jenny, H., 1989. In: Maher, D., Stuart, K. (Eds.), Hans Jenny, Soils Scientist, Teacher, and Scholar. Oral History (1979e1983). The Bancroft Library, University of California, Berkeley. Jenny, H., Arkley, R.J., Schultz, A.M., 1969. The pygmy forest-podsol ecosystem and its dune associates of the Mendocino Coast. Madron˜o 20, 60e71. Jobba´gy, E.G., Jackson, R.B., 2000. The vertical distribution of soil organic carbon and its relation to climate and vegetation. Ecological Applications 10, 423e446. Kaye, J., Resh, S., Kaye, M., Chimner, R., 2000. Nutrient and carbon dynamics in a replacement series of Eucalyptus and Albizia trees. Ecology 81, 3267e3273. Lawrence, G.B., Fernandez, I.J., Richter, D.D., Ross, D.S., Hazlett, P.W., Bailey, S.W., Ouimet, R., Warby, R.A.F., Johnson, A.H., Lin, H., Kaste, J.M., Lapenis, A.G., Sullivan, T.J., 2013. Measuring environmental change in forest ecosystems by repeated soil sampling: a North American perspective. Journal of Environmental Quality 42, 623e639. Lawrence, G.B., Fernandez, I.J., Hazlett, P.W., Bailey, S.W., Ross, D.S., Villars, T.R., Quintana, A., Ouimet, R., McHale, M.R., Johnson, C.E., Briggs, R.D., Colter, R.A., Siemion, J., Bartlett, O.L., Vargas, O., Antidormi, M.R., Koppers, M.M., 2016. Methods of soil resampling to monitor changes in the chemical concentrations of forest soils. Journal of Visualized Experiments 117, e54815. https://doi.org/10.3791/54815. Litton, C.M., Sandquist, D.R., Cordell, S., 2006. Effects of non-native grass invasion on aboveground carbon pools and tree population structure in a tropical dry forest of Hawaii. Forest Ecology and Management 231, 105e113. Logan, W.B., 2007. Dirt: The Ecstatic Skin of the Earth. Norton, New York. MarkInfo, 2018. Swedish Survey of Forest Soils. http://www-markinfo.slu.se/eng/index.html. ˚ gren, G.I., Karltun, E., 2013. Ortiz, C.A., Liski, J., Ga¨rdena¨sa, A.I., Lehtonen, C., Lundblad, M., Stendahl, J., A Soil organic carbon stock changes in Swedish forest soilsda comparison of uncertainties and their sources through a national inventory and two simulation models. Ecological Modelling 251, 221e231. Richter, D.B., Yaalon, D.H., 2011. “The changing model of soil” revisited. Soil Science Society of America Journal 76, 766e778. Richter, D.D., Markewitz, D., Trumbore, S.E., Wells, C.G., 1999. Rapid accumulation and turnover of soil carbon in a reestablishing forest. Nature 400, 56e58. Richter, D.D., Bacon, A.R., Billings, S.A., Binkley, D., Buford, M., Callaham, M.A., Curry, A.E., Fimmen, R.L., Grandy, A.S., Heine, P.R., Hofmockel, M., Jackson, J.A., Lemaster, E., Li, J.W., Markewitz, D., Mobley, M.L., Morrison, M.W., Strickland, M.S., Waldrop, T., Wells, C.G., 2014. Evolution of soil, ecosystem, and critical zone research at the Calhoun experimental forest. In: Hayes, D.C., Stout, S., Hoover, A., Crawford, R. (Eds.), Research for the Long-Term: The Interplay of Societal Need and Research on USDA Forest Service Experimental Forests and Ranges. Springer Verlag, NY. Richter, D.D., Bacon, A.R., Mobleym, M.L., Richardson, C.J., Andrews, S.S., West, L., Wills, S., Billings, S., Cambardella, C.A., Cavallaro, N., DeMeester, J.E., Franzluebbers, A.J., Grandy, A.S., Grunwald, S., Gruver, J., Hartshorn, A.S., Janzen, H., Kramer, M.G., Ladha, J.K., Lajtha, K., Liles, G.C., Markewitz, D., Megonigal, P.J., Mermut, A.R., Rasmussen, C., Robinson, D.A., Smith, P., Stiles, C.A., Tate III, R.L., Thompson, A., Tugel, A.J., van Es, H., Yaalon, D., Zobeck, T.M., 2011. Human-soil relations are changing rapidly: proposals from SSSA’s new cross-divisional working group on soil change. Soil Science Society of America Journal 75, 2079e2084. Richter, D.D., Bacon, A.R., Brecheisen, Z., Mobley, M.L., 2015. Soil in the Anthropocene. pp. 1e11. In: Soil Change Matters 2014. IOP Conf. Series: Earth and Environmental Science, vol. 25. https://doi.org/10.1088/ 1755-1315/25/1/012010. Richter, D.D., Billings, S.A., 2015. “One physical system”: Tansley’s ecosystem as Earth’s critical zone. New Phytologist 206, 900e912.
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Shaw, C.F., 1930. Potent factors in soil formation. Ecology 11, 239e245. Stewart, B.W., Capo, R.C., Chadwick, O.A., 2001. Effects of rainfall on weathering rate, base cation provenance, and Sr isotope composition of Hawaiian soils. Geochimica et Cosmochimica Acta 65, 1087e1099. van Breemen, N., 1993. Soils as biotic constructs favouring net primary productivity. Geoderma 57, 183e211. Weir, W.W., 1936. Soil Science: Its Principles and Practice. J.B. Lippincott, Chicago. Wilander, A., Lundin, L., 1999. Recovery of surface waters and forest soils in Sweden. In: Warfvinge, P., Bertills, U. (Eds.), Recovery from Acidification in the Natural Environment: Present Knowledge and Future Scenarios. Swedish Environmental Protection Agency Report 5034, Stockholm, pp. 53e66. Xu, H.-H., Berry, C.M., Stein, W.E., Wang, Y., Tang, P., Fu, Q., 2007. Unique growth strategy in the Earth’s first trees revealed in silicified fossil trunks from China. Proceedings of the National Academy of Sciences of the United States of America. https://doi.org/10.1073/pnas.1708241114. Zou, X.M., 1993. Tree species effects on earthworm density in tropical tree plantations in Hawaii. Biology and Fertility of Soils 15, 35e38.
