Stable isotopes to understand paddy soil biogeochemical processes under water-saving irrigation

To cope with climate change, the use of water-saving irrigation practices in rice production is increasingly promoted, instead of the traditional practice of continuous flooding. Yet, it is unclear how the periodic introduction of oxygen during drying events impacts some fundamental biogeochemical processes in paddy soils because of the difficulty to quantify them. This knowledge gap actually complicates the development of mechanistic biogeochemical models, which are key for the prediction of paddy soil nutrient dynamics over a range of edaphic, agronomic and climatic settings. However, stable isotopes may help to shed some light on dynamics of important soil nutrients, i.e., by exploiting their natural or artificial (by labelling) variations over different biogeochemical transformations. In an effort to improve their implementation in soil models, we experimentally investigated the effect of water-saving irrigation management on three prominent but understudied carbon and nitrogen transformations in paddy soils from Italy and Bangladesh by means of stable isotopes. In particular, we assessed the oxidation of methane (CH4) in situ, the decomposition of soil organic carbon (SOC) after the incorporation of organic residues, and the fixation and defixation of ammonium (NH4+) in clay interlayers, always as a function of irrigation management. As it turns out, the adoption of water-saving irrigation practices had the potential to relatively stimulate in situ CH4 oxidation, in spite of some methodological difficulties, which implies that the commonly observed lower CH4 emissions in paddy fields under water-saving irrigation probably do not only stem from lower CH4 production rates under less anaerobic conditions, but also from this mitigation mechanism. Secondly, the stimulation of native SOC dissolution after the incorporation of organic residues was restricted under water-saving irrigation, and redox-active iron oxides with sorbed SOC potentially play a role in this. However, this effect on SOC dissolution was not proportionally reflected in SOC-derived emissions of CH4 and carbon dioxide (CO2), so that irrigation management probably does not influence these emissions and the long-term SOC balance through the proposed mechanism. Thirdly, the effect of irrigation management on abiotic fixation and defixation dynamics of NH4+ into or from interlayers of clay minerals was rather limited, but these processes – which determine the bioavailability of NH4+ in paddy soil – were instead predominantly dictated by an equilibrium between bioavailable and clay-fixed NH4+ phases. Overall, with a well-considered experimental setup in which untargeted isotope contrasts can be quantified and the appropriate pools sampled, stable isotopes are apparently a powerful tool to quantify gross nutrient transformations in paddy soil.