Soil Organic Matter and Decomposition

,

Soil Organic Matter and Decomposition

Soil organic matter consists of plant and animal residue in various stages of decay and resynthesis. Inputs come from dead plant and animal tissues, often referred to as detritus, that accumulate at the soil surface. Except in old-growth forests, the organic matter in the surface layers of mineral soils generally comprises most of the total organic matter in forest ecosystems. All but the most recalcitrant fractions constantly u n d e r g o chemical b r e a k d o w n by soil microorganisms that d e c o m p o s e and resynthesize the material to form complex carbon-based c o m p o u n d s . Typically, the organic matter concentration of the surface of forest soils ranges from 0.5 to 5% by weight, but it can a p p r o a c h 100% in the organic soils comm o n to poorly d r a i n e d forests. A l t h o u g h organic matter comprises a small fraction of the soil by weight, it has p r o f o u n d effects on the chemical, physical, and hydrological p r o p e r t i e s of forest soils and plays a critical role in forest nutrition. The annual nutrient r e q u i r e m e n t s for most forests are met by nutrients released by organic matter decomposition, and the ability of soils to retain nutrients is strongly influenced by the cation exchange capacity, which is substantially greater for h u m u s m t h e n a m e given to the colloidal, carbon (C)-based polymers that are more resistant to decay than the original tiss u e m t h a n for clay minerals. Key physical characteristics of soil, such as structure, aggregation, and bulk d e n s i t y ~ w h i c h affect the soil water holding characteristics (see C h a p t e r 4 ) m a s well as gas and water transport in the soil, and r o o t growth, are affected by soil organic matter. Organic material on the forest floor increases water infiltration and minimizes overland flow that causes erosion. Lastly, soil organic matter is a major energy source for soil macro- and microinvertebrates and is an imp o r t a n t c o m p o n e n t of the global c a r b o n cycle. Given the n u m e r o u s i m p o r t a n t processes in which soil organic matter is involved, it is essential to u n d e r s t a n d the soil organic matter (or soil carbon) cycle (Fig. 6.1). For this, it is necessary to u n d e r s t a n d the factors 161

162

6. Soil Organic Matter and Decomposition

Figure 6.1 Schematic diagram of the soil C cycle. Major sources of C input to forest soils include the turnover of above- and below-ground plant tissues and to a lesser extent the death of animals. Plant and animal tissue is decomposed via physical and chemical processes (3) and degraded and resynthesized to form complex carbon polymers known as fulvic and humic acids and humin--collectively known as humus (4). Soil microbes derive their energy by decomposing the organic matter and add to it upon their death (2). Carbon losses from soil occur primarily as a gaseous loss when soil organic matter is converted to carbon dioxide (COz), water, and energy--this process is refered to as soil surface CO 2 flux (1). During pedogenesis, dissolved organic compounds move downward in the soil profile but are arrested in the lower soil hoizons.

that control carbon inputs (detritus production), transformations (decomp o s i t i o n a n d f o r m a t i o n o f v a r i o u s h u m u s f r a c t i o n s ) , a n d losses (soil surface C O ~ flux a n d l e a c h i n g ) . T h e w a t e r a n d n u t r i e n t cycles h a v e a p r o f o u n d e f f e c t o n t h e soil c a r b o n cycle b e c a u s e t h e y i n f l u e n c e t h e t y p e a n d quality of litter input, the r e s i d e n c e time of the litter, a n d the a c c u m u l a t i o n o r loss r a t e s o f c a r b o n in t h e soil. T h e n e t d i f f e r e n c e b e t w e e n t h e s e c a r b o n i n p u t s a n d losses d e t e r m i n e s w h e t h e r soil o r g a n i c m a t t e r is a c c u m u l a t i n g o r b e i n g l o s t f r o m f o r e s t soils. I n this c h a p t e r , we r e v i e w t h e m a j o r s o u r c e s a n d r a t e s o f s u r f a c e d e t r i tal i n p u t i n t o f o r e s t s , t h e f a c t o r s c o n t r o l l i n g d e c o m p o s i t i o n , r e s y n t h e s i s o f d e t r i t u s , soil C O ~ flux, t h e i m p o r t a n c e o f c a r b o n l e a c h i n g , a n d soil org a n i c m a t t e r c o n t e n t o f w o r l d f o r e s t s . W e also d i s c u s s t h e i m p a c t s o f forest m a n a g e m e n t p r a c t i c e s o n t h e v a r i o u s c o m p o n e n t s o f t h e soil c a r b o n cycle a n d t h e p o t e n t i a l i m p a c t t h a t c l i m a t e c h a n g e m a y h a v e o n soil carb o n cycles.

L Soil Carbon Content and Accumulation

163

I. Soil Carbon Content and Accumulation The organic matter content of a soil represents the integrated (net) balance between detrital i n p u t s m b o t h above- and b e l o w - g r o u n d m a n d organic matter losses in the form of CO 2 flux from the soil, which reflects organic matter d e c o m p o s i t i o n and root repiration, as well as by leaching and erosion (Fig. 6.1). Post et al. (1982) s u m m a r i z e d 2696 soil profiles from almost every terrestrial biome and calculated that forests contain 34% of the global organic carbon soil content. Soil C contents were generally lowest for warm temperate and very dry tropical forests (7.1 and 6.1 kg m -z, respectively) and highest for wet boreal and tropical forests (19.3 and 19.1 kg m -z, respectively). They r e p o r t e d that, in general, soil organic matter increased with increasing precipitation, decreasing temperature, and decreasing e v a p o t r a n s p i r a t i o n / p r e c i p i t a t i o n ratio. Similar relationships have b e e n observed in studies across local climatic gradients ( A m u n d s o n et al., 1990), but total soil organic matter can vary significantly within a biome or even a forest ecosystem because of differences in parent material, aspect, topography, and past stand history. For example, the range in soil organic matter in a coastal Douglas fir ecosystem (Stow et al., 1996) is similar to the range r e p o r t e d a m o n g the major terrestrial biomes (Post et al., 1982; Schlesinger, 1991). If detrital inputs exceed carbon losses, c a r b o n accumulates in the soil, and the converse obviously holds. It is difficult to d e t e r m i n e w h e t h e r the c a r b o n content of a soil is in steady state by measuring carbon losses and inputs because the i n p u t and o u t p u t fluxes may be an o r d e r of magnitude greater than their differences and there is significant uncertainty associated with each of them. The major losses are reflected in the soil CO2 flux (see Section IV), but m e a s u r e d fluxes include contributions from r o o t respiration, which can comprise a varying fraction of the total. Schlesinger (1991) summarized carbon a c c u m u l a t i o n rates m e a s u r e d in n u m e r o u s soil c h r o n o s e q u e n c e studies; using these data we calculated average rates of 8.7, 5.6, and 2.4 g C m -2 year -1 for boreal, temperate, and tropical forests, respectively (Table 6.1). These estimates of soil carb o n accumulation are substantially smaller than values r e p o r t e d for afforestation and in some cases reforestation studies. The R o t h a m s t e d study (Jenkinson et al., 1992) provides a rigorous, long-term assessment of the potential influence of land use practices on soil c a r b o n accumulation: Annual carbon accumulation rates r a n g e d from 26 to 48 g C m -2 year -1 for agricultural fields converted to forests, c o m p a r e d to - 2 g C m -2 year -1 for continuous c r o p p i n g with wheat. We n o t e d in C h a p t e r 5 that planting trees to sequester c a r b o n was limited by land availability and the p r o b l e m of what to do with the c a r b o n once it h a d b e e n fixed in vegetation biomass. Few studies examining the feasibility of planting trees to

164

6. Soil Organic M a t t e r a n d Decomposition

Table 6.1 Typical Carbon Accumulation Intervals (Year) and Carbon Accumulation Rates (g C m -2 year -1) for Contrasting Forest Biomes a

Biome type Boreal (C) Temperate C R, A Tropical C R, A

Accumulation interval

Rate of accumulation

1 5 0 - 3 , 5 0 0 (2415)

