Soil Microbial Community Structure Responses to an Organically Enhanced Nitrogen Fertilizer.

Interest in the use of alternate fertilizers has increased in recent years in order to improve soil health without compromising crop yields, but the effects of alternate fertilizers on the soil health have been poorly examined.  An organically enhanced nitrogen fertilizer was evaluated for its effect on the soil microbial community structure relative to conventional fertilizers ammonium sulfate and urea over three years and two separate locations using non-irrigated corn as the test crop on silt loam soils in western Tennessee, USA.  The three N fertilizer types were applied at 128 kg ha^-1. Soil at a 0-15 cm depth was analyzed for microbial structure shifts before treatment and after harvest from 2011 to 2013.  Soil microbial structures were more often affected by soil accumulated fertilizer effects than by fertilizer types.  In the few instances where fertilizer types had a significant impact on microbial structure shifts, the organically enhanced N fertilizer, ammonium sulfate, and urea tended to increase soil microbial biomass relative to the untreated control. The organically enhanced N fertilizer and ammonium sulfate both equally decreased arbuscular mycorrhizae populations relative to the untreated control.  The organically enhanced N fertilizer seemed to be more beneficial for eukaryotic microbial populations than the untreated control. With respect to accumulated fertilizer effects, actinomycetes and gram positive bacteria benefited from accumulated fertilizer effects. Eukaryotes and gram negative bacteria were negatively impacted by increasing fertilizer accumulation in the soil. Fungi seemed to benefit from accumulated fertilizer effects in the soil initially but later declined with continued fertilizer accumulation in the soil. Overall, the effects of the organically enhanced N fertilizer on soil microbial community structure are not steadily noticeable within three years of experimentation.