- Email: [email protected]
Measuring the environmental effects of converting cropland to short-rotation woody crops: A research approach
Pergamon PII:
Bb~wss oud Biounwg~~Vol. 13. Nos. 4/5, pp. 30-31 I. 1997 t_’1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain 096l-9534/97 $17.00 + 0.00 SO961-9534(97)10017-4
MEASURING THE ENVIRONMENTAL EFFECTS OF CONVERTING CROPLAND TO SHORT-ROTATION WOODY CROPS: A RESEARCH APPROACH J. tCollege
D. JOSLIN* and S. H.
*Tennessee Valley Authority, of Forest Resources. Mississippi
SCHOENHOLTZ?
Atmospheric Sciences, Norris, Forestry and Wildlife Research MS. U.S.A.
TN, U.S.A. Center. Mississippi
State,
Abstract-Conversion of cropland to short-rotation woody biomass crops (SRWC) has received increasing interest as biomass utilization technologies have improved and concerns for effects of fossil fuel emissions on global climate have developed. Effects of this conversion on erosion, hydrology, water quality and soil productivity may be significant. A large cooperative research project began in the spring of 1995 at three sites representative of the lower Tennessee Valley to compare the environmental effects of growing traditional row crops with the production of SRWCs over 3- to 5-year rotations. This paper presents the research approach be used to evaluate these effects and a few preliminary results from the initial 3 months of the study. Small watersheds cultivated in row crops: corn (Zea with small watersheds in tree crops: mr,ys L.) or cotton (Gossypium hirsutum L.), are being compared (Plaranus occidentalis L.). sweetgum (Liquidambar styrac@a L.) or eastern cottonwood sycamore (Populus deltoides Bartr.) with respect to: (1) erosion; (2) run-off quality (nutrients, pesticides) and quantity; (3) groundwater quality; (4) soil chemical changes (carbon, nutrients, pesticides); (5) soil physical changes (infiltration, bulk density. aggregate stability); (6) soil biological changes; and (7) wildlife populations. During the spring and summer of the first growing season, few differences in run-off quantity and erosion were observed between treatments. One exception was a tendency towards higher erosion under cotton than cottonwood. Larger differences are expected in later years as trees become established and a litter layer develops. At two sites during the first growing season, differences between row crops and SRWCs were observed in both the runoff and leaching of NOx-N, NHq-N. P, Ca, Mg and K in spring following fertilization of the row crops only at these two sites. Wildlife studies on small mammals and bird populations, as well as microfauna, are just getting under way. lc 1998 Elsevier Science Ltd. Keywords-Short-rotation deltoides; environmental
woody biomass crops; Plutanrrs occidentulis; Liquidambar styrac@a: impacts; erosion; water quality; soil quality.
Populu.s
(Populus sp.), eastern cottonwood (Populus deltoides Bartr.), sweetgum (Liquidambar styraciflua L.), sycamore (Plutanus occidentalis L.) and others, is to be become a significant component of power generation programs in the southeastern United States, hundreds of thousands of hectares of land currently in conventional agriculture will have to be planted with SRWCs. The impact of this conversion on water, air and land resources have received very limited evaluation.‘.2 The net environmental impact of conversion from cropland to SRWCs depends on prior land use, SRWC and crop management methods, soil type, local hydrology and climate. In general, conversions are expected to affect water quality by reducing soil erosion as well as levels of nitrate, phosphorus, pesticides and herbicides in surface run-off and groundin run-off and leaching water. ‘,2 Alterations quantities and their timing are also predicted
1. INTRODUCTION
Use of bioenergy crops as a feedstock in the generation of electricity is receiving considerable attention based chiefly on the development of improved technologies for biomass utilization in power generation, and pressures to reduce emissions of nitrogen oxides, sulfur dioxide and carbon dioxide from coal-fired power plants. One approach to reducing net carbon dioxide emissions is the development of closed-loop biomass systems to reduce the emission of additional carbon dioxide from factor in the fossils fuels.’ A secondary increased interest in bioenergy crops is the potential environmental benefits projected to occur with the conversion of conventional cropland to such crops. If conversion of short rotation woody bioenergy crops (SRWCs) to electricity using fastsuch as hybrid poplar growing species, 301
J. D. JOSLIN and S. H. SCHOENHOLZ
302
based on increased potential for rainfall interception and evapotranspiration by the perennial root systems of SRWCs. Effects of conversion on soil physical, chemical and biological properties are less certain and depend to some extent on biomass residue removal and cultivation intensity. Conversion to SRWCs may also provide improved wildlife habitat.3 All the above predicted impacts, however, are almost entirely based on what is generally known about row crop systems and short-rotation forestry systems. Controlled field comparisons using realistic operational methods have not been conducted in the United States to our knowledge. Such assessments are needed before embarking on largescale conversions of cropland to SRWCs. The primary objective of this paper is to describe an ongoing large-scale, cooperative research approach which is providing information on key environmental effects of conversion of agricultural row cropland to SRWCs in the southeastern U.S. This research comprised three coordinated projects at sites representing three distinct physiographic regions in the southeast. At these sites, a study was initiated in the spring of 1995 to evaluate intensively the effects of replacing cotton (Gossypium hirsutum L.) with eastern cottonwood, corn (Zea mays L.) with sweetgum and corn with sycamore. Environmental impacts are being assessed in the following order of priority: (1) erosion; (2) surface run-off water quality; (3) groundwater quality; (4) surface run-off quantity and timing; (5) soil physical and chemical properties; (6) soil biological properties; and (7) wildlife.
2. GENERAL RESEARCH
APPROACH
Because the intent of this paper is to present the broad outline of a research approach, the research methods described in this paper will focus on non-standardized measurement techniques and procedures of site selection, installation, etc. In general, specific details concerning sampling or analytical methods will not be presented. Many of the methods used to measure soil and sediment parameters can be found in Klute4 and Page et a1.,5 and most methods for chemical analysis of water are standard U.S. E.P.A. methods6 with the exception of bioavailable phosphorus.’
2.1. Site selection The three sites selected in this study were chosen to represent the three physiographic regions of the Tennessee Valley which economic analyses had previously determined to be the most viable for cost effective production of SRWCs.* Soils in these regions were projected to produce woody biomass at high yields on short rotations such that they could conceivably replace conventional row crops. The three regions selected were the upper Mississippi Delta, the loess belt of west Tennessee/northern Mississippi and the limestone valley region of northern Alabama. After choosing the physiographic regions to be represented, specific sites were selected based on the following criteria: (1) presence of land owned by an institution interested in cooperating in such a research effort; (2) presence of nearby supporting staff and facilities to manage the daily research operations and site maintenance; (3) current and recent historical use of the site for agricultural row crops; (4) a soil series and slope class appropriate for productive and intensive management of SRWCs; (5) an area large enough to accommodate at least six small (0.25-2 ha) replicated watersheds with relative uniformity of soil type, slope and past and current land use so that variability among watershed replicates would be minimal. The three sites selected for this study were: (1) the Delta Research and Extension Center at Stoneville, Mississippi operated by the Mississippi Agricultural and Forestry Experiment Station and Mississippi State University; (2) the Ames Plantation, located in the loess belt of west Tennessee near Grand Junction, site of the University of Tennessee Agricultural Experiment Station at Ames; and (3) the Alabama A and M University Agricultural Experiment Station at Hazel Green, Alabama. Aerial photographs, previous soil surveys, detailed topographic mapping of each site and on-site soil coring were all employed prior to the location of replicated watersheds. The establishment of replicated watersheds was essential for comparison of surface run-off quantity, timing and quality (including erosion) among the cropping treatments. While the replicated watersheds were established to examine surface run-off, they also became the basic experimental plot units for measurement
Environmental
effects of converting
of effects on groundwater chemistry and physical, chemical and biological properties the on-site soil resource.
on of
2.2. Treatments The primary comparison being made at each site is between row crop agriculture and short-rotation woody-crop silviculture. The row crop chosen for each site represents a common crop within the physiographic region, and cultivation practices were similarly chosen to be representative of modern conservation tillage practices. The lone exception was cotton at the Mississippi Delta site, where conventional tillage is the common practice. Tree crops were selected to represent the three most frequently recommended hardwood species for intensive short-rotation management in the southeastern U.S. (J. Tuscan and L. Wright 1994, U. S. Department of Energy Biofuels Feedstock Development Program, Oak Ridge National Laboratory, Oak Ridge, Tennessee, pets. commun.).’ Conveniently, a single tree species was considered most appropriate for the soil type selected at each location: (1) eastern cottonwood for comparison with cotton at the Mississippi Delta site (3-year rotation); (2) sycamore for comparison with silage corn at the Tennessee site (4-year rotation); and (3) sweetgum for comparison to corn at the limestone valley site at Hazel Green, Alabama (4year rotation). Table 1 summarizes site characand SRWC planting practices. teristics Silvicultural practices for each SRWC were selected to represent high-intensity management for maximum yield, i.e. extensive use of fertilizers, insecticides, herbicides and other weed control techniques, to be mechanized to the extent possible at an operational scale for forestry and to be cost-effective. One exception included an additional treatment at the Alabama site, where a “total weed control” treatment was added for comparison with a more “practical” cover crop, plus some weed control. At this site, a fourth treatment was also instituted for comparison with an important herbaceous bioenergy crop, i.e. switchgrass (Pa77icum virgatum L.). Table Site Stoneville. MS Hazel Green AL (A and M) Ames. TN
I. Site and planting Slope (X) 0.2-0.3 3-4 2
cropland
to woody
303
Small, artificially constructed watershed plots were created in three replicates of each treatment (crop vs SRWC) at the Mississippi Delta and Tennessee sites by enclosing 0.6 ha pentagonal plots with raised earthen berms. At the Mississippi site, treatments were assigned randomly. At the Tennessee site, the three SRWC plots were located contiguously to provide a large block of trees (approximately 5 ha total) to study wildlife impacts at a scale approaching that perceived to be realistic for silvicultural operations. At the Tennessee site, soils and slopes were particularly uniform, and hydrologic and soil comparisons are being conducted early in the study to establish any pre-treatment watershed differences. Four watershed treatments were established at the Alabama site: (1) corn; (2) sweetgum with total weed control; (3) sweetgum with cover crop; and (4) switchgrass. Each were replicated twice using bermed watersheds in two separate blocks. Additional studies at the Alabama site are being conducted to examine the impact of cover crop species and row width on erosion and tree productivity. At all three sites 15-m wide buffer strips, into which each row crop/SRWC extends beyond its individual watershed’s earthen berms separate the watersheds.
