Do alternative irrigation strategies for rice cultivation decrease water footprints at the cost of long-term soil health?

The availability of water is a growing concern for flooded rice production. As such, several water-saving irrigation (WSI) practices have been developed to reduce water requirements. Alternate wetting and drying (AWD) and mid-season drainage (MSD) have been shown to potentially reduce water requirements while maintaining rice yields when compared to continuous flooding (CF). With the removal of permanently anaerobic conditions during the growing season, WSI can also reduce COalt;subagt;2alt;/subagt; equivalent (COalt;subagt;2eqalt;/subagt;) emissions, helping reduce the impact of greenhouse gas (GHG) emissions. However, the long-term impact of WSI on soil organic carbon (SOC) – used here as an indicator of soil health and fertility – has not been explored. We therefore conducted a meta-analysis to assess effects of AWD and MSD on (i) SOC, and (ii) GHG emissions. Despite an extensive literature search, only 12 studies were found containing data to constrain the soil C balance in both CF and WSI plots, highlighting the limited understanding of long-term impacts of WSI on soil health and GHG emissions. AWD and MSD were found to reduce emissions of CHalt;subagt;4alt;/subagt; by 52.3% and increased those of COalt;subagt;2alt;/subagt; by 44.8%. COalt;subagt;2eqalt;/subagt; emissions were thereby reduced by 16.5% but the soil-to-atmosphere flux of carbon increased by 24% when compared to CF. AWD and MSD were also found to have a negative effect on both SOC, reducing concentrations by 5.2%, and soil organic nitrogen, potentially depleting stocks by more than 100kg N/ha per year. While negative effects of AWD and MSD on rice yield may not be visible in short-term experiments, care should be taken when assessing the long-term sustainability of these irrigation practices because WSI can decrease soil fertility. Strategies need to be developed for assessing the more long-term effects of these irrigation practices by considering trade-offs between water savings and other ecosystem services.