Phosphorus fractions and their transformation in coupling with organic carbon cycling after seven-year manure application in subtropical soil

Phosphorus (P) is a crucial macronutrient for crop yield. Repeated swine manure application can provide considerable amounts of P and organic carbon (C) for agricultural soils. However, a deeper understanding of the transformation of soil P fractions and their coupling with organic C cycling through chemical and biological processes is urgently needed to enhance P utilization efficiency and C sequestration. A seven-year swine manure application (SSMA) experiment was conducted at varying rates (0, 7.5, 15, 30, and 45 t·ha −1 per year) to investigate the changes in P fractions, the coupling of P and C transformation, and the driving factors in acidic soils. The results revealed that SSMA significantly increased soil total P, predominantly as inorganic P (P i ), whereas organic P (P o ) exhibited a limited increase and plateaued at 15 t·ha −1 manure application. The 15–45 t·ha −1 manure treatments dramatically enhanced the nonstable P fractions, particularly pH- and Ca-induced Ca 8 -P; moreover, Ca 8 -P had a greater impact on Olsen-P than Fe-P and Al-P. SSMA promoted P mobilization by increasing alkaline phosphatase activity and the abundance of P-cycling functional genes. However, the primary factors directly affecting nonstable P fractions were the elevated soil pH and soil organic carbon (SOC). Additionally, nonstable P fractions were positively correlated with O-aryl-C and ketone-C components. Swine manure applications altered organic C components by stimulating SOC-driven enzyme activities involved in organic C degradation. Organic C components were also influenced by available P and N, primarily through the abundance of genes involved in organic C fixation rather than C degradation. Furthermore, 45 t·ha −1 SSMA treatment restricted the increase in the abundance of P-cycling genes, most C-cycling genes, and dominant bacteria harboring P-cycling genes. This study provides critical insights into the coupling transformation mechanisms of P and C and highlights that excessive swine manure application impairs functional bacterial growth and organic C storage in addition to increasing the risk of P loss in agricultural soils. • SSMA significantly induced the Ca 8 -P formation in weakly acid soil. • Soil O-alkyl-C and ketone-C were associated with the non-stable P fractions. • Available P regulated organic C microbial fixation rather than degradation under SSMA. • Excessive SSMA can suppress bacterial growth trend. • SSMA: Seven-year swine manure.