3. SELECTED
3.1. Surface
BROAD PREDICTIONS: VS SRWCS
run-q#‘hydrolog?j
for short rotation
SRWC
Planting
crop
ROW CROPS
and erosion
During the first year there should be only small differences between treatments within a site in surface run-off hydrology, as both types of crops are planted anew in the spring and establish root systems gradually in the growing season. In subsequent years, however, tree crops should transpire a greater amount of water than row crops, because: (a) full canopy cover will be achieved by trees earlier in the growing season; and (b) perennial tree root systems should exploit a deeper portion of the soil profile and extract more water therefrom in spring and early summer when row crop root systems are still developing. We predict
characteristics
Cottonwood Sweetgum Sycamore
crops
material
Cuttings Seedlings Seedlings
woody
crops
Parent
material
Riverine sediments Limestone residuum Loess
Spacing 1.2 x 3.6 m 1.5 x 3.0 m 1.5 x 3.0 m
304
J. DJOSLIN
and S. H. SCHOENHOLZ
that tree crops will also improve infiltration capacity via improved litter cover, better soil structure and higher porosity (see predictions for “Soil Physical and Chemical Properties” below). Improved infiltration should result in reduced surface run-off, especially in spring and autumn, as well as in less “flashy” run-off hydrographs.’ During the first year of establishment, we predict little difference between treatments in erosion, as the amount of exposed soil will be comparable under row crops and trees.2 However, in subsequent years, improved litter coverlo’l ’ and reduced soil surface disturbance caused by cultural operations should reduce erosion on the SRWC plots substantially.g Improvements in litter cover are expected from both (1) the absence of removal of tree litter as opposed to considerable removal of crop litter during annual harvest operations, and (2) the building of minimal weed ground cover following canopy closure. Herbicide use will be discontinued once crown closure is achieved and the subsequent small build-up of weeds under the crown should provide some surface protection and retard the movement of litter by the wind. The reduction in run-off and flatter hydrographs predicted above should also reduce erosion on SRWC plots.’ 3.2. Chemistry of surface run-of water
and ground-
Because SRWCs generally require less fertilizer inputs than row crops,2,“,‘2 especially sweetgum and sycamore, we predict less runoff of most macronutrients, especially nitrogen and phosphorus, two major stream contaminants. The flux of cations should also be reduced in run-off because their export should be closely related to nitrate run-off, cations being required to balance negatively charged nitrate in run-off solutions. In the years following SRWC establishment, this effect could be particularly pronounced immediately following spring application of fertilizers, since SRWCs with their established root systems are probably more capable of rapid utilization of nutrients during spring leaf flush than row crops. ‘* Also, storage of nitrogen via immobilization in surface organic matter may enhance nitrogen retention later in SRWC rotations.‘3’14 Soil organic matter may also help reduce export of herbicides and pesticides by acting as adsorbance sites and by facilitating their microbial breakdown. Furthermore,
fewer pesticides will be used on SRWCs in this study than on row crops. All three tree species are native and pesticides will only be used on them in the case of serious infestations. As with surface run-off, we predict less movement of nutrients below the rooting zone of SRWCs. This prediction applies particularly to nitrogen (and attendant cations) since phosphorus is generally rather immobile in soils. Less fertilizer will be applied to SRWCs, and we expect established perennial tree root systems (after the first year) to be more efficient at recovering fertilizer than row crops, particularly in the spring when row crop root systems are not fully established. 3.3. Soil physical and chemical properties We predict improvements in soil porosity, bulk density and aggregate stability with SRWCs over time in the surface layers. This process may take several years for detection. Additions of organic matter should improve aggregate stability and bulk density,“-” and the development of root channels should improve porosity.‘8-2’ These impacts should in turn reduce penetrometer resistance and improve infiltration and aeration. Infiltration should also be improved because of better litter cover and reduced surface “puddling”.9~” We expect few large changes to occur in quantities of soil nutrients over the course of these short rotations. Changes in soil nutrient reserves of nitrogen and phosphorus will be controlled largely by the balance of fertilizer inputs and exports resulting from harvest and runoff. Loss of base cations and some micronutrients with nitrate run-off and leaching may result in some significant pool changes. We also expect at most small changes in soil organic matter, as it is traditionally measured, in the mineral soil horizons.‘5722 In the SRWC treatments, however, we do expect increases in both the surface litter layer resulting from tree litter and weed cover inputs,‘09” and in the coarse organic matter fraction within the mineral soil. The initial coarse organic matter fraction on all three sites is near zero, and the input of dead tree roots of greater diameter than crop roots should increase this component measurably. Soil biological changes are difficult to predict, but substrate changes resulting from introduction of considerable leaf litter input should favour earthworm population buildups “*23 as well as Opilionida and Isopoda.24
Environmental
effects of converting
cropland
These leaf litter inputs, plus the introduction of more coarse lignified root material, may also stimulate larger fungal populations.‘3.‘4 3.4. Wildlif It remains to be seen if areas as small as
5 ha will have significant impacts on avian and large mammal populations. Surely an increase in cover for both will be provided during the later years of rotation. Small mammal populations, on the other hand, should be affected over time by increases in ground cover and changes in food sources.3 The size and diversity of bird populations should be quite similar in the initial year or two, but should increase by the third year as a result of increased nesting sites, more ground and near-ground cover, and increasing insect populations.3*25726
AND MEASUREMENTS-AN OVERVIEW
crops
305
1 LJyJ C
I I
Upslope
Soil berm
4. METHODS
to woody
J
T
I
I
I
I
I
I
I
I
\
I
Lysimeter
‘1 Equipment building
4.1. Surface run-of hydrology and erosion
Fig. 1. Plot layout at Stoneville, MS site, illustrating schematically the location of pan lysimeters, H-flumes, equipment buildings and orientation of three SRWC (T) and three cotton (C) plots.
