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

Abstract

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 (N₂ O) 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 N₂ O emissions from leached NO₃ ^- that can be converted to N₂ 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) N₂ O 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 N₂ O, such that total direct + indirect N₂ 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.