Fertilizer management Effects on nitrate leaching and indirect nitrous oxide emissions in irrigated potato production

Potato (Solanum tuberosum L.) is a N-intensive crop, with high potential for nitrate (NO ₃ -) leaching, which can contribute to both water contamination and indirect nitrous oxide (N ₂O) emissions. Two approaches that have been considered for reducing N losses include conventional split application (CSA) of soluble fertilizers and single application of polymer-coated urea (PCU). The objectives of this study were to: (i) compare NO ₃ - leaching using CSA and two PCUs (PCU-1 and PCU- 2), which differed in their polymer formulations, and (ii) use measured NO ₃ - leaching rates and published emissions factors to estimate indirect N ₂O emissions. Averaged over three growing seasons (2007-2009), NO ₃ - leaching rates were not significantly different among the three fertilizer treatments. Using previously reported direct N ₂O emissions data from the same experiment, total direct plus indirect growing season N ₂O emissions with PCU-1 were estimated to be 30 to 40% less than with CSA. However, PCU-1 also resulted in greater residual soil N after harvest in 2007 and greater soil-water NO ₃ - in the spring following the 2008 growing season. These results provide evidence that single PCU applications for irrigated potato production do not increase growing season NO ₃ - leaching compared with multiple split applications of soluble fertilizers, but have the potential to increase N losses after the growing season and into the following year. Estimates of indirect N ₂O emissions ranged from 0.8 to 64% of direct emissions, depending on what value was assumed for the emission factor describing off-site conversion of NO ₃ - to N ₂O. Thus, our results also demonstrate how more robust models are needed to account for off-site conversion of NO ₃ - to N ₂O, since current emission factor models have an enormous degree of uncertainty.