Straw and biochar amendments improve cotton yield by regulating microbially mediated soil phosphorus cycling in arid cotton fields

Low soil phosphorus (P) availability limits P acquisition and cotton productivity in arid agroecosystems. Straw and biochar are two contrasting organic carbon amendments: straw provides readily decomposable carbon, whereas biochar is more stable and primarily regulates soil physicochemical properties. However, their differential impacts on soil P fractions, P-cycling microorganisms, functional genes, P uptake, and cotton yield remain unclear. A 10-year field experiment was conducted to compare conventional cropping control (CK), straw amendment (ST, 6 t ha⁻¹), and biochar amendment (BC, 3.7 t ha⁻¹). Compared with CK, ST increased cotton biomass, P uptake, and yield by 24.5%, 20.4%, and 5.1%, respectively, whereas BC increased these variables by 29.1%, 37.5%, and 12.7%, respectively. Both amendments enhanced soil available P (36.8% for ST and 60.1% for BC) and microbial biomass P (38.9% and 37.1%, respectively), while random forest analysis suggested that microbial biomass P was closely associated with soil P availability. Hedley-P fractionation showed that soil P was predominantly stored in recalcitrant inorganic forms, with D -HCl-Pi accounting for 75.3% of total P. Straw mainly increased moderately labile P (16.3%), whereas biochar promoted greater accumulation of labile P (4.2%), indicating distinct P transformation pathways. Bacterial P-cycling communities showed stronger responses than archaeal communities. Both amendments reshaped bacterial community composition, altered potential microbial association networks, and enriched the functional genes phoD and ppX-gppA , which are involved in organic P mineralization and polyphosphate metabolism. Partial least squares path modeling suggested that changes in soil environmental conditions and microbial biomass P were positively associated with soil P availability, which was further associated with cotton P uptake and yield formation. Collectively, straw and biochar improved P availability and cotton productivity through distinct microbially mediated P transformation pathways, providing mechanistic insights for optimizing P management in arid cotton production systems.