Microbial carbon use for incorporating biomass phosphorus drives CO2 emission in phosphorus-supplied subtropical forest soils

Abstract

Abstract. Subtropical forests store significant amounts of soil organic carbon (SOC) and are important in the global C cycle. Current understandings based on controlled experiments indicate that phosphorus (P) availability promotes SOC decomposition by alleviating microbial P limitation or rendering SOC available for microbial decomposition. While no alternative mechanism is currently known, it is uncertain if this mechanism holds across soils or P supply levels at the field scale. We formulated an alternative mechanism for acidic subtropical forest soils where organic C (OC) is bound to iron (Fe). Our hypothesis proposed that P supply would promote Fe-bound P formation and desorption of OC previously bound to Fe, and the microbial utilization of the desorbed OC for P-cycling contributes significantly to CO₂ emission. We tested our hypotheses by utilizing a forest P addition platform to explore C-dynamics, its regulators, and utilization across four P supply levels: 0, 25, 50, and 100 kg P ha^-1 yr^-1 (Con, P1, P2, and P3, respectively) for one year. Phosphorus supply significantly increased the periodic and cumulative dissolved OC (DOC) concentration, especially in P3, and was associated with increased iron (Fe)-bound P formation. With increased DOC following P addition, microbial biomass P (MBP) significantly increased, while MBC remained unchanged. The significantly positive relationship between MBP:MBC ratio and DOC, significant increase in MBP and carbon dioxide (CO₂) emission with P addition, and the reduction in CO₂ emission with increasing MBC:MBP ratio (0–10 cm) supports our results that the desorbed-C alleviated microbial C-limitation induced during P-cycling, particularly, MBP incorporation, to drive CO₂ emission. Structural equation modeling and multivariate analyses projected MBP as a critical factor inducing CO₂ emission. Besides, insignificant alterations in the relative abundance of C-degrading functional genes and reductions in P- and C-degrading enzyme activity indicated the sufficiency of desorbed OC for microbial use without further SOC degradation. Our study provides an alternative mechanism of P's impact on soil C-cycling processes in acidic subtropical forest soils vital for constraining process-based C models.