Special Feature - Forest Landscape Ecology Lab

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3Graduate Degree Program in Ecology, Colorado State University, Fort Collins, ... texture to areal patterns of decomposition rates in the U.S. Great Plains, explaining 30% ... 1999; accepted 26 November 1999; Special Feature accepted 2.
Ecology, 83(2), 2002, pp. 320–327 q 2002 by the Ecological Society of America

REGIONAL PATTERNS OF DECOMPOSITION AND PRIMARY PRODUCTION RATES IN THE U.S. GREAT PLAINS HOWARD E. EPSTEIN,1,3,5 INGRID C. BURKE,1,3,4

AND

WILLIAM K. LAUENROTH2,3,4

1Department of Forest Science, Colorado State University, Fort Collins, Colorado 80523 USA Department of Rangeland Ecosystem Science, Colorado State University, Fort Collins, Colorado 80523 USA 3Graduate Degree Program in Ecology, Colorado State University, Fort Collins, Colorado 80523 USA 4Natural Resource Ecology Laboratory, Colorado State University, Fort Collins, Colorado 80523 USA

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Abstract. Warmer regions generally exhibit greater rates of soil respiration and organic matter decomposition than colder regions. In the Great Plains of the United States, soil organic matter declines from the northern part of the region to the south, suggesting greater decomposition rates in areas with warmer temperatures. Our study used a regional data set of aboveground net primary production, soil organic carbon, soil texture, and climate to evaluate the environmental controls over areal patterns in decomposition rates, ( k; expressed as grams per year per gram of initial mass), throughout the U.S. Great Plains. We conducted multiple regression analyses of steady-state k with respect to mean annual temperature, mean annual precipitation, and percentage soil clay content to examine both the combined and individual effects of these independent variables on regional decomposition rates. Our results indicated that precipitation contributes more than either temperature or soil texture to areal patterns of decomposition rates in the U.S. Great Plains, explaining .30% of the areal variability in k. Decomposition rates increased with increasing precipitation and with decreasing soil clay content. Temperature explained ,8% of the regional variability in k. Ancillary analyses that related temperature and aboveground net primary production in the region indicated that plant productivity declines with increasing temperatures. This suggests that the reduction in soil organic matter to the south in the U.S. Great Plains may be due to reduced plant inputs rather than to increases in decomposition rates. The response of decomposition to temperature is probably constrained by moisture in this water-limited region. Therefore, changes in decomposition rates resulting from temperature dynamics are likely to be minimal unless they are accompanied by sufficient changes in precipitation. Key words: aboveground net primary production; areal patterns; carbon dynamics; decomposition; grasslands; Great Plains (USA); precipitation; regional scale; soil organic matter; soil texture; temperature.

INTRODUCTION Locations having greater temperatures, on average, exhibit greater rates of soil respiration (Raich and Schlesinger 1992, Lloyd and Taylor 1994) and organic matter decomposition than those having colder temperatures (Franz 1990, Johansson et al. 1995, Kirschbaum 1995, Trumbore et al. 1996). Positive effects of temperature on soil respiration have also been documented in numerous laboratory studies (e.g., Holland et al. 1995, MacDonald et al. 1995, Winkler et al. 1996, Lomander et al. 1998a). Soil respiration is also influenced by water availability (Schlentner and Van Cleve 1985, Naganawa et al. 1989, Zhang and Zak 1995), and the effects of temperature and water on soil respiration can be multiplicative (Lomander et al. 1998b). Trying to separate the effects of temperature and moisture on Manuscript received 31 March 1999; revised 16 November 1999; accepted 26 November 1999; Special Feature accepted 2 May 2001. For reprints of this Special Feature, see footnote 1, p. 305. 5 Present address: Department of Environmental Sciences, Clark Hall, University of Virginia, Charlottesville, Virginia 22904-4123 USA. E-mail: [email protected]

decomposition rates in the field is therefore problematic (Kirschbaum 1995). At regional to global scales, decomposition rates (k; expressed as grams per year per gram of initial mass), have been related to either mean air temperatures or average actual evapotranspiration (AET) (Meentemeyer 1978, 1984, Harrison et al. 1995, Johansson et al. 1995, Aerts 1997), an energy flux variable that combines the effects of both temperature and precipitation. Other studies have explicitly documented the control of moisture on areal patterns of decomposition and nutrient mineralization rates (Vitousek et al. 1994, Raich and Potter 1995, Burke et al. 1997). Johansson et al. (1995), however, found no significant relationships between precipitation variables and decomposition rates across a Scots pine forest region at northern latitudes of Europe. In models of global carbon (C) cycling, both temperature and precipitation variables generally are included as interactive factors controlling decomposition rates (Parton et al. 1987, 1995, Rastetter et al. 1991, Esser 1992, Raich and Potter 1995). Soil texture can also affect the decomposition rates of soil organic matter, with clay minerals, in particular,