1 - 1 2 (9)

1 0 0 0 - 1 0 , 0 0 0 (4260) 3 5 - 1 0 0 (75)

1 - 1 2 (6) 1 - 8 0 (36)

3 5 0 0 - 8 , 6 0 0 (6060) ---50 (50)

2 5 0 - 2 0 0 (103)

"The data indicate the range of values from studies on primary successional soil chronosequences (C) summarized by Schlesinger ( 1991) and reforestation and afforestation sites (R, A). (Data from I,ugo et al., 1986; Zarin and Johnson, 1995; Boone et al., 1988; Gholz and Fisher, 1982;Jenkinson et al., 1992; Huntington, 1995; Schiffman and.Johnson, 1990.) Means are in parentheses.

offset rising a t m o s p h e r i c CO,) concentrations consider carbon accumulation in the soil, but empirical data from afforestation studies suggest this can be b o t h significant and long term. T h e m e a n residence time of soil organic matter, the inverse of dec o m p o s i t i o n rate, is c o m m o n l y calculated as the ratio of soil organic matter c o n t e n t / o r g a n i c matter inputs (or losses); this calculation assumes that the soil organic matter pool is in steady state or equilibrium. Raich and Schlesinger (1992) used average soil carbon content and annual soil surface CO,) flux values to calculate m e a n carbon residence times for tropical lowland, temperate, and boreal forests of 38, 29, and 91 years, respectively. We used forest floor biomass and above-ground detritus p r o d u c t i o n data r e p o r t e d in the literature to calculate m e a n residence times for the detritus in the major forest biomes of the world. We f o u n d that it increases from tropical to boreal forests and is c o m m o n l y greater for evergreen than for d e c i d u o u s forests (Fig. 6.2). T h e rates are, of course, substantially smaller than those r e p o r t e d for total soil carbon. The greater m e a n residence time of surface litter in evergreen as o p p o s e d to deciduous forests may be because the litter of evergreen trees tends to have higher concentrations of secondary c o m p o u n d s , such as lignins and tannins, and lower concentrations of essential nutrients r e q u i r e d by decomposers. Vogt et al. (1986b) point out that excluding below-ground detritus input from calculations of the m e a n residence time of surface litter can cause overestimation of d e c o m p o s t i o n rates in forest ecosystems by 1 9 - 7 7 % . The testing of nuclear b o m b s in the early 1960s nearly d o u b l e d the rad i o c a r b o n c o n c e n t r a t i o n of a t m o s p h e r i c CO 2, which was absorbed by vegetation and transferred to the soil organic matter. This pulse of ra-

L Soil Carbon Content and Accumulation

165

d i o c a r b o n p r o v i d e s a n o t h e r way f o r scientists to e x a m i n e t h e r e s i d e n c e t i m e o f o r g a n i c m a t t e r in t h e soil ( J e n k i n s o n , 1963; R a f t e r a n d S t o u t , 1970; T r u m b o r e et al., 1989). S u c h a n a l y s e s c o n s i s t e n t l y y i e l d m e a n resid e n c e t i m e s f r o m 400 to 3000 y e a r s f o r soil c a r b o n ( J e n k i n s o n , 1963; C a m p b e l l et al., 1967; O ' B r i e n , 1984; S t e v e n s o n , 1982; J e n k i n s o n et al., 1992), w h i c h a r e o n e to two o r d e r s o f m a g n i t u d e m o r e t h a n t h o s e o f surface l i t t e r (Fig. 6.2). T h i s s u g g e s t s t h a t t h e r e is a small f r a c t i o n o f soil org a n i c m a t t e r t h a t is e x t r e m e l y r e c a l c i t r a n t o r i n e r t . T h e r e c a l c i t r a n t p r o p e r t i e s o f h u m u s (see b e l o w ) r e s u l t in l a r g e a c c u m u l a t i o n s o f h u m u s in t h e soil p r o f i l e so t h a t t h e mass o f h u m u s o f t e n e x c e e d s t h e c a r b o n cont e n t o f s u r f a c e d e t r i t u s a n d living b i o m a s s ( S c h l e s i n g e r , 1977). J e n k i n s o n a n d R a y n e r (1977) u s e d t h e r a d i o c a r b o n e v i d e n c e o f v e r y l o n g m e a n resi d e n c e t i m e s o f t h e i n e r t c a r b o n f r a c t i o n to r a t i o n a l i z e t h e f o r m u l a t i o n o f a t h r e e - p o o l soil o r g a n i c m a t t e r m o d e l ; this a p p r o a c h h a s b e e n w i d e l y a d o p t e d by o t h e r m o d e l e r s ( P a r t o n et al., 1987; C o m i n s a n d M c M u r t r i e , 1993). T h e c o n c e p t u a l s e p a r a t i o n o f soil o r g a n i c m a t t e r i n t o f r a c t i o n s o f d i f f e r e n t m e a n r e s i d e n c e t i m e has l e a d to a b e t t e r u n d e r s t a n d i n g o f soil o r g a n i c m a t t e r d y n a m i c s a n d b e t t e r e s t i m a t e s o f soil c a r b o n cycling rates.

100 MRT=20 -

10

8O

-

1+

60

E 8

4o

~

2o

21_

0 0

2

4

6

8

10

]

Litterfall (t ha-lyear -1) Figure 6.2 Average forest floor mass plotted against litterfall for (1) boreal needle-leaved evergreen (n = 16), (2) boreal broad-leaved deciduous (n = 7), (3) temperate needleleaved evergreen (n = 73), (4) temperate broad-leaved evergreen (n = 11), (5) temperate broad-leaved deciduous (n = 40), (6) tropical broad-leaved evergreen (n = 31), and (7) tropical broad-leaved deciduous (n = 2). Assuming the forest floor is in steady state, the average mean residence time (MRT, years) can be calulated as forest floor mass/litterfall mass. The lines indicate these values. Gower and Landsberg, submitted for publication.

166

6. Soil Organic Matter and Decomposition Table 6.2

Comparisonof Cation Exchange Capacity (CEC),

Surface Area, and pH Charge Dependency for Soil Organic Matter and Clay Mineralsa

Mineral type

Component Organic matter Allophane Vermiculite Montmorillonite Kaolinite

m w 2:1 2:1 1:1

CEC (meq 100 g - 1 soil)

Surface area (m z g - 1 soil)

pH dependence of charge

1 0 0 - 300 10-150 120-150 80-120 1 - 10

800-900 7 0 - 300 600-800 600-800 1 0 - 20

I,arge I,arge Small Small Medium

" (Adapted from Bohn et al., 1979).

Townsend et al. (1995) have shown that soil organic matter turnover rates based on a three-pool soil organic matter model gave rates three times slower than those p r o d u c e d by a single pool model. It also appears that the labile carbon pool in the soil is more sensitive to soil warming than the large recalcitrant pool (Updengraff et al., 1995). Even the most soluble c o m p o u n d s are rarely decomposed completely and the by-products often u n d e r g o further enzymatic and chemical reactions to form humus. The synthesis and composition of humus is complex and poorly u n d e r s t o o d (Stevenson, 1982; Tate, 1987), although there is general agreement regarding the probable structure. Humus is a very variable, a m o r p h o u s c o m p o u n d that consists of highly branched units with an aromatic ring skeleton linked by O, NH, N, and S bonds. Its amorphous nature and phenolic and organic acid groups provide humus with an extremely large cation exchange capacity, surface area, and waterholding capacity relative to clay minerals (Table 6.2).

