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 in corn and grain sorghum, and 0, 34 and 69 kg N ha^-1 in soybean and oats/clover) on microbial community structure was examined in 2014 and 2015 using fatty acid methyl ester (FAME) profiling. Soil sampling occurred over three different corn growth stages throughout the corn phase of each crop rotation. Total microbial biomass (TMB) responded similarly in 2014 and 2015, exhibiting a decrease with increasing N fertilizer and the greatest biomasses in the crop rotations with four crops. Fungal biomass, bacterial biomass, and arbuscular mycorrhizal fungi were also analyzed. Microbial community structure, analyzed through canonical discriminant analysis, differed among crop rotations, with each crop rotation having similar community structure across 2014 and 2015, and within sampling date. 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.