We investigated soil organic matter accumulations, soil respiration, and soil food webs in riparian grass filters on private farms in northern Story County, Iowa. We specifically compared soils beneath planted prairie grasses to soils beneath non-native forage grasses. Soil organic matter pools varied more between farms than among grassland types. Soil respiration rates were similar among old prairie-grass stands and long-established cool-season grasses. Soil food webs also were similar beneath the two grass types. Planting of native prairie species in filter strips is as effective as is planting non-native forage grasses, but provides greater grass production while perpetuating native species.
It is widely recognized that grass forages vary in quality as feedstuffs for grazing animals. On-farm research by our group suggests that grass species also differ in their impacts on soils, and that their effects may be predictable based on the well-known dichotomy between cool- and warm-season grasses. Specifically, our previous research suggests that cool-season grasses may generate high-quality, short-lived soil organic matter, and support high rates of nitrogen cycling by a diverse soil food web. Warm-season grasses, in contrast, may promote long-term soil organic matter accumulation, but have slower rates of nitrogen cycling, lower nitrate losses, and less diverse soil food webs. Our previously collected data, however, reflect short-term trends only. A key question that we seek to address on our Bear Creek Project, a National Restoration Demonstration Watershed, is whether warm-season or cool-season grasses are more effective over the long term for use in CRP and riparian buffer plantings.
The present emphasis on landscape sustainability, supported by such programs as the Conservation Reserve Program, has led to an estimated 14.7 million ha (36 million acres) of U.S. cropland being converted to perennial grasses as of 1995 (Barker et al. 1996). At the same time interest is increasing in providing farmers with carbon credits for increasing soil carbon sequestration. Our goal in this proposed research was to seek a better understanding of how planting of cool- versus warm-season grasses impacts long-term soil quality, as evidenced by rates of soil C cycling and soil organic matter accumulation. Identifying the long-term impacts of grass type on soil quality will help landowners and extension personnel select and manage grasslands to maximize their positive soil impacts. Our team, which includes ten farmers and landowners, has been at the forefront of buffer design, implementation and extension. The research we undertook provided for baseline measurements that can be continued into the future, to provide the long-term data needed to better define best management practices for riparian filter strips in the Midwest.
The species composition of grassland communities is increasingly influenced by human activities, but the consequences of species changes on soil ecosystems remain poorly understood. Widespread planting of grasses in filter strips, riparian buffers, and Conservation Reserve Program lands has been underway in the Midwest since 1985. The types of grasses that are established in these new grasslands is not a function of climate or soils, but of direct human management. Typically, either native, warm-season prairie grasses or non-native, cool-season forage grasses are established but, we contend, the type of grass being planted has important implications. The types of grasses established can be expected to influence soil properties (e.g., D’Antonio and Vitousek 1992, Vinton and Burke 1995, Burke et al. 1997, 1999). Cool-season forage and warm-season prairie grasses differ in physiology, phenology, phytochemistry, and nutrient use. What effects these functionally different grass types have on soil properties is not adequately known, and apparently varies with time since establishment (e.g., Corre et al. 1999). The information we seek is needed so that landowners, extension personnel, and policy makers can make informed decisions about land use options. It is also needed to improve the effectiveness of planted riparian buffers, and to evaluate the impacts of land use changes on regional biogeochemical cycles. We suggest that cool-season and warm-season grasses represent distinct functional groups whose differing characteristics generate predictable shifts in soil properties that may be important to land owners and landscape managers, and which influence the quality of the environment.
The research we undertook was driven largely by our preliminary work in riparian buffers, with previous funding from the EPA, USDA, USGS, SARE, Iowa Department of Natural Resources and Iowa’s Leopold Center for Sustainable Agriculture. In evaluating the effects of different riparian plant communities on soil properties and N processing, we have obtained intriguing data from our replicated stands of switchgrass (Panicum virgatum, a warm-season prairie grass) and mixed cool-season grasses (primarily Bromus inermis, Poa pratensis, Dactylis glomerata, Phleum pratense, and Phalaris arundinacea). End-of-season aboveground biomass was nearly three times greater in switchgrass plots (1600 g/m2) than in the cool-season plots (550 g/m2) (Tufekcioglu 2000). Live fine root biomass was also consistently greater in the switchgrass than in the cool-season plots (Tufekcioglu et al. 1999). However, total soil respiration was significantly greater in the cool-season plots than in the switchgrass plots (Tufekcioglu et al. 2001). We therefore had the paradox of apparently higher productivity and root biomass, but less soil respiration, in switchgrass than in cool-season grasses. These findings suggested substantial soil C sequestration in the switchgrass stands, but soil C pools were lower in switchgrass than in the cool-season plots (Marquez et al. 1999). Furthermore, surface-soil (0-15 cm) microbial biomass averaged >200 mgC/kg soil higher in the cool-season plots than in switchgrass (Pickle 1999), and protozoan populations were substantially higher on two sampling dates (unpublished data). These findings suggested faster rates of soil C and N turnover beneath cool-season grasses than beneath switchgrass. Differences among plots in soil ammonium and nitrate concentrations have not been consistent, but switchgrass soils have shown a propensity for net N immobilization during the winter months.
These different findings may reflect the fundamental differences between cool-season, C3, and warm-season, C4, grasses, and temporal differences in their effects on soils. The C4 photosynthetic system that typifies many of the dominant grasses of our native prairies includes a suite of biochemical, physiological, and morphological adaptations that influence the way that plants respond to environmental conditions (Pearcy and Ehleringer 1984, Ehleringer et al. 1997). Many publications compare the characteristics of C3 and C4 plants, with a focus on their aboveground physiological differences and habitat preferences (e.g., Black 1971, Teeri and Stowe 1976, Tieszen et al. 1979, Pearcy et al. 1981, Pearcy and Ehleringer 1984). Overall, the C4 pathway appears to be most favorable at high temperatures (Ehleringer 1978, Doliner and Jolliffe 1979, Boutton et al. 1980), and C4 prairie grasses are therefore called warm-season grasses. Cool-season, C3 grasses, in contrast, are favored under cooler conditions (e.g., Teeri and Stowe 1976, Boutton et al. 1980, Rundel 1980). In mixed grasslands, the relative dominance of cool-season grasses declines, and that of warm-season grasses increases, in late summer, when conditions favor C4 growth (Williams and Markley 1973, Redmann 1975, Boutton et al. 1980, Ode et al. 1980, Barnes et al. 1983).
