Soil erosion-related dynamics of soil bacterial communities and microbial respiration

Soil erosion can dramatically change physicochemical soil properties, but little is known about the responses of bacterial communities and microbial respiration to soil erosion. In this study, three sites (upslope, mid-slope and downslope) with different erosional and depositional characteristics were selected along three transects of abandoned land in the Qiaozi watershed of the Chinese Loess Plateau to evaluate the impacts of soil erosion on bacterial communities and microbial respiration. Samples of the topsoil (0-10 cm) and subsoil (10-20 cm), classified as Calcic Cambisols, were collected from these sites. The results showed that lower bacterial abundance was observed in the topsoil of the downslope site (7.58 x 10(8) copies g(-1) soil) relative to the upslope (9.32 x 10(8) copies g(-1) soil) and mid-slope (8.70 x 10(8) copies g(-1) soil) sites. However, no obvious change (P > 0.05) in the bacterial Shannon index and community composition was observed among the sites. Runoff-induced erosion and migration of sediment homogenized the bacterial communities along the eroded slopes. Soil microbial respiration in the topsoil of the downslope site (19.02 +/- 0.25 mg CO2-C kg(-1) soil d(-1)) was significantly (P < 0.05) higher than that of the upslope (15.12 +/- 1.07 mg CO2-C kg(-1) soil d(-1)) and mid-slope (17.75 +/- 0.73 mg CO2-C kg(-1) soil d(-1)) sites, indicating that the deposition of sediment and associated organic matter significantly increased the soil microbial respiration. Multiple stepwise regression analyses showed that available nitrogen was the main explanatory factor for the variation in soil microbial respiration in both the topsoil (60.2%, P = 0.009) and subsoil (80.3%, P = 0.002). Compared to the bacterial properties, the labile organic matter contributed more to the variation. Our work suggested that soil microbial respiration was primarily modulated by the quality of the organic matter.