Effect of water-saving irrigation management on soil organic matter decomposition in paddy soil

Predominately anaerobic conditions prevent the complete decomposition of organic matter (OM) in rice paddies, leading to emission of CH4. Irrigation practices, presence of reducible Fe and the addition of OM all determine the availability of electron donors vs. acceptors and hence the progression of soil reductive processes. As shown previously, the addition of crop residues can further increase the dissolution of native soil OM by co-release upon enhanced reductive dissolution of Fe and Mn oxyhydroxides. Yet, it is unknown if and how this positive feedback also stimulates native soil OM degradation and if this process is affected by irrigation management. In a six-week pot experiment with living rice plants, we therefore investigated the effect of irrigation management on the degradability of maize residues and the priming effect of its addition to two paddy soils under Alternate Wetting and Drying (AWD) irrigation and Continuously Flooded (CF) irrigation. The soils were taken from two Bangladeshi young floodplain paddy fields with contrasting SOM/Feox ratio. The δ13C values of the CH4 and CO2 emissions (measured by CRDS) were used to estimate the decomposition of added maize shoots (δ13C = -13.0 ‰) and soil OM (δ13C = -25.4 ‰ and -22.7 ‰). When maize was added, Eh was incrementally higher under AWD than under CF. Maize addition also clearly stimulated the reduction of Fe and Mn, and Fe2+ and Mn2+ were then more reoxidised under AWD compared to CF. Cumulative soil CH4 emissions were higher under CF. Soil CH4 emissions peaked after adding maize, with an initially increasing contribution of maize to C emissions followed by a gradual decrease. However, this contribution did not depend on irrigation management. Ongoing isotopic analyses will reveal to which extent dissolution of native soil OC bound onto pedogenic Fe and Mn was promoted by maize addition and if an interactive effect existed with irrigation management