Effect of water-saving technologies on nitrogen losses in rice fields: A meta-analysis

Nitrogen losses from intensified agricultural lands such as rice fields have posed severe water pollution. Thus, several water-saving technologies (WST) such as alternate wet and dry (AWD), controlled irrigation (CI), and shallow intermittent irrigation (SII) have been adopted to reduce nitrogen loss from rice fields. Research on WST effects has shown varied impacts on nitrogen losses, with limited consensus on the most efficient WST type or the most affected nitrogen component. This study synthesizes the current knowledge of the impact of WST on nitrogen loss in rice fields using meta-analysis. A total of 182 observations from 38 peer-reviewed studies were analyzed using meta-analytic and meta-CART models, with WST as treatment, and continuous flooding (CF) irrigation as control. The results revealed that nitrogen losses were significantly lower under WST compared to CF irrigation (p < 0.001), with reductions of 35 %, 28.2 %, and 25.2 % in ammonia (NH₄⁺-N), nitrate (NO₃^--N) and total nitrogen (TN) leaching, respectively. However, ammonia (NH₃) volatilization showed no significant difference between WST and CF irrigation, with response ratios close to zero. In contrast, nitrous oxide (N₂O) emissions were significantly higher under WST, particularly under AWD (RR = 0.37). NO₃^--N leaching was reduced 2.2 times more effectively with the application of controlled release fertilizer (CRF) under WST compared to urea (46 %N) (53.2 % vs. 24.3 %). Applying biochar combined with urea reduced nitrogen leaching by 42.7 %, outperforming urea alone but remaining less effective than CRF. Urea application increased N₂O emissions by 3.0 %, while CRF reduced them by 3.1 % under WST compared to CF irrigation. At higher nitrogen application rates (>225 kg N ha⁻¹), WST led to greater reductions in nitrogen leaching (1.9 times for NO₃^--N and 1.6 times for TN) compared to CF irrigation. Additionally, NO₃⁻-N leaching was 2.19 times effectively reduced in rice fields with higher seedling density (26–45 plants m⁻²) compared to lower density (10–25 plants m⁻²) under WST. The meta-analysis further showed no significant effect of WST on rice grain yield compared to CF irrigation. This study provides a quantitative synthesis of the effects of WST on nitrogen losses in rice fields, highlighting their variable effectiveness depending on WST type, nitrogen source, application rate, and seedling density. Future research should address the availability of CRF, explore alternatives for developing countries, and examine the relationship between water management, rice seedling density, and nitrogen losses.