One result of the CO2-concentrating mechanism present in C4 plants is their tendency to have less Rubisco in their leaves, and the nitrogen use efficiency of photosynthesis is typically higher in C4 (warm-season) than in C3 (cool-season) plants (Brown 1978, Pearcy and Ehleringer 1984). This, coupled with differences in leaf anatomy and greater C allocation to non-leaf structures (Akin 1989), results in warm-season grasses generally having lower-quality tissues than do cool-season grasses. In our sites, lignin concentrations were higher and N contents were lower in switchgrass than in the cool-season grasses. The mean N content of cool-season grass forages in the U.S. is 1.8% (n=43) and that of warm-season grasses is 1.3% (n=53). The mean lignin contents of cool-season and warm-season grasses are 7.7% (n=22) and 11.1% (n=21) (derived from data in Miller 1958). These values translate into lignin:N values that are almost twice as high in warm-season grasses than in cool-season grasses (8.4 vs. 4.3, respectively). Cool-season grasses are favored over warm-season grasses as forages because of their higher nutritive values. Lignin contents, and lignin:N, are also primary factors controlling rates of organic matter decomposition in terrestrial ecosystems (Meentemeyer 1978, Kaplan and Hartenstein 1980, Melillo et al. 1982, McClaugherty and Berg 1987, Parton et al. 1987). The same factors that allow cool-season grasses to be more easily digested by ruminant microbes also favor their consumption by soil organisms. We suggest that these differences have important ecosystem consequences. Specifically, we propose that physiological, morphological, phenological and phytochemical differences between cool-season and warm-season grasses modify soil detrital dynamics and alter soil properties, and that their effects on soils vary with time since establishment. These issues are currently important because grassland establishment is driven in part by their potentials for soil improvement, C sequestration, and the immobilization of nitrate in agricultural runoff.
This SARE-funded project provided funds to address three important questions concerning the comparative dynamics of grassland soils as influenced by either cool-season or warm-season grasses: (1) Do warm-season grasslands have a greater long-term potential for soil C sequestration than do cool-season grasslands? (2) Do cool-season grasslands support a more diverse soil food web than do warm-season grasslands? and (3) Are total soil respiration rates higher in cool-season than in warm-season grasslands? Our overarching goal was the continued development of the information base needed to provide meaningful recommendations to land owners and land managers interested in long-term soil improvement and soil quality restoration in the central United States.
Tilman and Wedin (1991) grew three cool-season species and two warm-season species in monoculture along a N gradient. They found no significant differences in aboveground mass after 3 yr, but the warm-season species had substantially higher root biomass than did the cool-season species. Wedin and Pastor (1993) found higher rates of net N mineralization beneath cool-season than beneath warm-season grasses, but Pickle (1999), using laboratory assays, found no such differences in our sites. Wedin (1995) measured mass loss and N dynamics in decomposing litter of one warm-season (Schizachyrium scoparium) and three cool-season grasses. Schizachyrium litter and roots had initially higher C:N than all cool-season species, but these differences disappeared after about 30% mass loss, and the C:N of all species approached 20 after 60% mass loss. However, 60% of the Schizachyrium litter remained after 2 y and immobilized N, whereas the cool-season species decomposed faster and did not immobilize N. These data are consistent with our own findings of faster C and N cycling in cool-grass sites, but lower detrital turnover rates in warm-season grasslands should promote C sequestration over the long term.
Federal government incentives have initiated large-scale conversion of cropland to grass filter strips, grass waterways and long-term pasture largely dominated by perennial grass species. Native warm-season grasses are being planted in riparian areas and CRP ground because their extensive root systems could potentially lead to substantial increases in SOC (Corre et al. 1999). A number of studies in the past 10 to 15 years have attempted to estimate new soil C inputs from warm-season vegetation growing in predominantly cool-season soils. One of the first to use this tool, Balesdent et al. (1987) estimated that 22% of the total soil C turned over after 13 years when corn (Zea mays, warm-season) was planted in soil with a predominantly cool-season signature. Gregorich et al. (1995) confirmed these data in their report that 30% of the total soil C in the plow layer (0-27 cm) had turned over after 13 years of corn growth in a forest-derived C3-soil. In perennial Australian warm-season grasslands planted into soils that had developed under subtropical rainforest vegetation, 33% of the original soil C had been replaced by grass-derived C after 35 years (Skjemstad et al. 1990). Wedin et al. (1995), working with monocultures of native tallgrass prairie bunchgrass (warm-season) growing in soils with a predominantly cool-season signature, reported that 22% of the total soil C was replaced by new C within 4 years. In contrast, only 10% of the total soil C turned over in 4 years for perennial cool-season species growing in the same soil. This suggests that native warm-season prairie grasses are more efficient at sequestering new C than cool-season grass species. Corre et al. (1999) utilized a change in vegetation to quantify the rate of accumulation of warm-season SOC in a C3-labeled soil. After 16-18 years, 53-72% of the total soil C had turned over. The potentially slow accumulation of warm-season-derived SOC is an important consideration when using warm-season grass to restore riparian and conservation areas.
Soil food webs consist of organisms that live all or part of their lives belowground. Energy and nutrients are transferred through the various trophic levels as organisms feed on one another. Many scientific studies in the past 25 years have demonstrated that microbial and faunal interactions have significant impacts on the cycling of carbon, nitrogen, phosphorus and sulfur in soils (Coleman et al. 1977, 1978, 1983; Parker et al., 1984; Whitford et al., 1983; Hunt et al., 1987; Moore et al., 1988; Gupta and Germida 1989). Soil protozoans have rapid generation times, turning over an average of 10 to 12 times in a growing season, which is significantly more rapid than other soil biota (Clarholm, 1985). Protozoa are important in N cycling because they excrete bacterially immobilized N as ammonia (Anderson et al. 1981). Numerous studies have shown that protozoan grazers stimulate C turnover and ammonium-N release in the soil (Coleman et al 1977; Woods et al. 1982; Anderson et al., 1985). Kuikman et al. (1990) further observed that N uptake by plants also increased from 9 to 17% when protozoa were present. Our preliminary sampling (unpublished data) suggests that protozoan populations are more than twice as high beneath cool-season grasses than beneath switchgrass.
