Abstract:

Objective: In this study, effects of long-term combined application of organic and inorganic fertilizers on soil carbon and nitrogen fixing microorganisms in Moso bamboo forests were investigated in order to provide theoretical basis for maintaining and improving soil fertility. Method: Soil samples were collected from surface (0 - 20 cm) and subsurface (20 - 40 cm) soil layers in moso bamboo forests intensively managed for 0(CK), 6, 10, 15, and 20 years. The fluorescence quantitative PCR (q-PCR), terminal restriction fragment length polymorphism (T-RFLP), and clone library of the cbbL and nifH genes, combined with redundancy analysis (RDA), were used to detect the differences in the community structure of CO₂- and N₂-fixing bacteria over time and reveal the effects of long-term combined application of organic and inorganic fertilizers on soil carbon and nitrogen fixation microbial community structure of Moso bamboo forests. Result: 1) The intensive management practice enhanced SOC and soil available N and K contents during 20 years, although they fluctuated during the process. The soil pH was generally stable between 5.0 and 5.5. 2) Compared with CK, the abundance of gene cbbL in topsoil decreased significantly at IM6 (P < 0.05), and then gradually recovered to the level of CK at IM20, while the abundance of gene nifH in management treatments was significantly decreased compared with CK (P < 0.05). Shannon index and evenness index of CO₂- and N₂-fixing bacteria decreased significantly after 10 years of management and increased significantly after 20 years of management (P < 0.05). The community structure of CO₂-fixing bacteria in CK was significantly different from that of all management treatments, while N₂- fixing bacterial community structure at IM20 was similar to that of CK. 3) Phylogenetic analysis showed that the dominant groups of CO₂- fixing microbials were Bradyrhizobium sp., Rhodospirillum centenum, Thioflavicoccus mobilis, Stappia meyerae, Mesorhizobium ciceri, and Starkeya novella, which all belonged to Proteobacteria, and the dominant group of N₂- fixing bacteria was Rhizobiales. 4) The community composition of CO₂- fixing bacteria was significantly correlated with contents of available phosphorus (AP) and NH₄⁺-N, and the change of δ¹³C. The community composition of N₂- fixing bacteria was significantly correlated with the changes of soil organic carbon (SOC), available phosphorus (AP), available potassium (AK), alkalytic nitrogen (N), NO₃^--N and C∶N. Conclusion: The long-term combined application of organic and inorganic fertilizer in Moso bamboo forests improves the soil nutrient content without causing soil acidification, which plays a positive role in improving the soil physicochemical properties. The changes in soil nutrient caused by the application of mineral fertilizer combined with manure is an important factor driving the changes of abundance, diversity and community structure of CO₂ -and N₂-fixing bacteria in the soil of Moso bamboo forests.

Key words: Moso bamboo forest, organic-mineral combined fertilzer, CO₂-fixing bacteria, N₂-fining bacteria