Phosphorus Fertilization Promotes Carbon Cycling and Negatively Affects Microbial Carbon Use Efficiency in Agricultural Soils: Laboratory Incubation Experiments

Soil organic carbon (SOC) loss from intensive agriculture represents a major global concern. Consequently, strategies to improve soil management to enhance carbon (C) sequestration are urgently needed. Nutrient availability, especially nitrogen (N) and phosphorus (P), regulates soil C cycling and storage. While N effects are well studied, less is known about how soil P status and different fertilizer types affects SOC dynamics. This laboratory incubation assessed how two common P fertilizers, diammonium phosphate (DAP) and single superphosphate (SSP), affected microbial activity and C immobilization in the fertilizer prillosphere across three contrasting agricultural soils (Inceptisol, Vertisol, Alfisol). Soils were amended with DAP or SSP granules and C turnover assessed with 14C-labeled glycine, malic acid or glucose, alongside unfertilized controls. After 3 weeks, pH, electrical conductivity (EC), Olsen-P and microbial C use efficiency (CUE) were measured. DAP increased pH in the Inceptisol (acidic soil), while SSP decreased pH in all soils. Both fertilizers increased EC and Olsen-P, but SSP enhanced Olsen-P more than DAP. Cumulative 14CO2 emissions were 19-20 % higher with P fertilizers compared to the control, with DAP stimulating faster initial C mineralization rates than SSP, except in the Alfisol. P addition reduced microbial CUE by 23-34 % across all soils and substrates versus the unfertilized control. We ascribe this reduction in CUE to an alleviation of nutrient limitation or a fertilizer-induced osmotic stress. The co-addition of N either in DAP or glycine did not alter the P-induced CUE response suggesting that P was more important than N in regulating microbial CUE in these soils. We conclude that P fertilization increased C turnover and may lead to reduced C storage in soil. Our findings imply that careful P management is needed to avoid C losses, and that microbial C dynamics should be considered when evaluating P effects on SOC storage.