Dynamics in Phosphorus Pools in Soils and Sediments along the Land-Freshwater Continuum of Agricultural Catchments

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Pronounced amounts of the precious nutrient phosphorus (P) are mobilized from arable soil due to erosion and surface run-off and are transferred into the receiving waters. Along its transport path, retention and interim storage of P occurs in soils and sediments of interconnected vegetated buffer strips, drainage channels and stream beds. Retention and storage of P constitutes an important ecosystem function; however, P can be re-released from respective storage pools due to biogeochemical processes, thereby continuously increasing P fluxes to downstream water bodies for years and decades. This does not only increase the risk of eutrophication, it also counteracts contemporary conservation measures aiming to reduce P inputs into the surface waters.If we are to mitigate P transfer from arable soil to surface waters, we must understand P pool dynamics not only in arable soil itself, but also within the interlinked P transfer continuum. Here, dynamic changes in the hydrological regime and redox conditions occur, which have in turn a pronounced influence on the storage of P within the different storage pools (e.g., P sorbed to Fe-minerals is partially released under reducing conditions). However, due to methodological limitations, changes of P pools in soils and sediments and transformation pathways involved still remain barely understood. The objectives of this study are therefore (i) to unravel the mechanisms underlying the retention and dynamic transfer of P between the different storage pools in drainage channels, vegetated buffer strips, and eroded arable soil; (ii) and to elucidate the significance of biotic and abiotic processes underlying the changes in P pools. Laboratory incubation experiments will be conducted based on an innovative isotopic labelling approach that uses the analysis of stable oxygen isotope ratios in inorganic phosphate (δ18OPi) in order to trace changes in P pools of soil and sediments under dynamic redox conditions. In summary, crucial information will be obtained regarding the cycling of P in agricultural catchments, which will allow predicting the stability of respective storage pools in the long-term in dependence on their position within the P transfer continuum and the prevailing environmental conditions therein.