Recovery of soil organic matter stocks from switchgrass plantings in agricultural landscapes
Agricultural activities have a major impact on global biogeochemical cycles, particularly nitrogen, phosphorus and carbon. Cultivation accelerates the decomposition of soil organic matter, which contributes to atmospheric carbon and decreases the ability of soils to retain nutrients. Planting perennial warm season grasses are a useful management alternative to row crop agriculture because they are effective at increasing soil carbon storage and may facilitate higher rates of nitrogen and phosphorus retention in agricultural watersheds. In this project, we examined the advantages of converting row crops to a native perennial warm season grass (Panicum virgatum, switchgrass) as a potential nutrient management and carbon sequestration strategy in agricultural systems within the Chesapeake Watershed. Soils were collected from transects in replicate fields of conventional row crop agriculture (corn/soybean rotation) and four ages of fields converted to switchgrass (1, 2, 3, and 4 years since establishment), and a single pasture set aside in the Conservation Reserve Program (CRP). Soil samples were sieved into three particle size classes (macro- and microaggregate and mineral associated fractions) and analyzed for carbon and nitrogen to. Additionally, soils were analyzed for mineralizable carbon and nitrogen pools through short term incubations, and microbial biomass using a chloroform fumigation extraction.
Contrary to what we expected, our results show an initial decline in soil organic matter in row crop soils converted to switchgrass. Soil organic carbon in the bulk soil ranged from 2.33 to 19.78 g C/ kg soil, and there was no significant effect of switchgrass planting age on carbon and nitrogen fractions. In the top 20 cm of the soil, the soil organic carbon in first year switchgrass was 19.04 g C/ kg soil, which was similar to 16.08 and 19.78 g C/ kg soil of the corn and CRP fields. The second year, third year, and fourth year fields exhibited a decline in soil organic matter of 9.00, 8.94, and 9.60 g C/ kg soil, respectively. Similar patterns were seen in swithgrass, corn, and CRP fields at the 20-40 cm and 40-60 cm soil depths. Although surprising, our results could be due to changes in the microbial community following conversion to perennial grasses and priming effects on soil organic matter stocks. Our results from this study will be used to understand the environmental and economic benefits of implementing warm season grass plantings within agricultural watersheds as a means to mitigate agriculturally-induced effects on carbon storage and nitrogen retention in soils.