Objectives The study investigated the changes in soil organic phosphorus fractions and the associated microbial community diversity and functional characteristics induced by different application rates of swine manure, providing a theoretical basis for the rational application of swine manure.

Methods A field experiment was conducted in Dujiangyan, Sichuan Province, in 2012, under a rice-wheat rotation system. The four swine manure rate treatments included a NPK fertilizer control (T1), and three annual application rates of swine manure at 10300 kg/hm² (moderate), 20600 kg/hm² (high), and 30900 kg/hm² (excessive), denoted as T2, T3, and T4, respectively. Surface soil samples were collected after the rice harvest in 2020 (the eighth year). Soil organic phosphorus was fractionated into labile organic phosphorus (LPo), moderately labile organic phosphorus (MLPo), moderately resistant organic phosphorus (MRPo), and highly resistant organic phosphorus (HRPo) using different extraction solutions. Soil microbial community diversity, the copy number of the phosphorus-solubilizing bacterial functional gene phoD, microbial biomass phosphorus (MBP) content, and alkaline phosphatase (ALPase) activity were measured.

Results Compared with T1, all three swine manure rates significantly increased the contents of the four soil organic phosphorus fractions (P<0.05). T4 treatment was recorded the highest content for all the four organic phosphorus fractions; T3 was recorded the second high LPo and HRPo contents but similar MLPo and MRPo contents with T2. However, the increase in LPo between T2 and T1 was as high as 159%, while the increases between T3 and T2, and between T4 and T3 were more gradual. Application of swine manure significantly increased the Chao and Shannon indices of soil bacteria while decreasing the Simpson index. Specifically, the Chao index of soil bacteria in T2 was significantly higher than in T3 and T4, whereas the Shannon and Simpson indices did not differ significantly among these three treatments, suggesting a more stable bacterial system in T2. There were significant differences in the Chao and Shannon indices of soil fungi among the swine manure rates, and the maximum values for both indices were observed in T3. The Simpson index values of fungi in T2 and T3 were significantly lower than those in T1, while those in T4 were significantly higher than those in T1, indicating that excessive swine manure application increased the richness of dominant fungi and reduced the stability of the fungal system. As the swine manure application rate increased, the copy number of the phoD gene and MBP content in the soil significantly increased. The soil MBP content increased with increasing swine manure application rates, but the increase between gradients decreased. Compared with T1, the soil MBP contents in T2, T3, and T4 significantly increased by 154%, 196%, and 222%, respectively, suggesting that moderate swine manure application could retain relatively high MBP levels. With increasing swine manure application rates, the copy number of the phoD gene in the soil showed a significant increasing trend, while ALPase activity first increased and then decreased, reaching a maximum in T2. Although T3 and T4 significantly decreased ALPase activity, it remained higher than that in T1. Redundancy analysis showed that the Simpson index of soil bacteria explained 67.10% of the variation in organic phosphorus fractions.

Conclusion Long-term application of swine manure significantly increased the contents of soil organic phosphorus fractions (LPo, MLPo, MRPo, HRPo), and the increase was positively correlated with the swine manure application rate. Moderate swine manure application (10300 kg/hm²) significantly improved soil ALPase activity and soil bacterial community diversity compared to high and excessive applications. Although high and excessive applications of swine manure more effectively increased soil phoD gene copy numbers and MBP content, they weakened the effect of moderate swine manure on enhancing ALPase activity, increased fungal community diversity and the richness of dominant fungi, and reduced system stability. Therefore, reasonable control of swine manure application rates is most beneficial for increasing soil microbial diversity, maintaining a balance among microbial diversity, and regulating the cycling and availability of soil organic phosphorus.