Forest soils in the Anthropocene
2 Dan Binkley*
School of Forestry, Northern Arizona University, Flagstaff, AZ, United States * Corresponding author
ABSTRACT The formation of forest soils continues across centuries and millennia. Many of the key characteristics of soils develop very slowly. The slow, long-term development of soils is coupled with much faster processes that substantially change soils over a period of years and decades. A century ago, soil scientists looked for explanations of soil development using concepts limited by simple assumptions and expectations. Quantification became more important as the 20th century advanced, with concepts retreating into the background. Case studies illustrate both these long-term and more-rapid processes and patterns of change. Detecting, monitoring, understanding and predicting change will be a key focus for forest soil science and management. In the Anthropocene, the breadth of human influences will be the major soil-forming factor in forest soils.
A world without soils Imagine a world without soil. Soil, as we know it, evolves from the interaction of life, chemistry and physics. For the majority of Earth’s history, there was no soil because there was no (or very little) biotic presence across the terrestrial face of our planet. The Earth had an abiotic skin, and the ecstatic nature of soil (Logan, 2007) has been possible for only 10% of Earth’s existence. Prior to the expansion of life into terrestrial geography, the skin of the Earth would have been substantially modified from its original geology. Primary minerals are generally unstable at the temperature, pressure, and oxygen concentrations at the Earth’s surface. Mineral weathering would have proceeded under the influence of oxidation and carbonic acid hydrolysis, both of which were facilitated by atmospheric legacies of the biogeochemistry of marine ecosystems. The hydrological cycle removed some of the products of geochemical reactions, leaving behind residual materials and secondary minerals. The texture of the regolith depended on the chemistry and physics of in situ reactions along with losses and deposition of transported material. The three-dimensional structure of the regolith would have been simply related to texture, with some additional features developing for sodic materials and shrink-swell clays. Long-term development of the characteristics of the regolith would have been the balance between processes increasing entropy (oxidation, mineral weathering, downhill flow of water) and geologic and hydrologic processes that provided continuing opportunities for entropic processes (geologic uplift, replenishment of atmospheric chemistry by marine ecosystems, and hydrology driven by sun and wind).
Global Change and Forest Soils. https://doi.org/10.1016/B978-0-444-63998-1.00002-1 Copyright Ó 2019 Elsevier B.V. All rights reserved.
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CHAPTER 2 Forest soils in the Anthropocene
The world of opportunities expanded when the one-way entropic stories of regolith formation were expanded into new dimensions by the influence of terrestrial life. Life brought three major potentials for turning mere regolith into soils. First, the coupling of plant life and decomposer communities created an engine that took carbon dioxide from the atmosphere and injected it into the soils at concentrations ten to one hundred times greater than the concentration in the atmosphere. The high partial pressure of carbon dioxide in soils pushed the carbonic acid equilibrium, dramatically increasing rates of mineral weathering. Second, undecomposed organic molecules had the potential to change patterns of ion transport within and beyond the soil. Much of the classic horizonation that lies at the heart of soil taxonomy derives from the influence of organic chemistry on the dissolution and precipitation of ions (particularly aluminum and iron). The third potential was perhaps the most important: the three-dimensional alteration of soils by plants and their biotic communities. Plant roots couple soils to the atmosphere through their water conducting systems, and with the movement of water comes the movement of nutrients and other ions. The death of plant tissues places these elements near the surface of the soil, essentially reversing the one-way downward flow driven by entropy. The persistence of organic matter in the soil is also a key aspect of the structure of soils, from the gluing together of particles into microaggregates, to macrochannels of decaying roots, to the mixing activities of soil animals. The full ensemble of chemistry, physics and life creates soils, with the potential for increasing the entropy in the universe by harnessing high-energy photons to split water and create stores of energy bound in organic molecules. These molecules embody the potential gradients that develop and operate in soils, with entropy increasing as energy-rich organic molecules oxidize to low-energy states of water and carbon dioxide. But why does this overall train of energy not run out as racing reactions in the soil increase entropy? Why do large, concentrated pools of plant-required nutrients remain in soils rather than dissipate loosely into the environment? Accumulation of these pools of organic matter and nutrientenriched upper soil layers might not be surprising if activity were slow and time spans were short. However, rates of turnover of pools are actually quite rapid relative to the time course over which soils develop. Could it be that soils somehow developed in concert with plant life such that interactions developed in mutualistic ways, as the evolution of an integrated, complex adaptive system? This idea was explored in a special issue of Geoderma, with the overall conclusion that the logic of selection at the scale of soil/plant systems did not appear tenable (van Breemen, 1993). Instead of simple high-entropy possibilities, we find ourselves contemplating truly complex, dynamic, and yet not totally unstable systems of soils interacting with plants that drive further changes in soils. The dynamics over timescales of seconds and days can be as large as those across seasons and years. Yet the long-term major changes in soils often result from very slow accumulations of very small net changes, such as the gradual reduction in soil hydraulic conductivity that can result from clay translocation or iron oxide deposition, which further reduces soil aeration, slows biogeochemical cycles, and lowers plant growth. This is the context humans encounter when engaging with soils to support growth of plants: a fascinating suite of interacting processes (inorganic and organic) across scales of time and space in ways that lead to very non-static outcomes. Forest soils began to evolve on Earth with appearance of trees in just the most recent 10% of Earth’s history (Xu et al., 2007). The evolution of forest soils across the Earth would have changed substantially with the evolution of trees and other life. Major influences of humans on forest soils developed only in the past few millennia, resulting from changing fire patterns (and subsequent tree species success), livestock grazing (altering tree species composition), harvesting, and converting forest soils into
Case study 1: one step on Hans Jenny’s ecological staircase
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agricultural soils. At the beginning of the Anthropocene most of Earth’s forested soils have a weak or strong legacy of human influences, and our influences will only increase. How will the forest soils of the future be shaped? The chapters of this book develop a wide range of insights. This chapter introduces five cases studies to illustrate some major themes of the changing forest soils of our planet: • • • • •
how very small changes lead to major differences in soil evolution; how soil change is not simple, because of confounding interacting factors; how conversion to agriculture and back to forest changed a particular soil; how intensive silviculture influenced soils across a large region; and how multiple challenges limit our ability to detect important soil changes at the scales that really matter.