II. S o u r c e s of Soil O r g a n i c Matter A. Detritus Production: Factors Controlling Sources and Quantity Detritus, which derives from the death of above- and below-ground plant tissue, death of organisms, and fecal material, is the dominant source of C input to the soil u n d e r normal conditions. Insects consume less than 10% of" net primary production (NPP) in forests (Schowalter et al., 1986), with only a fraction of this returning to the soil in the form of frass (insect feces). Vertebrates commonly comprise less than 1 or 2% of the total organic matter content of forests (Whittaker, 1975); their carbon input can be significant at a microscale, but averaged over a stand it is very small. Uncontrolled population growth of insects or vertebrates, however, can drastically alter the soil organic matter cycle of forest ecosys-

II. Sources of Soil Organic Matter

167

tems. Pastor et al. (1988) provide an interesting and elegant example of the effect that moose, mediated t h r o u g h soil microbes, can have in increasing the soil carbon and nutrient cycling rates of boreal forests. Similar examples exist for insects (Mattson and Addy, 1975; Schowalter et al., 1986; Romme et al., 1986), p r o m p t i n g ecologists to acknowledge the importance of insects in regulating net primary p r o d u c t i o n of terrestrial ecosystems (Romme et al., 1986). Most measurements of above-ground litterfall are restricted to small tissues such as foliage, twigs, and reproductive tissue, of which leaf detritus generally comprises the largest fraction (see Chapter 5). On a global scale, above-ground detritus p r o d u c t i o n follows the gradient of NPP, being generally highest in tropical forests and lowest in boreal forests (Fig. 6.2; see Chapter 5). Latitudinal trends have also been r e p o r t e d (Van Cleve et al., 1983; Schlesinger, 1977; Vogt et al., 1986b). Extreme environmental conditions such as severe d r o u g h t can cause abnormally high leaf litter p r o d u c t i o n (Linder et al., 1987; Raison et al., 1992), and during mast (flowering) years the p r o p o r t i o n of carbon allocated to reproductive tissue increases and leaf litter decreases (Burton et al., 1991). Litterfall does not appear to vary consistently among deciduous and evergreen forests in a similar climate, although the nutrient content of litter and nutrient use efficiency of the forests may be different (Chapter 7). Comparative studies in natural forests, aimed at examining the effects of species on carbon and nutrient cycling, are complicated by the fact that many variables that influence carbon cycling, such as climate, parent material, topography, and relief, are not held constant. Experimental plantations in replicated plots, commonly known as a c o m m o n garden study design, can be used to examine the effect of species on forest ecosystem processes. Alban et al. (1982) in Minnesota and Gower and Son (1992) in Wisconsin used c o m m o n garden studies to examine the effect of species on forest ecosystem processes and, in both cases, showed that leaf litter mass did not differ a m o n g species in which leaf longevity was typically very different (e.g., <1 to > 6 years), indicating that climatic and edaphic conditions are more important than species effects on aboveg r o u n d detritus inputs. The significance of coarse woody detritus to the soil carbon balance of forest ecosystems, as a factor in the rate of nutrient cycling, as habitat for vertebrates and invertebrates, and as an energy source for soil microorganisms, has only recently been fully appreciated ( H a r m o n et al., 1986). The large temporal and spatial variability of coarse woody detritus makes it difficult to quantify accurately, except by using large plots and collecting data for many years. The limited data available indicate that woody litterfall comprises 1 7 - 3 7 % of the total aboveground detritus production, the proportions being larger in tropical than in boreal forests (Vogt et al., 1986b).

"168

6. Soil Organic Matter and Decomposition

A l t h o u g h fine roots and mycorrhizae comprise a small fraction of the total forest ecosystem c a r b o n content, because of their rapid turnover they provide a significant fraction of the total annual i n p u t of carbon and nutrients to forest soils (Vogt et al., 1986b; Vogt, 1991; Gower et al., 1995). Most estimates of below-ground detritus are likely to be underestimates because of the difficulty of quantifying e p h e m e r a l fluxes such as mycorrhizal t u r n o v e r and r o o t exudates. Vogt et al. (1986b) summarized fine r o o t t u r n o v e r rates for the major forest biomes but the averages for some ecosystems were based on only a few studies, making it difficult to draw any general conclusions a b o u t the e n v i r o n m e n t a l or ecological controls on below-ground detritus p r o d u c t i o n . As m o r e data have become available, several trends are emerging, many s u p p o r t i n g those noted by Vogt et al. Above-ground detritus p r o d u c t i o n increases from boreal to tropical biomes for b o t h d e c i d u o u s and evergreen forests and, for a similar biome, above-ground detritus p r o d u c t i o n tends to be greater in deciduous than evergreen forests. Fine r o o t detritus p r o d u c t i o n also increases from boreal to warm temperate forests in both deciduous and evergreen forests, but it does not differ consistently between leaf habits for a similar climate. Last, the ratio of fine root:total (above-ground + fine root) detritus prod u c t i o n is larger in evergreen than d e c i d u o u s forests for a similar biome, b u t it is relatively constant across biomes for a similar leaf habit. Warmer soils p r e s u m a b l y increase root respiration costs ( C h a p t e r 5) and lead to faster fine r o o t t u r n o v e r rates (Table 6.3). Marshall and Waring (1986) f o u n d that fine roots of Douglas fir contained fixed a m o u n t s of carbohydrates so that the roots of seedlings grown in warm soils died more quickly than those grown in cooler conditions. H e n d r i c k and Pregitzer (1993) used a miniature video camera and m i n i r h i z o t r o n tubes to monitor fine r o o t turnover of two sugar maple stands on a n o r t h - s o u t h gradient in Michigan, from which they c o n c l u d e d that fine root turnover was greater in the s o u t h e r n than n o r t h e r n stand. E x t r e m e flucuations of the soil environment, such as d r o u g h t , can also cause rapid fine root t u r n o v e r (Eissenstat and Yanai, 1996).

B. Detritus Production: Chemical Composition The chemical c o m p o s i t i o n of detritus will affect soil carbon dynamics. Aber and Melillo ( 1991) summarized information a b o u t some of the maj o r carbon constituents of foliage, stem, and roots for a variety of boreal and t e m p e r a t e tree species. The c o m m o n c a r b o n a c e o u s constituents of plant tissue include sugars, cellulose and hemicellulose, and complex phenolics such as lignins, tannins, and suberin. The energy yield of these tissues varies considerably. If the chemical bonds are difficult for microbes to break, then the net energy gain from their d e c o m p o s i t i o n by m i c r o o r g a n i s m s is small. Sugar molecules, such as glucose, are simple and small and therefore an excellent source of energy for microbes. Car-

II. Sources of Soil Organic Matter Table 6.3

] 69

Above-ground and Fine Root Detritus Production and Fine Root:Total Detritus Ratio for Evergreen and Deciduous Forest in Contrasting Biomesa Above-ground (kg ha -1 year -1)

Biome leaf habit Boreal Evergreen (8) b Deciduous(5) Cold Temperate Evergreen (18) Deciduous (11) Warm Temperate Evergreen (4) Deciduous (3) Tropical Evergeen (4)

Fine root (kg ha -1 year -1)

Fine root:Total

mean

range

mean

range

1370 2520

(550-3000) (1660-3450)

1990 2338

(600-4110) (500-4390)

0.58 (0.41-0.77) 0.41 (0.17-0.68)

2940 3980

(1700-6010) (2840-5360)

5420 (1440-15880) 3500 ( 5 4 0 - 6 6 8 2 )

0.59 (0.27-0.89) 0.43 (0.11-0.59)

3032 4290

(250-5406) (3310-5300)

6154 5730

(2120-9506) (2190-9000)

0.70 (0.60-0.95) 0.54 (0.29-0.73)

4200 (1200-11170)

0.36 (0.18-0.52)

6200 (2430-10250)

mean

range

aData summarized from the literature by Gower and Landsberg (unpublished manuscript). bSample size.