Preliminary studies on our sites also showed that total microbial biomass-C was twice as high under cool-season grass filters (327 mg C kg soil-1) than under switchgrass (161 mg C/kg soil) (Pickle, 1999). It is believed that the continual litter input found under cool-season grass was the reason for the elevated biomass-C. The high C:N switchgrass roots are slow to decompose, and microbial growth is limited when compared to that found under cool-season grass. We have also found higher macroaggregation under the cool-season grass (Marquez et al. 1999). Macroaggregates are complexes of soil particles and humic compounds that are by-products of microbial biomass. Soil respiration rates were also higher under the cool-season grass than under the switchgrass (see below). These data are also consistent with our working hypothesis that cool-grass stands promote rapid soil C and N cycles, potentially at the expense of long-term C storage.
Soil respiration refers to the total amount of CO2 produced by a soil, including that produced by soil macrofauna, microbes, and live plant roots (Schlesinger 1977, Raich and Schlesinger 1992, Rustad et al. 2000). It is, therefore, a measure of the total biological activity within an aerobic soil. Measurements of soil respiration rate are particularly valuable as they reflect the interactions among a suite of characters that influence overall soil quality, including soil aeration, soil water and temperature regimes, root dynamics, and decomposer activities. Soil respiration is also the single greatest flux of carbon out of soils (Schlesinger 1977, Raich and Schlesinger 1992), and is therefore a key component of soil carbon budgets (e.g., Schlesinger 1977, Raich and Nadelhoffer 1989). Bremer et al. (1998) measured soil respiration in a variety of Kansas grasslands, and found that total soil respiration correlated positively with aboveground productivity. Among grasslands of the world, total soil respiration rates average 70% higher than aboveground productivity (Raich and Tufekcioglu 2000). In our Bear Creek study sites, soil respiration rates were significantly higher beneath cool-season grasses than beneath switchgrass, but aboveground biomass production and fine root biomass were greater in the switchgrass plots (Tufekcioglu et al. 1999).
The lower rates of soil respiration observed in our sites may reflect their lower soil C pools (Marquez et al. 1999). We suspect that the cool-season grasses produce much more labile detritus that decomposes very quickly, stimulating CO2 production, whereas the switchgrass produces long-lived, low-quality detritus that is more slowly processed by soil organisms but which, in the long term, sustains higher levels of soil organic matter. In other words, we believe that cool-season and warm-season grassland soils differ in their temporal dynamics. By producing high-N, low-lignin detritus, cool-season grasses may quickly stimulate soil C formation but, because of rapid C cycling, have low rates of soil C sequestration over the long term. Warm-season prairie grasses, in contrast, with long-lived root tissues, may more slowly contribute to the soil C pool, but may achieve overall higher levels of soil organic matter, as found in our native prairie soils. If this is true, both soil OM storage and soil respiration rates would be higher beneath cool-season grasses during the first years following establishment, but would be higher beneath warm-season grasses over the long term.
These ideas are consistent with data from the northeastern United States, where cool-season grasses were found to have higher SOM contents during the first decade after establishment, but switchgrass was believed to have higher soil C sequestration rates after 15 years (Corre et al. 1999). In Saskatchewan, 50-year-old successional prairies dominated by warm-season grasses had higher SOM, total soil N, and available soil N pools than did similarly aged cool-season grasslands (Christian and Wilson 1999). They speculated that lower soil C pools beneath the cool-season grass, which were planted over millions of hectares, left about 4 * 10^14 g of C in the atmosphere which would have been stored in soils had native, warm-season species been planted. Identifying the temporal dynamics of soil C storages and cycling are fundamental questions we wish to address in this current research. Unlike the NE or Saskatchewan, central Iowa’s environment is very favorable to warm-season grasses, and we expect that their effects on soils will be more quickly observable. Because most of the C lost from soils is in the form of CO2 (Schlesinger 1977; Raich and Nadelhoffer 1989), measurements of soil respiration are integral to the development of the long-term soil C budgets that are needed to address these issues.
Objective 1: To determine if cool-season and warm-season grasses differ in their soil organic carbon (SOC) sequestration potentials.
Hypothesis 1a: Soil organic carbon (soil organic matter) accumulations are greater beneath warm-season grasses than beneath cool-season grasses when the entire soil profile, to 1 m depth, is considered.
Hypothesis 1b: Surface-soil (0-35 cm) organic carbon pools are greater beneath cool-season grasses than beneath young warm-season grasses, but this difference declines as the warm-season grass stands mature.
Hypothesis 1c: Soil organic matter pools increase through time in warm-season grasslands.
Objective 2: To compare the soil food webs present beneath cool-season and warm-season grasses.
Hypothesis 2: Surface soils beneath cool-season grasses support larger populations of more diverse soil organisms than do those beneath warm-season grasses.
Objective 3: To compare overall soil quality, as quantified by total soil respiration rate, in cool-season and warm-season grasses.
Hypothesis 3: In situ soil respiration rates are higher beneath cool-season grasses than beneath warm-season grasses.
Objective 4: To develop best management practices for perennial cropping systems in reserved lands, filter strips, and riparian buffers.
Objective 1: Soil sampling was conducted on five private farms in northern Story County, Iowa. At each farm three plots were established in each grassland type present, and four intact soil cores were extracted with the aid of a jackhammer from each plot. Each soil-core location was referenced to its geographic coordinates using a hand-held GPS, and these locations were overlain onto digitized soil maps to verify that all samples were collected from the same soil map unit. A summary of all cores collected in presented in Table 1, below.
Table 1. Summary of 1-m deep soil cores collected as part of this study. Ages of the C3 plots are not known. All C4 plots were formerly cultivated to annual crops.
