Twenty years of overfertilization with organic materials increased total phosphorus cycling without changing the relative contributions of biotic and abiotic fluxes

Long-term inputs of phosphorus (P) and organic carbon from organic fertilizers may stimulate basal microbial mineralization and increase the biotic contribution to P availability. However, the effects of organic fertilization on basal soil P fluxes remain poorly understood, particularly for societal “wastes” (e.g. sewage sludge) which are gaining prominence as alternative P fertilizers. We quantified biotic and abiotic P fluxes using 33P radioisotopic techniques in soils from a field experiment after 20 years of overapplication of organic fertilizers (> 100 kg·ha−1a−1 of P), exceeding crop P requirements of ~ 25 kg·ha−1a−1. Biotic P fluxes were estimated after seven days of incubation and include gross P mineralization as well as net P mineralization, calculated after subtracting microbial P immobilization. Abiotic P fluxes reflect isotopically exchangeable P measured over 90 min and extrapolated to seven days. In soil treated with sewage sludge, gross and net P mineralization rates were approximately 4.5 times higher than those in the NPK treatment, while abiotic fluxes were 4.2 times higher. Although similar increases were also observed in the treatments with compost and cattle manure, in the sewage sludge treatment P mineralization occurred despite lower respiration and soil microbial biomass, suggesting that distinct mineralization pathways were stimulated. These results demonstrate that long-term organic fertilization can increase P fluxes in absolute terms, even though the balance between biotic and abiotic processes in overall P replenishment remained unchanged.