Microbial Community Shifts In Soil Profile In Response to Fall Conservation Deep Tillage.

Fall conservation deep tillage (FCDT) of a continuous maize irrigated cropping system led to the stabilization of C from crop residues deep into the profile over four years. We hypothesized that this change in cropping system management would be characterized by distinct changes in the soil microbial community. We used fatty acid methyl ester (FAME) profiling to assess changes in the soil microbial community in the 0-5, 5-15 and 15-30 cm soil depths of no till (2005) and three consecutive years (2006-2008) of FCDT. In general, soil microbial biomass (SMB) as measured by total FAMEs decreased with soil depth under no-till. While this trend continued during three years of FCDT, SMB at depth had increased over prior no-till reflecting increased residue inputs and stabilization of SMB at depth. Stepwise discriminant analysis selected 14 FAMEs that we used for canonical correlation analysis. The first canonical axis clearly separated microbial communities formed after three years of FCDT (2008) from no till and one or two years of FCDT. The second canonical axis demonstrated a shift in microbial community composition between no till and the first year after FCTD and was attributed largely to mixing of the soil during this first tillage event. By 2008, FCDT was characterized by increased loadings of FAME biomarkers for saprophytic and arbuscular mycorrhizal fungi.  Thus, FCDT led to a fungal dominated soil microbial community that may promote residue stabilization deep into the soil profile in this continuous maize cropping system.