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providing physical and chemical protection of organic material (Jenkinson 1977, Monreal et al. 1981, Oades 1988). Although soil texture has been related to quantities of soil organic matter at regional scales (Nichols 1984, Burke et al. 1989, Kern 1994, Davidson 1995), there are no explicit relationships between soil texture variables and decomposition rates for large areas. Burke et al. (1997) did suggest a significant, yet minor, relationship between soil texture and net nitrogen (N) mineralization rates in U.S. grasslands. Our first objective for this study was to compare the regional controls over decomposition rates with the regional controls of net primary production and soil organic carbon. Our goal was to examine the combined effects of temperature, precipitation, and soil texture on these ecosystem attributes across the U.S. Great Plains. Our second objective was to isolate the three independent environmental factors to determine how they individually affect areal patterns of organic matter decomposition. METHODS

k 5 soil organic matter (SOM) decomposed per unit of time/total SOM pool. (1) Assuming that Great Plains grasslands are at steady state with respect to SOM, the decomposition of SOM (outputs) will equal the inputs to the SOM pool. Longterm data suggest that soil organic carbon in the Great Plains is relatively stable for native prairie, in the absence of cultivation (Balesdent et al. 1988, Buyanovsky et al. 1997). Inputs to SOM are almost exclusively from plant litter; assuming steady state plant biomass, plant litter inputs to SOM are equivalent to net primary production (NPP) of new plant biomass (Raich and Schlesinger 1992, Kirschbaum 1995). Therefore, SOM decomposed (outputs) 5 NPP (inputs).

(2)

Reliable field estimates of total NPP for grasslands are infrequent, due to poor knowledge of belowground net primary production (BNPP). Milchunas and Lauenroth (1992), however, used a 14C turnover technique to estimate BNPP in a shortgrass steppe ecosystem; they found that the ratio of aboveground net primary productivity (ANPP) to BNPP is roughly 1:1. Aboveground to belowground NPP ratios are also ;1:1 for native tallgrass prairie ecosystems (Dahlman and Kucera 1965, Hayes and Seastedt 1987, Kucera 1992). These and other studies (Kucera et al. 1967, Sims et

al. 1971, 1978) suggest that the ratio between ANPP and BNPP does not vary along gradients of precipitation and temperature in the region. We consider a 1: 1 ratio to be the best estimate available and use it for the entire study area. Because regional-scale ANPP and soil organic matter data were available, our final calculation for k values at steady state was

k 5 2 ANPP/total SOM pool

(3)

(i.e., inputs, or outputs, divided by the total pool). If these grassland systems are actually a net source of carbon to the atmosphere (rather than at steady state C), then our calculations will have underestimated decomposition rates. If these systems are a net sink of carbon, then we will have overestimated k. Note that our estimations of k represent ‘‘actual’’ decomposition rates that are related to NPP and the actual inputs of dead organic matter. This is in contrast to ‘‘potential’’ decomposition rates that may be realized under optimal climatic conditions. We constructed our data set to include ANPP and soil organic matter data in order to calculate decomposition rates using Eq. 3. ANPP data were collected from USDA Natural Resource Conservation Service (NRCS) range site descriptions for 10 Great Plains states (see Epstein et al. 1996). NRCS range site descriptions represent the potential native plant community of well-managed grazing lands in the absence of abnormal disturbances or other management regimes (USDA 1967). Thus, the data set represents the natural potential vegetation, and any confounding effects of land use on k are eliminated. Range site descriptions include potential ANPP estimates for favorable, normal, and unfavorable years; we constructed our database using normal ANPP values for ;1700 range site descriptions throughout the Great Plains of the United States. Range site descriptions were made areally explicit through the use of a geographic information system (GIS, Arc/INFO 1992). Within the GIS, range site data were linked to USDA State Soil Geographic (STATSGO) polygonal databases (for more detailed descriptions, see Epstein et al. 1996, 1997a, b). Areal representations of ANPP using this method compare favorably to field estimates of ANPP, as well as to remotely sensed indices of ANPP (Epstein et al. 1997b, Paruelo et al. 1997). Soil organic carbon data to 20 cm depth were also obtained from the STATSGO databases. Substantial amounts of organic carbon exist below 20 cm in the soils of the Great Plains, probably due to slow rates of decomposition at depth. Many studies have illustrated that most decomposition occurs in soil layers close to the surface, because of the depth profiles of temperature, moisture, soil texture, and decomposer organisms (e.g., Weaver et al. 1935, Hunt 1977, Lomander et al. 1998a, Gill et al. 1999). The degree of aggregation in STATSGO makes this data set appro-