Direct measurement of sediment contained in surface run-off is the most accurate method of quantifying erosion from a site. It is also critical to the measurement of other potential impacts on streams including (1) potential inputs of nutrients, herbicides and pesticides, and (2) impacts on timing and quantity of stream flow. Run-off measurements are also the most accurate way to estimate removal of substances from a site by this pathway, as well as a very useful parameter for indirect measurement of infiltration. Measurement of surface run-off clearly requires a land area with well-defined boundaries and a control point where water can be monitored and sampled. Natural watersheds of appropriate size for the research described herein are both difficult to find and of great natural variability in topography, soils, etc. In their place, we created artificial “watersheds” using 0.5 m high berms to surround pentagonally-shaped land areas of approximately 0.6 ha (Fig. 1). The pentagons can be best visualized as rectangles, elongated in the downslope direction, with a triangle added at the downslope end to direct water to a control point. Each control point has a 2-m flume approach section and a 0.5-m “H-flume” equipped with a flow meter and an automated flow-proportional sampler. The flow meter continually measures volume and timing of flow during
each storm. The flow proportional sampler collects water samples for later analyses of concentrations of sediment, nutrients, herbicides and pesticides on a flow-proportional basis. Fluxes of these substances in surface flow can be calculated for each storm. Fluxes of sediment, nutrients and pesticides can also be determined for portions of individual storm hydrographs if desired. In addition to sediment, specific nutrients being measured in run-off include N (separately as nitrate and ammonium), P (separately as orthophosphate, total P, and bioavailable P), Ca, Mg, K, Na, B, Fe, Mn, Cu, Zn and Al. Except for P, for which sediment-related concentrations are being continually measured, estimates of nutrient fluxes contained in sediment will be made via analysis of periodic sediment sub-samples. Similarly, export of selected herbicides and pesticides will be estimated from periodic samples of water and sediment, since continuous monitoring of these substances is too expensive. Special attention is given to P in surface run-off because it is a major factor in the eutrophication of bodies of water in the southeastern U.S.7,27 Measurement of surface run-off and its chemistry is the most expensive component of the evaluations attempted in this research pro-
306
J. D. JOSLIN and S. H. SCHOENHOLZ
ject. On unique sites where surface run-off may be a very minor component of the hydrologic budget (e.g. sites with highly permeable soils and low rainfall intensity), one may wish to consider whether the value of these measurements justifies their cost. Also, in any such study employing watershed comparisons, the hydrology of individual watersheds ideally should be examined for at least 1 year prior to treatment initiation in order to “calibrate” the watersheds for pre-existing hydrologic differences.28 In practice, funding requirements for such additional data collection and delays produced in obtaining results made this impractical for our study. 4.2. Chemistry of surface run-of and groundwater
The contribution of a given watershed plot to below-ground water storage areas requires measurement of the flux of water and chemicals below the rooting zone. Once water travels below the rooting zone there is usually very limited movement upward into plants or back into the atmosphere via evapotranspiration. Two major exceptions to this are: (1) lateral flow which reappears at the surface downslope; and (2) water refluxed to the surface in soils, where the water table periodically reappears at the surface. While both of these circumstances present considerable problems for measuring the downward flux of water and nutrients to groundwater, selection of sites that are well-drained with permeable soils reduces these concerns. To measure the downward flux of water, nutrients and agrochemicals, we installed pan lysimeters (91 cm length x 61 cm width x 8 cm depth) to capture the flow of zero-tension water. Such lysimeters not only collect a representative sample of such water, but also allow an estimation of volume of the downward flux of water.29 Four pan lysimeters were installed in the mid-slope region of each watershed plot at each site. At the Tennessee and Alabama sites, pan lysimeters were installed at 1.5 m. This depth was chosen as the practical base of the rooting zone. While selection of such a depth is arbitrary and very specific to the soil and crop being examined, 1.5 m was believed to be a practical limit for a large percentage of the roots for the short-rotation tree-cropping period being examined and for the particular soils involved. Whereas some roots may reach beyond this depth by the end of the study,30
the bulk of the root system of most trees lies within 1 m of the surface,“T3’ and the amount of water and nutrients extracted below this level should be typically be quite minor. Because the Mississippi site may have water table fluctuations to within 1 m of the surface, lysimeters were installed at this site at approximately 80 cm to avoid problems with the reflux of groundwater. Lysimeters were installed into the up-slope faces of soil pits which were excavated with a backhoe. Disturbance of the area immediately above each pan was minimized. Such pan lysimeters should provide water samples with chemistry that is representative of water flowing through the soil profile (i.e. zero-tension water).32’33The volume of water collected by such pans, however, has high spatial variability.29.34 Consequently, water budgets for each watershed must be maintained. One alternative to using pan lysimeters is the use of tension lysimeters. These lysimeters are much easier to install and, thus, make examination of spatial variability less difficult. The major disadvantage of such lysimeters is that they sample water under tension. During periods between storms, these lysimeters collect water with longer residence times and under higher tension, so that water chemistry from tension lysimetersmay differ from thatofzero-tensionwater. 4.3. Soil physical and chemical properties The soil physical changes which we believe to be of greatest interest if impacted by conversion to SRWCs are those relating to (1) infiltration rate, (2) root penetration and (3) water-holding capacity. Specific soil physical properties being measured related to these characteristics are (a) aggregate stability, (b) porosity, (c) bulk density, (d) hydraulic conductivity and (e) penetrometer resistance. Pretreatment measurements of these parameters will be compared with mid-rotation. pre-harvest and post-harvest measures. Whereas the above measures provide indicators of soil change that may relate to infiltration, measurement of infiltration rates per se are needed. Direct and indirect measures of infiltration being used include (a) double ring infiltrometry3s%36 and (b) the use of run-off fractions to calculate infiltration.3s For a given storm, this calculation is determined from the difference between precipitation and the sum of interception (surface evaporation) losses and surface run-off losses.
Environmental
effects of converting
Soil chemical changes to be examined in this study include the buildup or release of nutrient stores and organic matter (including coarse organic matter, i.e. particles not passing through a 2 mm sieve). Soils have been intensively sampled by depth and soil nutrient reserves determined to a depth of 3 m. Changes in the size of soil nutrient pools will be determined at the end of the rotation following remeasurement. Changes in size of these nutrient pools may be small over the short rotation period being studied (3-4 years) so that run-off and groundwater leaching estimates may be more accurate measurements of change. Changes in the size of soil organic matter pools are the most likely avenue by which water-holding capacity (WHC) of soils might be altered. The role of organic matter in the WHC of soils is well-established,““6.‘9 whereas the other two major factors influencing WHC, texture and type of clay,16 are not likely to be influenced by the treatments or the time period of this study. Buildup of litter layers on the plots will be carefully measured, as this factor relates to water-holding capacity, infiltration, erosion potential and surface evaporation. Coarse organic matter (i.e. organic matter not passing through a 2 mm sieve) content of the mineral soil horizons is also being monitored. This component is often overlooked in soil organic matter determinations which employ sieving prior to analysis, but it may be significant different between soils in which row crops have been grown and those supporting SRWCs. Soil biological parameters that are being compared between conventional crops and SRWCs include populations of bacteria, fungi, actinomycetes and other microfauna and mesofauna, including arthropods, nematodes and earthworms. Introduction of earthworms to selected areas to examine the potential of their populations will be build-up attempted. 4.4. Wildlife Effects on larger vertebrate wildlife are also being examined at the Tennessee site where the SRWC area is largest. Avian populations will be monitored in the spring, summer and winter throughout the study on both row crop and SRWC areas. Small mammal populations will also be monitored during these same seasons of the year. Visitations to the two types of plots by larger mammals will be period-
cropland
to woody crops
ically monitored dors for tracks.
307
by examining cleared corri-
5. SELECTED
PRELIMINARY
RESULTS
Whereas the major objective of this paper is to describe a research approach to evaluating the environmental effects of converting agricultural row cropland to short rotation woody crops, at the time of writing some data relating to the hydrology and chemistry of surface runoff and groundwater were available. These results pertain only to the initial 3 months of the first growing season of the study. We present them here to provide examples of the type of data that will be generated by this approach and to give some preliminary indication of effects during the establishment phase. 5.1. Surface run-ofl hydrology and erosion Few differences were expected between SRWCs and row crops in runoff quantity or erosion during the first 3 months of the study, since both types of crops have a period of establishment during which soil exposure is similar. This expectation was met at the Tennessee and Alabama sites, but at the Mississippi site sediment losses tended to be higher under cotton (Fig. 2). This higher erosion rate is probably the result of intensive cultivation required to create elevated beds and to provide weed control for cotton production. Run-off volume was also slightly greater under cotton than under cottonwood, perhaps indicating an early improvement in infiltration under the cottonwood trees. This hypothesis was confirmed by the fact that the fraction of precipitation which became run-off during late spring/early summer averaged 0.40 under trees and 0.53 under cotton. The run-off fraction increased dramatically, to as high as 0.99 in cotton, when two precipitation events were closely spaced, implying that the soil surface was effectively sealed during the first event by puddling. 5.2. Chemistry
of surface
run-ofl
and ground-
water
run-off data from both the Spring Tennessee site (Figs 3 and 4) and the Alabama site (Fig. 5) reflect the impact that the application of fertilizers to row crops only had on nitrogen and phosphorus export from crops and SRWCs at those
J. D. JOSLIN and S. H. SCHOENHOLZ
308
Sediment in Runoff 600
500
400 iii f UI 5 E 300 2 ._ z *
200
100
0
1
2
1
3
Cotton Plots
2 Tree Plots
Fig. 2. Total quantity (kg ha-‘) of sediment lost through flumes via surface run-off for each cotton SRWC (Tree) plot in runoff at Stoneville, MS site during event of 31 May 1995.
two sites. Based on foliar colour and size, tree survival and growth rates and the relatively high concentrations of nutrients in soil water samples collected during the
3
and
spring and summer compared with typical concenforest plantation or “natural” trations, trees at these two sites (sycamore and sweetgum) appeared to obtain adequate
Nitrate in Runoff
1
2
Corn Plots
3
1
2
3
Tree Plots
Fig. 3. Total quantity (kg ha-‘) of nitrate N lost through flumes via surface run-off SRWC (tree) plot at Ames, TN site during the period 4 May-3 August
for each corn and 1995.
Environmental
effects of converting
cropland
to woody
crops
309
Ammonium in Runoff 1.500
1.250
s i3
1.000
Y
z o.750 2 =
0.500
0.250
0 000
1
2 Corn Plots
3
1
2 Tree Plots
3
Fig. 4. Total quantity (kg ha-‘) of ammonium N lost through flumes via surface run-off for each corn and SRWC (tree) plot at Ames, TN site during the period 4 May-3 August 1995.
nitrogen and phosphorus during the first growing season from residual soil supplies of those nutrients. These supplies are apparently adequate as a result of both soil sto-
rage in long-term reserves and recent contributions from previous year’s crop fertilization. With the lack of fertilizer additions to the soil surface of SRWCs at
Bioavailable P in Runoff 2.000 1.800 --
1.600 --
1.400 -G f 2
1.200 -~
n. g 1.000 -m % 2 0.800 -.o a 0.600 --
Block 1
Block 2
Fig. 5. Total quantity (kg ha-‘) of bioavailable P lost through flumes via surface run-off for each corn, switchgrass. and SRWC plot at Hazel Green, AL site during the period 1 May-5 August 1995.
J. D. JOSLIN and S. H. SCHOENHOLZ
310
these two sites, run-off of nitrogen and phosphorus during the first 3 months was near zero. During the second year, trees at these two sites will be fertilized with nitrogen and phosphorus, but at levels approximately half those of the row crops. In addition to lower foliar requirements for most nutrients, trees in later years will have established root systems, which should initially recover spring-applied fertilizer more efficiently than newly-developing root systems of row crops. Similar to run-off loss of nutrients, spring concentrations of nitrogen (nitrate and ammonium) in water moving downward through the soil was higher under row crops than under trees at the Tennessee (sycamore) site (Fig. 6). The elevation of nitrate in the soil water also resulted in 2-3-fold increases in calcium, magnesium and potassium concentrations below 1.4 m at this site. Large differences between SRWCs and row crops in nutrient movement to groundwater are also expected during subsequent years, for the same reasons given for runoff, i.e. smaller applications of fertilizer and more efficient utilization of nutrients by the trees.
