Role of autotrophic microbes in organic matter accumulation in soils degraded by erosion

Atmospheric CO2 fixation by autotrophic microbes has been investigated in various habitats, but little is known about the microbial CO2 fixation capacity in soil degraded by erosion. To fill the knowledge gap about the role of autotrophic microbes in organic matter accumulation in degraded soil, the microbial CO2 fixation rate was estimated in nondegraded, lightly degraded, and severely degraded soils using C-13 stable isotope tracing technique. The influences of soil physical properties, nutrient contents, and autotrophic bacterial community on the microbial CO2 fixation capacity were investigated using a structural equation model. The results showed that soil degradation significantly reduced the autotrophic bacterial abundance. However, a higher species diversity and a more complex cooccurrence network of autotrophic bacterial taxa were observed in the lightly degraded soil than in the nondegraded and severely degraded soils. Interestingly, the microbial CO2 fixation rates in the lightly and severely degraded soils were 2.15- and 1.52-times higher than those in the nondegraded soil, respectively. The annual organic carbon fixed by autotrophic microbes exceeded 2.05% of the total organic carbon in the degraded soil, suggesting that autotrophic microbes play a vital role in organic matter accumulation in degraded soil. The structural equation model showed that soil nutrients other than the autotrophic bacterial community are the key factor affecting the microbial CO2 fixation capacity. The negative effect of soil nutrients on the CO2 fixation capacity indicates that autotrophic microbes exert a more important function at the initial stage of the restoration of degraded soil.