Does Inorganic N Fertilizer Affect Soil Organic Matter Mineralization?.

Uncertainty about the effect of N fertilizer on soil organic matter (SOM) mineralization has created intense debate regarding the long-term sustainability of cereal cropping systems. Long-term N fertilizer application may increase SOM by increasing crop residue input and therefore maintain soil fertility. Alternatively, inorganic N fertilizer may enhance soil N mineralization, thereby decreasing SOM. Our objective was to quantify the effects of inorganic N fertilizer application on SOM mineralization via measurement of gross ammonification. We measured gross ammonification rates in long-term N fertilizer rate experiments at two sites located in central and southern Iowa. In 2015, plots with continuous maize that had received one of three historic N fertilizer rates (insufficient, optimum or excessive) for the previous 15 years, were split into two subplots that received either the agronomic optimum N rate (AONR) that was empirically determined from maize response to N from 1999-2014 or zero N fertilizer. Gross ammonification was measured at the V5 and V12 maize growth stages. Across all historical N fertilizer rates at the V5 maize growth stage, N fertilizer input at the AONR significantly reduced gross ammonification rates by 15 and 12% as compared to zero N at the central and southern Iowa sites, respectively; however there was no effect at the V12 stage. At both sites, the effect of N fertilizer on gross N ammonification rate decreased with an increase in historic N application rate from zero to excessive. Because C and N mineralization occur in tandem, our results are inconsistent with the hypothesis that N fertilizer application reduces soil organic matter.