Farm Year # of C4 Total # of # of C3 Total # of
Planted Plots C4 cores Plots C3 cores
R. Risdal 1990 2 8 6 24
L. Strum 1994 3 12 2 8
L. Tesdal 1997 3 12 2 8
J. Risdal 1999 3 12 0 0
I. Larson 2001 3 12 0 0
Each core collected was returned intact to the laboratory, and was dissected into 9 segments/samples: 0-15 cm, 15-25 cm, 25-35 cm, 35-50 cm, 50-60 cm, 60-75 cm, 75-85 cm, and 85-100 cm. Each segment was passed through a 2.00-mm sieve to remove gravel, the volume of gravel collected from each segment was measured by volume displacement in water, and roots were hand-picked from the remaining soil. Subsamples of soil were dried at 65 C for nutrient analyses, and separate subsamples were dried at 105 C for dry-weight determinations. Soil bulk density was determined from the total dry soil mass per segment divided by the volume of that segment after correction for gravel volume. Samples dried at 65 C were finely ground in a roller mill prior to analysis of their carbon and nitrogen contents with a Carlo Erba NA1500 CNS analyzer. Subsamples were tested for the presence of carbonates via application of acid. Carbonate-containing samples were acidified to remove carbonates prior to analysis for organic C contents. Carbonate contents were then determined by difference (i.e., total C – organic C = carbonate C). The acidified samples are still awaiting analysis, so we provide only total C data in this report.
Objective 2: Samples for soil food web analysis were collected seven times during this study from each of three C4 grasslands and three C3 grasslands. Data for one C4 grassland is not included here because it was discovered to have been planted onto old meadow. In each grassland we collected 10 randomly located surface-soil (0-15 cm) samples with a push tube and composited them into a single sample that was shipped overnight express to Oregon for analysis by Soil Foodweb Incorporated (http://www.soilfoodweb.com/). There, each sample was analyzed for total bacterial biomass, active bacterial biomass, total fungal biomass, active fungal biomass, and protozoan and nematode population densities.
Objective 3: We utilized a LI-COR 6400 gas exchange system attached to an LI-6400-04 soil respiration chamber to measure soil respiration rates in each farm. Four-inch-diameter, 5-cm-tall PVC rings were cut into place 24 hours prior to measurements, and all measurements were conducted between 10 a.m. and 2 p.m. to minimize variability due to time of day. Ring locations were changed prior to each measurement date. We used standard procedures as recommended by LI-COR, Incorporated, to collect measurements and to analyze the data collected. Soil respiration was monitored on all plots at least monthly throughout the year, and twice monthly during the active growing season. In each of the three plots per farm, we took measurements at five locations on each day. On two dates we measured soil respiration on an hourly basis for 25 hours, to determine diel patterns in respiration rates.
Soil organic matter pools.
The purpose of our soil sampling was to estimate total soil C pools in planted C4-dominated riparian grass filter strips on a variety of farms in northern Story County, Iowa, so that we could resample those same farms and locations at a future date, to quantify directly rates of soil C (and N) accumulation through time. We further compared these data to samples collected from formerly grazed C3-dominated riparian grass filters on the same farms. We further tested the possibility that, by sampling planted filter strips of different ages, we might be able to estimate soil C accumulation rates as a function of grassland age.
Although all soil cores were collected from a single soil type, there were observable differences in mean soil properties among farms. In general, the older C3 grasslands had lower soil bulk density values throughout the surface 60 cm of soil, than did any of the more recently planted C4 grasslands (Table 2). Even after 11 years in C4 grasses, formerly cropped soils had soil bulk densities that averaged about 0.05 g/cm3 greater than those beneath older C3 grass plantings. Soil bulk densities were highest in the C4 filter strips planted in 2001 and 1994, largely because of observably sandier soils at those farms. These data are useful because they allow the estimation of total soil C storage, which depends both upon soil organic matter content (%) and soil mass.
Table 2. Soil bulk density (g/ cm3) by depth for five C4 grasslands and all C3 grassland sites, as well as the overall mean for all locations sampled. Years refer to the date of grassland establishment, which is not known for the C3 grasslands.
Depth C4 C4 C4 C4 C4 C3 Overall
(cm) 2001 1999 1997 1994 1990 Combined Mean
0 to 15 1.23 1.12 1.13 1.23 1.14 1.03 1.15
15 to 25 1.30 1.12 1.23 1.35 1.19 1.12 1.22
25 to 35 1.32 1.20 1.21 1.34 1.16 1.13 1.23
35 to 50 1.33 1.30 1.15 1.29 1.18 1.12 1.23
50 to 60 1.36 1.31 1.24 1.27 1.22 1.17 1.26
60 to 75 1.39 1.35 1.23 1.22 1.27 1.20 1.27
75 to 85 1.48 1.38 1.29 1.24 1.36 1.25 1.33
85 to 100 1.59 1.43 1.18 1.32 1.36 1.26 1.36
Variations in gravel contents among cores and farms were very high, so it is difficult to place statistical certainty on our findings. However, soils beneath our oldest C4 grass filters generally had higher gravel volumes than did any other sites (Table 3. Even so, average gravel volumes in soils at all of our farms and at all depths were <4% of total soil volume, so they did not substantially impact our estimates of total soil C pools.
Table 3. Soil gravel volume (%) by depth for five C4 grasslands and all C3 grassland sites, as well as the overall mean for all locations sampled. Years refer to the date of grassland establishment, which is not known for the C3 grasslands.
Depth C4 C4 C4 C4 C4 C3 Overall
(cm) 2001 1999 1997 1994 1990 Combined Mean
0 to 15 0.2 0.3 0.1 2.4 0.9 0.3 0.7
15 to 25 0.6 0.5 0.2 0.8 1.2 0.7 0.7
25 to 35 0.3 0.7 0.1 0.1 1.8 0.6 0.6
35 to 50 0.3 0.8 0.1 0.1 3.7 0.6 0.9
50 to 60 0.3 0.8 0.1 0.1 3.3 0.8 0.9
60 to 75 0.4 0.8 0.1 0.1 3.7 1.7 1.1
75 to 85 0.6 0.7 0.1 0.1 3.8 1.6 1.1
85 to 100 1.1 1.6 0.1 0.2 3.4 1.1 1.2
Although our study has focused on C for a variety of reasons (e.g., soil organic matter is just great to have in soils), we also analyzed nitrogen (N) contents in all of our samples. Nitrogen is, of course, a very important soil-derived nutrient. Soil C:N contents (0-50 cm depths) were on average lower in the oldest C4 grass filters and in the C3 grasslands, and generally increased with soil depth in most sites (Table 4).
Table 4. Soil C:N by depth for five C4 grasslands and all C3 grassland sites, as well as the overall mean for all locations sampled. Years refer to the date of grassland establishment, which is not known for the C3 grasslands.