6. SUMMARY
AND CONCLUSIONS
The conversion of large land areas in the southeastern U.S. from row crops to short rotation woody crops should have substantial effects on the environment. Long-term field trials are needed to evaluate the direction and magnitude of the effects of conversion on erosion, on surface run-off quantity and quality, on groundwater quality, on soil physical, chemical and biological properties and on wildlife populations. This paper describes a research approach to such an evaluation that uses replicated bermed watersheds of approximately 0.6 ha in size as the basic study plot unit. Results from the initial growing season of this study support the working hypothesis that few differences will be seen in run-off quantity or quality between row crops and SRWCs during the establishment phase for the trees. Exceptions during the initial 3 months include a trend toward higher erosion under cotton than cottonwood, and higher run-off and leaching of nitrate under fertilized corn than under unfertilized sycamore or sweetgum. Within 3-5 years after SRWC establishment, we will evaluate the working hypotheses that SRWCs compared with row crops will have: (1) higher infiltration rates
Soil Solutions 16
14
Nota:Ca Valuea are
(mglO.lL)
6
T Nitrate - N
NH4
L w
4
K
Ca
Fig. 6. Mean nutrient (nitrate N, ammonium N, Mg, K and Ca) concentrations in soil solutions collected by pan lysimeters at Ames, TN site on 17 May 1995. Note that Ca concentrations should be multiplied by 10 to obtain equivalent concentrations (mg I-‘) of other nutrients.
Environmental
effects of converting
and less surface run-off and erosion; (2) less run-off and groundwater contamination with nutrients; (3) improved soil porosity, bulk density and aggregate stability; and (4) greater biological diversity.
REFERENCES I. OTA (U.S. Congress, Office of Assessment), Potential Environmental Impacts of Bioenergy Crop Production Background Paper, OTA-BP-E-118 Washington DC: U.S. Govt. Printing Office, 1993. 2. Ranney. J. W. and Mann, L. K., Environmental conBiomass siderations in energy crop production, Bioenergy. 1994, 6, 2 I l-228. D. P., Hoffman, W., Hanowski, J. M.. 3. Christian, Niemi, G. J. and Beyea, J., Bird and mammal diversity on woody biomass plantations in North America. Biomass and Bioenergy. in press. 4. Klute, A.. Methods of Soil Analysis Part 1: Physical and Mineralogical Methods. 2nd edn, No. 9, Part I. American Society of Agronomy, Madison, WI., 1986. 5. Page, A. L., Miller, R. H. and Keeney, D. R., Methods of Soil Analysis Part 2: Chemical and Microbiological Properties. 2nd edn, No. 9, Part 2. American Society of Agronomy, Madison, WI, 1982. 6. American Public Health Association (APHA), Standard Methods for the E.uamination of Water and Wastewater. 17th edn, American Public Health Association, Washington, DC, 1989,. 7. Sharpley, A. N.. Troeger, W. W. and Smith, S. J., The measurement of bioavailable phosphorus in agricultural runoff, J. Environ. Qual., 1991. 20, 235-238. 8. Downing, M. and Graham, R. L., Evaluating a biomass resource: the TVA region-wide biomass resource assessment model. In Proc. 1st Biomass Conf: of the Americas: Energy, Environment, Agriculture, and Industry. Burlington, VT, 30 August-2 September, 1993. 9. Schwab, G. 0.. Frevert. R. K., Edminster, T. W. and Soil and Water Conservation Barnes, K. K., Engineering, 2nd edn. John Wiley and Sons. New York, 1966. IO. Bray. J. R. and Gorham, E., Litter production in forests of the world. In Advances in Ecological Research, 2., ed. J. B. Cragg. pp. 101-157. Academic Press, New York, 1964. I1 Pritchett, W. L., Properties and Management of Forest Soils. John Wiley and Sons, New York, 1979. 12 Ericsson, T., Rytter, L. and Linder, S., Nutritional dynamics and requirements of short rotation forests. In Ecophysiology of Short Rotation Forest Crops, eds C. P. Mitchell, J. B. Ford-Robertson, T. Hinckley and L. Sennerby-Forsse, pp. 35565. Elsevier Applied Science, London, 1992. M., introduction to Soil Microbiology. 13 Alexander, John Wiley and Sons, New York, 1977. 14 Brady, N. C., The Nature and Properties of Soils. MacMillan. New York, 1990. R. F., Rolfe, L. and Eastburn, R. P., 15 Fisher, Productivity and organic matter distribution in a pine plantation and an adjacent old field, Ill. Agr. Exp. Sta. For. Res. Rep., 1975, 75(l). 3. 16 Hillel, D., Soil and Water, Physical Principles and Properties. Academic Press, New York, 1971. 17 McCall, J. G. and Gressel, N., Forest soil organic matter: Characterization and modern methods of analysis. In Carbon Forms and Functions in Forest Soils, eds W. W. McFee and J. M. Kelly, pp. 13-32. Soil Sci. Sot. Amer., Madison, Wisconsin, U.S.A., 1995.
cropland
to woody crops
311
18. Armson, K. A.. Forest Soils: Properties and Processes. University of Toronto Press, Toronto, Ontario, Canada. 1977. H. 0. and Brady, N. C., The Nature and 19. Buckman, Properties @Soils. MacMillan Co., New York, 1969. P., Sustaining 20. Heilman, production: Nutrient dynamics and soils. In Ecophysiology of Short Rotation Forest Crops, eds C. P. Mitchell, J. B. FordRobertson, T. Hinckley and L. Sennerby-Forsse, pp. 216-230. Elsevier Applied Science, London, 1992. G. S., Soil organic matter: A link between 21. Henderson, forest management and productivity. In Carbon Forms und Functions in Forest Soils, eds W. W. McFee and J. M. Kelly, pp. 419436. Soil Sci. Sot. Amer., Madison, Wisconsin, U.S.A., 1995. D. F. and Berguson, W. E., Soil carbon 22. Grigal. changes associated with short-rotation systems. Biomass and Bioenergy., in press. 23. Nielson, G. A. and Hole, F. D., Earthworms and the development of coprogenous Al horizons in forest soils of Wisconsin, Soil Sci. Sot. Am. Proc., 1964, 28, 426430. F., Short rotation plantations of poplars 24. Makeschin, and willows on formerly arable land: sites, nutritional status, biomass production and ecological effects. In Proc. 5th E. C. Conf. on Biomass ,for Energy and Industry, eds G. Grassi, G. Gosse and G. dos Santos, Lisbon, Portugal. Elsevier Applied pp. 398403. Science, 1989. P. A. and Sotherton, N. W., Arable energy 25. Robertson, coppice as a wildlife habitat. In Wood Energy and the pp. 1433147. Environment., ed. G. E. Richards, Harwell Laboratories, Oxfordshire, U.K., 1992. 26. Warwick, S., Warwick, J., Bowes, D. and Bowes, D., Bird ringing study in short rotation coppice. In Wood Energy and the Environment., ed. G. E. Richards, pp. 115-l 19. Harwell Laboratories, Oxfordshire, U.K., 1992. 27 Hynes, H. B. N., The Biology of Polluted Waters. University of Toronto Press, Toronto, Canada, 1974. H. M. and 28 Brooks, K. N., Folliott. P. F., Gregersen, Thames, J. L., Hydrology and the Management of Watersheds, Iowa State University Press, Ames, IA, 199 I. 29 Tyler, D. D. and Thomas, G. W., Lysimeter measurement of nitrate and chloride losses from conventional and no-tillage corn, J. Environ. Qual., 1977, 6, 63366. 30 Stone, E. L. and Kalisz, P. J., On the maximum extent of tree roots, For. Ecol. Manage., 1991, 46, 59-102. 31 Schulz, R. P., Root development of intensively cultivated slash pine, Soil Sci. Sot. Amer. Proc,.. 1972, 36(l), 158-162. 32 Barbee. G. C. and Brown, K. W., Comparison between suction and free-drainage soil solution samplers, Soil Science, 1986, 141, 149-154. 33 Cronan. C. S., April, R., Bartlett, R. J.. Bloom, P. R.. Driscoll, C. T.. Gherini, S. A., Henderson, G. S.. Joslin, J. D.. Kelly, J. M., Newton, R. M., Parnell, R. A., Patterson, H: H., Raynal, D. J., Schaedle, M.. Schofield. C. L.. Sucoff. E. I.. TeDDer. H. B. and Thornton, F. C.. Aluminum toxicity in forests exposed to acidic deposition: the ALBIOS results, Water, Air & Soil Pollut., 1989, 48, 181-192. R. and Sollins, P., Improved performance 34. Radulovich, of zero-tension lysimeters, Soil Sci. Sot. Am. J.. 1987, 51, 13861388. 35. Bertrand, A. R., Rate of water intake in the field. In Methods of Soil Analysis, Part I, Physical and Mineralogical Properties, ed. C. A. Black, pp. 1977 209. American Society of Agronomy, Madison, Wisconsin, USA, 1965. to 36. Haise. H. R.. The use of cvlinder infiltrometers determine the intake characteristics of irrigated soils. USDA-ARS Res. Rept., 41-7. 1956.
