Coordination of the Research Unit and development of a conceptual model of paddy soil evolution

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Paddy soils may originate from many different types of soil but are highly modified by human activities. They are mostly managed under submerged conditions, leading to a unique agroecosystem in terms of element cycling. However, climate change and competition from other sectors will severely affect water availability for rice cropping. Thus, a joint international workshop, organised by two DFG Research Units focussing on paddy rice production and associated paddy soil development aims at gaining more insight into processes of C and N balances due to different management systems. The DFG Research Unit 995 (Biogeochemistry of Paddy Soil Evolution) focusses on the identification of soil forming processes responsible for paddy soil evolution. To understand paddy soil evolution, we have to consider the dynamics of hydrologically and microbially mediated redox processes to the dynamics of soil minerals and soil organic matter. This is strongly related to the question of the microbial accessibility of organic carbon (OC) and nitrogen (N), but also of iron (Fe). Different initial natural conditions may require different management practices for rice cultivation, and these may determine the direction of paddy soil evolution. The direct comparison of paddy soils and non-flooded agricultural soils allows to identify and to quantify management induced differences of biogeochemical properties in different soil types. These investigations are strongly related to the DFG Research Unit 1701 (Introducing Non-Flooded Crops in Rice Dominated Landscapes: Impact on Carbon, Nitrogen and Water Cycles, ICON). This Research Unit explores and quantifies the ecological consequences of crop diversification in rice dominated landscapes. Diversion from two flooded rice crops per year to rotations with one or two non-flooded crops, such as maize or non-flooded (aerobic) rice, entails pronounced implications in terms of hydrology, element cycling and ecosystem functioning. The higher OC content in rice/rice rotations as compared to rice/maize rotations only partially compensates for higher methane (CH4) emissions. Reduced CH4 emissions in non-flooded crops are at least in part offset by increased N2O emissions (pollution swapping). Addressing these aspects trans-disciplinary will help understanding and quantifying the short and long term dynamics of carbon, nitrogen and water in up-coming rice ecosystems. Thus, integrating the results and knowledge of both Research Units as well as the exchange of experiences with international scientists with professional expertise on paddy rice production will contribute greatly to deepen our insights concerning management-depending biogeochemical processes in paddy soils.