This experiment studied the effects of long-term synergistic application of organic materials and chemical fertilizers on soil bacterial community and enzyme activity, and revealed the relationship between soil nutrients, extracellular enzyme activity and bacterial community, so as to provide a theoretical basis for formulating long-term and reasonable fertilization strategies under wheat-maize rotation system in fluvo-aquic soil.

Based on a 10-year located experiment, five treatments were set up, including no fertilization (NF), chemical fertilizer (NPK), chemical fertilizer with straw return (NPKS), 50% chemical fertilizer with 6 000 kg·hm^-2 pig manure (NPKP), and 50% chemical fertilizer with 6 000 kg·hm^-2 cow manure ( NPKC ).

(1) The combined application of organic materials and chemical fertilizers ( NPKS, NPKP and NPKC ) could significantly improve soil fertility and extracellular enzyme activity, among which NPKC treatment had the most significant effect. Compared with NPK treatment, the contents of organic matter, total nitrogen, available nitrogen, available phosphorus and alkaline phosphatase activity were increased by 13.8%-15.4%, 9.7%-15.5%, 7.2%-15.9%, 13.6%-38.5%和2.5%-13.1%. (2) Long-term combined application of organic and inorganic fertilizer significantly changed the bacterial community structure and composition. In the wheat season, compared with NPK treatment, NPKS treatment significantly increased the abundance of Aggregatilinea and Parachlamydia, NPKP treatment significantly increased the abundance of Pseudomonas, Nonomuraea and Flexilinea, while NPKC treatment only significantly increased the abundance of Luteitalea. In the maize season, compared with NPK treatment, NPKS treatment significantly increased the abundance of Phycisphaera and Syntrophothermus, NPKP treatment significantly increased the abundance of Gemmatimonas, and NPKC treatment significantly increased the abundance of Aquipuribacter and Desulfosoma. (3) The results of functional prediction showed that combined application of organic and inorganic fertilizers could promote soil carbon and nitrogen cycling compared with long-term single application of chemical fertilizer. In particular, the NPKC treatment had a strong effect on nitrification, ureolysis, aromatic compound degradation, xylanolysis and cellulolysis. (4) Mental analysis showed that soil pH was the main factor regulating bacterial community structure and ecological function in fluvo-aquic soil.

Long-term application of organic and inorganic fertilizers (especially chemical fertilizers combined with cow manure) could improve soil fertility and extracellular enzyme activity, increase the abundance of beneficial bacteria, significantly change the structure and composition of bacterial communities, and promote the circulation of carbon, nitrogen and phosphorus, thus construct an environment suitable for crop and bacterial growth in fluvo-aquic soil.