Bb~wss oud Biounwg~~Vol. 13. Nos. 4/5, pp. 30-31 I. 1997 t_’1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain 096l-9534/97 $17.00 + 0.00 SO961-9534(97)10017-4
MEASURING THE ENVIRONMENTAL EFFECTS OF CONVERTING CROPLAND TO SHORT-ROTATION WOODY CROPS: A RESEARCH APPROACH J. tCollege
D. JOSLIN* and S. H.
*Tennessee Valley Authority, of Forest Resources. Mississippi
SCHOENHOLTZ?
Atmospheric Sciences, Norris, Forestry and Wildlife Research MS. U.S.A.
TN, U.S.A. Center. Mississippi
State,
Abstract-Conversion of cropland to short-rotation woody biomass crops (SRWC) has received increasing interest as biomass utilization technologies have improved and concerns for effects of fossil fuel emissions on global climate have developed. Effects of this conversion on erosion, hydrology, water quality and soil productivity may be significant. A large cooperative research project began in the spring of 1995 at three sites representative of the lower Tennessee Valley to compare the environmental effects of growing traditional row crops with the production of SRWCs over 3- to 5-year rotations. This paper presents the research approach be used to evaluate these effects and a few preliminary results from the initial 3 months of the study. Small watersheds cultivated in row crops: corn (Zea with small watersheds in tree crops: mr,ys L.) or cotton (Gossypium hirsutum L.), are being compared (Plaranus occidentalis L.). sweetgum (Liquidambar styrac@a L.) or eastern cottonwood sycamore (Populus deltoides Bartr.) with respect to: (1) erosion; (2) run-off quality (nutrients, pesticides) and quantity; (3) groundwater quality; (4) soil chemical changes (carbon, nutrients, pesticides); (5) soil physical changes (infiltration, bulk density. aggregate stability); (6) soil biological changes; and (7) wildlife populations. During the spring and summer of the first growing season, few differences in run-off quantity and erosion were observed between treatments. One exception was a tendency towards higher erosion under cotton than cottonwood. Larger differences are expected in later years as trees become established and a litter layer develops. At two sites during the first growing season, differences between row crops and SRWCs were observed in both the runoff and leaching of NOx-N, NHq-N. P, Ca, Mg and K in spring following fertilization of the row crops only at these two sites. Wildlife studies on small mammals and bird populations, as well as microfauna, are just getting under way. lc 1998 Elsevier Science Ltd. Keywords-Short-rotation deltoides; environmental
woody biomass crops; Plutanrrs occidentulis; Liquidambar styrac@a: impacts; erosion; water quality; soil quality.
Populu.s
(Populus sp.), eastern cottonwood (Populus deltoides Bartr.), sweetgum (Liquidambar styraciflua L.), sycamore (Plutanus occidentalis L.) and others, is to be become a significant component of power generation programs in the southeastern United States, hundreds of thousands of hectares of land currently in conventional agriculture will have to be planted with SRWCs. The impact of this conversion on water, air and land resources have received very limited evaluation.‘.2 The net environmental impact of conversion from cropland to SRWCs depends on prior land use, SRWC and crop management methods, soil type, local hydrology and climate. In general, conversions are expected to affect water quality by reducing soil erosion as well as levels of nitrate, phosphorus, pesticides and herbicides in surface run-off and groundin run-off and leaching water. ‘,2 Alterations quantities and their timing are also predicted
1. INTRODUCTION
Use of bioenergy crops as a feedstock in the generation of electricity is receiving considerable attention based chiefly on the development of improved technologies for biomass utilization in power generation, and pressures to reduce emissions of nitrogen oxides, sulfur dioxide and carbon dioxide from coal-fired power plants. One approach to reducing net carbon dioxide emissions is the development of closed-loop biomass systems to reduce the emission of additional carbon dioxide from factor in the fossils fuels.’ A secondary increased interest in bioenergy crops is the potential environmental benefits projected to occur with the conversion of conventional cropland to such crops. If conversion of short rotation woody bioenergy crops (SRWCs) to electricity using fastsuch as hybrid poplar growing species, 301
J. D. JOSLIN and S. H. SCHOENHOLZ
302
based on increased potential for rainfall interception and evapotranspiration by the perennial root systems of SRWCs. Effects of conversion on soil physical, chemical and biological properties are less certain and depend to some extent on biomass residue removal and cultivation intensity. Conversion to SRWCs may also provide improved wildlife habitat.3 All the above predicted impacts, however, are almost entirely based on what is generally known about row crop systems and short-rotation forestry systems. Controlled field comparisons using realistic operational methods have not been conducted in the United States to our knowledge. Such assessments are needed before embarking on largescale conversions of cropland to SRWCs. The primary objective of this paper is to describe an ongoing large-scale, cooperative research approach which is providing information on key environmental effects of conversion of agricultural row cropland to SRWCs in the southeastern U.S. This research comprised three coordinated projects at sites representing three distinct physiographic regions in the southeast. At these sites, a study was initiated in the spring of 1995 to evaluate intensively the effects of replacing cotton (Gossypium hirsutum L.) with eastern cottonwood, corn (Zea mays L.) with sweetgum and corn with sycamore. Environmental impacts are being assessed in the following order of priority: (1) erosion; (2) surface run-off water quality; (3) groundwater quality; (4) surface run-off quantity and timing; (5) soil physical and chemical properties; (6) soil biological properties; and (7) wildlife.
2. GENERAL RESEARCH
APPROACH
Because the intent of this paper is to present the broad outline of a research approach, the research methods described in this paper will focus on non-standardized measurement techniques and procedures of site selection, installation, etc. In general, specific details concerning sampling or analytical methods will not be presented. Many of the methods used to measure soil and sediment parameters can be found in Klute4 and Page et a1.,5 and most methods for chemical analysis of water are standard U.S. E.P.A. methods6 with the exception of bioavailable phosphorus.’
2.1. Site selection The three sites selected in this study were chosen to represent the three physiographic regions of the Tennessee Valley which economic analyses had previously determined to be the most viable for cost effective production of SRWCs.* Soils in these regions were projected to produce woody biomass at high yields on short rotations such that they could conceivably replace conventional row crops. The three regions selected were the upper Mississippi Delta, the loess belt of west Tennessee/northern Mississippi and the limestone valley region of northern Alabama. After choosing the physiographic regions to be represented, specific sites were selected based on the following criteria: (1) presence of land owned by an institution interested in cooperating in such a research effort; (2) presence of nearby supporting staff and facilities to manage the daily research operations and site maintenance; (3) current and recent historical use of the site for agricultural row crops; (4) a soil series and slope class appropriate for productive and intensive management of SRWCs; (5) an area large enough to accommodate at least six small (0.25-2 ha) replicated watersheds with relative uniformity of soil type, slope and past and current land use so that variability among watershed replicates would be minimal. The three sites selected for this study were: (1) the Delta Research and Extension Center at Stoneville, Mississippi operated by the Mississippi Agricultural and Forestry Experiment Station and Mississippi State University; (2) the Ames Plantation, located in the loess belt of west Tennessee near Grand Junction, site of the University of Tennessee Agricultural Experiment Station at Ames; and (3) the Alabama A and M University Agricultural Experiment Station at Hazel Green, Alabama. Aerial photographs, previous soil surveys, detailed topographic mapping of each site and on-site soil coring were all employed prior to the location of replicated watersheds. The establishment of replicated watersheds was essential for comparison of surface run-off quantity, timing and quality (including erosion) among the cropping treatments. While the replicated watersheds were established to examine surface run-off, they also became the basic experimental plot units for measurement
Environmental
effects of converting
of effects on groundwater chemistry and physical, chemical and biological properties the on-site soil resource.
on of
2.2. Treatments The primary comparison being made at each site is between row crop agriculture and short-rotation woody-crop silviculture. The row crop chosen for each site represents a common crop within the physiographic region, and cultivation practices were similarly chosen to be representative of modern conservation tillage practices. The lone exception was cotton at the Mississippi Delta site, where conventional tillage is the common practice. Tree crops were selected to represent the three most frequently recommended hardwood species for intensive short-rotation management in the southeastern U.S. (J. Tuscan and L. Wright 1994, U. S. Department of Energy Biofuels Feedstock Development Program, Oak Ridge National Laboratory, Oak Ridge, Tennessee, pets. commun.).’ Conveniently, a single tree species was considered most appropriate for the soil type selected at each location: (1) eastern cottonwood for comparison with cotton at the Mississippi Delta site (3-year rotation); (2) sycamore for comparison with silage corn at the Tennessee site (4-year rotation); and (3) sweetgum for comparison to corn at the limestone valley site at Hazel Green, Alabama (4year rotation). Table 1 summarizes site characand SRWC planting practices. teristics Silvicultural practices for each SRWC were selected to represent high-intensity management for maximum yield, i.e. extensive use of fertilizers, insecticides, herbicides and other weed control techniques, to be mechanized to the extent possible at an operational scale for forestry and to be cost-effective. One exception included an additional treatment at the Alabama site, where a “total weed control” treatment was added for comparison with a more “practical” cover crop, plus some weed control. At this site, a fourth treatment was also instituted for comparison with an important herbaceous bioenergy crop, i.e. switchgrass (Pa77icum virgatum L.). Table Site Stoneville. MS Hazel Green AL (A and M) Ames. TN
I. Site and planting Slope (X) 0.2-0.3 3-4 2
cropland
to woody
303
Small, artificially constructed watershed plots were created in three replicates of each treatment (crop vs SRWC) at the Mississippi Delta and Tennessee sites by enclosing 0.6 ha pentagonal plots with raised earthen berms. At the Mississippi site, treatments were assigned randomly. At the Tennessee site, the three SRWC plots were located contiguously to provide a large block of trees (approximately 5 ha total) to study wildlife impacts at a scale approaching that perceived to be realistic for silvicultural operations. At the Tennessee site, soils and slopes were particularly uniform, and hydrologic and soil comparisons are being conducted early in the study to establish any pre-treatment watershed differences. Four watershed treatments were established at the Alabama site: (1) corn; (2) sweetgum with total weed control; (3) sweetgum with cover crop; and (4) switchgrass. Each were replicated twice using bermed watersheds in two separate blocks. Additional studies at the Alabama site are being conducted to examine the impact of cover crop species and row width on erosion and tree productivity. At all three sites 15-m wide buffer strips, into which each row crop/SRWC extends beyond its individual watershed’s earthen berms separate the watersheds.
