- Agronomic: corn
- Soil Management: general soil management
Conservation Reserve Program (CRP) restorations can benefit soil quality and fertility, but few evaluations of soil biological aspects of CRP lands exist. Measurement of soil microbial communities and activities may be useful in environmental evaluations of CRP prairie restorations. We examined microbial community composition under adjacent prairie- and corn-based agroecosystems, to test the general hypothesis that microbial community structure and activity would be distinct under varying vegetation and management treatments. Soil samples were taken in August of 2008 from two Conservation Reserve Program (CRP) prairie restorations and adjacent corn fields, on working farms in Dane County, WI. Principal component analysis (PCA) of the lipid biomarkers revealed soil microbial community structure within CRP prairie treatments displayed significant differences from the corn-based treatments. We found elevated abundances of arbuscular mycorrhizal fungi in the soils of prairie treatments, whereas under corn-based treatments we found elevated relative abundance of gram-positive bacteria, and increased ratios of gram-positive:gram-negative bacteria. We also found, in corn-based treatments, elevated production of cyclopropyl lipids compared to their precursor lipids, possibly indicating more physiological stress among bacterial communities in agricultural systems. These findings may reflect a more rhizosphere-dominated soil habitat within prairies, with greater microbial utilization of labile carbon sources, than in corn-based systems.
We suggest that the use of three types of lipid ratios, fungal:bacterial, Gm+:Gm-, and cyclopropyl/precursor, are most useful in distinguishing the effects of prairies from corn-based systems on soil microbial communities.
Preservation of soil quality, including soil fertility and structure, is a major goal of the growing number of public and private incentive-based ecosystem restoration projects. The U.S. Department of Agriculture’s Conservation Reserve Program (CRP) has provided incentives to landowners since 1985 to take environmentally sensitive land out of agricultural production and restore native vegetation. Much of this ‘sensitive’ land is classified as such due to high erodibility potential, such as steeply sloped land, or land especially prone to wind erosion. Some land is also enrolled in the program due to its suitability for wildlife habitat, protection of riparian areas, or other environmental benefits (Smith 2000). Currently, about 31 million acres are enrolled in the program, U.S.-wide (USDA 2009a). This comprises about 2% of land in the contiguous U.S., while cropland comprises about 19% of national land-use (USDA NRCS 2007). In the state of Wisconsin CRP-enrolled acres have decreased from a high of 700,000 in the mid-1990’s to the 2009 level of approximately 461,000 (USDA 2009b).
The future of the CRP program is uncertain. Under the Food, Conservation, and Energy Act of 2008, total acres allowed to be maintained in CRP were reduced from 39.2 million acres to 32 million acres nationwide, beginning in fiscal year 2010. Some have advocated replacing the program with a harvestable biomass program in order to supply the U.S. with cellulosic bioenergy feedstocks for alternative energy projects. For example, in a U.S. Department of Energy assessment of potential U.S. acreage that could be dedicated to bioenergy production (U.S. DOE 2005), 50% of CRP biomass productivity (17-28 million dry tons) was assumed to be directed towards biofuel production, under the category of “idle cropland”. Many soil and climate scientists are advocating not for replacement of the CRP program, per se, but for expansion of governmental and market-based incentive programs to encompass prairie species-based restorations that could also potentially be used as harvestable cellulosic feedstock, and/or carbon sinks as part of a U.S. cap-and-trade program (Fargione et al. 2009).
Numerous studies have quantified the benefits of CRP restorations for wildlife habitat, soil quality and fertility, reduced soil erosion potential, and soil carbon accumulation (Gebhart et al., 1994; Staben et al. 1997, Baer et al. 2002; but, see Kucharik 2007). However, the biological activity of the soils in CRP restorations has largely been ignored. Staben et al. (1997) did quantify microbial parameters, including potential enzyme activities, in CRP soils, but did not assess microbial community composition. Soil microorganisms mediate important soil functions such as decomposition and cycling of nitrogen and phosphorus, and play a crucial role in development of soil structure. Additionally, recent evidence suggests that indicators of soil microbial community composition may be important predictors of soil processes and services, including carbon and nutrient cycling rates and dynamics (Smithwick et al. 2005, Carney et al. 2007).
Indicators of microbial community composition, in addition to total microbial biomass, may be suitable for assessing soil quality in CRP and other types of restoration sites. Biomarker compounds, such as phospholipid fatty acids (PLFAs), are one type of community structure indicator (Vestal and White 1989, Zelles 1999). Phospholipids are produced as part of microbial cell membranes, and are present in fairly constant amounts. Furthermore, their abundance reflects viable microbial biomass, as they are lost rapidly upon cell death (White 1993). Generally, PLFA data are useful in describing broad microbial community structure differences among different ecosystem and land-use types (Baath et al., 1995, Zelles et al. 1999, Burke et al. 2003).
In this study, we characterized soil microbial community composition and soil attributes within CRP prairie restorations and adjacent reduced-tillage cropland plots, on two working farms in south Dane County, Wisconsin. Our aims were: to determine whether CRP prairie restorations could affect soil microbial community composition; to determine whether soil carbon and soil aggregate-size distribution, important regulators of microbial communities, were affected by CRP restoration, compared to no-till corn management; and, to explore the use of microbial indicators that might differentiate between these two systems and be useful in future soil quality evaluations.
1) Evaluate the effects of CRP management on microbial biomass, soil microbial community composition and diversity. Relate microbial indices to soil quality parameters (especially soil carbon) that may respond to CRP and post-CRP cultivation practices.
2) Quantify soil carbon differences between paired CRP and cultivated sites. Examine the relationship between soil aggregates and soil carbon within CRP and cultivated sites.
3) Increase farmer knowledge of soil quality, and increase the ability of farmers to preserve long-term soil quality through dissemination of educational materials and decision-making tools.