bohydrate polymers, such as starch, are slightly more difficult to break down because the longer molecules must first be cleaved into sugar units, which can then be metabolized. Sugars and starches occur in small amounts ( 1 - 7 % by weight) in plant litter. The most important constituent of plant tissue on a weight basis is cellulose. It is the primary c o m p o u n d of the cell wall in plants and comprises 4 0 - 8 0 % of plant tissue (Aber and Melillo, 1991). Cellulose consists of a series of sugar units that are linked between the 1 and 4 carbon of adjacent sugar molecules. Unlike simple sugars, cellulose cannot be remobilized and retains its original form until decomposed. N u m e r o u s soil microorganisms are capable of decomposing cellulose and h e m i c e l l u l o s e m a highly b r a n c h e d cellulose-like c o m p o u n d very similar in decomposibility to cellulose. Fungi are the primary decomposers of cellulose in h u m i d soils, whereas bacteria are the main decomposers of cellulose in semiarid for-ests (Alexander, 1977). Various fungi, bacteria, and actinomycetes can decompose hemicellulose. Lignins are also a b u n d a n t in plant tissue. The molecules are highly variable in structure but are generally composed of aromatic rings containing primarily carbon, hydrogen, and oxygen. The exact role of lignin is not fully understood: It is highly resistant to enzymatic decay, which has led to speculation that it is the most i m p o r t a n t structural constituent of plants, with a major role in deterring herbivory (Coley et al., 1985; H o r n e r et al., 1988). Many basidiomycetes, a type of fungi, and some aerobic bacteria can break down lignin, but the reaction is very slow because

170

6. Soil Organic Matter and Decomposition

Figure 6.3 Diagram illustrating the relative composition and rate of mass loss of plant tissue during decomposition. Initially, mass loss is rapid as soluble and easily decomposable constituents are degraded. In the later stages of decomposition, mass loss is largely controlled by lignin mass loss (adapted from CotSteaux et al., 1995). of the high density of aromatic rings, numerous side chains, and complex interlinkages between lignins. It is interesting to note that lignin must be decomposed during the papermaking p r o c e s s m a n expensive process that requires the use of extremely caustic chemicals. Paper chemists are currently trying to identify and isolate "super fungi" that can degrade lignin so that papermaking will be cheaper and require fewer envionmentally hazardous chemicals. Tannins consist of aromatic rings and are t h o u g h t to deter herbivory by their low digestibility and energy return (see H o r n e r et al., 1988). The collective effect of the various carbonaceous constituents on decomposition of organic matter is shown in Fig. 6.3. Although all the components are constantly decomposing, their relative importance differs during decomposition. Mass loss of detritus is characterized by an initial rapid weight loss of soluble compounds, followed by an intermediate weight loss of cellulose and hemicellulose and finally the slow decomposition oflignin and lignin-derivative c o m p o u n d s (Berg et al., 1984). A negative exponential decay function is often used by ecologists to describe the weight loss of organic matter in terms of time (Olson, 1963; see Box 6.1).

III. Litter Decomposition A. The Role of Organisms Soon after plant and animal tissue are incorporated into the soil, microorganisms begin decomposing the material to obtain energy. A great many invertebrates and microorganisms are involved (Alexander, 1977;

IlL Litter Decomposition

Box 6.1

171

Estimating Decomposition Constants

Several t e c h n i q u e s are used to calculate d e c o m p o s i t i o n ; the m e t h o d of choice is largely d e t e r m i n e d by the type of material of interest and question to be answered. The simplest a p p r o a c h is the mass-balance m e t h o d . T h e d e c o m p o s i t i o n constant, k, is calculated by dividing annual litterfall or detritus p r o d u c t i o n by organic matter content in the forest floor or total soil organic matter. Detritus p r o d u c t i o n is expressed as a r a t e m m a s s area -1 t i m e - a (where the units of time are usually years) a n d detritus mass is expressed as mass area -a so that k has units of t i m e - l : k = (detritus p r o d u c t i o n / d e t r i t u s mass) A critical a s s u m p t i o n u n d e r l y i n g this a p p r o a c h is that the detritus is in steady state. Excluding fine root t u r n o v e r a n d coarse woody debris will result in an u n d e r e s t i m a t i o n of k (Vogt et al., 1982, 1986b). A second a p p r o a c h is the use of litter d e c o m p o s i t i o n bags. This m e t h o d consists of placing a known mass of plant litter (ml), usually leaves or fine roots, into nylon mesh bags, collecting the bags at intervals (At), and m e a s u r i n g weight loss (Am1). A simple negative e x p o n e n t i a l decay m o d e l is usually sufficient to describe the fractional weight loss over time. Written in terms of the fraction of ml r e m a i n i n g after t = EAt, (days, months, or years), ml (t) = cl exp ( - kt). k is o b t a i n e d from the plot of ln(ml) against t, f r o m which k = - ( l n m l - In Cl)/t, where In is the natural l o g a r i t h m , cis the intercept, and if it is set to unity (the fraction r e m a i n i n g at the beginning of the exercise is one) t h e n k = - ( l n m l ) / t . D e c o m p o s i t i o n of large woody tissue, such as stems a n d large branches, is c o m m o n l y estimated as the c o m b i n e d v o l u m e loss a n d decrease in specific gravity (Grier, 1978) as m 1 =

(V o X 0o)

--

(V(t)

X O(t))

,

where ml is weight loss, Vo and 0o are v o l u m e a n d specific gravity, respectively, at time zero, and Vt a n d o(t) are v o l u m e a n d specific gravity at time t, respectively. T h e m e a n residence time a n d the time r e q u i r e d to achieve 95 a n d 99% of the steady-state forest floor mass can be calculated as 1/k, 3/k, a n d 4.6/k, respectively (Olson, 1963).

172

6. Soil Organic Matter and Decomposition

Paul and Clark, 1989). Invertebrates f r a g m e n t the litter, increasing the surface area and providing greater o p p o r t u n i t i e s for microbial colonization, and i n c o r p o r a t e the small pieces of litter into the mineral soil (Swift et al., 1979; Seastedt and Crossley, 1980). I m p o r t a n t soil invertebrate groups include nematodes, collembola, mites, earthworms, and termites. Nematodes, collembola, and mites tend to be m o r e prevalent in conifer forests, whereas earthworms are m o r e c o m m o n in t e m p e r a t e deciduous and tropical forests (Swift et al., 1979; Phillipson et al., 1978). Termites are most prevalent in warm t e m p e r a t e a n d tropical forests (Gentry and Whitford, 1982). T h e r e are very large n u m b e r s of soil invertebrates in forest soils, but they comprise a small fraction (generally < 5 % ) of the total organic matter of a forest ecosystem (Ugolini and E d m o n d s , 1983; Anderson and Domsch, 1980). T h e biomass of soil invertebrates tends to increase from boreal to warm t e m p e r a t e and tropical forests, except where d e c o m p o s i t i o n is so rapid that little or no forest floor exists (Table 6.4). T h e effect of invertebrates on s u b s e q u e n t d e c o m p o s i t i o n is greatest for low-quality litter (i.e., high C / N ratio). During the d e c o m p o s i t i o n process, microorganisms can act as sinks (immobilization) or sources (mineralization) of carbon and nutrients and t h e r e f o r e control the availability of nutrients to vegetation. Bacteria, fungi, and actinomycetes are the most i m p o r t a n t microorganisms; they can be classified on the basis of n u m e r o u s structural or functional characteristics, but are often separated into two broad g r o u p s m a u t o t r o p h s and h e t e r o t r o p h s m b a s e d on the way they obtain their energy. Autotrophs obtain their energy from e i t h e r sunlight ( p h o t o a u t o t r o p h s ) or f r o m the oxidation of inorganic c o m p o u n d s ( c h e m o a u t o t r o p h s ) . Chemoa u t o t r o p h s are limited to a few species of bacteria, each using a very specific c o m p o u n d (Paul and Clark, 1989), but they play an i m p o r t a n t role in the cycling of many elements ( C h a p t e r 7). H e t e r o t r o p h s , the organisms responsible for most litter d e c o m p o s i t i o n , use p r e f o r m e d organic matter as a source of energy, which is obtained by cleaving chemical bonds. As in the case of soil invertebrates, the biomass of fungi and bacteria increases from boreal to tropical forests (Table 6.5). Fungi dom-