Depth C4 C4 C4 C4 C4 C3 Overall
(cm) 2001 1999 1997 1994 1990 Combined Mean
0 to 15 11.8 12.3 12.7 43.4 11.4 11.7 17.2
15 to 25 11.9 12.8 14.3 39.7 11.4 11.3 16.9
25 to 35 12.3 13.2 17.6 12.2 11.5 11.8 13.1
35 to 50 12.7 14.5 17.3 12.4 12.3 12.3 13.6
50 to 60 13.2 17.2 23.7 12.8 16.7 13.9 16.2
60 to 75 13.9 14.4 18.6 13.0 19.7 16.8 16.1
75 to 85 13.9 15.3 23.7 15.1 38.6 18.0 20.8
85 to 100 14.5 14.5 24.7 17.0 25.2 23.9 20.0
We specifically tested the hypothesis that soil carbon (soil organic matter) accumulations are greater beneath warm-season grasses than beneath cool-season grasses when the entire soil profile, to 1 m depth, is considered. This was not the case. There were large variations in soil properties among farms (i.e., among locations), and among plots within farms. On average, total soil C storage to a depth of 1 m was greatest in the C4 grasslands planted in 1997, and in the older C3 grass filters. The lowest soil C pools to 1 m depth were observed in the youngest (planted in 2001) grass filters, which had just been taken out of cultivation and which was not yet an established grass filter. However, it is likely that these differences among sites were due not to time-related changes in soil organic matter accumulations, but rather to the inherent spatial variation that exists within the soil type investigated.
We also tested the hypothesis that soil C accumulations would be greater in C3 than in C4 grass filters, when only the surface 0-35 cm of soil was considered. Farms differed substantially in their surface-soil C accumulations. The C4 grass filters planted in 1999 had greater surface-soil C accumulations than did C4 grass filters planted in 1994 and 2000. The old C3 grass filters and the oldest (1990) C4 grass filters had intermediate levels of surface-soil C. These data are particularly interesting because there is virtually no carbonate in the surface 35 cm of these soils, so they are accurate representations of soil organic matter accumulations.
The purpose of this study was to collect baseline data, allowing us to return to the same locations in 5-10 years to resample soils, and measure directly their carbon (and nitrogen) contents again, for direct comparison to our original data (collected in this research). Overall, our data suggest that the alternative chronosequence approach (i.e., sampling different locations to estimate changes in soil C pools through time) was not useful, even though sampling was restricted to a single soil type. Even with soil types, there is spatial variability in soil properties that confounds comparisons made among locations. That is not surprising, but does call into question the validity of information derived from studies that compare different sites to estimate vegetation affects on soil C dynamics. To overcome that problem, we collected accurate GPS coordinates for each plot sampled, and for each soil core collected. This will allow us to come back to the very same locations in the future, to collect additional cores for identical analyses, and thereby to quantify directly changes in soil properties through time. We also collected additional soil cores from just inside the still-cultivated crop fields that are adjacent to each of our grass filters. When analyzed, data from those cores will provide a paired “time-zero” estimate of soil C pools for each location. However, because we did not budget for those analyses in this current grant, they are not yet complete. We suspect that this paired sampling design will be a much more powerful approach for investigating the impacts of riparian filter strip establishment on soil properties.
Soil food webs.
Although Pickle (1999) found higher microbial biomass in soils beneath cool-season grasses than beneath switchgrass, our study suggests that soil food webs beneath relatively mature (7-11-year-old) C4 grasslands are similar to those beneath well-established C3 grasslands composed of forage-grass species (Table 5). Total protozoan and total nematode abundances were slightly higher in the C3 grasslands based on annual means, but variability was high among sampling dates, and both grasslands exhibited similar ranges in values. The proportions of individuals in different functional groups of protozoans (i.e., flagellates, amoebae, and ciliates), and in five different trophic groups of nematodes, were also similar among the grassland types. Seasonal trends in active bacterial biomass, total bacterial biomass, active fungal biomass, total fungal biomass, active:total ratios, and bacterial:fungal ratios were all similar among the grassland types. In fact, after all that work, we cannot discern any single way in which the soil food webs of the two grassland types differed substantially. We did, on two dates, sample a native prairie preserve to place our numbers into perspective. In general, on the dates sampled, the native prairie had fewer total protozoans, more nematodes, far more active fungal biomass, more total fungal biomass, and more active fungi:active bacteria than did our agricultural grassland sites. These data cannot be evaluated statistically, however; whereas we sampled three of each type of agricultural grassland, we had only a single prairie to sample.
Why, then, did we not find greater microbial biomass beneath C3 grasses, as did Pickle (1999)? Pickle’s work was based on chloroform incubation of soil samples. In that case all microbes, including bacteria and protozoans and nematodes, are considered a part of microbial biomass. Our analyses were done via direct counts of individual soil organisms. So in fact the comparison of our current results with those of Pickle (1999) is like apples to oranges. On the other hand, it may be that chloroform incubation provides meaningful information than is derived from the more reductionist approach we applied. Second, Pickle sampled similar C3 sites as did we, but her C4 sites were younger at the time of her sampling, and it is possible that the older age of our C4 sites has allowed them to develop larger soil microbial abundances.
Table 5. Mean annual soil microorganism biomass and abundance data in planted riparian grasslands in northern Story County, Iowa. Each datum represents the mean of seven sample dates over two years, in three plots of each grassland type.
Grass Type Bacterial Fungal
Biomass Biomass Protozoans Nematodes
(mg/kg) (mg/kg) (#/g) (#/g)
warm-season (C4) 169 183 13,000 2.1
cool-season (C3) 174 194 14,800 2.4
Soil respiration rates.
We collected nearly 1.5 years of soil respiration from a variety of grasslands. In our summary below we present a fairly complete picture of the first full year of data, which has been more fully analyzed. Tufekcioglu et al. (2001) found that soil respiration rates beneath C3 grasses were, on average, higher than those beneath C4 grasses (switchgrass). We did not find this to be the case, when old C4 grass filters were compared to old C3 grass filters (Table 6). We believe that this discrepancy is due to the different surface litter masses present during Tufekcioglu’s study, and ours. Our sites were all burned in the spring of 2001, as part of filter strip management. As a result, the litter layer was completely burned off, leaving the soil exposed, and a dense litter layer had not developed even by the first winter. Tufekcioglu’s measurements were done in C4 stands that had not been burned in several years, and had a very dense litter mat that created an ice sheet that persisted well into late spring. As a result, they observed very low rates of soil respiration throughout the spring; we did not.
