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

With this project we estimate the rate at which Phosphorus (P) is released, that means becomes bioavailable, from primary bedrock at two ICP Level II Forest sites in South Germany (Bavarian Forest and Black Forest). Additionally, we fingerprint the source and depth at which plants take up P. Both approaches can be achieved with the application of innovative geochemical tools such as meteoric and in-situ produced ¹⁰Be, and traditional source tracers like radiogenic strontium isotopes and the elemental ratio Ca/Sr.

We sample bedload sediment at the outlet of the catchment since it integrates over the entire catchment area. On bedload sediment we measure in-situ ¹⁰Be concentrations from which we obtain a total denudation rate. Also, we determine total denudation rates on topsoil samples which are distributed across the whole catchment in order to check for spatial heterogeneity.

Once a total denudation rate is determined we apply chemical depletion fractions (CDF) in order to divide total denudation into chemical weathering and physical erosion. In order to estimate P release rates we determine the degree of P depletion (τ_P) in soils referenced to unweathered bedrock. Having τ_P at hand we combine it with CDF and the total denudation rate, derived from in-situ ¹⁰Be concentrations.

In order to determine both CDF and tau values unweathered fresh bedrock is required. The most unaltered bedrock was sampled by 20 to 30 m deep drilling cores (Fig. 1). Both boreholes at the hillslope in the Black Forest and the Bavarian Forest was constructed to groundwater monitoring wells in close collaboration with the Department of Hydrology of the University of Freiburg. Additionally, we sunk a well at the ridge in the Black Forest to check for spatial heterogeneity and to avoid periglacial cover beds.

.The Phosphorus sources of plants, such as apatite hosted in primary bedrock, secondary soil minerals, organic litter or dust, will be fingerprinted with ⁸⁷Sr/⁸⁶Sr. Thus, we measure radiogenic strontium isotopes on unweathered bulk bedrock, mineral separates of unweathered bedrock, weathered bulk soil, easily extractable soil phases, soil, stream and groundwater and plant tissues like stem wood, leaves and needles. The same method will be used for the elemental ratio Ca/Sr and the isotopic ratio ¹⁰Be (meteoric)/ ⁹Be. On behalf of the latter, we leach the reactive soil phase with a sequential extraction method. By definition, the reactive soil phase consist of amorphous and crystalline oxides. Both isotopic ratios and the elemental ratio serves to disclose the depth of P nutrient uptake by plants. While the isotopic ratios doesn’t fractionate during nutrient uptake by plants at the soil-water-vegetation interface it does for the elemental ratio Ca/Sr due to the high affinity of Ca for plants. To overcome this issue we will determine a “foliar discrimination factor”.