Table 6.4

Fungi, Bacteria, and Microfauna Biomass (kg ha-l) for Selected Forest Biomes

kg ha-1 Biome type Boreal and temperate conifer Temperate deciduous Warm temperate broad-leafed and tropical

Fungi 836-4620 890-1290 4500

B a c t e r i a Microfauna 1-110 1-265 1100

84-282 83-786 84

173

III. Litter Decomposition Table 6.5 Decomposition Coefficients (k, years) for Foliage and Fine and Coarse Wood for Deciduous and Evergreen Forests in Contrasting Environments

Tissue Component Biome

Leaf habit

Foliage

Fine wood

Coarse wood

Boreal

Decidious Evergreen Deciduous Evergreen Deciduous Evergreen Deciduous Evergreen

0.39-0.702 0.223-0.446 0.28-0.85 0.140-0.693 0.441-2.465 0.162-0.751 0.62-4.16 0.162-2.813

0.058-0.120 -0.10-0.38 m ~ ~ ~ ~

0.022-0.29

Cold temperate Warm temperate Tropical

m 0.011 - 0.060 0.03-0.27 0.04 0.115-0.461

inate in well-aerated soils, w h e r e a s b a c t e r i a are m o r e c o m m o n in a n a e r o bic soils. F u n g i also t e n d to be m o r e p r e v a l e n t t h a n b a c t e r i a in acidic soils b e c a u s e b a c t e r i a are less t o l e r a n t o f low p H ( A l e x a n d e r , 1977). B. E n v i r o n m e n t a l Controls

D e c o m p o s i t i o n can be d i v i d e d into t h r e e processes: f r a g m e n t a t i o n o f organic matter, l e a c h i n g , a n d m i c r o b e - m e d i a t e d c h e m i c a l d e g r a d a t i o n , with t h e latter p r o c e s s p r o d u c i n g C O 2, water, a n d t h e e n e r g y u s e d by the microorganisms: C 6 H 1 2 0 6 + 6 0 2 ---> 6 C O 2 -+- 6 H 2 0 + energy.

(6.1)

N u m e r o u s t e c h n i q u e s are u s e d to e s t i m a t e d e c o m p o s i t i o n rates, alt h o u g h n o single m e t h o d p r o v i d e s a c o m p l e t e p i c t u r e of the factors controlling the d e c o m p o s i t i o n o f surface litter a n d soil o r g a n i c matter. Methods i n c l u d e m e a s u r i n g the w e i g h t loss o f the tissue o f i n t e r e s t (see Box 6.1), m e a s u r i n g soil C O 2 e v o l u t i o n (discussed b e l o w ) , or m e a s u r i n g ATP, a p r o x y for m i c r o b i a l activity (Vogt et al., 1980). E a c h m e t h o d p r o v i d e s useful i n f o r m a t i o n a n d t h e r e are c o n s i d e r a b l e a d v a n t a g e s in u s i n g combinations of different methods Table 6.5 s u m m a r i z e s d e c o m p o s i t i o n coefficients for a wide variety o f tissues a n d e n v i r o n m e n t a l c o n d i t i o n s . In g e n e r a l , d e c o m p o s i t i o n rates are g r e a t e s t for foliage, slowest for w o o d , a n d i n c r e a s e f r o m b o r e a l to t r o p i c a l forests. D e c o m p o s i t i o n o f o r g a n i c m a t t e r is s t r o n g l y c o n t r o l l e d by e n v i r o n m e n t a l c o n d i t i o n s that affect the activity o f soil i n v e r t e b r a t e s a n d m i c r o o r g a n i s m s . As a g e n e r a l rule, m i c r o b i a l activity i n c r e a s e s 2.4fold with a t e m p e r a t u r e i n c r e a s e o f 10~ across t h e n o r m a l t e m p e r a t u r e r a n g e o f soils (i.e., d e c o m p o s i t i o n t e n d s to have a Qa0 o f a b o u t 2.4; R a i c h a n d Schlesinger, 1992). H o w e v e r , e x t r e m e t e m p e r a t u r e s a n d m o i s t u r e c o n d i t i o n s can d e c r e a s e the efficiency o f m o s t soil m i c r o o r g a n i s m s , re-

174

6. Soil Organic Matter and Decomposition

sulting in a d e c r e a s e in d e c o m p o s i t i o n rates a n d t h e r e f o r e an increase in litter a c c u m u l a t i o n (Ino a n d Monsi, 1969). M e e n t e m e y e r (1978) p r o p o s e d that d e c o m p o s i t i o n c o u l d be e s t i m a t e d f r o m a n n u a l estimates o f e v a p o t r a n s p i r a t i o n (which h e called AET), calc u l a t e d using the T h o r n t h w a i t e f o r m u l a t i o n that i n c o r p o r a t e s , to some extent, the effects o f t e m p e r a t u r e , p r e c i p i t a t i o n , a n d growing d e g r e e d a y s - - a n integral of t e m p e r a t u r e . T h e success o f this r e l a t i o n s h i p , o n an a n n u a l basis, is illustrated using d e c o m p o s i t i o n data for Scots pine n e e d l e s (Fig. 6.4). H o w e v e r , the A E T m o d e l is only a s u r r o g a t e for the actual regulators o f d e c o m p o s i t i o n a n d is n o t as useful at the local scale because of a r a n g e o f o t h e r factors that i n f l u e n c e d e c o m p o s i t i o n rates. It is i m p o r tant to n o t e that a n n u a l averages of AET d o n o t a c c o u n t for seasonal dist r i b u t i o n o f p r e c i p i t a t i o n a n d t e m p e r a t u r e or for e x t r e m e t e m p e r a t u r e a n d m o i s t u r e c o n d i t i o n s that adversely affect d e c o m p o s i t i o n rates (Fogel a n d C r o m a c k , 1977; E d m o n d s , 1979). W h i t f o r d et al. (1981) n o t e d that the A E T - d e c o m p o s i t i o n r e l a t i o n s h i p was a p o o r p r e d i c t o r o f litter dec o m p o s i t i o n rates in r e c e n t l y clear-cut forests; they s u g g e s t e d that this was b e c a u s e the m o d e l did n o t a c c o u n t for m a r k e d c h a n g e s in microclimate that adversely affect m i c r o b i a l activity. E r i k s o n et al. (1985) also n o t e d that s u s p e n d e d w o o d y litter in clear-cut forests d e c o m p o s e d m o r e slowly t h a n w o o d y litter in c o n t a c t with the s o i l - - p r e s u m a b l y because c o n d i t i o n s at the soil surface were m o r e moist t h a n in s u s p e n d e d

60 50

1

I

I

-

I

o

I

I

o

-

0 40-

o

9

0~

_.o ,~

30

~;

20

,

-

/f-. ~

9

9

o 9

9

100

300

I

400

L

L

500 600

J

L

700

800

Evapotranspiration

I

900 1000

(mm)

Figure 6.4 First-year mass loss of Sc~)ts pine (Pinus sylvestris) needles versus actual evapotranspiration. Openl symbols represent Scots pine sites ill a Scandanavian-NW continental transect and pine sites near the European west coast or exposed to Atlantic coast influence. Solid symbols are tk)r sites around the Mediterranean, in Poland, and the eastern United States [adapted from (;o6teaux et al. (1995). Data sources include Meentcmeyer and Berg (1986) and Berg et al. (1993)].