3. SELECTED
3.1. Surface
BROAD PREDICTIONS: VS SRWCS
run-q#‘hydrolog?j
for short rotation
SRWC
Planting
crop
ROW CROPS
and erosion
During the first year there should be only small differences between treatments within a site in surface run-off hydrology, as both types of crops are planted anew in the spring and establish root systems gradually in the growing season. In subsequent years, however, tree crops should transpire a greater amount of water than row crops, because: (a) full canopy cover will be achieved by trees earlier in the growing season; and (b) perennial tree root systems should exploit a deeper portion of the soil profile and extract more water therefrom in spring and early summer when row crop root systems are still developing. We predict
characteristics
Cottonwood Sweetgum Sycamore
crops
material
Cuttings Seedlings Seedlings
woody
crops
Parent
material
Riverine sediments Limestone residuum Loess
Spacing 1.2 x 3.6 m 1.5 x 3.0 m 1.5 x 3.0 m
304
J. DJOSLIN
and S. H. SCHOENHOLZ
that tree crops will also improve infiltration capacity via improved litter cover, better soil structure and higher porosity (see predictions for “Soil Physical and Chemical Properties” below). Improved infiltration should result in reduced surface run-off, especially in spring and autumn, as well as in less “flashy” run-off hydrographs.’ During the first year of establishment, we predict little difference between treatments in erosion, as the amount of exposed soil will be comparable under row crops and trees.2 However, in subsequent years, improved litter coverlo’l ’ and reduced soil surface disturbance caused by cultural operations should reduce erosion on the SRWC plots substantially.g Improvements in litter cover are expected from both (1) the absence of removal of tree litter as opposed to considerable removal of crop litter during annual harvest operations, and (2) the building of minimal weed ground cover following canopy closure. Herbicide use will be discontinued once crown closure is achieved and the subsequent small build-up of weeds under the crown should provide some surface protection and retard the movement of litter by the wind. The reduction in run-off and flatter hydrographs predicted above should also reduce erosion on SRWC plots.’ 3.2. Chemistry of surface run-of water
and ground-
Because SRWCs generally require less fertilizer inputs than row crops,2,“,‘2 especially sweetgum and sycamore, we predict less runoff of most macronutrients, especially nitrogen and phosphorus, two major stream contaminants. The flux of cations should also be reduced in run-off because their export should be closely related to nitrate run-off, cations being required to balance negatively charged nitrate in run-off solutions. In the years following SRWC establishment, this effect could be particularly pronounced immediately following spring application of fertilizers, since SRWCs with their established root systems are probably more capable of rapid utilization of nutrients during spring leaf flush than row crops. ‘* Also, storage of nitrogen via immobilization in surface organic matter may enhance nitrogen retention later in SRWC rotations.‘3’14 Soil organic matter may also help reduce export of herbicides and pesticides by acting as adsorbance sites and by facilitating their microbial breakdown. Furthermore,
fewer pesticides will be used on SRWCs in this study than on row crops. All three tree species are native and pesticides will only be used on them in the case of serious infestations. As with surface run-off, we predict less movement of nutrients below the rooting zone of SRWCs. This prediction applies particularly to nitrogen (and attendant cations) since phosphorus is generally rather immobile in soils. Less fertilizer will be applied to SRWCs, and we expect established perennial tree root systems (after the first year) to be more efficient at recovering fertilizer than row crops, particularly in the spring when row crop root systems are not fully established. 3.3. Soil physical and chemical properties We predict improvements in soil porosity, bulk density and aggregate stability with SRWCs over time in the surface layers. This process may take several years for detection. Additions of organic matter should improve aggregate stability and bulk density,“-” and the development of root channels should improve porosity.‘8-2’ These impacts should in turn reduce penetrometer resistance and improve infiltration and aeration. Infiltration should also be improved because of better litter cover and reduced surface “puddling”.9~” We expect few large changes to occur in quantities of soil nutrients over the course of these short rotations. Changes in soil nutrient reserves of nitrogen and phosphorus will be controlled largely by the balance of fertilizer inputs and exports resulting from harvest and runoff. Loss of base cations and some micronutrients with nitrate run-off and leaching may result in some significant pool changes. We also expect at most small changes in soil organic matter, as it is traditionally measured, in the mineral soil horizons.‘5722 In the SRWC treatments, however, we do expect increases in both the surface litter layer resulting from tree litter and weed cover inputs,‘09” and in the coarse organic matter fraction within the mineral soil. The initial coarse organic matter fraction on all three sites is near zero, and the input of dead tree roots of greater diameter than crop roots should increase this component measurably. Soil biological changes are difficult to predict, but substrate changes resulting from introduction of considerable leaf litter input should favour earthworm population buildups “*23 as well as Opilionida and Isopoda.24
Environmental
effects of converting
cropland
These leaf litter inputs, plus the introduction of more coarse lignified root material, may also stimulate larger fungal populations.‘3.‘4 3.4. Wildlif It remains to be seen if areas as small as
5 ha will have significant impacts on avian and large mammal populations. Surely an increase in cover for both will be provided during the later years of rotation. Small mammal populations, on the other hand, should be affected over time by increases in ground cover and changes in food sources.3 The size and diversity of bird populations should be quite similar in the initial year or two, but should increase by the third year as a result of increased nesting sites, more ground and near-ground cover, and increasing insect populations.3*25726
AND MEASUREMENTS-AN OVERVIEW
crops
305
1 LJyJ C
I I
Upslope
Soil berm
4. METHODS
to woody
J
T
I
I
I
I
I
I
I
I
\
I
Lysimeter
‘1 Equipment building
4.1. Surface run-of hydrology and erosion
Fig. 1. Plot layout at Stoneville, MS site, illustrating schematically the location of pan lysimeters, H-flumes, equipment buildings and orientation of three SRWC (T) and three cotton (C) plots.
