Nitrogen fertilizer suppresses mineralization of soil organic matter in maize agroecosystems

Authors: MAHAL, NAVREET - Iowa State University; OSTERHOLZ, WILLIAM - Iowa State University; MIGUEZ, FERNANDO - Iowa State University; POFFENBARGER, HANNA - Iowa State University; SAWYER, JOHN - Iowa State University; Olk, Daniel - Dan; ARCHONTOULIS, SOTIRIOS - Iowa State University; CASTELLANO, MICHAEL - Iowa State University

Submitted to: Frontiers in Ecology and Evolution
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/18/2019
Publication Date: 3/13/2019
Citation: Mahal, N.K., Osterholz, W.R., Miguez, F.E., Poffenbarger, H.J., Sawyer, J.E., Olk, D.C., Archontoulis, S.V., Castellano, M.J. 2019. Nitrogen fertilizer suppresses mineralization of soil organic matter in maize agroecosystems. Frontiers in Ecology and Evolution. https://doi.org/10.3389/fevo.2019.00059.
DOI: https://doi.org/10.3389/fevo.2019.00059

Interpretive Summary: Nitrogen fertilizer is a necessary input to most crop production fields, and it is a large expense for farmers. Information is lacking on how its addition to soil affects the availability of much larger amounts of organic nitrogen already bound in the soil. Here we found that addition of nitrogen fertilizer to soil slowed the release of soil-bound nitrogen into forms that can nourish the crop, in contrast to earlier reports that nitrogen fertilizer can increase the release of soil-bound nitrogen. The inhibition appears to involve activity of soil microorganisms. These results demonstrate the need to account for interactions between fertilizer nitrogen and the soil when planning long-term rates of nitrogen fertilizer application and to account for activity of soil microorganisms when predicting nitrogen cycling in soil. The results are useful to researchers of soil nitrogen cycling or management and to farm producers who seek optimal management of soil-bound nitrogen.

Technical Abstract: Inorganic nitrogen (N) fertilizer is critical to support grain production in high-input, low-diversity agroecosystems that occupy an increasingly large proportion of global land use. Here, we test the hypotheses that N fertilizer enhances soil organic matter (SOM) mineralization and, as a result, the ‘N difference’ method overestimates fertilizer N use efficiency because it underestimates N availability from zero-N controls. We measured the effect of inorganic N fertilizer on SOM mineralization via gross ammonification at two long-term N fertilizer rate experiments in central and southern Iowa, USA. In both experiments, plots with continuous maize (Zea mays L.) received one of three ‘historical’ N fertilizer rates (zero, moderate or high) for 15 years (1999-2014). In 2015, we split the historical N fertilizer rate plots into two subplots that received either the site-specific empirically determined agronomic optimum N rate or zero N fertilizer. At the onset of the maximum rate of maize N uptake, N fertilizer reduced gross ammonification by 12-15%. A companion laboratory experiment rejected the hypothesis that differences in net primary production (NPP) and soil moisture between N-fertilized and zero-N control treatments explain the negative effect of N fertilizer on SOM mineralization. In contrast, N fertilizer had a direct suppressive effect on ammonification. By demonstrating that the direct effect of N fertilizer on microbial activity exceeds the indirect effects of N fertilizer through changes in NPP, these results highlight the importance of adding microbial activity into ecosystem process models.