Special Feature

We collected data from a variety of sources to construct an areal database of decomposition rates ( k), mean annual precipitation (MAP), mean annual temperature (MAT), and soil texture for the U.S. portion of the North American Great Plains. There are no comprehensive data sets on decomposition rates for the U.S. Great Plains; we therefore had to calculate these rates from other sources of data. Decomposition rates can be calculated as

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TABLE 1. Collinearity (r) among the variables mean annual precipitation (MAP), mean annual temperature (MAT), and percentage soil clay content (CLAY) within the U.S. Great Plains. Variable

MAP

MAT

CLAY

MAP MAT CLAY

1.00 0.57 0.08

0.57 1.00 0.18

0.08 0.18 1.00

priate for regional-scale studies (Davidson 1995, Lathrop et al. 1995). We used daily climate data from 296 weather stations throughout the region (CLIMATEDATA 1988). Annual precipitation and annual temperature were averaged for each station over the period 1969–1988. Mean annual temperature (MAT) isotherms of 0.258C intervals and mean annual precipitation (MAP) isohyets of 10-mm intervals were generated using the GIS. Values for MAT and MAP in the areas between contour lines were calculated by averaging the values of the surrounding contours. Soil texture data (percentage sand, silt, and clay) were generated from the texture classification of the surface soil layer for each STATSGO polygon (Burke et al. 1991). Thus, any geographic point in the Great Plains could be associated with values for ANPP, mean annual temperature, mean annual precipitation, and soil texture. We used the GIS to generate a set of random points throughout the U.S. Great Plains, such that for any stratification by climate or soil texture, we would have adequate observations (;30) for regression analyses. Our final data set, in its entirety, was composed of ;700 random points. To achieve our first objective, we performed a stepwise multiple regression analysis on k using the entire data set, with the independent variables of mean annual precipitation, mean annual temperature, and percentage soil clay content. These environmental variables have some areal collinearity in the U.S. Great Plains (Table 1). There have been several areal analyses of primary production in the Great Plains (Sala et al. 1988, Epstein et al. 1997a). To make a direct comparison, we conducted the same stepwise multiple regression analysis with ANPP as the dependent var-

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iable. In addition, we conducted the analysis with soil organic carbon (SOC) as the dependent variable. After examining the results of our multiple regression analyses, we chose to stratify the Great Plains by precipitation and soil texture values in order to isolate the effect of temperature on decomposition rates (our second objective). We subdivided the region into areas of 50-mm MAP intervals from 250–300 mm to 1100– 1150 mm. We additionally subdivided the region into 5% intervals of percentage soil clay content (CLAY) from 5–10% to 55–60%. Within each MAP interval, we related k to mean annual temperature and CLAY; within each CLAY interval, we related k to mean annual temperature and MAP. We examined how the variance explained by each independent variable (r2) changed along the precipitation and soil texture gradients. RESULTS Mean annual precipitation, mean annual temperature, and percentage soil clay content together explained 51% of the areal variance in k, 62% of the variance in ANPP, and 34% of the variance in SOC for the U.S. Great Plains (Table 2). MAP alone explained 31% of the variation in k and 57% of the variation in ANPP. MAT explained only 7% of the variation in k and 6% of the variation in ANPP, yet it explained 17% of the variation in SOC. Decomposition rates increased with increasing precipitation and increasing temperature, and declined as soil clay content increased. ANPP also increased with increasing precipitation, yet declined as temperatures became greater. ANPP also declined with increasing clay content in the Great Plains. Soil organic carbon increased with increasing precipitation and clay content, and declined with increasing temperatures. Along a gradient of precipitation, with MAP held constant in 50-mm intervals, clay content consistently accounted for more of the variability in decomposition rates than did MAT (Fig. 1). In many cases, CLAY explained .50% of the variation in k when precipitation was held relatively constant. MAT explained .20% of the variation in k at only three levels of MAP, all of which were .900 mm. Along a clay content gradient, MAT generally explained more of the vari-

