Spatial patterns of nitrogen runoff from Chinese paddy fields

Fertilizer additions (Nrate) have increased surface runoff of nitrogen (RTN) from croplands, which is eventually transferred to rivers. This pollution further contributes to ecological degradation and and health risks of drinking waters. Despite this recognition, little is known about the spatial pattern of RTN from croplands and the drivers of its variation. On the basis of 210 site-years of measurements at 41 sites in Chinese paddy fields, we examined the nonlinear response of RTN to Nrate and the effects of environmental factors on RTN, RTN per unit nitrogen fertilizer additions (RR), and background N runoff (R0). The results show that (i) RTN-Nrate relationship deviates from linearity and the parameters vary by climate and soil attributes; (ii) Observed variation of RTN is better explained by precipitation and clay content (48%) than Nrate and its interaction (17%); (iii) The RTN is 1.09 ± 0.36 Tg N yr−1 for Chinese paddy fields in 2008, to which R0 contributes more than 50%, and the corresponding average RR is 6.8 ± 1.7% (1 − σ), 30% lower than the linear model. This study therefore suggests that the future policies for agricultural N runoff need to account for local environmental conditions rather than solely attempting to reduce N fertilizer applications.