Case study 1: one step on Hans Jenny’s ecological staircase Geologic uplift is an important process in rejuvenating soils, bringing fresh low-entropy materials into contact with oxygen and life. Hans Jenny’s favorite example of uplift and soil development was an “ecological staircase” on the Pacific Coast of northern California (Jenny et al., 1969; Jenny, 1989). Gradual uplift provided ecosystems and soils that developed on the same parent material, but with varying distance along the dimension of time. One of the steps in the staircase has soils that have been developing for half a million years or so. The portions of the step that are close to the edge of the terrace sustain productive forests of redwood and Douglas-fir, on fertile, well-drained soils (Fig. 2.1). Farther from the edge of the terrace are stunted forests of Bishop pine, Bolander pine, and Mendocino cypress, eking an existence from poorly drained, infertile soils. Each square meter of the terrace has received something like 500 million liters of rainwater over the eons. The water drained away near the edges of the terrace, but greater distances away from the terrace edges led to somewhat more poorly drained conditions leading to a series of biogeochemical developments. The geochemical and organic weathering of minerals generated iron and aluminum oxides, and a large portion of these oxides was transported away from the well-drained soil. The poorly drained backsides of the terrace experienced an accumulation of precipitated oxides which formed iron pans not far below the surface. The iron pans restrain biological activity by sealing off the nutrient supply in the deeper soil from access by the roots, and further ponding of water (especially in winter) leads to poor oxygen supply for roots and soil biota. This ancient step on the ecological staircase supports two very different sorts of soils, not because of differences in parent material, climate, or the length of time for soil development. The differences stem from the spatial topographic effect of how well water could drain in relation to distance from the terrace edge. These differences were substantial across the soil profile, and even deeper into what is referred to as the Critical Zone: ecological processes in the upper soil alter biogeochemistry deep below the soil surface in ways that can feedback and alter vegetation and the upper soil (Richter and Billings, 2015). Biology always plays a major role when soils differ across topographic (and other) gradients. Factors that alter the species composition and productivity of plant communities in turn have soils reshaped by those vegetation differences. Hans Jenny once thought that soil development could
FIG. 2.1 Location: the difference a few meters can make. The ecological staircase along the coast of northern California (Jenny et al., 1969) includes this “step” that has been developing soil for about half a million years since uplift raised the terrace above the ocean. Well-drained conditions near the edge of the terrace (left) allowed the development of deeper soils with higher nutrient supplies for trees (redwood and Douglas-fir). Poorer drainage away from the terrace edge (right) restricted removal of weathered iron and aluminum, leading to a feedback effect of increasing limitation on drainage as a cemented iron pan formed. Current soil condition in the poorly drained portions of the terrace support stunted vegetation of Bishop pine, Bolander pine, and Mendocino cypress. Figure from Binkley (2015). Photos: pygmy vegetation from Sarah Bisbing, others from Ron Amundson.
Case study 2: rainfall and time in Hawaii
13
be quantified by the independent, non-interacting influence of factors (Jenny, 1941, 1961), but the circularity of cause and effect always challenges single-factor stories of the development of soils over long periods of time over landscapes.
Case study 2: rainfall and time in Hawaii Under warm tropical conditions in Hawaii, the leaching losses of calcium (Ca) from the original lava parent material might be expected to increase with increasing rainfall. This was indeed a pattern found after 170,000 years of soil development (Fig. 2.2): half or more of the original Ca remained for sites with <500 mm yr1 rainfall, and sites with more than 1500 mm yr1 lost almost all the original Ca (Chadwick et al., 2003). This simple pattern might seem to fit Jenny’s hope that soil-forming factors operate independently, but the actual story of soil development on these lava flows was not so simple. Organic matter accumulation increased with increasing rainfall, up to about 1500e2000 mm yr1 and then declined. Soils receiving about 3000 mm yr1 of rain had about the same amount of organic matter as those receiving only 500 mm yr1. Non-crystalline clay-sized particles increased asymptotically with rainfall, coupled with a decline in poorly crystallized clay-sized particles. Cation exchange capacity increased almost linearly with rainfall, but exchangeable base cations showed a threshold with a major drop off above 1200 mm yr1. The picture of soil development becomes even more complicated when considering changes over time as well as in relation to rainfall (Stewart et al., 2001). The rate of mineral weathering was initially high when the lava began weathering, especially with high rainfall (Fig. 2.2). Depletion of weatherable minerals from wet locations in the first 100,000 years of soil development resulted in subsequent weathering rates that were higher for drier portions of the rainfall gradient. The curves in Fig. 2.2 probably do not represent the actual quantitative relationships that happened over 170,000 years, because the climate was probably drier for much of that time.
FIG. 2.2 170,000 years of rainfall in Hawaii removed calcium that was originally present in a lava, with higher rainfall leading to greater losses (left). The pattern of Ca removal over time would depend on the twofold effect of rain; higher rainfall leads to higher weathering and leaching, but after some time the depletion of the original material substantially reduces weathering rates (right). The effect of rainfall was not linear for Ca depletion or for Ca weathering, and the dynamics were not uniform across time. (Left) From data in Chadwick et al. (2003). (Right) From data of Stewart et al. (2001).
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CHAPTER 2 Forest soils in the Anthropocene
Quantitative studies of soil development can provide good representations of how soils change over time in response to aspects of climate, vegetation, and other soil forming factors. These quantitative estimates may have rather limited scope because so many factors vary greatly with variable interactions. Parent material is a term that covers hundreds to thousands (or more) possibilities. The characteristics of one lava flow are not the same as another, especially as magma bodies age. One limestone is never quite like another, and there are so many kinds of granitic rock that it may not be clear where the class of granitic igneous rocks stops and other classes begin. The influence of rainfall is not consistent across temperature gradients, and the patterns probably differ based on major vegetation types. Broad scale patterns may be interesting and somewhat useful to think about, but the value for any single location, environment and time may be quite small. The characteristics of soils shift across space and through time, and we can explain some of the variation in quantitative terms, but the wonderful diversity is only partially captured in either simple or complex models.
