Do high nitrogen use efficiency rice cultivars reduce nitrogen losses from paddy fields?

Several high nitrogen use efficiency (high-NUE) rice cultivars have been developed to meet the increased food demand and urgent environmental concerns, but the effect of these cultivars on reducing N losses from paddy fields is not well documented. A two-year field experiment was conducted in the Taihu Lake region of China to evaluate the advantages of high-NUE rice on yields and N losses in 2011 and 2012. Two “high-NUE” rice cultivars and a conventional cultivar were compared at the same fertilizer N input of 200 kg N ha−1. Ammonia (NH3) volatilization, N2O emission, N leaching and runoff losses were monitored during the entire rice growing season within a rice-wheat rotation. The high-NUE rice cultivars Wuyunjing 23 (W23) and Zhendao 11 (Z11) achieved higher grain yields, and took up more N with a higher NUE than the conventional cultivar Wuyujing 3 (W3). There was a trend toward smaller N losses with the high-NUE cultivars W23 and Z11 (N leaching, N2O emissions and NH3 volatilization decreased by 12-23%, 5-10% and 2-8%, respectively), but the total reductions for the growing season compared to W3 were not statistically significant. Even though the high-NUE cultivars did not significantly decrease the absolute losses of N from the field, they did show lower N losses expressed on a yield-scaled basis than the conventional cultivar due to higher grain yields. Cultivars W23 and Z11 produced 21-27% less NH3 volatilization, 23-26% less N2O emission, 23-33% less N leaching, and 13-24% less N in runoff per ton of rice grain harvested than those of W3. Another cultivar Aoyusi 386 (A386), known to have a higher N uptake at early growth stages than W23 and Z11 was also planted in 2012. This cultivar significantly reduced N losses compared with the conventional variety. It was concluded that improving N uptake rate of high-NUE cultivars in the early stages of growth would be most effective in decreasing N losses and that the main part of N fertilizer applications should be later in the growing season than is currently practiced in the region.