Objectives In order to reveal the influence of different organic fertilizer sources on the stability of soil microbial carbon, nitrogen and phosphorus content, soil extracellular enzyme activity and stoichiometric ratio of soil microbial biomass under the combination of fertilizer and different organic materials were studied by long-term positioning experiment of fertilizer.

Methods The study was based on a long-term field experiment located in the Agricultural Technology Center of Ningxiang City (since 1986). Five fertilization treatments were selected including no fertilizer (CK), chemical fertilizer (NPK), 30% pig manure organic fertilizer replaces fertilizer nitrogen (30%OM), 60% pig manure organic fertilizer replaces fertilizer nitrogen (60%OM) and chemical fertilizer with rice straw (NPKS), the planting system is three crops per year, early rice-late rice-barley rotation. Soil samples (0−20 cm) were collected at the mature stage of late rice in September 2022. Soil organic carbon (SOC), total nitrogen (TN), total phosphorus (TP) contents, microbial biomass carbon, nitrogen, phosphorus (MBC, MBN, MBP) contents, and β-glucosidase (βG), β-N-acetylglucosaminidase (NAG) and acid phosphatase (ACP) activities were determined, and the stoichiometric ratio was calculated.

Results Long-term fertilization significantly increased SOC, TN and TP contents in soil, and 60%OM treatment had the highest increases (P<0.05). The increase of 30%OM was also significantly higher than that of NPK treatment, while the increase of TN in NPKS treatment was similar to that in NPK treatment. There was no significant difference in soil C/N among the three combined treatments, while the C/P and N/P of NPKS treatment were significantly higher than those of 60%OM and 30%OM treatment. There was no significant difference in MBC content among the three organic and inorganic treatments, which was significantly higher than that of NPK treatment. The content of MBN and MBP in 60%OM treatment was the highest, significantly higher than that in other treatments, followed by 30%OM. Meanwhile, the MBN content in the NPKS treatment was higher than that in the NPK treatment, whereas the MBP content was equivalent to that in the NPK treatment. Therefore, the MBC/MBN and MBC/MBP in NPKS treatment were significantly higher than that in both pig manure treatments, and lower than that in NPK treatment. Compared with CK, NPK treatment only increased βG activity, while organic and inorganic application increased βG, NAG and ACP activities in soil. The βG/NAG and βG/ACP in soil treated with 30%OM were significantly higher than those treated with NPK, while those treated with 60%OM were significantly lower than those treated with NPK. Correlation analysis showed that soil microbial biomass and its stoichiometry as well as βG and ACP enzyme activities were positively correlated with soil carbon, nitrogen and phosphorus content and their stoichiometry. Further redundancy analysis showed that soil TP content and N/P were the key factors affecting soil microbial biomass and its stoichiometric changes.

Conclusions Long-term combined application of chemical fertilizer and organic fertilizer effectively increased the contents of organic carbon, total nitrogen and total phosphorus in paddy soil, and increased microbial biomass and extracellular enzyme activity. In particular, 60% pig manure instead of fertilizer nitrogen had the most significant effect on increasing soil carbon, nitrogen and phosphorus contents, and further increased the contents and ratio of carbon, nitrogen and phosphorus in microbial biomass, but decreased the extracellular enzyme stoichiometric ratio. And maintain a low extracellular enzyme stoichiometric balance. Therefore, the combined application of pig manure and NPK can maintain a relatively stable accumulation by increasing the input of organic carbon, nitrogen and phosphorus in soil, thus improving soil fertility and having a high ability to adapt to environmental changes. The combined application of nitrogen, phosphorus and potassium with straw may limit soil phosphorus and reduce the supply capacity of soil nutrients.