Effects of Additives Applied with Urea on Nitrate Leaching and Potato Tuber Yield and N Uptake.

Typically grown in sandy soils, the potato has large nitrogen (N) and water requirements and a shallow root system, which result in a high potential for groundwater contamination by nitrate (NO₃^--N). We conducted a two-yr field experiment with irrigated potato grown in a Hubbard loamy sand to assess the effects of urea applied at 67% and 100% of the recommended N rate (RNR) with and without the addition of varying combinations of inhibitors and other amendments, including the nitrification inhibitors 3,4-dimethylpyrazol phosphate (DMPP) and dicyandiamide (DCD), the urease inhibitor N-(nbutyl) thiophosphoric triamide (NBPT), the N-fixing microorganisms (NFM) Azotobacter vinelandii and Clostrium pasteurianum, and the microbial enhancer (ME) L-amino acids, chitin, chitosan and glucosamine. Effects on growing season nitrate leaching, tuber yield, N uptake, and residual soil N concentration after harvest were evaluated. In the 2015 growing season, NO₃^--N leaching and residual soil N concentration were greater, while tuber yield and N uptake were less, compared to 2016. Tuber yield and N uptake increased with increasing N rate but were not affected by inhibitors and microbes or enhancers under the conditions of this study. In 2015, the combination of urea+DMPP+NBPT at 67% of RNR resulted in 13%, 23% and 25% less NO₃^--N leaching than urea alone, urea+DCD+NBPT, and urea+ NFM, respectively, while at 100% of RNR this combination resulted in the least NO₃^--N leached. In 2016, urea+DMPP+NBPT, urea+DCD+NBPT, NFM, and NFM+ME resulted in 44%, 60%, 51%, and 40% less NO₃^--N leaching compared to urea+DMPP at 67% of RNR, respectively, while urea+DCD and urea+DMPP+NBPT had 44% and 39% less NO₃^--N leaching than reported in urea alone at 100% of RNR, respectively. Conversely, the combination of urea+DCD+NBPT and urea+NFM at 67% of RNR resulted in 10% and 13% greater NO₃^--N leaching than urea alone in 2015, respectively, while at 100% of RNR, urea+DCD+NBPT and urea+NFM+ME resulted in 14% and 10% greater NO₃^--N leaching than urea alone, respectively. Overall, the results indicate that applying the dual inhibitors DMPP+NBPT with urea can be an effective strategy to reduce NO₃^--N leaching. The effect of NFM and ME was inconsistent, indicating that more research is needed to elucidate the effectiveness of these microorganism-enhancing additives for reducing N losses.