Direct measurement of sediment contained in surface run-off is the most accurate method of quantifying erosion from a site. It is also critical to the measurement of other potential impacts on streams including (1) potential inputs of nutrients, herbicides and pesticides, and (2) impacts on timing and quantity of stream flow. Run-off measurements are also the most accurate way to estimate removal of substances from a site by this pathway, as well as a very useful parameter for indirect measurement of infiltration. Measurement of surface run-off clearly requires a land area with well-defined boundaries and a control point where water can be monitored and sampled. Natural watersheds of appropriate size for the research described herein are both difficult to find and of great natural variability in topography, soils, etc. In their place, we created artificial “watersheds” using 0.5 m high berms to surround pentagonally-shaped land areas of approximately 0.6 ha (Fig. 1). The pentagons can be best visualized as rectangles, elongated in the downslope direction, with a triangle added at the downslope end to direct water to a control point. Each control point has a 2-m flume approach section and a 0.5-m “H-flume” equipped with a flow meter and an automated flow-proportional sampler. The flow meter continually measures volume and timing of flow during
each storm. The flow proportional sampler collects water samples for later analyses of concentrations of sediment, nutrients, herbicides and pesticides on a flow-proportional basis. Fluxes of these substances in surface flow can be calculated for each storm. Fluxes of sediment, nutrients and pesticides can also be determined for portions of individual storm hydrographs if desired. In addition to sediment, specific nutrients being measured in run-off include N (separately as nitrate and ammonium), P (separately as orthophosphate, total P, and bioavailable P), Ca, Mg, K, Na, B, Fe, Mn, Cu, Zn and Al. Except for P, for which sediment-related concentrations are being continually measured, estimates of nutrient fluxes contained in sediment will be made via analysis of periodic sediment sub-samples. Similarly, export of selected herbicides and pesticides will be estimated from periodic samples of water and sediment, since continuous monitoring of these substances is too expensive. Special attention is given to P in surface run-off because it is a major factor in the eutrophication of bodies of water in the southeastern U.S.7,27 Measurement of surface run-off and its chemistry is the most expensive component of the evaluations attempted in this research pro-
306
J. D. JOSLIN and S. H. SCHOENHOLZ
ject. On unique sites where surface run-off may be a very minor component of the hydrologic budget (e.g. sites with highly permeable soils and low rainfall intensity), one may wish to consider whether the value of these measurements justifies their cost. Also, in any such study employing watershed comparisons, the hydrology of individual watersheds ideally should be examined for at least 1 year prior to treatment initiation in order to “calibrate” the watersheds for pre-existing hydrologic differences.28 In practice, funding requirements for such additional data collection and delays produced in obtaining results made this impractical for our study. 4.2. Chemistry of surface run-of and groundwater
The contribution of a given watershed plot to below-ground water storage areas requires measurement of the flux of water and chemicals below the rooting zone. Once water travels below the rooting zone there is usually very limited movement upward into plants or back into the atmosphere via evapotranspiration. Two major exceptions to this are: (1) lateral flow which reappears at the surface downslope; and (2) water refluxed to the surface in soils, where the water table periodically reappears at the surface. While both of these circumstances present considerable problems for measuring the downward flux of water and nutrients to groundwater, selection of sites that are well-drained with permeable soils reduces these concerns. To measure the downward flux of water, nutrients and agrochemicals, we installed pan lysimeters (91 cm length x 61 cm width x 8 cm depth) to capture the flow of zero-tension water. Such lysimeters not only collect a representative sample of such water, but also allow an estimation of volume of the downward flux of water.29 Four pan lysimeters were installed in the mid-slope region of each watershed plot at each site. At the Tennessee and Alabama sites, pan lysimeters were installed at 1.5 m. This depth was chosen as the practical base of the rooting zone. While selection of such a depth is arbitrary and very specific to the soil and crop being examined, 1.5 m was believed to be a practical limit for a large percentage of the roots for the short-rotation tree-cropping period being examined and for the particular soils involved. Whereas some roots may reach beyond this depth by the end of the study,30
the bulk of the root system of most trees lies within 1 m of the surface,“T3’ and the amount of water and nutrients extracted below this level should be typically be quite minor. Because the Mississippi site may have water table fluctuations to within 1 m of the surface, lysimeters were installed at this site at approximately 80 cm to avoid problems with the reflux of groundwater. Lysimeters were installed into the up-slope faces of soil pits which were excavated with a backhoe. Disturbance of the area immediately above each pan was minimized. Such pan lysimeters should provide water samples with chemistry that is representative of water flowing through the soil profile (i.e. zero-tension water).32’33The volume of water collected by such pans, however, has high spatial variability.29.34 Consequently, water budgets for each watershed must be maintained. One alternative to using pan lysimeters is the use of tension lysimeters. These lysimeters are much easier to install and, thus, make examination of spatial variability less difficult. The major disadvantage of such lysimeters is that they sample water under tension. During periods between storms, these lysimeters collect water with longer residence times and under higher tension, so that water chemistry from tension lysimetersmay differ from thatofzero-tensionwater. 4.3. Soil physical and chemical properties The soil physical changes which we believe to be of greatest interest if impacted by conversion to SRWCs are those relating to (1) infiltration rate, (2) root penetration and (3) water-holding capacity. Specific soil physical properties being measured related to these characteristics are (a) aggregate stability, (b) porosity, (c) bulk density, (d) hydraulic conductivity and (e) penetrometer resistance. Pretreatment measurements of these parameters will be compared with mid-rotation. pre-harvest and post-harvest measures. Whereas the above measures provide indicators of soil change that may relate to infiltration, measurement of infiltration rates per se are needed. Direct and indirect measures of infiltration being used include (a) double ring infiltrometry3s%36 and (b) the use of run-off fractions to calculate infiltration.3s For a given storm, this calculation is determined from the difference between precipitation and the sum of interception (surface evaporation) losses and surface run-off losses.
Environmental
effects of converting
Soil chemical changes to be examined in this study include the buildup or release of nutrient stores and organic matter (including coarse organic matter, i.e. particles not passing through a 2 mm sieve). Soils have been intensively sampled by depth and soil nutrient reserves determined to a depth of 3 m. Changes in the size of soil nutrient pools will be determined at the end of the rotation following remeasurement. Changes in size of these nutrient pools may be small over the short rotation period being studied (3-4 years) so that run-off and groundwater leaching estimates may be more accurate measurements of change. Changes in the size of soil organic matter pools are the most likely avenue by which water-holding capacity (WHC) of soils might be altered. The role of organic matter in the WHC of soils is well-established,““6.‘9 whereas the other two major factors influencing WHC, texture and type of clay,16 are not likely to be influenced by the treatments or the time period of this study. Buildup of litter layers on the plots will be carefully measured, as this factor relates to water-holding capacity, infiltration, erosion potential and surface evaporation. Coarse organic matter (i.e. organic matter not passing through a 2 mm sieve) content of the mineral soil horizons is also being monitored. This component is often overlooked in soil organic matter determinations which employ sieving prior to analysis, but it may be significant different between soils in which row crops have been grown and those supporting SRWCs. Soil biological parameters that are being compared between conventional crops and SRWCs include populations of bacteria, fungi, actinomycetes and other microfauna and mesofauna, including arthropods, nematodes and earthworms. Introduction of earthworms to selected areas to examine the potential of their populations will be build-up attempted. 4.4. Wildlife Effects on larger vertebrate wildlife are also being examined at the Tennessee site where the SRWC area is largest. Avian populations will be monitored in the spring, summer and winter throughout the study on both row crop and SRWC areas. Small mammal populations will also be monitored during these same seasons of the year. Visitations to the two types of plots by larger mammals will be period-
cropland
to woody crops
ically monitored dors for tracks.
307
by examining cleared corri-
5. SELECTED
PRELIMINARY
RESULTS
Whereas the major objective of this paper is to describe a research approach to evaluating the environmental effects of converting agricultural row cropland to short rotation woody crops, at the time of writing some data relating to the hydrology and chemistry of surface runoff and groundwater were available. These results pertain only to the initial 3 months of the first growing season of the study. We present them here to provide examples of the type of data that will be generated by this approach and to give some preliminary indication of effects during the establishment phase. 5.1. Surface run-ofl hydrology and erosion Few differences were expected between SRWCs and row crops in runoff quantity or erosion during the first 3 months of the study, since both types of crops have a period of establishment during which soil exposure is similar. This expectation was met at the Tennessee and Alabama sites, but at the Mississippi site sediment losses tended to be higher under cotton (Fig. 2). This higher erosion rate is probably the result of intensive cultivation required to create elevated beds and to provide weed control for cotton production. Run-off volume was also slightly greater under cotton than under cottonwood, perhaps indicating an early improvement in infiltration under the cottonwood trees. This hypothesis was confirmed by the fact that the fraction of precipitation which became run-off during late spring/early summer averaged 0.40 under trees and 0.53 under cotton. The run-off fraction increased dramatically, to as high as 0.99 in cotton, when two precipitation events were closely spaced, implying that the soil surface was effectively sealed during the first event by puddling. 5.2. Chemistry
of surface
run-ofl
and ground-
water
run-off data from both the Spring Tennessee site (Figs 3 and 4) and the Alabama site (Fig. 5) reflect the impact that the application of fertilizers to row crops only had on nitrogen and phosphorus export from crops and SRWCs at those
J. D. JOSLIN and S. H. SCHOENHOLZ
308
Sediment in Runoff 600
500
400 iii f UI 5 E 300 2 ._ z *
200
100
0
1
2
1
3
Cotton Plots
2 Tree Plots
Fig. 2. Total quantity (kg ha-‘) of sediment lost through flumes via surface run-off for each cotton SRWC (Tree) plot in runoff at Stoneville, MS site during event of 31 May 1995.
two sites. Based on foliar colour and size, tree survival and growth rates and the relatively high concentrations of nutrients in soil water samples collected during the
3
and
spring and summer compared with typical concenforest plantation or “natural” trations, trees at these two sites (sycamore and sweetgum) appeared to obtain adequate
Nitrate in Runoff
1
2
Corn Plots
3
1
2
3
Tree Plots
Fig. 3. Total quantity (kg ha-‘) of nitrate N lost through flumes via surface run-off SRWC (tree) plot at Ames, TN site during the period 4 May-3 August
for each corn and 1995.
Environmental
effects of converting
cropland
to woody
crops
309
Ammonium in Runoff 1.500
1.250
s i3
1.000
Y
z o.750 2 =
0.500
0.250
0 000
1
2 Corn Plots
3
1
2 Tree Plots
3
Fig. 4. Total quantity (kg ha-‘) of ammonium N lost through flumes via surface run-off for each corn and SRWC (tree) plot at Ames, TN site during the period 4 May-3 August 1995.
nitrogen and phosphorus during the first growing season from residual soil supplies of those nutrients. These supplies are apparently adequate as a result of both soil sto-
rage in long-term reserves and recent contributions from previous year’s crop fertilization. With the lack of fertilizer additions to the soil surface of SRWCs at
Bioavailable P in Runoff 2.000 1.800 --
1.600 --
1.400 -G f 2
1.200 -~
n. g 1.000 -m % 2 0.800 -.o a 0.600 --
Block 1
Block 2
Fig. 5. Total quantity (kg ha-‘) of bioavailable P lost through flumes via surface run-off for each corn, switchgrass. and SRWC plot at Hazel Green, AL site during the period 1 May-5 August 1995.
J. D. JOSLIN and S. H. SCHOENHOLZ
310
these two sites, run-off of nitrogen and phosphorus during the first 3 months was near zero. During the second year, trees at these two sites will be fertilized with nitrogen and phosphorus, but at levels approximately half those of the row crops. In addition to lower foliar requirements for most nutrients, trees in later years will have established root systems, which should initially recover spring-applied fertilizer more efficiently than newly-developing root systems of row crops. Similar to run-off loss of nutrients, spring concentrations of nitrogen (nitrate and ammonium) in water moving downward through the soil was higher under row crops than under trees at the Tennessee (sycamore) site (Fig. 6). The elevation of nitrate in the soil water also resulted in 2-3-fold increases in calcium, magnesium and potassium concentrations below 1.4 m at this site. Large differences between SRWCs and row crops in nutrient movement to groundwater are also expected during subsequent years, for the same reasons given for runoff, i.e. smaller applications of fertilizer and more efficient utilization of nutrients by the trees.