III. Litter Decomposition

175

samples. The AET m o d e l also c a n n o t account for the p r e s e n c e of soil invertebrates such as termites that hasten d e c o m p o s i t i o n (Whitford et al., 1981; Santos and Whitford, 1982). A second major factor affecting d e c o m p o s i t i o n rate is litter quality. For example, T a n n e r (1981) r e p o r t e d that mass loss of foliage in 1 year r a n g e d from 27 to 96% for 15 tree species in a J a m a i c a n m o n t a n e rainforest and Cuevas a n d M e d i n a (1986) r e p o r t e d that the time r e q u i r e d for 95% of mass loss to occur r a n g e d from 0.4 to 13.6 years for h u m i d tropical tree species in a Venezuelan tropical forest. These large interspecies differences in d e c o m p o s i t i o n rate can be e x p l a i n e d by differences in c o m p o s i t i o n of c a r b o n constituents (Table 6.3) a n d m i n e r a l nutrients. It is interesting to note that certain tree species have e x t r e m e l y high concentrations of extractives that have p r e s u m a b l y evolved to d e t e r pest and p a t h o g e n attack and it is no c o i n c i d e n c e that m a n y of these same species are highly valued t i m b e r species because of their resistance to rot. T h e d e c o m p o s i t i o n rate of plant tissue is inversely correlated with the p r o p o r t i o n of lignin (Berg et al., 1984; M c C l a u g h e r t y et al., 1985) and is also i n f l u e n c e d by the c o n c e n t r a t i o n of m i n e r a l elements, especially those that limit plant and microbial growth (e.g., N a n d P). Melillo et al. (1982) d e m o n s t r a t e d that the ratio of l i g n i n / n i t r o g e n concentration explained 8 2 - 8 9 % of the observed variation in d e c o m p o s i t i o n rate for tree species in two contrasting climates (Fig. 6.5). However, some

120

I

I

I

I

100 80

"

ffl

o

60

_

ex,,x,NorthCarolina \ Q

O3

40

20

X~ew Hampshire

-

0

I

I

10

20

30

40

50

60

Initial lignin /Initial nitrogen Figure 6.5 Relationship between percentage weight loss during the first year and the lignin/nitrogen ratio of leaf litter for temperate broad-leafed deciduous tree species in North Carolina (closed circles) and New Hampshire (open circles) (redrawn from Melillo et al., 1982).

176

6. Soil Organic Matter and Decomposition

plant tissues do not d e c o m p o s e at the rate predicted by their size or tissue biochemistry. Fine roots, which tend to have high concentrations of suberin--lignin-like c o m p o u n d laid down as roots mature and become r i g i d m d e c o m p o s e more slowly than their size or nutrient concentration would suggest. The influence of litter quality on decomposition rates is not u n i f o r m across climates; it has the greatest influence in tropical envir o n m e n t s and the smallest effect in boreal climates (Meentemeyer, 1978; Baragali et al., 1993). A third factor influencing d e c o m p o s i t i o n rate is the surface area/volume ratio of the tissue. All other factors being equal, tissue with a small surface a r e a / v o l u m e ratio (e.g., stems) will d e c o m p o s e more slowly than tissue with a larger ratio (Berg, 1984; Erickson et al., 1985; H a r m o n et al., 1986), which emphasizes the i m p o r t a n c e of soil microinvertebrates as agents for fragmenting litter. It follows that site preparation practices that reduce the size of logging debris should facilitate decomposition and nutrient release from logging residue left on the site.

IV. Carbon Losses from Forest Ecosystems The efflux of COz from the soil is often referred to as soil respiration or soil surface CO2 flux; the f o r m e r term is technically incorrect because the efflux of COz from the soil surface is derived from soil organic matter dec o m p o s i t i o n and root respiration (Box 6.2). Soil CO 2 flux is c o m m o n l y the second largest term in forest carbon budgets (Gower et al., 1996c) and of similar importance in the global carbon cycle (Raich and Schlesinger, 1992). The p r o d u c t i o n of COz by microbial and root respiration also plays an important role in nutrient cycling because CO x dissolves in water forming carbonic acid (a weak acid) that can dissociate to p r o d u c e a bicarbonate ion (HCO:~-) and H+. Anions such as bicarbonate influence weathering rates and leaching losses of nutrients (see Chapter 7). Because climate strongly influences decomposition, it is a major factor controlling soil CO~ f l u x m a by-product of the oxidation of organic matter. Fung et al. (1987) r e p o r t e d a positive correlation between monthly soil COz flux and air t e m p e r a t u r e for a diverse g r o u p of terrestrial biomes, and annual soil COz flux has also been correlated to mean annual t e m p e r a t u r e (Raich and Schlesinger, 1992). Using data summarized by Raich and Nadelhoffer (1989), we calculated average annual soil CO 2 fluxes for deciduous and evergreen forests in boreal, temperate, and tropical biomes (Fig. 6.6) and found that, for a given forest type (e.g., deciduous or evergreen), they increased from high to low latitudes. In tropical and temperate regions, soil CO 2 flux was greater in evergreen

IV. Carbon Lossesfrom Forest Ecosystems

177

Figure 6.6 Average annual soil surface C O 2 fluxes for different forest biomes. Open columns are deciduous, shaded are evergreen, and the dark shaded column (temperate) is mixed forests (adapted from Raich and Schlesinger, 1992).

than in d e c i d u o u s forests but the reverse was true for boreal forests, presumably because of the negative effects of low soil t e m p e r a t u r e s a n d p o o r drainage on microbial activity in lowland boreal conifer forests (Flanagan and Van Cleve, 1983; U p d e n g r a f f et al., 1995). Correlations between microbial activity and t e m p e r a t u r e exhibit considerable variation, and attempts to u n d e r s t a n d the factors that control microbial activity have i n c l u d e d field e x p e r i m e n t s in which microbial activity has b e e n m a n i p u l a t e d by varying n u t r i e n t a n d c a r b o n supply in the soil. These studies have shown that microbial biomass is highest in the s u m m e r and lowest in the w i n t e r m c o r r e s p o n d i n g to seasonal soil CO 2 f l u x e s - - a n d that the effects of c a r b o n a n d n u t r i e n t additions on respiration are site specific. Strong positive correlations have b e e n established between net p r i m a r y p r o d u c t i o n a n d b o t h microbial biomass (Zak et al., 1994) a n d soil CO 2 flux (Myrold et al., 1989; Raich a n d Schlesinger, 1992), implying that microbial biomass a n d activity are d e p e n d e n t on detrital i n p u t and may be c a r b o n limited. Microbial activity a n d biomass are constrained in forests with large organic m a t t e r a c c u m u l a t i o n s because of low soil t e m p e r a t u r e s a n d / o r p o o r litter quality (Beare et al., 1990; B r i d g h a m a n d Richardson, 1992; Sugai a n d Schimel, 1994). Foster et al. (1980) a d d e d c a r b o n a n d nutrients to a boreal pine forest (P. b a n k s i a n a ) a n d conc l u d e d that microbial activity was mainly limited by available c a r b o n b u t also, to a lesser extent, by nitrogen. In warm t e m p e r a t e u p l a n d forests where d e c o m p o s i t i o n is m o r e c o m p l e t e a n d rapid, microbial biomass a n d soil CO 2 flux a p p e a r to be m o r e strongly related to nutrient availability. Soil n i t r o g e n availability c o m m o n l y limits microbial biomass (Zak et al., 1990; Beare et al., 1990; Wardle, 1992), whereas p h o s p h o r u s can limit mi-

178

6. Soil Organic Matter and Decomposition

B o x 6.2

Soil C O 2 F l u x M e a s u r e m e n t s

Several techniques are used to measure soil surface C O 2 f l u x (root mycorrhize respiration + microbial respiration). It is extremely difficult to partition soil surface CO 2 flux into autotrophic and h e t e r o t r o p h i c respiration. To date, the most c o m m o n m e t h o d used to measure soil surface CO 2 flux was the static alkali absorption technique (Edwards, 1982; Raich and Nadelhoffer, 1989), but there is increasing evidence that this m e t h o d underestimates soil CO 2 flux, especially at high rates (Ewel et al., 1987, Haynes and Gower, 1995). Because global estimates of soil surface CO 2 flux are largely based on static alkali absorption technique (see Raich and Schlesinger, 1991), data collected in this way may be underestimates. Portable COz analyzers e q u i p p e d with a special chamber ( N o r m a n et al., 1992; Haynes and Gower, 1995) and eddy correlation systems located near the soil surface (Baldocchi et al., 1996) have also been used to estimate soil CO 2 flux. The latter approach has the advantage that it integrates over a large area, but it may be unreliable because of the low wind speeds and p o o r (aerodynamic) mixing c o m m o n l y e n c o u n t e r e d beneath forests canopies, particularly at night.

