Soil Microbial Community Response to Nitrogen in Minimum till, High Yield Corn/Soybean Rotation.

Soil microbial community composition as measured by fatty acid methyl ester analysis (FAME) changes in response to crop rotation and nitrogen rate (0,50, 100, 150 and 300 kg N ha‾¹) in a long-term fertility study in eastern NE.  A subset of bacterial FAMEs representing 16-30% of the total microbial biomass increase with increasing N rate.  Biomarkers for saprophytic fungi are not responsive to N rate.  The remaining eukaryotic markers decline with increasing N rate.  The largest changes are seen in the marker for arbuscular mycorrhizal fungi (AMF) which is most abundant at 0 kg N ha‾¹.  Soil microbial biomass as measured by total FAME concentration is also highest where no N was applied.  Increasing N does not promote increases in microbial biomass in spite of higher crop residue inputs.  The large contribution of AMF biomass where no N was applied highlights the importance of AMF as a carbon sink into the soil and encourages the inclusion of AMF into current soil carbon models.