TABLE 2. Stepwise multiple regression analyses of aboveground net primary production (ANPP), soil organic carbon (SOC), and decomposition rate (k) against the independent variables mean annual precipitation (MAP), mean annual temperature (MAT), and soil clay content (CLAY). Independent variable MAP MAT CLAY Total r2

ANPP

k

SOC

Coefficient

r2

Coefficient

r2

Coefficient

r2

0.59 28.70 20.49

0.556 0.058 0.002 0.616

0.006 20.484 0.070

0.086 0.173 0.079 0.338

0.0001 0.0053 20.0022

0.310 0.073 0.129 0.512

Notes: All coefficients are statistically significant (P , 0.01). Intercepts are 56.51 for ANPP, 6.623 for SOC, and 0.0268 for k.

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FIG. 1. Proportion of variance in k explained (r2) by mean annual temperature (MAT) and soil clay content (CLAY) along a gradient of mean annual precipitation (MAP).

DISCUSSION There are few estimates of decomposition rates (k) for total soil organic matter in the U.S. Great Plains. Kelly et al. (1996) estimated k of total soil C for a sandy loam soil in northeastern Colorado, using both field measurements and model simulations. The location of their study was the Central Plains Experimental Range (CPER), the shortgrass steppe Long Term Ecological Research site, which receives on average 321 mm of annual precipitation and has a mean annual temperature of 8.68C (Lauenroth and Sala 1992). Based on these soil and climate data, our regression model predicts decomposition rates for total soil C ranging from 0.039/yr to 0.052/yr (assuming that a sandy loam can have a clay content of 0–20%). The field estimates of k from Kelly et al. (1996) range from 0.020 to 0.041/ yr. Simulated estimates of k at the CPER using the CENTURY model (Parton et al. 1987, 1988) range from 0.024 to 0.026/yr (Kelly et al. 1996). Our estimates of k at the CPER are slightly greater than those of Kelly et al. (1996), possibly because the latter were made from soils that were manipulated with reduced plant inputs to soil organic matter. With no reduction in plant inputs, the CENTURY model simulates k for

total soil C on a sandy loam at the CPER to be ;0.046/ yr (R. H. Kelly and H. E. Epstein, unpublished data), which falls right in the middle of our estimated range. Decay rates of soil organic matter pools with intermediate turnover times (compared to more active or passive soil organic matter) have been calculated in a range from 0.003 to 0.12/yr (van Veen and Paul 1981, Parton et al. 1987, Jenkinson 1990, Townsend et al. 1995), a range that essentially matches the spread of our estimates for the U.S. Great Plains. Therefore, our methods for estimating k generate reasonable and expected values for the region. Results for our first objective do not support the idea that temperature explains regional-scale patterns of decomposition rates in the Great Plains, and suggest that moisture and soil texture have greater explanatory power. Burke et al. (1997) also found that areal patterns in net N mineralization rates throughout the Great Plains, as simulated by the CENTURY model, were largely explained by mean annual precipitation (r2 5 0.93), with very little of the variance explained by mean annual temperature (r2 5 0.001). These results seem to contradict significant decreases in soil organic carbon and nitrogen with increasing temperatures in the Great Plains (Burke et al. 1989). Previously, it had been assumed that temperature has no effect on areal patterns of aboveground net primary production (i.e., potential litter inputs to soil organic matter) in grassland ecosystems (Lauenroth 1979, Sala et al. 1988). Therefore, a decline in soil organic matter with increasing temperatures was assumed to be due to increases in the organic matter decay rate, k (Jenny 1930, Burke et al. 1989). In this study, however, we found that rates of aboveground net primary production decline with increasing temperatures in the Great Plains (see also Epstein et al. 1997a). Our results suggest that decreases in soil

FIG. 2. Proportion of variance in k explained (r2) by mean annual temperature (MAT) and mean annual precipitation (MAP) along a gradient of clay content (CLAY).