crobial biomass in some forests (Scheu, 1990). The relative abundance of fungi in relation to bacteria can influence N cycling rates because of the potentially large differences in their C / N ratios (Paul and Clark, 1989). It is difficult to separate the contribution of microbial (or heterotrophic) respiration and root (or autotrophic) respiration because microbial respiration is influenced by root e x u d a t e s m l a b i l e carbon that leaks from roots. N u m e r o u s a p p r o a c h e s have been used to partition soil CO 2 flux into the two components, including c o m p a r i n g COz flux from rootfree t r e n c h e d and control plots (Ewel et al., 1987; Haynes and Gower, 1995), c o m p a r i n g soil COz flux from intact forests and recent clear-cuts (Nakane, 1984), 14COz-labeling and tracing studies (Cheng et al., 1993), and process-based models. The contribution of root respiration to total soil COz flux averages 0.45 but is moderately variable, ranging from 0.33 to 0.62 (Table 6.6). The average value is 15% greater than the value assumed by Raich and Schlesinger (1992). T h e r e does not appear to be any relationship between the ratio of r o o t / t o t a l soil CO 2 flux and climate; however, Raich and Nadelhoffer (1989) r e p o r t e d that annual soil CO 2 flux was positively correlated to annual litterfall. As we noted earlier, they suggested that a carbon balance a p p r o a c h (i.e., carbon loss, as

IV. Carbon Losses from Forest Ecosystems

Table 6.6

179

The Fraction of Total Soil Surface C O 2 Flux Attributable to Root Respiration

Forest type Cold temperate Mixed hardwoods Pinus resinosa Pinsu densiflora

Warm temperate Mixed hardwoods Pinus elliottii

Floodplain forest Temperate broad-leaved evergreen Nothofagus sup. Tropical deciduous Mixed

Root/total soil CO 2 flux

Source

0.33 0.57 0.50

Bowden et al. (1993) Haynes and Gower (1995) Nakane et al. (1983)

0.35 0.58-0.62 0.55

Edwards and Sollins (1973) Ewel et al. (1987) Pulliam (1993)

0.22

Tare et al. (1993)

0.50

Behera et al. (1990)

reflected by soil CO 2 flux, equals above- plus below-ground detritus input, assuming no change in soil C ) can be used to set an u p p e r limit on the total a m o u n t of carbon that can be allocated to root turnover, respiration, and exudates (see Chapter 5). In addition to being lost as carbon dioxide to the atmosphere, carbon can be leached below the rooting zone. This process is particularly important for the soil surface layer, and accumulation in lower horizons is important in soil development, especially for Spodosols (Dawson et al., 1978; Ugolini et al., 1988). The dissolved organic matter consists mostly of high-molecular-weight polymers originating from the canopy and surface detritus layer. Tree species differ greatly with respect to their production of soluble organic substances (Pohlman and McColl, 1988), and concentrations of dissolved organic carbon in the u p p e r soil vary seasonally in conjunction with microbial activity. High dissolved organic carbon concentrations can occur in the spring, coinciding with snowmelt and leaching of organic substances from the forest floor (Antweiler and Drever, 1983). As fulvic and humic acids move downward in the soil, they accumulate in the lower horizons; hypotheses p r o p o s e d to explain the arrest of these substances in these soil horizons include the metal-fulvate theory (Schnitzer, 1979), flocculation (DeConinck 1980), polymerization or decomposition by microorganisms, or adsorption on the surface of clay minerals (Jardine et al., 1989; Dahlgren and Marrett, 1991). Whatever the mechanism(s) responsible for the arrest of these solutes, their concentrations in the soil solution below the rooting zone of upland forests are l o w m c o m m o n l y less than 2 mg liter -1 (Sollins and McCorison, 1981; McDowell and Likens, 1988). However, dissolved organic

180

6. Soil Organic Matter and Decomposition

~ 14 >" E O ~

12-

o

8

lO

Swamp an lowland

-

X

O o

6 I/ ~_ /

"E

4

(3

2 0

Ip

0

I

Temperateupland forests ~ "

f

50

I

I

I

I

100 150 Annual runoff (cm)

L

J

200

Figure 6.7

Relationship between carbon export and annual runoff for rivers draining lowland boreal and temperate tloodplain forests compared to upland fl)rests (redrawn from Mulholland and Kuenzler, 1979).

c a r b o n concentrations are often substantially higher in streams draining lowland boreal and t e m p e r a t e floodplain forests than u p l a n d forests (Fig. 6.7).

V. Influence of Forest Management on Soil Carbon Dynamics Forest m a n a g e m e n t decisions, including "no m a n a g e m e n t " m o r simply allowing the forest to develop u n i n t e r u p t e d m c a n affect many of the processes controlling carbon inputs and losses and, hence, soil organic m a t t e r content. Carbon and nutrient cycles are tightly coupled (Chapter 7); therefore, any natural or a n t h r o p o g e n i c disturbance of the carbon cycle is likely to affect nutrient cycles as well. J o h n s o n (1992) reviewed the literature and found no discernable effect of harvesting on soil organic matter content; most of the studies he reviewed r e p o r t e d a net change in soil organic matter of less than _+10% (we n o t e d earlier that large spatial variability of the surface litter and soil organic m a t t e r content makes it difficult to detect small changes). In general, the m o r e intensive the postharvest site preparation, the greater the loss of soil organic matter (Johnson, 1992). Cultivation is the most ext r e m e site p r e p a r a t i o n practice and can result in large losses of soil organic matter (Mann, 1986; Detwiler, 1986), a l t h o u g h these are generally smaller than in the case of agricultural crops because, in the case of forests, cultivation occurs only once d u r i n g the stand rotation and the

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vegetation regrows immediately after planting. Organic matter losses as a consequence of site p r e p a r a t i o n are caused by increased erosion, accelerated d e c o m p o s i t i o n resulting from h i g h e r soil temperatures, i m p r o v e d i n c o r p o r a t i o n of organic matter into the soil facilitating microbial immobilization, and deterioration of soil aggregate structure. Prescribed b u r n i n g is an i m p o r t a n t m a n a g e m e n t tool used to remove logging debris before replanting, reduce fuel loads to minimize the chance of catastrophic fire, and control the invasion of undesirable competing species. In general, prescribed b u r n i n g appears to have little effect on soil organic matter content (Wells, 1971), but a long-term study has shown that f r e q u e n t fires cause redistribution of the surface organic matter to the u p p e r mineral soil (Binkley et al., 1994). However, Sands (1983) r e p o r t e d that a prescribed b u r n decreased soil organic matter by 4 0 50%. Differences in results o b t a i n e d from experiments involving prescribed b u r n i n g may be caused by factors such as fire intensity, the a m o u n t and c o n d i t i o n of the material in the litter layer, and the composition of that material. Fertilization generally increases soil carbon storage ( J o h n s o n , 1992), a l t h o u g h the reasons for this are not fully u n d e r s t o o d ; it may affect the carbon dynamics of roots and soil microorganisms, tending to increase leaf litter p r o d u c t i o n and decrease the relative allocation of c a r b o n to fine root net p r i m a r y p r o d u c t i o n (see C h a p t e r 5). The varying effects of fertilization on d e c o m p o s i t i o n may be due, in part, to the different types of fertilizer used and their effects on soil microorganisms (Fog, 1988). For example, u r e a can raise the p H of the soil immediately s u r r o u n d i n g the fertilizer pellet to 8.0, whereas a m m o n i u m nitrate increases soil acidity. Haynes a n d Gower (1995) m e a s u r e d soil CO 2 flux in t r e n c h e d and u n t r e n c h e d plots in control and fertilized red pine plantations in Wisconsin and f o u n d that soil CO 2 flux did not differ significantly between t r e n c h e d plots in control and fertilized stands but was significantly greater in the u n t r e n c h e d control than the fertilized stand for all 3 years of the study. These results suggest that fertilization had little effect on decomposition. J o h n s o n (1991) cited n u m e r o u s studies that e x a m i n e d the influence of various forest m a n a g e m e n t practices on soil c a r b o n content, a l t h o u g h few of them e x a m i n e d the effects of forest m a n a g e m e n t on the processes that control soil carbon content, such as soil CO 2 flux, h u m u s formation, and decomposition. However, it is necessary to u n d e r s t a n d how managem e n t practices affect the processes so that those practices can be altered, if necessary, before they have detrimental effects on long-term site productivity. The effects of harvesting on soil respiration are inconsistent (Fig. 6.8) but may be explained by the varying impact of harvesting on the structure of forests and, hence, on the environmental conditions af-

