Phosphorus mobilization in acid forest soils as affected by interactions of water regime, fertilization and growth of beech

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In naturally structured soils with their inherent small-scale heterogeneity, soil moisture is the chief environmental factor. Together with the availability of organic carbon it co-determines the occurrence of microbial hot spots and moments and thereby, also decomposition rates and nutrient cycling on a larger scale. Therefore, soil moisture, and in particular the alternation of periods with dry and wet conditions, influences leaching, microbial immobilization, and plant uptake of P. More extreme variations in soil moisture, as a possible consequence of climate change, may increase the P loss from forest soils. In addition, soil moisture governs redox-sensitive abiotic reactions involving iron compounds and P bound to them, in particular in strongly aggregated subsoils.Our general objective is to investigate how different water regimes interact with P mobilization and immobilization processes in acid forest soils, and how this will affect leaching and availability of P. In a first experiment, we will address P mobilization in the topsoil as a consequence of differing organic matter mineralization. We propose column experiments with naturally structured soils taken from three core sites of the SPP (Lüss, Vessertal, Bad Brückenau) that are planted with juvenile beeches taken from the same sites. The mesocosms will be subjected to different water regimes involving three levels of constant soil moisture combined with rewetting pulses. We will assess how these different regimes (i) influence P leaching and P uptake by beech, (ii) affect the heterogeneity of the P mobilization potential at the microscale, and (iii) govern the competition for P between microbes and beech. In a second experiment with soils from the aggregated B horizon in Vessertal, we will study the P mobilization and fixation in the subsoil, as a consequence of differing redox conditions in soil aggregates. The convective discharge and recharge of aggregate surfaces by macropore flow, as well as their diffusive recharge and discharge from and to the inside of the aggregates will be determined in a sequential percolation experiment. In addition we will integrate part of the fertilizing variants of the central SPP field experiment in our column experiments. By including N fertilization as an additional variant in the topsoil experiment, we will test the hypothesis that the effect of continuing high N deposition on P mobilization depends on the soil water status. In the experiment with subsoil, the convective-diffusive recharge of intra-aggregate P pools simulates the possible P input with leaching from the topsoil. Finally, with field measurements of P mobilization potential on the microscale at two moisture levels, we will contribute to the assessment of N and P addition effects tested with the common fertilization experiment.