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ation in k than did MAP at the low end of the gradient (i.e., sandier soils); at the high end of the CLAY gradient, MAP consistently explained more of the variation in k than did MAT (Fig. 2). Rates of soil organic matter decomposition in the U.S. Great Plains ranged from 0.003/yr to 0.12/yr, with mean annual temperature held constant at 108C. In the sandiest areas, they ranged from 0.06/yr to 0.12/yr, increasing with precipitation, whereas in areas with the most clay, k ranged from 0.003/yr to 0.06/yr, also increasing with precipitation.

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organic matter with increasing temperatures in the Great Plains may be caused by reduced plant inputs to soil organic matter, rather than by increases in decomposition rates. If we assume a constant decomposition rate of 0.06/yr across temperatures, then a decline in ANPP of 8.70 g·m22·yr21 for every 18C increase in MAT (Table 2) will yield a reduction in soil organic carbon of 145 g C/m2 to 20 cm depth (Eq. 3). Extrapolating this over the 208C range in mean annual temperatures across the U.S. Great Plains, we find that reduced plant inputs due to higher temperatures could account for differences in soil organic C of .2800 g C/m2 across the entire temperature gradient. The reason for the lack of a strong correlation between temperature and decomposition rates is the water-limited nature of the Great Plains grasslands. There are consistent findings that regional- to global-scale patterns of decomposition rates correlate well with estimates of actual evapotranspiration (Meentemeyer 1984, Johansson et al. 1995, Aerts 1997). In the Great Plains, however, deficits dominate the water balance (Lauenroth et al., in press), and AET is essentially equivalent to precipitation (Lauenroth and Burke 1995). Additionally, the effects of temperature and moisture are considered to be multiplicative in many process models of decomposition (Lomander et al. 1998b); therefore, it is not surprising that we found precipitation to control decomposition rates in a waterlimited region. When we held precipitation constant at 50-mm intervals (our second objective), temperature explained more of the variation in k at greater levels of MAP, essentially when precipitation became less limiting. Similarly, Gifford (1992) proposed that increasing temperature would not lead to a reduction in soil C on the largely water-limited continent of Australia. Whitford et al. (1981) suggested that decomposition rates in arid systems will exceed those predicted based on AET (Meentemeyer 1978) because of the adaptations of decomposers to hot, dry conditions. Macro-decomposers (e.g., termites) can account for most of the litter decomposition in desert systems (Santos and Whitford 1981), whereas micro-decomposers will concentrate in the most humid of soil microsites. Soil texture explained much of the areal variation in decomposition rates, particularly when precipitation was held constant. This may be because soil texture influences both the level of soil moisture throughout the soil profile (Cosby et al. 1984) and the physical and chemical availability of organic substrate to decomposers (Jenkinson 1977, Monreal et al. 1981, Sorenson 1981). When soil texture (CLAY) was held constant, temperature explained more of the variance than did precipitation on coarser textured soils, whereas precipitation explained more of the variance on finer textured soils. A caveat of our approach is that we are describing correlations and, therefore, only potential causal mechanisms. The environmental factors used in our analysis

