Co-application of DMPSA and NBPT with urea mitigates both nitrous oxide emissions and nitrate leaching during irrigated potato production.

Potato (Solanum tuberosum L.) production in irrigated coarse-textured soils requires intensive nitrogen (N) fertilization which may increase reactive N losses. Biological soil additives including N-fixing microbes (NFM) have been promoted as a means to increase crop N use efficiency, though few field studies have evaluated their effects, and none have examined the combined use of NFM with microbial inhibitors. A 2-year study (2018-19) in an irrigated loamy sand quantified the effects of the urease inhibitor NBPT, the nitrification inhibitor DMPSA, NFM, and the additive combinations DMPSA + NBPT and DMPSA + NFM on potato performance and growing season nitrous oxide (NO) emissions and nitrate (NO) leaching. All treatments, except a zero-N control, received diammonium phosphate at 45 kg N ha and split applied urea at 280 kg N ha. Compared with urea alone, DMPSA + NBPT reduced NO leaching and NO emissions by 25% and 62%, respectively, and increased crop N uptake by 19% in one year, although none of the additive treatments increased tuber yields. The DMPSA and DMPSA + NBPT treatments had greater soil ammonium concentration, and all DMPSA-containing treatments consistently reduced NO emissions, compared to urea-only. Use of NBPT by itself reduced NO leaching by 21% across growing seasons and NO emissions by 37% in 2018 relative to urea-only. In contrast to the inhibitors, NFM by itself increased NO by 23% in 2019; however, co-applying DMPSA with NFM reduced NO emissions by ≥ 50% compared to urea alone. These results demonstrate that DMPSA can mitigate NO emissions in potato production systems and that DMPSA + NBPT can reduce both NO and NO losses and increase the N supply for crop uptake. This is the first study to show that combining a nitrification inhibitor with NFM can result in decreased NO emissions in contrast to unintended increases in NO emissions that can occur when NFM is applied by itself.