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 (N₂O) emissions, nitrate (NO₃⁻) leaching, and crop performance in contrasting irrigated and rainfed maize systems over two growing seasons. We also used published emission factors to estimate indirect N₂O emissions from leached NO₃⁻ that can be converted to N₂O. 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) N₂O emissions, most consistently for treatments that contained NI which reduced emissions by 24%–77%. However, these beneficial effects were counteracted by increased NO₃⁻ leaching, which occurred most consistently with UI or NFM applied as single additives or with NI. In these treatments, NO₃⁻ leaching increased during at least one growing season, and at both sites, by factors of 2–7. In three site-years, increased NO₃⁻ leaching with NFM and NI + NFM offset large reductions in direct N₂O, such that total direct + indirect N₂O 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.