P dynamics on temperate forest soils as revealed by isotopes (33P and 18O in phosphate)

The current view on the phosphorus (P) cycle relies mostly on measurements and correlations of pools, and to a lower extent on measurement of fluxes. We have no direct insight into the processes phosphate goes through at the ecosystem level, and into the relative importance of organic and mineral pools in sustaining P nutrition of plants.

Assessing the relevance of inorganic and biological processes controlling P cycling requires the use of appropriate approaches and tracers. Within the framework of The German Priority Program “Ecosystem Nutrition: Forest Strategies for limited Phosphorus Resources” we studied the P cycle in temperate beech forest soils along a gradient of decreasing soil P availability.

We followed the fate of P from the litter into the soil pools, using a multi-isotope approach (O in water and phosphate plus 33P), relying on measurements in experimental forest sites and on laboratory incubations. Using isotopic dilution techniques we were able to quantify gross and net mineralization rates and immobilization of P by microbial biomass.
The results highlight great differences between P-poor and P-rich soil. Contrary to what is found in most agricultural soils, the importance of biological processes is greater than inorganic processes in the P-poor soil, both in terms of relative size of the microbial P pool compared to the others and in terms of fluxes.