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may be related to other potential controlling variables that we did not consider. One such variable that has been shown to influence decomposition rates is the nature of the decomposing substrate. Site- to global-scale analyses show decomposition rates to be related to a variety of litter and organic matter chemistry parameters, including lignin and cellulose concentrations, nitrogen and phosphorus (P) concentrations, lignin:N ratio, and C:N and C:P ratios (Meentemeyer 1978, 1984, Melillo et al. 1982, McClaugherty et al. 1985, Dyer et al. 1990, Vitousek et al. 1994, Aerts 1997). Gradients of increasing temperature in the Great Plains correspond to increases in lignin concentration and lignin: N ratios of plant litter (Murphy et al. 1996). In areas of the Great Plains with precipitation ,500 mm, the polysaccharide content of soil organic matter declines with increasing temperatures (Amelung et al. 1997). It is possible that the potential positive effect of greater temperatures on decomposer activity is offset by a concomitant decline in the decomposability of organic matter in this region. Lignin concentration and lignin:N ratios of plant litter also increase with precipitation in this region (Murphy et al. 1996), suggesting that we may actually be underestimating the influence of precipitation on decomposition rates in the Great Plains. Other potential controls of k are the length of the decomposition season (i.e., the number of days with soil temperatures and moisture conditions conducive for decomposition) and the influence of litter and snow on insulation of the soil. Considering that patterns of winter temperatures differ from those of summer temperatures in the Great Plains (Borchert 1950), the areal distribution of mean annual temperature may not represent well the distribution of length of the decomposition season. Our study uses data on the potential natural systems of the Great Plains and therefore does not consider the effects of land use on decomposition. Over 30% of the Great Plains is used for crops, with much of this cropland occurring in the cool, wet portion of the region, essentially the northeastern states of Kansas, Nebraska, South Dakota, and North Dakota (Lauenroth et al. 1994). Increased rates of decomposition associated with cultivation (Burke et al. 1989, 1997) may offset some, if not all, of the positive effects of a cool, moist climate on soil carbon storage. Understanding the relationships between environmental variables and decomposition rates is crucial for predicting the response of soil carbon storage to global warming. One general assumption has been that increasing temperatures will lead to increases in both net primary productivity and organic matter decomposition (Kirschbaum 1995). Based on our results, it is possible that neither part of this assumption holds for the U.S. Great Plains, and may not for other similar regions as well. Our analyses suggest that increases in temperature will enhance the water deficit in the region (Lauenroth et al., in press) and will lead to declines in aboveground

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net primary production. Because of the water-limited nature of the region, decomposition rates may increase only slightly with increasing temperatures. The net effect in response to a temperature increase will still be a reduction in soil organic matter storage. Changes in precipitation may have similar directional effects for both ANPP and k. General circulation models predict an increase in precipitation for the Great Plains (Hansen et al. 1983, Manabe and Wetherald 1987, Wilson and Mitchell 1987), which could lead to increases in both plant productivity and organic matter decomposition. The net effect of increased precipitation on soil C storage will be determined by which ecosystem function (productivity or decomposition) is influenced more by changes in moisture. ACKNOWLEDGMENTS

LITERATURE CITED Aerts, R. 1997. Climate, leaf litter chemistry and leaf litter decomposition in terrestrial ecosystems: a triangular relationship. Oikos 79:439–449. Amelung, W., K. W. Flach, and W. Zech. 1997. Climatic effects on soil organic matter composition in the Great Plains. Soil Science Society of America Journal 61:115– 123. Arc/INFO. 1992. Arc/INFO. Version 7.0. Environmental Systems Research Institute, Redlands, California, USA. Balesdent, J., G. H. Wagner, and A. Mariotti. 1988. Soil organic matter in long-term field experiments as revealed by 13C natural abundance. Soil Science Society of America Journal 52:118–124. Borchert, J. R. 1950. The climate of the central North American grassland. Annals of the Association of American Geographers 40:1–39. Burke, I. C., T. G. F. Kittel, W. K. Lauenroth, P. Snook, C. M. Yonker, and W. J. Parton. 1991. Regional analysis of the Central Great Plains. BioScience 41:685–692. Burke, I. C., W. K. Lauenroth, and D. G. Milchunas. 1997. Biogeochemistry of managed grasslands in central North America. Pages 85–102 in E. A. Paul, K. Paustian, E. T. Elliott, and C. V. Cole, editors. Soil organic matter in temperate agroecosystems: long-term experiments in North America. CRC Press, Boca Raton, Florida, USA. Burke, I. C., W. K. Lauenroth, and W. J. Parton. 1997. Regional and temporal variation in net primary production and nitrogen mineralization in grasslands. Ecology 78: 1330–1340. Burke, I. C., C. M. Yonker, W. J. Parton, C. V. Cole, K. Flach, and D. S. Schimel. 1989. Texture, climate, and cultivation effects on soil organic matter content in U.S. grassland soils. Soil Science Society of America Journal 53:800–805. Buyanovsky, G. A., J. R. Brown, and G. H. Wagner. 1997. Sanborn field: effect of 100 years of cropping on soil parameters influencing productivity. Pages 205–225 in E. A.

