Dose effect of pig manure addition on cbbL-harboring bacterial community in a paddy soil

CO₂ fixation by autotrophic microbes has a significant effect on C cycle in agricultural filed. Organic fertilization is an eco-friendly strategy for environmental protection and agricultural sustainability via ameliorating soil microbial community. However, the information of the dose effect of organic fertilizer on soil CO₂ fixing microbes has been less documented. In this study, the cbbL gene, a key gene in the Calvin-Benson-Bassham cycle, was used to characterize the CO₂-fixing microbes under the gradient rates of pig manure (PM) addition (0, 1400, 2800, 5600, 11,200, 22,400, and 44,800 kg ha⁻¹) in a 5-year field experiment with a double-rice cropping system in the red soil region of China. The results showed that Proteobacteria and Actinobacteria accounted for ∼99.4 % of total cbbL-harboring bacteria in average. High PM doses (22,400 and 44,800 kg ha⁻¹), rather than Low PM doses (11,200 kg ha⁻¹ and less), would significantly change the cbbL-harboring bacterial community attributes. Specifically, high PM doses decreased the alpha-diversity of cbbL-harboring bacterial community and the relative abundances of some lineages belonging to Proteobacteria and Actinobacteria, such as Bradyrhizobium Methylibium, and Variovorax, but increased the proportions of some stress-tolerant taxa (i.e. Hydrogenophaga and Methyloferula). Redundancy analysis (RDA) and partial least squares path modeling (PLS-PM) showed that pH, AP, and metals (Cu and Zn) negatively influenced the cbbL-harboring bacterial diversity and components through PM addition. Collectively, our study demonstrated that high PM addition would potentially threaten the soil cbbL-harboring microbes, and a rational application of manure-derived organic fertilizer should be addressed in agriculture for maintaining soil bio-diversity in future.