Influence of digestate application on rice yield and soil organic matter pools in a climate change scenario

Increasing temperature and decreasing precipitation represent a serious threat to the agricultural sustainability of paddy soils, not only because rice (Oryza sativa L.) is the main irrigated crop worldwide, but also because they represent the largest anthropogenic wetland which, as such, is involved in carbon (C) sequestration. The application of anaerobic digestate represents an enormous potential to increase both soil C accrual, and thus mitigate climate change, and soil fertility. However, there is still a lack of knowledge on the relative distribution of digestate into soil organic matter (SOM) pools under increasing temperatures. The main aims of our research are: a) to investigate the effects of increased temperature (~2°C) and reduced water levels (by 30%) on the amount and quality of SOM pools; and b) to determine how digestate application affects organic C (OC) stability and distribution into SOM fractions, with or without climate manipulation. The ability of digestate to mitigate the negative influence of climate changes on rice yields will be also assessed. The experimental design consists of 3 factors - amendment application (unamended control, UN; digestate, DS), climate manipulation (ambient temperature, AM; warming, WM) and water management (normal flooding, NF; reduced flooding, RF). To capture SOM protection mechanisms, soil samples were fractionated by size following aggregate dispersion, thus resulting in a particulate (POM) and a mineral-associated organic matter (MAOM) fraction. Preliminary results reveal that, in AM and NF conditions, digestate application slightly increased rice yield compared to UN. In general, increased temperature and reduced water supply result in a crop yield loss ranging between 60 and 70%. Moreover, while in UN the water regime (NF vs. RF) seems to have no effect on rice yield in both AM and WM conditions, in DS the NF results in significantly higher rice yield. The main differences in the POM fraction are observed between DS and UN; in particular, reduced flooding conditions in UN decrease the OC content by ca. 3× compared to normal flooding. In the MAOM pool, OC is not significantly affected by flooding level and digestate application, while both digestate application and warming, as well as their interaction, affect total N content. This research has the potential to generate soil management recommendations required to address global change challenges and recycle organic wastes while improving SOM content.