Soil microbial community response to maize grown for high yield in rotation with soybeans

The size and structure of the soil microbial community was measured under maize grown in rotation with soybeans using fatty acid methyl ester (FAME) analysis. Soil microbial community composition changed almost continually during the growing season in response to environmental factors, changes in root exudates with plant growth stage, and residue decomposition. With soil moisture treated as a covariate and removed from the statistical model, temporal changes were still significant.;Tillage used to apply fertilizer or prepare for irrigation, caused a temporary decline in soil microbial biomass (SMB) and altered the composition of the microbial community in favor of bacteria. The decline in fungi was likely due to disruption of hyphae in the soil.;The response of the SMB to N shows that bacteria and fungi are contrasting subsets of the microbial community. Soil fungi must also be divided between saprophytic and arbuscular mycorrhizal (AM). Saprophytic fungi respond to rotational differences in residue amount and composition but not to increasing N rate, maize residue amount or N content. AM fungi in soil decline with increasing N rate. FAME biomarkers for fugivores such as protozoa and collembola decline with increasing N, likely responding to fungal hyphae concentration in the soil. Many saturated FAMEs associated with bacteria increase with increasing N rate.;The lack of SMB response to increased crop residue inputs suggest that a soil type in combination with environmental factors may have a "carrying capacity" for SMB that is mediated by the soil mineral component. Increased C inputs without increased SMB promote conditions for soil C storage. The response of AM fungi to applied N also has implications for soil C storage. High concentrations of AM fungal marker C16:1c11 were measured where no N had been applied to continuous maize. This represents a large C sink into the soil and encourages including AM fungi in soil C models. AM fungal hyphae promote aggregate formation which improves soil structure and would protect C in the soil from loss.