Nitrogen removal rate and its controlling factors across different ditch levels using meta-analysis

Excess nutrient loads from agricultural production are transported through drainage ditches and gradually move downstream into nearby water bodies. During this process, ditches at different levels play different roles in reducing nitrogen (N) loads. However, the variation in N removal rates across ditch levels and the relative importance of influencing factors remain unclear. In this study, a meta-analysis of 24 studies with 1,121 observations was conducted to evaluate the removal rates of total nitrogen (TN), nitrate nitrogen (NO 3 -–N), and ammonium nitrogen (NH 4 +–N) across different ditch levels and to quantify the key factors controlling N removal. The results show that ditches play a significant role in N removal, with average removal rates of TN, NO 3 -–N, and NH 4 +–N of 16.24%, 13.89%, and 22.61%, respectively. Both sublateral and head ditches show significant N removal, except for NO 3 -–N in sublateral ditches, while branch ditches show no significant removal effect. Among all ditch levels, head ditches have the highest removal rates for TN (20.93%) and NO 3 -–N (20.90%), while sublateral ditches are most effective for NH 4 +–N (24.62%). The removal of TN and NH 4 +–N in sublateral ditches is primarily driven by external factors, including antecedent precipitation during previous 7 days, temperature, and soil moisture. With increasing ditch level, control shifts from external influences to internal factors, particularly influent concentration. Across all levels, NO 3 -–N removal is influenced mainly by internal drivers, especially influent concentration and flow speed. These findings highlight the critical role of ditches in N removal and provide insights for optimizing mitigation strategies by identifying key controlling factors at different ditch levels.