Case study 3: can trees heal soils? The Calhoun forest experience Major characteristics of soil form over very long periods of time, such as the half-a-million years of development of the terrace soils in the ecological staircase. How dynamic are forest soils over a shorter time period, such as the career of a forest scientist? Unfortunately we have little insight into the rates of changes of forest soils at a time scale of one to a few decades. This is the timescale for the next three case studies in this chapter. The small number of case studies currently available (Lawrence et al., 2013) may not be able to provide the robust insights we need, but they do suffice to give us a key overall insight: forest soils commonly change in major ways at times scales of decades. A working group focused on examining rates of change in forest soils will provide much greater insights in coming decades (see Richter et al., 2011; Lawrence et al., 2016). One of the best characterized case studies comes from the Calhoun Experimental Forest in the Sumter National Forest in eastern South Carolina, USA (Richter et al., 1999, 2014; Fig. 2.3). The historical forests that developed on this site over recent millennia were composed of various hardwood species (oaks, hickories, and others), with some conifers (including shortleaf pine and loblolly pine). European colonization of the landscapes led to forest clearing and two centuries of intensive soil exploitation for cotton, tobacco, corn, wheat, and livestock. The soil suffered massive erosion, and even subsistence-level agriculture was untenable or only marginally successful by the mid-20th century: “Nowhere in the country have hydrological processes in the soil been altered by past land use to a greater extent than in the South Carolina Piedmont” (Lou Metz, quoted in Richter et al., 2014). The USDA Forest Service established the Calhoun Experimental Forest to develop knowledge that would support restoration of soils and forests. Could the degraded soils of the southeastern US provide the water and nutrients required by a planted forest, and could the trees restore soil fertility? Seedlings of loblolly pine were planted, and soil samples were collected and analyzed. The scientists had the great foresight to archive samples that allow the changes in the soil to be traced across decades. The pine forest grew very well, attesting the trees’ ability to cope with challenging conditions. The repeated soil sampling and archiving provided a suite of insights, only some of which might have been predicted: •
The upper mineral soil did accumulate organic matter and carbon (C), but these gains were matched by losses from the deeper soil;
Case study 3: can trees heal soils? The Calhoun forest experience
15
FIG. 2.3 Soils degraded by two centuries of agricultural land use (left photo of pine seedlings planted in the Calhoun Experimental Forest) can support reforestation. Five decades of forest growth (such as in the forest on the right) substantially change the soils, but trees could not miraculously reverse all aspects of soil degradation. (Left) From archives of the USDA Forest Service; Richter et al. (2014).
• • •
• •
• •
•
Within each mineral soil horizon, relatively small changes in soil C pools over time did not indicate a lack of activity, but rather a close balance between high rates of C input and C loss; The overall change in soil carbon (C) storage in the soil came from the development of a massive O horizon; Reforestation led to strong soil acidification of the mineral soil, largely from the net movement of soil cations into trees and the O horizon. Iron and aluminum increasingly dominated the exchange complex of the mineral soil, and relatively strong-acid organic compounds dominated in the O horizon; The accumulation of calcium (Ca) in the trees and O horizon was offset by depletion of exchangeable Ca in the mineral soil; In contrast to Ca, potassium (K) accumulation in the trees and O horizon totaled about 20 times the exchangeable K in the soil, indicating that weathering replenished this pool as rapidly as it was used; One of the most important elements driving soil change was iron (Fe), with oxidation, reduction, and solution movement reshaping the soil; The mass balance of nitrogen (N) provided one of the best tests of whether unexpected sources or losses of N might alter ecosystems, and no evidence was found for unexpected fluxes; and finally, Sampling only portions of the forest (such as the trees) and soils (such as the 0e30 cm depth mineral soil) would have missed much of the biogeochemical changes that occurred in 5 decades.
How much value do these insights provide for understanding forest soil change in other places and in other conditions? The statistical power would be near 0, as the replication of plots within a single site provides no degrees of freedom to assess a population of inference that is larger than this single site. The non-statistical value, in contrast, is extremely high. The observed changes were related to processes that explained the overall changes. For example, the depletion of soil exchangeable Ca
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CHAPTER 2 Forest soils in the Anthropocene
clearly indicated that depletion should be expected when mineral phases are not present to restock the supplies through weathering. Where weatherable elements are present, rates of weathering can keep up with accumulation rates in trees (as for K in this case). We can strongly infer that sustaining long-term fertility of the soils, even for low-demanding pine trees, will depend on how the nutrient capital of the site is conserved. The changes in soil C storage have high value in underscoring that O horizons are often the most important horizon in forest soils. Further, relatively small changes in C storage do not reflect robust, stable pools of soil C, but the close balance between high rates of input and loss. High flux rates indicate we should not be surprised if soil C changed rapidly or slowly, as the net size of the pools of stored C could be changed by relatively small changes in these fluxes.
