Effects of Summer Catch Cropping on Nitrogen Accumulation and Loss in Farmland with Meta-Analysis

In vegetable production, excessive nitrogen fertilizer application leads to extremely low NUE and substantial reactive nitrogen losses. Given the abundant rainfall and the high residual soil nitrogen in soil after harvest, there is a pronounced risk of nitrate leaching during the summer fallow period. This study conducted a meta-analysis to systematically evaluate the effects of catch crops on nitrogen loss and soil nitrogen accumulation during the summer fallow season. The results showed that catch crops significantly reduced total N leaching loss by 58.10% through decreasing leachate volume by 24.54% and reducing leachate concentrations of TDN (43.36%), NO3−–N (34.47%), and NH4+–N (56.05%), respectively. Catch crops reduced soil NO3−–N storage by 36.28% and SIN storage by 40.61%, while increasing SON storage by 69.79% and MBN by 21.47%, thereby achieving a transformation from readily available mineral N to more stable organic N. Catch crops also enhanced ammonification, mineralization, and nitrification by 126%, 142%, and 42%, respectively, and altered the microbial community structure. The effectiveness of catch crops varied significantly among taxa: Poaceae exhibited the best overall performance, the genus Zea (especially sweet corn) showed the most comprehensive performance in integrated N control, while Fabaceae increased soil NH4+-N. Environmental factors, including soil depth, pH, organic matter, total nitrogen, initial NO3−–N, rainfall, and soil texture, collectively regulated the effectiveness of catch crops. In summary, a rational selection of catch crop species tailored to site-specific soil and environmental conditions can effectively reduce nitrogen leaching during the summer fallow period and support precision nitrogen management.