Paul, K. Paustian, E. T. Elliott, and C. V. Cole, editors. Soil organic matter in temperate agroecosystems: long-term experiments in North America. CRC Press, Boca Raton, Florida, USA. CLIMATEDATA. 1988. U.S. West Optical Publishing. Denver, Colorado, USA. Cosby, B. J., G. M. Hornberger, R. B. Clapp, and T. R. Ginn. 1984. A statistical analysis of the relationships of soil moisture characteristics to the physical properties of soils. Water Resources Research 20:682–690. Dahlman, R. C., and C. L. Kucera. 1965. Root productivity and turnover in native prairie. Ecology 46:84–89. Davidson, E. A. 1995. Spatial covariation of soil organic carbon, clay content, and drainage class at a regional scale. Landscape Ecology 10:349–362. Dyer, M. L., V. Meentemeyer, and B. Berg. 1990. Apparent control of mass loss rate of leaf litter on a regional scale: litter quality versus climate. Scandinavian Journal of Forest Research 5:311–323. Epstein, H. E., W. K. Lauenroth, and I. C. Burke. 1997a. Effects of temperature and soil texture on ANPP in the U.S. Great Plains. Ecology 78:2628–2631. Epstein, H. E., W. K. Lauenroth, I. C. Burke, and D. P. Coffin. 1996. Ecological responses of dominant grasses along two climatic gradients in the Great Plains of the U.S. Journal of Vegetation Science 7:777–788. Epstein, H. E., W. K. Lauenroth, I. C. Burke, and D. P. Coffin. 1997b. Productivity patterns of C3 and C4 functional types in the Great Plains of the U.S. Ecology 78:722–731. Esser, G. 1992. Implications of climate change for production and decomposition in grasslands and coniferous forests. Ecological Applications 2:47–54. Franz, E. H. 1990. Potential influence of climate change on soil organic matter and tropical agroforestry. Pages 109– 120 in H. W. Scharpenseel, M. Schomaker, and A. Ayoub, editors. Soils on a warmer Earth. Elsevier, Amsterdam, The Netherlands. Gifford, R. M. 1992. Implications of the globally increasing atmospheric CO2 concentrations and temperature for the Australian terrestrial carbon budget: integration using a simple model. Australian Journal of Botany 40:527–543. Gill, R., I. C. Burke, D. G. Milchunas, and W. K. Lauenroth. 1999. Relationships between root biomass and soil organic matter pools in the shortgrass steppe of eastern Colorado. Ecosystems 2:226–236. Hansen, J. C., G. Russel, D. Rind, P. Stone, A. Lacis, S. Lebedeff, R. Ruedy, and L. Travis. 1983. Efficient threedimensional global models for climate studies: models I and II. Monthly Weather Review 3:609–662. Harrison, A. F., P. J. A. Howard, D. M. Howard, D. C. Howard, and M. Hornung. 1995. Carbon storage in forest soils. Forestry 68:335–348. Hayes, D. C., and T. R. Seastedt. 1987. Root dynamics of tallgrass prairie in wet and dry years. Canadian Journal of Botany 65:787–791. Holland, E. A., A. R. Townsend, and P. M. Vitousek. 1995. Variability in temperature regulation of CO2 fluxes and N mineralization from five Hawaiian soils: implications for a changing climate. Global Change Biology 1:115–123. Hunt, H. W. 1977. A simulation model for decomposition in grasslands. Ecology 58:469–484. Jenkinson, D. S. 1977. Studies on the decomposition of plant material in soil. V. Journal of Soil Science 28:424–434. Jenkinson, D. S. 1990. The turnover of carbon and nitrogen in soil. Philosophical Transactions of the Royal Society of London B 329:361–368. Jenny, H. 1930. A study on the influence of climate upon the nitrogen and organic matter content of the soil. Missouri Agricultural Experiment Station Bulletin 152. Johansson, M.-B., B. Berg, and V. Meentemeyer. 1995. Litter

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This research was supported by several National Science Foundation awards (BSR 91-06183 for development of the Great Plains ANPP database and BSR 90-11659 for data analysis and manuscript preparation, in addition to the NSF Shortgrass Steppe Long Term Ecological Research Program and the NSF Presidential Faculty Fellows award to Ingrid Burke). We thank Martha Coleman, Caroline Yonker, and Weihong Fan for their assistance in the development of the Great Plains database, and the USDA Natural Resource Conservation Service for providing the range site descriptions. We also wish to thank John Blair, Olof Andre´n, and two anonymous reviewers for helping us to improve the manuscript.

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