Case study 4: soil changes from intensive silviculture across Brazil, with a side trip to Hawaii Even the most fascinating insights from a well-developed case study cannot tell us about rates of change that occur in other forests. This sort of knowledge requires sampling of a larger population of sites across landscapes or regions. Perhaps the “hardest working” forest soils in the world are found in Brazil, where intensive silviculture produces crops of 30þ m tall trees in just 6 years (Fig. 2.4). The quantity of wood that might be harvested from the Calhoun forest pine plantations once every 50 years would be less than the wood removed every 6 years in Eucalyptus plantations in Brazil. Few forest soils could supply the nutrients for this rate of growth over multiple generations, and intensive fertilization is required. If fertilization restocks the soil nutrient supplies, can the fast-growing trees sustain soil organic matter and C? Cook et al. (2014) tracked changes in total soil C in the 0e30 cm depth of over 300 sites across a 1200-km latitudinal gradient in Brazil for 20 years (3 rotations). The overall rate of change averaged 0.2 Mg ha1 yr1, out of a total C stock of 29 Mg ha1. The trend was not uniform across rotations or regions. The most tropical region (Bahia) showed no net change. In the two subtropical regions, one showed no change, and the other a more substantial loss (0.9 Mg ha1 yr1). Most of the loss appeared to occur in the first rotation, with no significant change in later rotations. Across all sites and periods, the best predictor of change in soil C was the amount of rain during the dry season. An increase of 100 mm yr1 in dry-season rain was associated with a 0.5 Mg ha1 yr1 increase in soil C. The documented pattern leaves many open questions about the processes that led to the overall average outcome of small changes in soil C in response to intensive silviculture. Did the pattern result from small inputs of C to the soil balanced by small outputs, or were the fluxes offsetting but large? Would the effect have been the same if intensive fertilization was not included in the silviculture? Would the effect have been different for another tree species? How much of the C story was determined by the trees themselves or by the soil biotic community? The network study was not designed to answer questions about processes behind the patterns, but again some insights are possible from a single site where large budgets supported detailed investigations. A plantation of Eucalyptus trees was established in Hawaii on soils that were formerly cropped with sugarcane. The carbon isotope ratios of cane-derived organic matter differ from organic matter derived from trees, allowing the change in soil C to be partitioned between loss of old soil C (labeled naturally by the cane organic matter) and the gain of new C from trees. The low spatial variability in
Case study 4: soil changes from intensive silviculture across Brazil
17
FIG. 2.4 Intensive silviculture of Eucalyptus plantations in Brazil (upper) yield an average of 250 m3 ha1 of stemwood in 6-year rotations, across very different soil types (sandy Entisol lower left, clayey Ultisol middle). Permanently marked locations (lower right) in over 300 stands were used by Cook et al. (2014) to measure changes in soil C over the course of 3 rotations.
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CHAPTER 2 Forest soils in the Anthropocene
the ash-derived soil allowed very precise measurement of changes in soil C over a single 8-year rotation (Binkley et al., 2004). The initial soil content of C was very high, with 13.8 kg m2 in the O horizon plus 0e45 cm depth mineral soil. No net change occurred over 8 years, because a small rate of loss of old soil cane-derived C (0.144 kg m2 yr1) was matched by a gain of new tree-derived soil C (0.136 kg m2 yr1). Very heavy additions of inorganic nitrogen (N) fertilizer had precisely no effect on these fluxes. However, a nearby plantation on the same soil type showed that soil C changes differed very much with choice of tree species planted. Soils beneath N-fixing Falcataria trees accumulated 0.9 kg C m2 yr1 compared to Eucalyptus plots (Kaye et al., 2000). About 0.5 kg C m2 yr1 accumulated from C recently added by the N-fixing trees, and an additional 0.4 kg m2 yr1 remained from old cane-derived C. Why did the N-fixing trees increase soil C when inorganic N fertilization did not? The microbial communities were very different beneath the two tree species, with higher biomass of fungi and lower counts of bacteria under Eucalyptus. The tree species were also associated with very different soil animal communities; earthworm densities were several-fold higher under the N-fixing Falcataria (Zou, 1993). The Hawaii experiments demonstrate how a pattern can be explained by investigating processes, but then the insights from those processes become patterns themselves in want of further process-based investigation. The statistical inferences from the Eucalyptus study with 300 sites were far stronger than in the pine reforestation study or the comparison of Eucalyptus and N-fixing Falcataria at single sites. The in-depth investigations that were possible at the single sites were not feasible (at least budgetarily) across the 300 Eucalyptus sites. Forest soil science clearly benefits from experimental designs at both ends of the spectrum, from single sites to networks across a thousand km. But would it be possible to develop an experimental design that would have some of the power of both single-site intensity and many-site representativeness?
Case study 5: detecting soil change across Sweden The pine and spruce forests of Sweden (Fig. 2.5) take an average of about a century to accumulate the mass of wood found in a 6-year-old Eucalyptus stand in Brazil or a 50-year-old pine stand in South Carolina. The forest soils of Sweden are all very young and many are shallow, forming after the melting of glaciers about 10,000 years ago. Swedish soils tend to be quite stony, posing major challenges for quantitative sampling of soil characteristics. The Swedish Survey of Forest Soils ambitiously samples soils at the permanent plots used for forest inventory across the country (MarkInfo, 2018). About 2000 forest sites are resampled on a decadal basis, with some records extending 5 decades. A wide range of information is collected at each site, including vegetation, O horizon, and mineral soils. Much of the data is available online, including maps of soil characteristics and some changes over time. The work has been quite valuable, including providing an empirical basis for testing models that predict storage and dynamics of forest soil C (Ortiz et al., 2013). An example of the information derived from this work shows that C concentrations in B horizons generally decline from the southwest to the northeast (Fig. 2.5). The pH of forest horizons tends to increase across the same geographic gradient as base saturation of the exchange complex increases. This level of mapping can show associations among variables, but would not indicate if the pattern of pH derived from the influence of carbon or base saturation, from the influence of C accumulation on base saturation (through higher exchange capacity), or from other factors.
Case study 5: detecting soil change across Sweden
19
FIG. 2.5 Forest soils Sweden with high concentrations of C in the B horizon (left) tend to have lower pH (middle) and lower base saturation (right). National Forest Soils Inventory datasbase, MarkInfo (2018), http://www-markinfo.slu.se/, reprinted from Binkley and Ho¨gberg (2016).
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CHAPTER 2 Forest soils in the Anthropocene
Repeated sampling allows investigation into rates of soil change, and issues of quality assurance in sampling and analysis are fundamentally important. An analysis of change in the pH of O horizons over 3 decades indicated substantial acidification of forest soils across the country (Wilander and Lundin, 1999), and most of the changes appeared to warrant statistical confidence (Fig. 2.6). A thorough assessment of the patterns of change over time can look at possible confounding factors in a resampling program. In this case, the decline in pH was associated with an increase in base saturation, which would not be expected if acidification was driven by acidic deposition (Binkley and Ho¨gberg, 2016). What could drive down pH while increasing base saturation? This combination of changes would indicate that the “acid strength” of the soil surfaces increased, and indeed the C concentration of the O horizon samples increased over time. An apparent strong change in the acid strength of organic
FIG. 2.6 The pH of O horizons appeared to decrease in about half of the forests in Sweden over three decades, with the large declines in southern, west central, and north eastern Sweden (significance of change: *, p < 0.05; **, p < 0.01; ***, p < 0.001, with red [gray in print version] indicating declines and blue [dark gray in print version] indicating increase). MarkInfo (2018), National Forest Soils Inventory datasbase, http://www-markinfo.slu.se/, reprinted from Binkley and Ho¨gberg (2016).