Table 6. Annual soil respiration rates and aboveground net primary production (OM = organic matter) for three C4 grasslands differing in their date of planting, and one C3 grassland in Central Iowa, U.S.A. Values within parentheses are the standard error of the mean, n = 3 plots/site.
Farm Annual Soil Annual Aboveground
Respiration Net Primary Production
C3 1990 1298 (105) 706 (180)
C4 1994 1256 (28) 1825 (244)
C4 1999 937 (43) 764 (185)
C4 2001 726 (7) 934 (62)
We also compared directly the soda lime technique used by Tufekcioglu et al. (2001) with the LI-COR approach we utilized. There was a direct linear relationship between soil CO2 emissions as measured with the soda lime and LI-COR techniques:
Rsoil(LI-COR) = 1.84 * Rsoil(soda lime) – 1.12
In this equation, Rsoil refers to the soil respiration rate in gC per m2 per day. Over the majority of observed flux rates the LI-COR system measured higher soil-CO2 emissions rates than did the soda lime technique, but at very low fluxes (< 1 gC per m2 per day) the soda lime technique produced higher estimates of soil CO2 emissions.
We did find both dramatic seasonal trends in soil respiration rates in all sites, different seasonalities of soil respiration in the C3 sites in comparison with the oldest C4 sites, and different annual carbon fluxes from soils in the different sites (Table 6). In these same sites we undertook seasonal aboveground biomass harvest, and those were used to calculate annual aboveground grassland productivity (ANPP, Table 3). Based on the three age-classes of C4 filter strips investigated, there was an increase in annual soil respiration rates as stands matured. The youngest stand (C4 2001 in Table 6) was dominated by large, fast-growing weeds.
We also undertook measurements of soil CO2 emissions over whole 24-hour periods, to determine how biased our measurements, collected between 10:00 and 14:00, might be. On average, measurements collected between 10:00 and 14:00 were about 5% higher than were mean daily soil-CO2 emissions calculated over 24 hours. We did not correct our data to account for that slight bias, given the large spatial variability in soil respiration rates that was observed within sites.
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Our study demonstrated, if anything, that non-native, C3, cool-season forage grasses and native, C4 prairie grasses have similar soil carbon pools, soil food webs, and soil CO2 production rates. Thus, either grass type may be useful for use in riparian filter strips planted as part of the Conservation Reserve Program, or for other purposes. We did not investigate surface-soil erosion rates, water infiltration rates, or many other parameters that might be important to consider in planting filter strips, but the data we did collect did not provide any strong arguments for recommending one grass type over the other. Longer-term responses of the sites to continued management of the existing grass covers should be very interesting, because the cool-season grasses have been in place for much longer than have even the oldest prairie-grass filters that we studied. Thus, future studies on these same plots has great potential to address directly the question of whether one grass type or the other is better able to fulfill the long-term conservation and environmental improvements sought under the CRP program.
We have been closely involved with riparian buffer strip and grass filter plantings, however, and there seems to be a visible shift away from planting non-native, cool-season grasses, to more frequent and widespread use of native prairie grasses with mixed prairie forbs. These latter plantings are in many cases outstandingly beautiful throughout the summer, because they incorporate a diversity of flowering plant species, and a diversity of grasses that flower at different times, and grow with different forms. This shift in planting may be driven by greater availability of mixed prairie seed, or by the greater information now available on planting and management options for native-species seed mixtures. However, it most certainly is driven in part by the appearance of the plantings themselves, coupled with landowner attempts to improve the beauty of their land while promoting streamside conservation. At a minimum, our data demonstrate that mixed native-species plantings are in no way inferior to grass filters composed of cool-season forage grasses.
This was a research project, not one designed to address economic issues, and we did not.
In 2001 the top four practices for enrollment in the CRP in the State of Iowa were CP02-Native Grasses (62724 acres), CP21-Filter Strips (41833 acres), CP04-Wildlife Habitat (36226 acres), and CP01-Introduced Grasses (34177 acres). Just two years earlier, in 1999, these same four practices were enrolled in the order of Wildlife Habitat – Introduced Grasses – Filter Strips – Native Grasses. Note the complete shift from a primary focus on planting of native grasses in 1999 (and previous years) to a dominant practice of planting native species (2001).
The top five CREP (Conservation Reserve Enhancement Program) practices, in terms of total acreages enrolled in the U.S. through fiscal year 2001, were Riparian Buffers (88,234 acres), Filter Strips (85,195 acres), Wetland Restoration (64,713 acres), Non-native Grasses (49,800 acres) and Wildlife Planting (29,954 acres).
Educational & Outreach Activities
Objective 4 focused on extension activities. In the first year of activity we were invited to give 29 presentations to community and student groups, we provided 17 tours of the Bear Creek watershed to groups ranging from individuals to >100 people. We held six workshops on riparian management systems. We had 11 Field Days, involving students, NRCS and ARS personnel, and landowners. We also had 14 planning meetings involving diverse groups. Each of these activities involves many people; Farm field days, for instance, typically attract >100 individuals, most of whom come from surrounding communities. We undertook a similar amount of extension and outreach activities in the second year of our grant.
In addition we have revised one extension bulletin (Maintenance of Riparian Buffers, Iowa State Extension Bulletin #PM1626C), which is available through the Iowa State University Extension Office, and through our Web site (see below). Although we originally proposed to create a CD describing establishment and management procedures for riparian buffers and grass filters, for dispersal to landowners, we became convinced that world wide web access to this information provided a more efficient means of information transfer, and allowed for more up-to-date information to be incorporated as it came available. We therefore developed a web-based introduction to riparian management systems, at http://www.buffer.forestry.iastate.edu/. This site is devoted to riparian buffers and grass filters. This has turned out to be a very effective way to share information with the public (see, for example, our FAQ section).
