Effects of alfalfa planting with different cultivating ages on abundance of soil carbon-sequestration bacteria and labile organic carbon fractions in rainfed Loess Plateau

This study aimed to reveal the effects of alfalfa planting on the abundance of soil carbon-sequestration bacteria and labile organic carbon fractions. We performed an experiment in rain-fed areas of the Loess Plateau by using real-time fluorescence quantitative PCR to analyze the variation characteristics of soil carbon-sequestration bacteria abundance in alfalfa (Medicago sativa) artificial grassland with different cultivation ages (2, 9, 16, and 18 a) and farmland maize (Zea mays). Additionally, we examined the relationship between the abundance of soil carbon-sequestration bacteria, labile organic carbon fractions, and RubisCO enzyme activity. Furthermore, we analyzed the important factors affecting the abundance of the cbbL gene and soil labile organic carbon fractions by coupling with soil physicochemical properties. The results showed that compared with farmland, 18 and 16 a alfalfa significantly increased soil total organic carbon and three kinds of labile organic carbon fractions (P < 0.05), among which total organic carbon, readily oxidizable organic carbon, soluble organic carbon, and microbial biomass organic carbon increased by 18.15% and 14.60%, 130.00% and 76.43%, 22.87% and 11.17%, 127.03% and 133.49%, respectively. Moreover, 9 a alfalfa improved the three types of soil labile carbon fractions, while 2 a alfalfa only significantly increased the content of microbial biomass carbon (P < 0.05). The copy number of cbbL gene ranged from 1.12 × 10⁸ to 1.96 × 10⁸ copies·g⁻¹ in dry soil, which showed that alfalfa treatments were significantly higher than farmland (P < 0.05). The abundance of the cbbL gene increased with increasing number of alfalfa planting years. Correlation analysis showed that the abundance of cbbL gene was positively correlated with soil C, N, and pH, and negatively correlated with soil water, P, and RubisCO enzyme activity. Furthermore, stepwise regression analysis showed that soil total nitrogen and pH were key factors affecting the abundance of the cbbL gene. The results of redundancy analysis (RDA) showed that soil moisture and total nitrogen were the main factors affecting the total organic carbon and labile organic carbon fractions. In conclusion, understanding the response of soil cbbL bacterial abundance and labile organic carbon fractions to alfalfa cultivation ages provides a reference for further investigating the microbial mechanism of soil carbon sequestration in rain-fed areas of the Loess Plateau.