Navreet K. Mahal*, Fernando E. Miguez, Hanna J. Poffenbarger, John E. Sawyer, William R. Osterholz and Michael J. Castellano
Department of Agronomy, Iowa State University, Ames, Iowa, USA (50011), *[email protected]
The effect of N fertilizer on soil organic matter mineralization is uncertain. This uncertainty has led to suggestions that inorganic N fertilizer may enhance soil N mineralization, thereby decreasing soil organic matter. This uncertainty has also led to confusion about how to measure crop system fertilizer N use efficiency (FNUE). Two options exist to measure FNUE: The indirect ‘N difference’ method compares N uptake in a zero N control plot compared to N uptake in a fertilized plot (FNUE = (N uptake in fertilized – N uptake in control)/N fertilizer input). The direct ‘15N tracer’ method uses isotopically labeled N fertilizer to directly measure the uptake of individual N atoms. The ‘N difference’ method always measures higher FNUE and this result has been hypothesized to be due to a ‘priming effect’ or enhancement of added-N on soil N mineralization. The objective of this study was to quantify the effects of inorganic N fertilizer application on soil organic matter 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 corn that had received one of three historic N fertilizer rates for the past 15 years: 0, 202 (long-term AONR) and 269 kg N/ha (highest rate) at the central Iowa site and 0, 224 (rate increment just below the AONR) and 269 (long-term AONR, at the highest rate) at southern Iowa site, were split into two subplots that received either the agronomic optimum N rate (AONR) 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; whereas 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 highest. Because C and N mineralization occur in tandem, our results are inconsistent with the hypothesis that N fertilizer application reduces soil organic matter. Moreover, our results suggest that the ‘N difference’ method is more accurate than the ‘15N tracer’ method for measurement of FNUE, because gross N mineralization is positively associated with soil C mineralization.