Influence of reduced water flooding and increased temperature on SOM dynamics and crop yield in amended paddy soils

Paddy soils represent a globally significant agroecosystem, not only as the primary environment for irrigated rice (Oryza sativa L.) production, but also as extensive anthropogenic wetlands, playing a key role in soil organic carbon (SOC) sequestration. In the context of climate change, rising temperatures and reduced water availability threaten soil health and fertility, as well as the whole resilience of this agroecosystem. This study evaluates how anaerobic digestate application influences soil organic matter (SOM) dynamics and crop yield in paddy soils under simulated climate stresses. A factorial field experiment tested three factors: amendment application (digestate, DS; unamended control, UN), temperature (ambient, AM; warming, ~2 °C, WR), and water regime (normal flooding, NF; reduced flooding, -30%, RF). SOM was fractionated into particulate (POM) and mineral-associated (MAOM) pools to assess C stabilization. Comparing data obtained in 2024 and 2023, a SOC decline has been observed in both DS plots (~13.8 to 12.0 g/kg) and UN plots (~18.2 to 14.8 g/kg), with WR plots losing up to 15% more SOC than AM. At the same time, digestate application under NF conditions partially mitigated SOC losses. The C/N ratio also declined more in DS (11 to 9) than in UN (15 to 11), reflecting enhanced N availability. Rice yield trends over the two-year period demonstrated clear responses to both organic amendment and climate stresses. In 2023, increased temperature and reduced flooding level resulted in a crop yield loss around 60-70%, whereas digestate application did not seem to mitigate the observed differences. By 2024, yields declined across all treatments, with warming reducing productivity by 40-60% compared to ambient conditions. This highlights the strong negative effect of elevated temperature on rice yield. While digestate improved N availability and supported higher yields under favorable conditions, its benefits were markedly constrained under warming, especially in combination with normal flooding. These findings indicate that digestate can enhance productivity and nutrient cycling under current climates, while its use needs to be integrated with tailored water management to sustain yields under warming scenarios.