Changes in phosphorus turnover when soils under long-term P management are amended with bio-based fertiliser

Understanding available phosphorus (P) turnover could improve sustainable P management. An isotope tracing ³³P was used to measure daily P turnover rates and exchangeable P (E) in P deficient, balanced, and excess P soils from a long-term P site. Turnover under P deficient conditions was characterised by the lowest P flux predominantly from the available into the exchangeable P pools (P efflux). The P efflux in the P deficient soil was 3.7 ± 0.6 mg P/L day⁻¹ and the inverse flux (P influx) was 1.6 ± 0.4 mg P/L day⁻¹. Turnover rates were more than twofold higher under P balanced and surplus conditions, exhibiting an equilibrium between influx and efflux rates. The contribution of abiotic processes to P turnover was predominant in the excess P soil, whereas biotic processes dominated turnover rates under P deficient conditions. Changes in P turnover were measured following application of single superphosphate (SSP) and dairy processing sludge (DPS). Both fertiliser types increased P turnover rates and availability across all soils. After SSP application, E values plateaued between 238 and 297 mg P/L regardless of initial P status. Slower P release from DPS was evidenced by a wider range of E values (97–160 mg P/L) with slower turnover.