Seasonal dynamics and turnover of microbial phosphorusin a permanent grassland

Microbial activity is known to be high under permanent grassland, but consequences for soil phosphorus (P) dynamics and availability are not well understood. Our main objective was to assess the microbial P turnover derived from the seasonal fluctuations in microbial P (measured as hexanol-labile P (Phex) at 13 sampling times during 9months) in a permanent grassland in Switzerland as affected by different P fertilization treatments (P inputs of 0 (NK) or 17kgPha−1year−1 in the form of superphosphate (NPK) or dairy slurry (DS)). Plant P uptake, available inorganic P measured as resin-extractable P (Pres), potential organic P mineralization indicated by acid phosphomonoesterase activity and climatic conditions were also recorded. Despite significant differences in plant P uptake and Pres (NPK > DS > NK), the turnover rate of Phex was similar in all treatments (approximately once per growing season). Thus, the seasonal P flux through Phex was similar to the stock of Phex, which was about 18, 25 and 37kgPha−1 in NK, NPK and DS, respectively, and larger than the corresponding seasonal plant P uptake of 6, 17 and 12kgPha−1. The estimate of Phex turnover based on seasonal dynamics did not confirm previous tracer-based findings of a much faster Phex turnover under low availability of inorganic P, and the magnitude of Phex turnover depended on the number of sampling points taken into account. Fluctuations in Pres and Phex were related to soil moisture and indicated competition between plants and microorganisms for available P