6. SUMMARY
AND CONCLUSIONS
The conversion of large land areas in the southeastern U.S. from row crops to short rotation woody crops should have substantial effects on the environment. Long-term field trials are needed to evaluate the direction and magnitude of the effects of conversion on erosion, on surface run-off quantity and quality, on groundwater quality, on soil physical, chemical and biological properties and on wildlife populations. This paper describes a research approach to such an evaluation that uses replicated bermed watersheds of approximately 0.6 ha in size as the basic study plot unit. Results from the initial growing season of this study support the working hypothesis that few differences will be seen in run-off quantity or quality between row crops and SRWCs during the establishment phase for the trees. Exceptions during the initial 3 months include a trend toward higher erosion under cotton than cottonwood, and higher run-off and leaching of nitrate under fertilized corn than under unfertilized sycamore or sweetgum. Within 3-5 years after SRWC establishment, we will evaluate the working hypotheses that SRWCs compared with row crops will have: (1) higher infiltration rates
Soil Solutions 16
14
Nota:Ca Valuea are
(mglO.lL)
6
T Nitrate - N
NH4
L w
4
K
Ca
Fig. 6. Mean nutrient (nitrate N, ammonium N, Mg, K and Ca) concentrations in soil solutions collected by pan lysimeters at Ames, TN site on 17 May 1995. Note that Ca concentrations should be multiplied by 10 to obtain equivalent concentrations (mg I-‘) of other nutrients.
Environmental
effects of converting
and less surface run-off and erosion; (2) less run-off and groundwater contamination with nutrients; (3) improved soil porosity, bulk density and aggregate stability; and (4) greater biological diversity.
REFERENCES I. OTA (U.S. Congress, Office of Assessment), Potential Environmental Impacts of Bioenergy Crop Production Background Paper, OTA-BP-E-118 Washington DC: U.S. Govt. Printing Office, 1993. 2. Ranney. J. W. and Mann, L. K., Environmental conBiomass siderations in energy crop production, Bioenergy. 1994, 6, 2 I l-228. D. P., Hoffman, W., Hanowski, J. M.. 3. Christian, Niemi, G. J. and Beyea, J., Bird and mammal diversity on woody biomass plantations in North America. Biomass and Bioenergy. in press. 4. Klute, A.. Methods of Soil Analysis Part 1: Physical and Mineralogical Methods. 2nd edn, No. 9, Part I. American Society of Agronomy, Madison, WI., 1986. 5. Page, A. L., Miller, R. H. and Keeney, D. R., Methods of Soil Analysis Part 2: Chemical and Microbiological Properties. 2nd edn, No. 9, Part 2. American Society of Agronomy, Madison, WI, 1982. 6. American Public Health Association (APHA), Standard Methods for the E.uamination of Water and Wastewater. 17th edn, American Public Health Association, Washington, DC, 1989,. 7. Sharpley, A. N.. Troeger, W. W. and Smith, S. J., The measurement of bioavailable phosphorus in agricultural runoff, J. Environ. Qual., 1991. 20, 235-238. 8. Downing, M. and Graham, R. L., Evaluating a biomass resource: the TVA region-wide biomass resource assessment model. In Proc. 1st Biomass Conf: of the Americas: Energy, Environment, Agriculture, and Industry. Burlington, VT, 30 August-2 September, 1993. 9. Schwab, G. 0.. Frevert. R. K., Edminster, T. W. and Soil and Water Conservation Barnes, K. K., Engineering, 2nd edn. John Wiley and Sons. New York, 1966. IO. Bray. J. R. and Gorham, E., Litter production in forests of the world. In Advances in Ecological Research, 2., ed. J. B. Cragg. pp. 101-157. Academic Press, New York, 1964. I1 Pritchett, W. L., Properties and Management of Forest Soils. John Wiley and Sons, New York, 1979. 12 Ericsson, T., Rytter, L. and Linder, S., Nutritional dynamics and requirements of short rotation forests. In Ecophysiology of Short Rotation Forest Crops, eds C. P. Mitchell, J. B. Ford-Robertson, T. Hinckley and L. Sennerby-Forsse, pp. 35565. Elsevier Applied Science, London, 1992. M., introduction to Soil Microbiology. 13 Alexander, John Wiley and Sons, New York, 1977. 14 Brady, N. C., The Nature and Properties of Soils. MacMillan. New York, 1990. R. F., Rolfe, L. and Eastburn, R. P., 15 Fisher, Productivity and organic matter distribution in a pine plantation and an adjacent old field, Ill. Agr. Exp. Sta. For. Res. Rep., 1975, 75(l). 3. 16 Hillel, D., Soil and Water, Physical Principles and Properties. Academic Press, New York, 1971. 17 McCall, J. G. and Gressel, N., Forest soil organic matter: Characterization and modern methods of analysis. In Carbon Forms and Functions in Forest Soils, eds W. W. McFee and J. M. Kelly, pp. 13-32. Soil Sci. Sot. Amer., Madison, Wisconsin, U.S.A., 1995.
cropland
to woody crops
311
18. Armson, K. A.. Forest Soils: Properties and Processes. University of Toronto Press, Toronto, Ontario, Canada. 1977. H. 0. and Brady, N. C., The Nature and 19. Buckman, Properties @Soils. MacMillan Co., New York, 1969. P., Sustaining 20. Heilman, production: Nutrient dynamics and soils. In Ecophysiology of Short Rotation Forest Crops, eds C. P. Mitchell, J. B. FordRobertson, T. Hinckley and L. Sennerby-Forsse, pp. 216-230. Elsevier Applied Science, London, 1992. G. S., Soil organic matter: A link between 21. Henderson, forest management and productivity. In Carbon Forms und Functions in Forest Soils, eds W. W. McFee and J. M. Kelly, pp. 419436. Soil Sci. Sot. Amer., Madison, Wisconsin, U.S.A., 1995. D. F. and Berguson, W. E., Soil carbon 22. Grigal. changes associated with short-rotation systems. Biomass and Bioenergy., in press. 23. Nielson, G. A. and Hole, F. D., Earthworms and the development of coprogenous Al horizons in forest soils of Wisconsin, Soil Sci. Sot. Am. Proc., 1964, 28, 426430. F., Short rotation plantations of poplars 24. Makeschin, and willows on formerly arable land: sites, nutritional status, biomass production and ecological effects. In Proc. 5th E. C. Conf. on Biomass ,for Energy and Industry, eds G. Grassi, G. Gosse and G. dos Santos, Lisbon, Portugal. Elsevier Applied pp. 398403. Science, 1989. P. A. and Sotherton, N. W., Arable energy 25. Robertson, coppice as a wildlife habitat. In Wood Energy and the pp. 1433147. Environment., ed. G. E. Richards, Harwell Laboratories, Oxfordshire, U.K., 1992. 26. Warwick, S., Warwick, J., Bowes, D. and Bowes, D., Bird ringing study in short rotation coppice. In Wood Energy and the Environment., ed. G. E. Richards, pp. 115-l 19. Harwell Laboratories, Oxfordshire, U.K., 1992. 27 Hynes, H. B. N., The Biology of Polluted Waters. University of Toronto Press, Toronto, Canada, 1974. H. M. and 28 Brooks, K. N., Folliott. P. F., Gregersen, Thames, J. L., Hydrology and the Management of Watersheds, Iowa State University Press, Ames, IA, 199 I. 29 Tyler, D. D. and Thomas, G. W., Lysimeter measurement of nitrate and chloride losses from conventional and no-tillage corn, J. Environ. Qual., 1977, 6, 63366. 30 Stone, E. L. and Kalisz, P. J., On the maximum extent of tree roots, For. Ecol. Manage., 1991, 46, 59-102. 31 Schulz, R. P., Root development of intensively cultivated slash pine, Soil Sci. Sot. Amer. Proc,.. 1972, 36(l), 158-162. 32 Barbee. G. C. and Brown, K. W., Comparison between suction and free-drainage soil solution samplers, Soil Science, 1986, 141, 149-154. 33 Cronan. C. S., April, R., Bartlett, R. J.. Bloom, P. R.. Driscoll, C. T.. Gherini, S. A., Henderson, G. S.. Joslin, J. D.. Kelly, J. M., Newton, R. M., Parnell, R. A., Patterson, H: H., Raynal, D. J., Schaedle, M.. Schofield. C. L.. Sucoff. E. I.. TeDDer. H. B. and Thornton, F. C.. Aluminum toxicity in forests exposed to acidic deposition: the ALBIOS results, Water, Air & Soil Pollut., 1989, 48, 181-192. R. and Sollins, P., Improved performance 34. Radulovich, of zero-tension lysimeters, Soil Sci. Sot. Am. J.. 1987, 51, 13861388. 35. Bertrand, A. R., Rate of water intake in the field. In Methods of Soil Analysis, Part I, Physical and Mineralogical Properties, ed. C. A. Black, pp. 1977 209. American Society of Agronomy, Madison, Wisconsin, USA, 1965. to 36. Haise. H. R.. The use of cvlinder infiltrometers determine the intake characteristics of irrigated soils. USDA-ARS Res. Rept., 41-7. 1956.