Moving into the future of forest soils
21
compounds in the O horizon may have resulted from an inconsistent identification of the boundary between the O and A horizons across the decades. The apparent acidification of the O horizon over time may not indicate that the chemistry of the O horizon itself actually changed; the apparent pattern may be only an artifact of the challenge of keeping sampling techniques consistent across decades.
Moving into the future of forest soils A look through the history of soil science shows that conceptualizations were at the heart of the science’s development. The “science” developed in the late 1800s and early 1900s when actual information on real soils was limited. In the absence of strong evidence, concepts were developed to explain how soils developed, and why different parts of the Earth’s surface developed such different soils. Simple assumptions are often appealing, but are not always helpful. Early concepts about soil development were sometimes expressed in ways that gave them the aroma of science, but without the substance. The soil scientist Vasily Dokuchaev (1899, 1951) appears to be the first person to represent soil development in the form of an equation: S [ f ðcl; o; pÞto
where S ¼ soil, cl ¼ climate, o ¼ organisms, p ¼ geologic substrate, and to is a measure of relative age. Three decades later, an ecologist (Shaw, 1930) wrote about “Potent Factors of Soil Formation” in the journal Ecology, and he created his own equation (without reference to Dokuchaev): S [ MðC D VÞT D D
where S ¼ soil, M ¼ parent material, C ¼ climate, V ¼ vegetation, T ¼ time and D ¼ deposition or erosion. Shaw may not have expected that the equation would actually be quantifiably tested. Wilbert Weir’s classic text (1936) highlighted soil formation and the “conjoint” action of soil forming agents, but he did not seem to expect that expression of soil development in the form an equation would have actual value. Hans Jenny’s 1941 book on soil formation had the strongest confidence in an equation-format for soil development. Jenny proposed a modification of Dokuchaev’s equation, which he felt could be quantified because the soil forming factors did not interact with each other. This non-interacting assumption probably reflected his understanding of the state of mathematical approaches, where differential equations would have limited the opportunity for examining interactions even if they were important. Two decades after Jenny’s book, Marlin Cline’s classic paper on The Changing Model of the Soil (1961) concluded “I would cite the increasingly quantitative character of our model as the accomplishment of greatest impact over the past 25 years.” Cline was not referring to hopeful equations that covered the full suite of soil development, but less ambitious, quantifiably measurable features. Five decades later Richter and Yaalon (2011) updated Cline’s essay, and they focused very heavily on quantifiable aspects of soils. Their single issue of a conceptual nature was that we can no longer conceive of soils as “natural bodies,” but as complex systems that are now under increasing human influence. We have essentially reached a point where applications and advances in forest soil science can be based on quantification across time and space, without expectations that nebulous concepts will be a key sources of insight.
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CHAPTER 2 Forest soils in the Anthropocene
How will forest soils change into the future? The answers to this question will develop in the context of the soil processes and patterns described in this chapter, and a wealth of details follow in later chapters. Three broad issues are likely to be important for a majority of the world’s forest soils: changing climate, changing species, and changing rates of nutrient inputs and outputs. Future climates will be different from those that shaped the soils we inherited. We know that chemical reactions generally speed up when temperature increases, so it might seem that warming climates will increase rates of decomposition and decrease stocks of carbon in soils. Forests are not so simple, of course. Increasing temperatures will change growth rates as well as decomposition rates, and most of the C that accumulates in soils is stabilized in a variety of ways that may or may not respond to temperature (Binkley and Fisher, 2020). Perhaps current patterns of soil C across geographic gradients in temperature could give a simple, general expectation for warmer future soils. Global-scale tallies of the C content of tropical, temperate, and boreal soils clearly show that warmer sites store more soil C (Jobba´gy and Jackson, 2000). At the scale of a single country, the C content of Swedish forest soils shows a similar trend, declining by half from the warmer southern portions of the country to the colder northern landscapes (Berggren Kleja et al., 2010). This pattern of increasing soil C with increasing temperature does not seem to be universal, however. A temperature gradient in Hawaii (Giardina et al., 2014) and a comparison of hardwood and pine forests across the eastern United States found no relationship between soil C and temperature (Fissore et al., 2009). Across the contiguous United States, forest soils show lower C with increasing temperature (Domke et al., 2017), contrary to the global pattern. The forest soils across Brazil do show a significant decline in carbon concentration of the 0e20 cm mineral soil (Fig. 2.7), with the temperature effect appearing to become weaker with higher temperatures. However, this correlation between temperature and soil C across Brazil should not be interpreted as a causal relationship, especially as temperature simply seems to covary with factors (rainfall and clay content) that account for a much higher proportion of the variation across the 1400 locations. Perhaps the most defensible conclusion about future stocks of C in forest soils is that the evidence is too variable to support inferences of a simple, overall pattern with any change in temperature, either across sites or within sites. A second major driver of the development of future forest soils comes from the changing species composition in response to human activities, both intended and unintended. Tree species differ substantially in their influences on forest soils (Binkley and Fisher, 2020), and managed forests typically aim for large areas dominated by one species (or in some cases, a few species). The intentional choice of species in managed forests will lead to soils that reflect the dominant species rather than the suite of species that historically shaped soils. Two unintentional changes in species composition are the extirpation of native species and invasion of exotic species. Introductions of exotic insects and diseases have dramatically reduced some of the dominant species of forests in the eastern United States, including losses of American chestnut, American Elm, eastern hemlock, and various species of ash. Forest soils undoubtedly change when dominant species are removed, but these changes are complex and not well understood. Invasive species have great potential to change forest soils, especially when the species increase fire (such as exotic grass in Hawaii, Litton et al., 2006), reduce fire (exotic cattle grazing in the American Southwest, Covington and Moore, 1994), or substantially increase inputs of nitrogen (such as invasive Falcataria moluccana in Hawaii, Hughes and Denslow, 2005).
