Subsurface fluxes dominate the dissolved nitrogen losses from rice paddies in central China

Nitrogen losses in rice paddy fields pose a high risk to surrounding aquatic environment. While the surface N losses have garnered considerable attention in recent years, subsurface flux, including vertical leaching and nearditch lateral seepage, has barely been quantified. To understand how it impacts the N run off, we collected threeyear high-frequency and systematic field observations and adopted a two-dimensional quantitative model coupling water, nitrogen dynamics and crop growth. We quantified the magnitude and dynamics of subsurface N loss during rice growing season. Our results showed that subsurface N loss, reaching 6.2 kg N ha-1 and 4% of total applied N in rice growing seasons, was 2-fold larger than surface N runoff, in which lateral seepage contributed 33%. The driving factors include hydraulic gradients brought by ponded water depth and ditch water level. Based on our study, optimized irrigation management and conservation tillage were priority in mitigating subsurface N loss, with 41% and 32% reduction potential, respectively, and improved ditch-pond system combined with irrigation-drainage management could also be an effective solution. Together, these findings highlight an important role of subsurface flux including lateral seepage, which could be instrumental for future waterquality improvements in rice paddy-ditch landscape.