Response of organic matter pools in paddy soils to digestate application, changes in flooding level and increased temperatures

Changing climatic conditions represent a threat to the agricultural sustainability of paddy soils, not only because rice (Oryza sativa) is the main irrigated crop worldwide, but also because they represent the largest anthropogenic wetlands. Their multiple key wetland ecosystem functions also include carbon (C) sequestration. The application of anaerobic digestate represents a great potential to increase soil C sequestration. However, there is still a lack of knowledge on the fate of applied digestate and its incorporation into SOM under increasing temperatures, and about SOM stabilization processes promoted by iron (Fe) (hydr)oxides. Our research aims at gaining a deeper insight into these processes. Specific objectives are: a) to investigate the effects of increased temperature (~2°C) and reduced water levels on the amount and quality of SOM pools; b) to determine how digestate application affects SOC stability and distribution into SOM fractions; c) to assess if changes in wetting and drying cycles dictated by climate changes may alter Fe mineral dissolution and linked implications on colloidal stability and isotopic composition. The experimental design consists of 3 factors - amendment application (unamended control and digestate), climate manipulation (ambient temperature, and warming) and water management (normal supply and reduced by 30%). To capture SOM protection mechanisms, soil samples were fractionated by size following aggregate dispersion, thus resulting in a particulate organic matter (POM) and a mineral-associated organic matter (MAOM) fraction. Preliminary results revealed that MAOM fraction was significantly affected by the flooding level and digestate application, but flooding level and warming did not significantly affect C content in the POM fraction. Our investigations of the Fe water-dispersible colloids and isotopic compositions will help to elucidate the differential mechanisms involved in paddy soil Fe cycling under variable land management and increasing temperatures. This research has also the potential to generate the policy-relevant soil management recommendations required to underpin international programs needed to address global-change challenges and recycle organic wastes while preserving SOC.