Contrasting effects of partial organic substitution and its inoculation with phosphate-solubilizing bacteria on stoichiometric imbalances and phosphorus fractions accumulation after six-year application in a rice paddy

Organic substitution for chemical fertilizer and inoculation of phosphate-solubilizing bacteria (PSB) benefit sustainable agriculture, yet their individual or combined effects on soil stoichiometry and phosphorus (P) fractions accumulation remains poorly understood. Here, a 6-year field experiment was set up with four fertilization regimes, including no fertilizer (CK), chemical fertilizer (CF), partial organic substitution (POS) of chemical fertilizer with compost manure, and POS together with PSB inoculants (MOF). Compared with the CF, both POS and MOF increased rice yield and P uptake but had weak effects on soil total carbon (C), nitrogen (N) and P contents after six years. The MOF significantly increased dissolved organic C and decreased microbial biomass C:P ratio, resulting in an increased C:P imbalance between microorganisms and their resources compared with POS. Though both POS and MOF increased the accumulation of soil recalcitrant P fraction compared with the CK, MOF significantly decreased the accumulation of residual P and NaHCO3-Pi in comparison with the POS treatment. Solution 31P NMR spectra analysis revealed that MOF remarkably increased phosphate monoesters accumulation and their proportion compared to the CF. POS increased the relative abundances of the functional genes and enzyme activities involved in cellulose and hemicellulose degradation, while MOF increased those of organic P mineralization. Partial least squares path modeling suggested that changes in C:P imbalance play a key role in affecting P accumulation by affecting microbial composition, the organic C and P related degradation genes and enzymes activities. Our study suggests that partial organic substitution and its inoculation with PSB induced divergent effects on P fractions accumulation by changing C and P related function, providing insight into the potential mechanisms of organic management on P mobilization in future agriculture production.