Dynamics in Phosphorus Pools in Soils and Sediments along the Land-Freshwater Continuum of Agricultural Catchments
Final Report Abstract
The aim of the project was to understand dynamics in the interim storage of phosphorus (P) in agricultural catchments. Here, the transfer of P between different pools in soils and sediments remains still barely understood, especially under the influence of changing redox conditions. Fresh soil and sediment samples were labeled with dissolved 18O-enriched inorganic P (Pi) and incubated for 24 weeks at 20 and 5°C under different hydrological conditions, comprising short and long pulsed drying-rewetting cycles as well as permanently water-logged conditions. Samples were harvested after certain time intervals and immediately treated with a modified five step P sequential extraction protocol. Our approach revealed pronounced dynamics in P pools over time. While in stream bed sediments mainly labile P pools were affected in the long-term, more stable pools were important for vegetated buffer strips and arable soil. In general, we observed that organic P pools played an unexpected role in the mid-term storage of P, which was accompanied by an intense enzymatic turnover of the applied P by microorganisms. Dryingrewetting had a pronounced impact on the fate of P; while vegetated buffer strip soil represented a resilient sink for P, drainage channel sediments were not able to retain additional P in the longterm. Furthermore, the increase in moderately labile Pi in the long-term in drainage channel sediments as well as arable soil highlights the potential of re-mobilization. As such, our outcomes hold important implications regarding the management of P in agricultural landscapes in a changing climate. They confirm and underline the important role of vegetated buffer strip soils as sink for P derived from agriculture. Furthermore, the role of drainage channel sediments as interim storage for P in agricultural catchments was found less than expected. Our results also highlight that arable soil undergoes significant changes when subject to drying-rewetting, which is important when soil particles are eroded and enter adjacent drainage channels and stream, or when flooding occurs. Thus, transfer of dissolved or particulate P from arable soil into drainage channel sediments needs to be avoided in order to mitigate the export of P to receiving surface waters.