Partners in our Bear Creek Riparian Management work include:
Iowa Agriculture and Home Economics Experiment Station
Iowa State University Extension
Leopold Center for Sustainable Agriculture
Ron and Sandy Risdal Farm
Loren Tjernagel, Radcliffe farms
USDA Forest Service/IDNR Stewardship Incentive Program
USDA/USEPA Agriculture in Concert with the Environment Program
USDA National Research Initiative Competitive Grants Program
USDA Natural Resources Conservation Service
USDA-ARS National Soil Tilth Laboratory
USEPA/IDNR Section 319 Nonpoint Source Management Program
USGS Water Resources Research Program
A more complete view of all of our extension and outreach activities is available on our home page. Citable outcomes of this and related research are detailed below.
Publications (arranged alphabetically)
Brewer, M.J. and J.P. Colletti. 2001. Financial agents, water quality and riparian forest buffers. In Proceedings of the Association for Temperate Agroforestry Convention, Regina, Saskatchewan, Canada.
Dornbush, M. E., T. M. Isenhart and J. W. Raich. 2002. Quantifying fine-root decomposition: An alternative to buried litterbags. Ecology 83: 2985-2990.
Lee, K., T.M. Isenhart, R.C. Schultz, and S.K. Mickelson. 1999. Sediment and nutrient trapping abilities of switchgrass and bromegrass buffer strips. Agroforestry Systems 44:121-132.
Lee, K., T.M. Isenhart, R.C. Schultz, and S.K. Mickelson. 2000. Multispecies riparian buffers trap sediments and nutrients during rainfall simulations. J. Environ.Qual. 29:1200-1205
Raich, J. W., C. S. Potter and D. Bhagawati. 2002. Interannual variability in global soil respiration, 1980-1994. Global Change Biology 8: 800-812.
Raich, J. W. and A. Tufekcioglu. 2000. Vegetation and soil respiration: Correlations and controls. Biogeochemistry 48: 71-90.
Schultz, R.C., J.P.Colletti, T.M. Isenhart, C.O. Marquez, W.W. Simpkins, and C.J. Ball. 2000. Riparian forest buffer practices. Pages 189-281. In: H.E.Garrett, W.J. Rietveld and R.F. Fisher (Eds.) North American Agroforestry: An integrated Science and Practice. American Society of Agronomy, Madison, WI. 402 pp.
Simpkins, W.W., T.R. Wineland, R.J. Andress, D.A. Johnston, G.C. Caron, T.M. Isenhart, and R.C. Schultz. 2002. Hydrogeological constraints on riparian buffers for reduction of diffuse pollution: examples from the Bear Creek Watershed in Iowa, USA. Water Science and Technology 45(5):61-68.
Tufekcioglu, A., J.W. Raich, T.M. Isenhart, and R.C. Schultz. Biomass, carbon and nitrogen dynamics of multi-species riparian buffer zones within an agricultural watershed. Agroforestry Systems, in press.
Tufekcioglu, A., J. W. Raich, T. M. Isenhart, and R. C. Schultz. 2001. Soil respiration within riparian buffers and adjacent crop fields. Plant and Soil 229: 117-124.
Symposium and Meeting Presentations and Abstracts (arranged alphabetically):
Cambardella, C. A., C. O. Marquez, J. E. Pickle, T. M. Isenhart, R. C. Schultz, J. W. Raich and A. Tufekcioglu. Soil C storage potential in riparian buffer zones. Poster, Carbon: Exploring the Benefits to Farmers and Society, Des Moines, Iowa, 29-31 August 2000.
Dornbush, M. E. and J. W. Raich. Do litterbags underestimate fine root decomposition? Oral presentation, ESA Annual Meeting, 2001.
Garcia, V. J., C. O. Marquez, C. A. Cambardella, R. C. Schultz, and T. M. Isenhart. A conceptual model to study soil aggregate dynamics. Agronomy Abstracts. s03-garcia211819-O.pdf. 2001.
Haake, D.M., J.L. Nelson, R.C. Schultz, T.M. Isenhart. 2000. Denitrification and microbial biomass in soils under riparian forests, pastures and crop fields of three northern Missouri streams. Agriculture and the Environment Conference, March 5, 2001, Ames, IA (poster).
Haake, D.M., J.L. Nelson, R.C. Schultz, T.M. Isenhart. 2000. Denitrification and microbial biomass in soils under riparian forests, pastures and crop fields of three northern Missouri streams. Pp 382. Abstracts 2000 Annual Meetings of the American Society of Agronomy, Crop Science Society of America and the Soil Science Society of America, Nov 5-9, 2000, Minneapolis, MN (poster).
Isenhart, T.M., R.C. Schultz, J.W. Raich, W.W. Simpkins, T.B. Parkin. 2000. Nitrogen transformations and fate within re-established multi-species riparian buffers in central Iowa. Pp 382. Abstracts 2000 Annual Meetings of the American Society of Agronomy, Crop Science Society of America and the Soil Science Society of America, Nov 5-9, 2000, Minneapolis, MN (poster).
Lee, K.H., T.M. Isenhart, R.C. Schultz, S.K. Mickelson. 2000. Multi-species riparian buffers as traps for sediment and nutrients in surface runoff. Pp 382. Abstracts 2000 Annual Meetings of the American Society of Agronomy, Crop Science Society of America and the Soil Science Society of America, Nov 5-9, 2000, Minneapolis, MN (poster).
Maiers, R.P. R.C. Schultz, S.D. Hinz, S.D. Logsdon. 2000. Surface runoff comparisons of row-crop, pasture, forest and cool-season grass land-use practices along riparian zones in three central Iowa watersheds. Pp. 381. Abstracts 2000 Annual Meetings of the American Society of Agronomy, Crop Science Society of America and the Soil Science Society of America, Nov 5-9, 2000, Minneapolis, MN (poster).
Marquez, C. O., C. A. Cambardella, V. J. Garcia, R. C. Schultz, and T. M. Isenhart. Seasonal dynamics of soil macroaggregates in different vegetations. Agronomy Abstracts. s03-213438-P.pdf. 2001.
Marquez, C.O., C.A. Cambardella, T.M. Isenhart, R.C. Schultz. 2000. Seasonal trends in soil organic matter and aggregates in multi-species riparian buffers, forests, and crop fields. Pp 384. Abstracts 2000 Annual Meetings of the American Society of Agronomy, Crop Science Society of America and the Soil Science Society of America, Nov 5-9, 2000, Minneapolis, MN (poster).
