The Impact of Increasing Diversity of Crop Rotations on Soil Microbial Communities Under Variable Rates of Nitrogen Fertilization.

Crop rotation is a growing management strategy that can increase soil quality and positively impact ecosystem services. The physicochemical mechanisms by which crop rotations increase soil quality have been extensively studied.  For example, crop rotations can increase soil organic carbon leading to improved macroaggregation and higher levels of plant available N and P. Less is known about how crop rotations impact the structure and function of soil microbial communities, particularly in response to N fertilization. Thus, the interaction between crop rotation (continuous corn (CC), corn-soybean (C-SB), corn-soybean-sorghum-oat/clover (C-SB-SG-OC), corn-oat/clover-sorghum-soybean (C-OC-SG-SB)) and N fertilization (0, 90, and 180 kg N ha^-1) on microbial community structure was examined in 2014 and 2015 after after seven years of no-till using fatty acid methyl ester (FAME) profiling. In 2014, the greatest microbial biomass was observed in CC (98.1 nmol FAME g^-1), followed closely by C-OC-SG-SB (95.9 nmol g^-1) and C-SB-SG-OC (95.8 nmol g^-1), while the lowest was in C-SB (82.0 nmol FAME g^-1), averaged across N treatments. Trends were similar for the ratio of saprophytic fungi to bacteria (F:B), in which the greatest ratio was observed in C-OC-SG-SB (7.0), followed closely by C-SB-SG-OC (6.8) and CC (6.6), while the lowest was in C-SB (5.9). Nitrogen fertilization rate had a significant negative impact on the biomass of arbuscular mycorrhizal fungi in the soil. Understanding how diverse crop rotations influence soil microbial communities under variable rates of nitrogen fertilization can help design crop rotations that result in both profitable yields and increased soil quality.