Elevated CO2 increased phosphorous loss from decomposing litter and soil organic matter at two FACE experiments with trees

Sustained increased productivity of trees growing in elevated CO2 depends in part on their stoichiometric flexibility, i.e., increasing their nutrient use efficiency, or on increased nutrient uptake from the soil. Phosphorus (P) may be a nutrient as limiting as nitrogen (N) in terrestrial ecosystems and may play a key-process in global terrestrial C storage. For this study archived litter and soil samples of two free air CO2 enrichment (FACE) experiments were analyzed for C, N and P. Populus euramericana, nigra and alba and Betula pendula, Alnus glutinosa and Fagus sylvatica were grown in ambient and elevated CO2 at respectively the Euro- and BangorFACE experiments. At EuroFACE, aboveground litter accumulated in L, F and H layers, while at BangorFACE almost all aboveground litter was incorporated into the mineral soil due to bioturbation. At EuroFACE, more P was lost from the F and H litter layers due to trees growing in elevated CO2, while at BangorFACE more P was lost from the mineral soil. Results of this study imply that trees growing in elevated CO2 were P limited at both experiments. Therefore, with increasing atmospheric CO2, P may play a more pronounced role than previous thought in regulating secondary forest growth. Moreover, increased atmospheric CO2 and ample N may allow a larger pool of P to become available for uptake due to, for instance, increased phosphatase activity resulting in increased organic matter turnover and biogenic weathering. Therefore, it may be postulated that under non-N-limited conditions, e.g., during regrowth, under high N deposition or in systems with high N-2-fixation, increased P availability and uptake may allow P-limited forests to sustain increased growth under increasing atmospheric CO2.