Direct-seeded rice increases nitrogen runoff losses in southeastern China

methods: (1) conventional, manually transplanted seedling rice (CTSR); (2) mechanically transplanted seedling rice (MTSR); (3) dry direct-seeded rice (DDSR); and (4) wet direct-seeded rice (WDSR). Runoff volumes; runoff concentrations of NH4+-N, NO3−-N, and total N (TN); N uptake; and grain yield were measured. The seasonal runoff volumes varied from 775 to 2397 m3 ha−1 in 2013 and 2014. Compared with CTSR, WDSR and DDSR significantly increased the total runoff volumes by 76% and 30%in 2013, and by 46% and 26% in 2014, respectively. The seasonal TN losses in runoff from the rice fields ranged from 1.99 to 10.18 kg N ha−1, accounting for 0.74-3.77% of the seasonal N input during the two years. In contrast with CTSR, WDSR increased TN losses by 169% in 2013 and by 143% in 2014, while DDSR increased TN losses by 31% in 2013 and by 84% in 2014. Direct-seeded rice significantly increased runoff N losses during the early growth period largely by increasing runoff volumes and/or the N concentrations in runoff water. The main forms of N lost in runoff were NH4+-N for CTSR, MTSR and WDSR and NO3−-N for DDSR. The grain yield, N uptake at mid-tillering and panicle initiation stages were similar in WDSR and DDSR and significantly lower than those under CTSR and MTSR. The results show that direct-seeded rice increased N runoff losses from intensively cultivated rice fields in southeastern China.