How will forest soils change into the future?
23
FIG. 2.7 Across the vast country of Brazil, forests experience mean annual temperatures from about 16 to 28 C. The temperature gradient is associated with a very gradual decline in the C concentration of the 0e20 cm soil depth (upper graph) across 1400 sampling locations. A temperature increase from 20 to 25 C is associated with a decline from 1.6% C to 1.5% C. However, the apparent temperature relationship “effect” is only a result of covariation with factors that account for a great proportion of the variation among the sites, such as the combined influence of annual rainfall and clay concentration (lower graph). Any conclusion about geographic patterns in temperature driving changes in soil C would be spurious. Map and data provided by Clayton Alcalde Alvarez, Suzano Pulp and Paper.
The third major factor shaping future forest soils is the addition and removal of nutrients. The biogeochemistry of forests entails relatively small inputs and outputs of nutrients, and the growth of most forests is limited by the supply of one or more nutrient elements (Binkley and Fisher, 2020). Intensive forest management often includes fertilization; a typical application of phosphorus fertilizer represents the equivalent of a century or millennium of normal input of P. The burning of fossil fuels has an unintended consequence of increasing the atmospheric deposition of N to forests by several-fold across very large portions of the Earth. Forest harvest entails some amount of removal of an ecosystem’s nutrient capital, in excess of historical background rates of loss. These issues of nutrient gains and losses will be important in shaping the future soils of most forests, but the various processes and rates of nutrient changes in forests are too complex and variable among forests for any simple generalization (local details will always be important). At a broader scale, the quantitative domain of forest soil science will change into the future. It will be enriched by ever increasing data on soil conditions, both across landscapes and over time. It will
24
CHAPTER 2 Forest soils in the Anthropocene
give us more powerful representations of average patterns (how soil conditions tend to be shaped), as well as the details needed for understanding which drivers might lead a particular soil to be above or below the average expectation. However, we will never have all the knowledge needed to understand each part of a landscape in detail. Soil management will always find value in acknowledging some irreducible kernel of uncertainty, and responding with some humility. The future forest soil science will have explicit consideration of how humans are shaping the soils as we accelerate into the Anthropocene (Richter et al., 2015).
Acknowledgments The chapter was substantially improved by insights provided by Matt Busse, Chris Johnson, Dale Johnson, Dave Morris, and Dan Richter.
References Berggren Kleja, D., Olsson, M., Syensson, M., Jansson, P.-E., 2010. Soil C dynamics in Swedish forest soils e gradients from south to north. In: Jandl, R., Olsson, M. (Eds.), Cost Action 639, Greenhouse Gas Budget of Soils under Changing Climate and Land Use (BurnOut). Federal Research and Training Centre for Forests, Natural Hazards and Landscape (BFW), Vienna, pp. 81e84. Binkley, D., 2015. Commentary: ecosystems in four dimensions. New Phytologist 206, 883e885. Binkley, D., Fisher, R.F., 2020. Ecology and Management of Forest Soils, fifth ed. Wiley, New York. Binkley, D., Ho¨gberg, P., 2016. Revisiting the influence of nitrogen deposition on Swedish forests. Forest Ecology and Management 368, 222e239. Binkley, D., Kaye, J., Barry, M., Ryan, M.G., 2004. First rotation changes in soil carbon and nitrogen in a Eucalyptus plantation in Hawaii. Soil Science Society of America Journal 68, 1713e1719. Chadwick, O.A., Gavenda, R.T., Kelly, E.F., Ziegler, K., Olsond, C.G., Elliott, W.C., Hendricks, D.M., 2003. The impact of climate on the biogeochemical functioning of volcanic soils. Chemical Geology 202, 195e223. Cline, M.G., 1961. The changing model of soil. Soil Science Society of America Proceedings 25, 442e446. Cook, R.L., Stape, J.L., Binkley, D., 2014. Soil carbon dynamics following reforestation of tropical pastures. Soil Science Society of America Journal 78, 290e296. Covington, W.W., Moore, M.M., 1994. Southwestern ponderosa forest structure: changes since Euro-American settlement. Journal of Forestry 92, 39e47. Dokuchaev, V.V., 1899. Report to the Transcaucasian Statistical Committee on Land Evaluation in General and Especially for the Transcaucasia. Horizontal and Vertical Soil Zones. Tipogr. kantselyarii Glavnonachal’stvuyushchego grazhdanskoi chast’yu na Kavkaze, Tiflis [in Russian]. Dokuchaev, V.V., 1951. Writings, vol. 6. Akademia Nauk, Moscow, p. 381. Domke, G.M., Perry, C.H., Walters, B.F., Nave, L.E., Woodall, C.W., Swanston, C.W., 2017. Toward inventory-based estimates of soil organic carbon in forests of the United States. Ecological Applications 27, 1223e1235. Fissore, C., Giardina, C.P., Swanston, C.W., King, G.M., Kolka, R.K., 2009. Variable temperature sensitivity of soil organic carbon in North American forests. Global Change Biology 15, 2295e2310. Giardina, G.P., Litton, C.M., Crow, S.E., Asner, G.P., 2014. Warming-related increases in soil CO2 efflux are explained by increased below-ground carbon flux. Nature Climate Change 4, 822e827. Hughes, R.F., Denslow, J.S., 2005. Invasion by a N2-fixing tree alters function and structure in wet lowland forests of Hawaii. Ecological Applications 15, 1615e1628.
