Microbial CO2 assimilation is not limited by the decrease in autotrophic bacterial abundance and diversity in eroded watershed

The impacts of soil erosion on soil structure, nutrient, and microflora have been extensively studied but little is known about the responses of autotrophic bacterial community and associated carbon (C)-fixing potential to soil erosion. In this study, three abandoned croplands (ES1, ES2, and ES3) and three check dams (DS1, DS2, and DS3) in the Qiaozi watershed of Chinese Loess Plateau were selected as eroding sites and depositional sites, respectively, to evaluate the impacts of soil erosion on autotrophic bacterial community and associated C-fixing potential. Lower abundance and diversity of autotrophic bacteria were observed in nutrient-poor depositional sites compared with nutrient-rich eroding sites. However, the relative abundances of obligate autotrophic bacteria, such as Thiobacillus and Synechococcus, were significantly enhanced in depositional sites. Deposition of nutrient poor soil contributed to the growth of obligate autotrophic bacteria. The maximum microbial C-fixing rate was observed in DS1 site (5.568 ± 1.503 Mg C km-2 year-1), followed by DS3 site (5.306 ± 2.130 Mg C km-2 year-1), and the minimum was observed in ES2 site (0.839 ± 0.558 Mg C km-2 year-1). Soil deposition significantly enhanced microbial C-fixing rate. Assuming a total erosion area of 1.09 × 107 km2, microbial C-fixing potential in eroded landscape can range from 0.01 to 0.06 Pg C year-1. But its effect on the C pool recovery of degraded soil is limited. Dissolved organic C (DOC) was the main explanatory factor for the variation in soil microbial C-fixing rate (72.0%, P = 0.000).