Intended and unintended impacts of nitrogen-fixing microorganisms and microbial inhibitors on nitrogen losses in contrasting maize cropping systems.

Efforts to mitigate the nitrogen (N) footprint of maize production include using N-fixing microbes (NFM) and/or microbial inhibitors. We quantified the effects of NFM, the nitrification inhibitor (NI) 2-(N-3,4-dimethyl-1H-pyrazol-1-yl) succinic acid isomeric mixture, and the urease inhibitor (UI) N-(n-butyl) thiophosphoric triamide, each applied by itself or paired with another additive, on nitrous oxide (N O) emissions, nitrate (NO ) leaching, and crop performance in contrasting irrigated and rainfed maize systems over two growing seasons. We also used published emission factors to estimate indirect N O emissions from leached NO that can be converted to N O. Agronomic effects were relatively small; the NI + NFM treatment increased N use efficiency and grain yield and protein content in some cases by 11%-14% relative to a treatment receiving only urea. Most of the additive treatments reduced direct (in-field) N O emissions, most consistently for treatments that contained NI which reduced emissions by 24%-77%. However, these beneficial effects were counteracted by increased NO leaching, which occurred most consistently with UI or NFM applied as single additives or with NI. In these treatments, NO leaching increased during at least one growing season, and at both sites, by factors of 2-7. In three site-years, increased NO leaching with NFM and NI + NFM offset large reductions in direct N O, such that total direct + indirect N O emissions were not different from that in the urea only treatment. These unintended effects may have resulted from unfavorable rainfall timing, varying crop N demand, and declining additive effectiveness. Use of these soil additives requires caution and further study.