The oxygen isotope composition of bioavailable phosphate in soil reflects the oxygen isotope composition in soil water driven by plant diversity effects on evaporation. (1st March 2019) - Hacker, Nina
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 (δ 18 OPi ). 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 δ 18 O values of phosphate and of soil water as well as on the processes involved in O incorporation. We added 18 O-enriched and 18 O-depleted water to separated subplots of 27 study sites differing in plant species richness and plant functional group composition. δ 18 O 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). δ 18 O values of bioavailable phosphate indicated microbial P processing i.e., turnover and subsequent release of P in soil. Plant species richness was related to δ 18 O 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 δ 18 O 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 δ 18 O values of bioavailable phosphate in soil indirectly by affecting δ 18 O 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.