Isotope geochemical determination of phosphorus weathering sources and fluxes in forest ecosystems

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The primary source of phosphorus, an essential nutrient to all terrestrial ecosystems, is its release by weathering of rock. As rock weathering is an inherently slow process, the rate at which phosphorus is supplied to ecosystems is ultimately limited by the rock weathering rate, and by the ability of soils to retain the released phosphorus. Forest ecosystems respond to slow supply by increasing their phosphorus recycling rate. Hence a full quantification of the phosphorus cycle through ecosystems requires knowing these initial release rates. Yet such rates are still not generally available. This project will make use of several innovative isotope geochemical tools to determine these fluxes at four upland “Level 2” sites of the DFG Priority Programme “Ecosystem Nutrition”. 1) In two sites of granitoid/gneissic bedrock, the rare in situ-produced cosmogenic isotope 10Be will be used in combination with element mass balances to determine phosphorus fluxes and quantify phosphorus depletion in soil. 2) In two sites underlain by volcanic substrates that do not contain the quartz required for measuring in situ-produced 10Be we will use the a new system to obtain the same parameters: the 10Be(meteoric)/9Be isotope ratio in combination with weathering mass balances. 3) We will measure the radiogenic 87Sr/86Sr isotope ratio in primary phosphorus-containing minerals such as apatite, in soil water, in the exchangeable soil phase, and in tree stem wood and leaves to determine phosphorus sources. To evaluate the retention behavior of phosphorus in soils we will compare these primary fluxes with the phosphorus discharge by streams and groundwater and the preferential flow paths this discharge takes as determined in other SPP projects. Ultimately, we will close the phosphorus flux loop determine the ecosystems‘ phosphorous recycling efficiency by comparing litterfall fluxes with phosphorus weathering fluxes, and the sensitivity of fluxes ad recycling to change by comparing the dissolved phosphorus discharge with the primary input flux through weathering.