Microbial FAMEs Analysis of Marginal Soils Cultivated for Biofuel Production in Eastern Nebraska.

Microbial fatty acid methyl ester (FAME) analysis was conducted on a marginal Aksarben silty clay loam (fine, smectitic, mesic Typic Argiudoll) used for biofuel production in eastern Nebraska. Samples were taken in November 2011, after harvest. Three crop types were studied: continuous corn, soybean, and switchgrass. Corn and switchgrass plots have been continuously cropped since 1998. Soybean plots were cropped in switchgrass through fall of 2009, and were planted in soybeans in the spring of 2010 and 2011. Nitrogen fertilizer was applied as NH4NO3 in one of four rates (0, 60, 120 or 180 kg ha-1). Corn plots were harvested in one of two ways: 50 percent stover removal or no stover removal. Switchgrass plots had two harvest treatments: pre-killing frost and post-killing frost. Soybean plots had no harvest treatment. Total microbial biomass was strongly influenced by crop type, as well as by a combination of crop, N application rate, and harvest. Switchgrass had the highest total biomass by far (287 nmol/g), followed by soybean (198 nmol/g), and corn (163 nmol/g). Crop types each had significantly different soil community compositions. Switchgrass and soybean plots had the most similar compositions, while corn and switchgrass had the least similar community compositions. Arbuscular mycorrhizal fungi (AMF) and saprophytic fungi were most common in switchgrass plots. Between corn and soybean plots, AMF were more common in soybean while saprophytic fungi were more common in corn. Soybean plots had stronger representation from bacterial biomarkers than fungal markers. However, the high total biomass and similarity in community composition to switchgrass suggests that the crop history of the soybean plots plays a significant role in the structure of the microbial community.