The possibility that N fertilizer increases soil organic matter (SOM) mineralization and, as a result, reduces SOM stocks has led to a great debate about the long-term sustainability of maize-based agroecosystems as well as the best method to estimate fertilizer N use efficiency (FNUE). Much of this debate is because synthetic N fertilizer can positively or negatively affect SOM mineralization via several direct and indirect pathways. Here, we test a series of hypotheses to determine the direction, magnitude, and mechanism of N fertilizer effect on SOM mineralization and discuss the implications for methods to estimate FNUE. We measured the effect of synthetic N fertilizer on SOM mineralization via gross ammonification at two long-term experiments in central and southern Iowa, USA with replicated plots of continuous maize that received one of three “historical” N fertilizer rates (zero, moderate or high) from 1999 to 2014. In 2015, prior to our measurements, we split the historical N fertilizer rate plots into two subplots that received either the site-specific agronomic optimum N rate or zero N fertilizer. At the onset of rapid maize N uptake, N fertilizer reduced gross ammonification by 13–21% (2–5 kg NH4-N ha−1 d−1). A companion laboratory experiment rejected the hypothesis that differences in net primary productivity between fertilized and unfertilized treatments explained the negative effect of N fertilizer on SOM mineralization. Moreover, the NH4+ pool size was negatively correlated with the gross ammonification rate (r2 = 0.85, p < 0.001). Thus, we conclude that NH4+ -N fertilizer had a direct suppressive effect on SOM mineralization. These results demonstrate that the direct effect of N fertilizer on microbial activity can exceed the indirect effects of N fertilizer via large changes in NPP that alter organic matter inputs, soil temperature and moisture content. The magnitude of this effect and specificity to NH4+ -N has significant implications for fertilizer management as well as the measurement and modeling of agroecosystem N dynamics including FNUE.

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Published in Frontiers in Ecology and Evolution, v. 7, article no. 59, p. 1-12.

Copyright © 2019 Mahal, Osterholz, Miguez, Poffenbarger, Sawyer, Olk, Archontoulis and Castellano.

This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

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This study was funded by the United States Department of Agriculture National Institute of Food and Agriculture grant number 2014-67019-21629 and the Iowa State University Plant Sciences Institute Faculty Scholars Program.

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The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fevo.2019.00059/full#supplementary-material

Data_Sheet_1.docx (322 kB)
Supplementary Material