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Figure 6.8 Comparison of soil surface carbon dioxide flux for control (hatched columns) and clear-cut (open columns) forests during the first year after harvest. Sources of data are (1) Populus tremuloides forest in Alaska (Schlentner and Van Cleve, 1985), (2) P. densiflora forest in Japan (Nakane et al., 1984), (3) P. elliottii plantation in Florida (Ewel et al., 1987), (4) Quercus-Carya forest in Tennessee (Edwards and Ross-Todd, 1983), and (5) mixed broad-leaved deciduous forest in West Virginia (Mattson and Smith, 1993). Soil surface COz flux values for the West Virginia study are based on only 142 days.

fecting organic matter d e c o m p o s i t i o n (Binkley, 1986). It has been variously r e p o r t e d that the d e c o m p o s i t i o n rates of organic matter in clearcuts are slower (Whitford et al., 1981; Erickson et al., 1985), about the same, and faster than in control forests (Gholz et al., 1985b; O'Connell, 1988). In forests where d e c o m p o s i t i o n is strongly temperature limited, such as boreal forests, removal of the canopy increases the solar radiation reaching the soil surface and stimulates decomposition. Thus, the effect of clear-cutting on organic matter d e c o m p o s i t i o n and soil CO 2 flux is site specific. The uncertainties can be r e s o l v e d m o r at least various scenarios can be e x p l o r e d m b y the use of process-based d e c o m p o s i t o n models (Chapter 9). The r e p l a c e m e n t of native vegetation by forest plantations is widespread in some regions of the world (Chapter 2), but there is little information about the effects of different tree species on soil organic matter. Altering species or land use practices is likely to influence the soil carbon cycle because detritus p r o d u c t i o n , carbon allocation, and decomposition rates differ a m o n g species. Nitrogen-fixing species c o m m o n l y increase soil organic matter because they lead to e n h a n c e d detritus p r o d u c t i o n (Johnson, 1992; see also C h a p t e r 7). As discussed earlier, cultivation decreases soil carbon; therefore, it should not be surprising that reforestation or afforestation increases the soil C content of agricultural or a b a n d o n e d soils (Table 6.1). The effects of reforestation of previously forested soil are less clear. Alban et al. (1982) c o m p a r e d soil properties

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in four 40-year-old tree plantations established in a c o m m o n g a r d e n design (i.e., adjacent plots on a similar soil) and f o u n d that forest floor organic matter content did not differ between the species, but the soil C content did: soil C b e n e a t h the aspen plantation was significantly less (31 and 18% at two sites) than u n d e r white spruce, red pine, and jack pine. In a similar c o m m o n g a r d e n study design, Gower and Son (1992) f o u n d that forest floor biomass differed by 80% (8.7-42.8 mg ha -I) a m o n g 30-year-old E u r o p e a n larch, red and white pine, red oak, and Norway spruce plantations in southwestern Wisconsin. Few studies have examined the effect of species on the m o r e stable carbon constituents in forest soils.

VI. Role of Forest Soils in the Global Carbon Budget An estimated 1400 • 1015 g C are sequestered in the form of organic matter in soils across the earth, of which 34% is in forest soils (Post et al., 1982). As discussed earlier, the carbon content of forest soils is subject to large changes caused by changes in land use. H o u g h t o n et al. (1983) calculated the change in global soil and vegetation carbon content over, approximately, the past century ( 1 8 6 0 - 1 9 8 0 ) using land use statistics to calculate agricultural expansion and deforestation. They estimated that the release of carbon in 1980 from land, including soil, was between 1.8 and 4.7 • 1015 g C year -1, c o m p a r e d to 5 • 1015 g C year -1 released by the c o m b u s t i o n of fossil fuel (Rotty and Masters, 1985). However, as J o h n s o n (1992) points out, the H o u g h t o n et al. estimate is based on the assumption of 35% c a r b o n loss following deforestation, and empirical data do not s u p p o r t this assumption. These large discrepancies make it difficult to balance the global carbon b u d g e t and illustrate the n e e d for better u n d e r s t a n d i n g of the effects of land use practices, including forestry, on soil carbon cycling processes. One likely effect of global warming is accelerated d e c o m p o s i t i o n , resulting in a greater emission of CO 2 into the atmopshere. This would further exacerbate global warming. Experimental soil warming studies have shown that increasing the soil t e m p e r a t u r e by 5~ above ambient caused annual CO 2 flux from a n o r t h e r n h a r d w o o d forest to increase from 712 to 1250 g C m -2 year -1 (Peterjohn et al., 1994). J e n k i n s o n et al. (1992) used the R o t h a m s t e d soil carbon cycling m o d e l to examine the a m o u n t of carbon that could be released u n d e r different climate change scenarios. They estimated that an average t e m p e r a t u r e rise of 0.03~ across the globe would increase CO 2 release by 61 • 1015 g C over the next 60 y e a r s m e q u i v a l e n t to a b o u t 25% of the CO 2 that will be released from the c o m b u s t i o n of fossil fuel if fuel c o n s u m p t i o n remains u n c h a n g e d

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over the next 60 years. Their analysis assumes annual detritus inputs will remain u n c h a n g e d and that t e m p e r a t u r e increase will be uniform worldw i d e - - b o t h of which are highly suspect assumptions (see Chapter 5). Nevertheless, such analyses are useful because they force evaluation of the influence of abiotic and biotic factors on soil carbon cycle and its impact on the global C cycle.

VII. Concluding Remarks Soil organic matter is often the largest carbon pool in forest ecosystems, with many biological, chemical, and physical properties that have beneficial effects on tree growth. Therefore, forest managers should be aware of how forest m a n a g e m e n t practices affect the processes responsible for carbon inputs to and losses from forest soils. Unfortunately, our u n d e r s t a n d i n g of the effect of natural factors and m a n a g e m e n t practices on these processes is incomplete, a l t h o u g h it is clear that m a n a g e m e n t practices that alter the interrelationships between primary producers and d e c o m p o s e r s could alter the long-term productivity of forest soils. There is a great need for better data on soil carbon contents and better u n d e r s t a n d i n g of microbial processes as a basis for the development of more realistic soil carbon and nutrient cycling models. Because of the large a m o u n t of carbon stored in soils and its susceptibility to climate warming and land use, soil carbon dynamics have significant implications for global ecology.

Recommended Reading Johnson, D. W. (1992). Effects of forest management oil soil carbon storage. Water Air Soil Pollut. 64, 83-120. Harmon, M. E. et al. (1986). The ecology of coarse woody debris in temperate ecosystems. Adv. Ecol. Res. 15, 133-302. Paul, E., and (;lark, (1989). "Soil Microbiology and Biochemistry." Academic Press, San Diego, CA. Stevenson, F.J. (1982). "Humus Chemistry." Wiley, New York. Raich, J. W., and Schlesinger, W. H. (1992). The global carbon dioxide tlux in soil respiration and its relationship to vegetation and climate. 7~llus 44B, 81-99.