Marquez, C. O., C. A. Cambardella, T. M. Isenhart, R. C. Schultz. Seasonal trends in soil organic matter and aggregates in multi-species riparian buffers, forests, and crop fields. Agronomy Abstracts. p. 384. 2000.
Marquez, C.O., V. J. Garcia, C. A. Cambardella, R. C. Schultz, and T. M. Isenhart. Assessing soil degradation after conversion of native ecosystems to agricultural production. Agronomy Abstracts. s03-marquez210736-0.pdf. 2001.
Pickle, J.E. A. Tufeckcioglu, J.W. Raich, T.M. Isenhart, R.C. Schultz. 2000. Seasonal changes in microbial biomass and soil respiration in re-established multi-species riparian buffers and adjacent crop fields. Pp 382. Abstracts 2000 Annual Meetings of the American Society of Agronomy, Crop Science Society of America and the Soil Science Society of America, Nov 5-9, 2000, Minneapolis, MN (poster).
Raich, J.W. and M.E. Dornbush. Plant phenology affects seasonal patterns of soil respiration in central Iowa grasslands. Oral presentation, Ecological Society of America (ESA) Annual Meeting, 2002.
Raich, J. W., C. S. Potter and D. Bhagawati. Global soil respiration in a warmer world. Oral presentation, E cological Society of America Annual Meeting, 2000.
Schultz, R.C., T.M. Isenhart, W.W. Simpkins, J.W. Raich, J.P. Colletti. 2000. Research on pollution control by multi-species riparian buffers in the Cornbelt Ecoregion of the United States. Pp. 381 Abstracts 2000 Annual Meetings of the American Society of Agronomy, Crop Science Society of America and the Soil Science Society of America, Nov 5-9, 2000, Minneapolis, MN (symposium oral presentation).
Simpkins, W. W. Hydrogeologic Controls on the Efficiency of Nitrate Removal Beneath Multi-Species Riparian Buffers in the Bear Creek Watershed, Central Iowa. Presented at the Annual Geological Society of America Meeting, October 27, 2002, in Denver, Colorado.
Simpkins, W.W., R.A. Andress, B.A. Spear, D.A. Johnston, T.R. Wineland, T.M. Isenhart, T.B. Parkin, R.C. Schultz. 2000. Assessing the role of geology for nitrate fate and transport in groundwater beneath riparian buffers. Pp 382. Abstracts 2000 Annual Meetings of the American Society of Agronomy, Crop Science Society of America and the Soil Science Society of America, Nov 5-9, 2000, Minneapolis, MN (poster).
Simpkins, W.W., T.R. Wineland, R.J. Andress, D.A. Johnston, G.C. Caron, T.M. Isenhart, and R.C. Schultz. 2001. Hydrogeological constraints on riparian buffers for reduction of diffuse pollution: examples from the Bear Creek Watershed in Iowa, USA. Fifth International Conference on Diffuse Pollution, Milwaukee, Wisconsin, p. 81.
Simpkins, W. W., T.R. Wineland, T.M. Isenhart and R.C. Schultz, 2003. Hydrogeologic setting controls NO3-N removal in groundwater beneath multi-species riparian buffers. AWRA Agricultural Hydrology Conference.
Simpkins, W.W., T.R. Wineland, T.M. Isenhart, and R.C. Schultz, 2001. Utilizing hydrogeologic setting for siting riparian buffers in areas of intensive agriculture. Geol. Soc. Am. Absts. with Progs. 33(6): A-301.
Thimmesch, C.A., W.W. Simpkins, R.C. Schultz, and T.M. Isenhart, 2002. Groundwater quality trends in three riparian buffers in the Bear Creek watershed (1996-2002). Abstracts of the Iowa Academy of Science, p. 29.
Tufekcioglu, A. J.W. Raich, T.M. Isenhart, R.C. Schultz. 2000. Above- and below-ground C and N dynamics in multi-species riparian buffers and adjacent crop fields. Pp 381. Abstracts 2000 Annual Meetings of the American Society of Agronomy, Crop Science Society of America and the Soil Science Society of America, Nov 5-9, 2000, Minneapolis, MN (poster).
Wineland, T.R., W.W Simpkins, I.A. Beresnev, T.M. Isenhart, and R.C. Schultz, 2001. Hydrogeology and water quality beneath multi-species riparian buffers in the Bear Creek watershed, central Iowa. Geol. Soc. Am. Absts. with Progs. 33(6): A-423.
Wineland, T.R., W.W. Simpkins, I.A. Beresnev, T.M. Isenhart, and R.C. Schultz. 2001. Geological and geophysical characterization as a component of riparian buffer design. Geol. Soc. Am. Absts. with Progs. 33(4): A-46.
Wineland, T.R., W.W. Simpkins, I.A. Beresnev, R.C. Schultz, and T.M. Isenhart, 2002. Hydrogeologic controls on the efficiency of nitrate removal beneath multi-species riparian buffers in the Bear Creek watershed, central Iowa. Geol. Soc. Am. Absts. with Progs. 34(6): 57.
Zaimes, G.N., R.C. Schultz, T.M. Isenhart. 2000. Stream bank erosion adjacent to differing land-use practices in central Iowa. Pp 382. Abstracts 2000 Annual Meetings of the American Society of Agronomy, Crop Science Society of America and the Soil Science Society of America, Nov 5-9, 2000, Minneapolis, MN (poster).
Zaimes, G.N., R.C. Schultz, T.M. Isenhart. 2000. Stream bank erosion adjacent to differing land-use practices in central Iowa. Agriculture and the Environment Conference, March 5, 2001, Ames, IA (poster).
Zhang, H., C. O. Marquez, C. A. Cambardella, R. C. Schultz, T. M. Isenhart, and J. L. Nelson. Soil aggregation dynamics under riparian pastures, forest, and croplands in NE Missouri. Agronomy Abstracts. 2002.
Areas needing additional study
This research project was able to accomplish our goals, but much more is needed. Understanding better the reasons why more landowners in more locations do not take advantage of opportunities to improve streamside management on their lands, despite significant government financial support, remains a mystery. Understanding better the long-term management approaches that minimize landowner time and monetary expenditures while maximizing benefits of conservation lands is important. Additional experimental studies where variables are controlled is of the utmost importance, because private lands are subject to many unpredictable activities that make research results difficult to interpret. Government incentives are multiplying far faster than is objective information; defining best management options therefore lags far behind demand for that information.