The oxygen isotope composition of bioavailable phosphate in soil reflects the oxygen isotope composition in soil water driven by plant diversity effects on evaporation

Abstract

Abstract Despite the importance of phosphorus (P) for organisms, the contributions of different processes delivering P into the bioavailable pool in soil have not yet been unraveled mainly due to methodological constraints in separating physico-/geochemical from biological processes. Stable O isotope analyses of phosphate might help to disentangle these processes and recent method improvements now allow an application in terrestrial ecosystems. Plant diversity influences not only plant P uptake but also biological P release mechanisms in soil thereby potentially affecting stable isotope ratios of bioavailable P (δ18OPi). We calculated the degree of enzyme-mediated O incorporation into phosphate, determined the contributions of different biological processes (Pi mobilization from soil organic matter, microbial P turnover), and tested for plant diversity effects on δ18O values of phosphate and of soil water as well as on the processes involved in O incorporation. We added 18O-enriched and 18O-depleted water to separated subplots of 27 study sites differing in plant species richness and plant functional group composition. δ18O values of phosphate and soil water were measured before and six days after labeling. We calculated mixing models based on isotope fractionation published so far. The O incorporation factor a reflecting the proportional incorporation of O atoms from labeled water into phosphate molecules under field conditions averaged 0.33 (0.15–0.70). δ18O values of bioavailable phosphate indicated microbial P processing i.e., turnover and subsequent release of P in soil. Plant species richness was related to δ18O values of soil water via evaporation. Because of the complete exchange of O atoms in a phosphate molecule with ambient water during microbial P turnover in soil, plant species richness effects were also visible in δ18O values of bioavailable phosphate. In conclusion, our approach yielded new insights into the role of microorganisms in supplying bioavailable P. For the first time we could show that plant-community induced microclimate shapes δ18O values of bioavailable phosphate in soil indirectly by affecting δ18O values of soil water with which phosphate exchanges O. Therefore, stable O isotope analyses in phosphate represent a useful tool for unraveling complex plant-soil-microorganism interactions in the P cycle.