Agricultural ditches and ponds potentially enhance nitrogen sink function of paddy system

Agricultural ditches and ponds are important chain for water and nitrogen (N) cycle in paddy system, and deep understanding of their N component dynamics and buffering capacity are paramount importance to control N pollution. In this study, the high-frequency water quality monitoring of different surface water types (paddy field, ditch, and pond) was conducted in a typical paddy system. Dynamic characteristics of total N and its components of different surface water were analyzed, and the corresponding N export dynamics and N sink function of paddy system were identified. Results showed that ditches and ponds had remarkable N buffering capacities for paddy field draiange, with 36.23 %–50.35 % and 57.41 %–69.11 % N concentration reduction during the rice-growing season, respectively. N buffering capacity increased as the water flowed downstream during drainage events, with high N concentration fluctuation in the field ditch water and relative stability in the collector ditch and pond waters. High heterogeneities changes of N components in different rice-growing season were identified in different surface water types. Ammonium N (NH₄⁺-N) and organic N (ON-N) was the dominant form in the regreening stage (44.09 %) and the remaining stages (43.50 %) in the field ponding water, respectively, whereas nitrate N (NO₃^--N) was the dominant form (50.87 %–64.23 %) in the collector ditch water and pond water. The comparison of N fluxes with water movement at different scales demonstrated that ditches and ponds enhanced N sink function of the paddy system, with approximately 50.38 % N runoff reduction of paddy field drainage. These findings revealed that fully utilizing the N sink and buffering capacity of agricultural ditches and ponds acted as a key strategy for water pollution control in paddy field watershed.