References
25
Jenny, H., 1941. Factors of Soil Formation: A System of Quantitative Pedology. McGraw-Hill, New York. Jenny, H., 1961. Derivation of state factor equations of soils and ecosystems. Soil Science Society of America Proceedings 25, 385e388. Jenny, H., 1989. In: Maher, D., Stuart, K. (Eds.), Hans Jenny, Soils Scientist, Teacher, and Scholar. Oral History (1979e1983). The Bancroft Library, University of California, Berkeley. Jenny, H., Arkley, R.J., Schultz, A.M., 1969. The pygmy forest-podsol ecosystem and its dune associates of the Mendocino Coast. Madron˜o 20, 60e71. Jobba´gy, E.G., Jackson, R.B., 2000. The vertical distribution of soil organic carbon and its relation to climate and vegetation. Ecological Applications 10, 423e446. Kaye, J., Resh, S., Kaye, M., Chimner, R., 2000. Nutrient and carbon dynamics in a replacement series of Eucalyptus and Albizia trees. Ecology 81, 3267e3273. Lawrence, G.B., Fernandez, I.J., Richter, D.D., Ross, D.S., Hazlett, P.W., Bailey, S.W., Ouimet, R., Warby, R.A.F., Johnson, A.H., Lin, H., Kaste, J.M., Lapenis, A.G., Sullivan, T.J., 2013. Measuring environmental change in forest ecosystems by repeated soil sampling: a North American perspective. Journal of Environmental Quality 42, 623e639. Lawrence, G.B., Fernandez, I.J., Hazlett, P.W., Bailey, S.W., Ross, D.S., Villars, T.R., Quintana, A., Ouimet, R., McHale, M.R., Johnson, C.E., Briggs, R.D., Colter, R.A., Siemion, J., Bartlett, O.L., Vargas, O., Antidormi, M.R., Koppers, M.M., 2016. Methods of soil resampling to monitor changes in the chemical concentrations of forest soils. Journal of Visualized Experiments 117, e54815. https://doi.org/10.3791/54815. Litton, C.M., Sandquist, D.R., Cordell, S., 2006. Effects of non-native grass invasion on aboveground carbon pools and tree population structure in a tropical dry forest of Hawaii. Forest Ecology and Management 231, 105e113. Logan, W.B., 2007. Dirt: The Ecstatic Skin of the Earth. Norton, New York. MarkInfo, 2018. Swedish Survey of Forest Soils. http://www-markinfo.slu.se/eng/index.html. ˚ gren, G.I., Karltun, E., 2013. Ortiz, C.A., Liski, J., Ga¨rdena¨sa, A.I., Lehtonen, C., Lundblad, M., Stendahl, J., A Soil organic carbon stock changes in Swedish forest soilsda comparison of uncertainties and their sources through a national inventory and two simulation models. Ecological Modelling 251, 221e231. Richter, D.B., Yaalon, D.H., 2011. “The changing model of soil” revisited. Soil Science Society of America Journal 76, 766e778. Richter, D.D., Markewitz, D., Trumbore, S.E., Wells, C.G., 1999. Rapid accumulation and turnover of soil carbon in a reestablishing forest. Nature 400, 56e58. Richter, D.D., Bacon, A.R., Billings, S.A., Binkley, D., Buford, M., Callaham, M.A., Curry, A.E., Fimmen, R.L., Grandy, A.S., Heine, P.R., Hofmockel, M., Jackson, J.A., Lemaster, E., Li, J.W., Markewitz, D., Mobley, M.L., Morrison, M.W., Strickland, M.S., Waldrop, T., Wells, C.G., 2014. Evolution of soil, ecosystem, and critical zone research at the Calhoun experimental forest. In: Hayes, D.C., Stout, S., Hoover, A., Crawford, R. (Eds.), Research for the Long-Term: The Interplay of Societal Need and Research on USDA Forest Service Experimental Forests and Ranges. Springer Verlag, NY. Richter, D.D., Bacon, A.R., Mobleym, M.L., Richardson, C.J., Andrews, S.S., West, L., Wills, S., Billings, S., Cambardella, C.A., Cavallaro, N., DeMeester, J.E., Franzluebbers, A.J., Grandy, A.S., Grunwald, S., Gruver, J., Hartshorn, A.S., Janzen, H., Kramer, M.G., Ladha, J.K., Lajtha, K., Liles, G.C., Markewitz, D., Megonigal, P.J., Mermut, A.R., Rasmussen, C., Robinson, D.A., Smith, P., Stiles, C.A., Tate III, R.L., Thompson, A., Tugel, A.J., van Es, H., Yaalon, D., Zobeck, T.M., 2011. Human-soil relations are changing rapidly: proposals from SSSA’s new cross-divisional working group on soil change. Soil Science Society of America Journal 75, 2079e2084. Richter, D.D., Bacon, A.R., Brecheisen, Z., Mobley, M.L., 2015. Soil in the Anthropocene. pp. 1e11. In: Soil Change Matters 2014. IOP Conf. Series: Earth and Environmental Science, vol. 25. https://doi.org/10.1088/ 1755-1315/25/1/012010. Richter, D.D., Billings, S.A., 2015. “One physical system”: Tansley’s ecosystem as Earth’s critical zone. New Phytologist 206, 900e912.
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CHAPTER 2 Forest soils in the Anthropocene
Shaw, C.F., 1930. Potent factors in soil formation. Ecology 11, 239e245. Stewart, B.W., Capo, R.C., Chadwick, O.A., 2001. Effects of rainfall on weathering rate, base cation provenance, and Sr isotope composition of Hawaiian soils. Geochimica et Cosmochimica Acta 65, 1087e1099. van Breemen, N., 1993. Soils as biotic constructs favouring net primary productivity. Geoderma 57, 183e211. Weir, W.W., 1936. Soil Science: Its Principles and Practice. J.B. Lippincott, Chicago. Wilander, A., Lundin, L., 1999. Recovery of surface waters and forest soils in Sweden. In: Warfvinge, P., Bertills, U. (Eds.), Recovery from Acidification in the Natural Environment: Present Knowledge and Future Scenarios. Swedish Environmental Protection Agency Report 5034, Stockholm, pp. 53e66. Xu, H.-H., Berry, C.M., Stein, W.E., Wang, Y., Tang, P., Fu, Q., 2007. Unique growth strategy in the Earth’s first trees revealed in silicified fossil trunks from China. Proceedings of the National Academy of Sciences of the United States of America. https://doi.org/10.1073/pnas.1708241114. Zou, X.M., 1993. Tree species effects on earthworm density in tropical tree plantations in Hawaii. Biology and Fertility of Soils 15, 35e38.