Soil Labile Organic Carbon and Nitrate Nitrogen are the Main Factors Driving Carbon-Fixing Pathways During Vegetation Restoration in the Loess Plateau, China

Abstract

Microbial fixation of CO2 is a crucial pathway in the regulation of soil carbon cycling, however, the microbial carbon fixation pathways and their driving factors among different vegetation types remain unclear. Therefore, soil physicochemical properties, soil organic carbon (SOC), carbon-fixing microorganisms, and carbon sequestration genes in a farmland (FL) and four typical vegetation types (Robinia pseudoacacia (RP), Caragana korshinskii (CK), Prunus sibirica (PS), and grassland (GL) in the Wuliwan watershed were identified using macro-genome sequencing and other analytical methods. The results showed that SOC content increased in the following order: RP>CK>PS~GL>FL (P <0.05). The proportion of recalcitrant organic carbon (ROC) increased, whereas that of labile organic carbon (LOC) decreased. Among the six carbon fixation pathways identified, the DC/4-HB cycle had the highest relative abundance of 25.10%-25.52%. Changes in the type of vegetation restoration caused a significant difference (P<0.05) in the relative abundance of carbon-fixing microorganisms, with Actinobacteria and Proteobacteria being the most prevalent. From a carbon perspective, LOC has a closer relationship with carbon-fixing microorganisms and carbon-fixing pathways than does ROC. Furthermore, there was a significant positive correlation (P<0.05) between the composition and function of these microorganisms and soil nutrients like soil total nitrogen (STN), soil ammonium nitrogen (SAN), and soil nitrate nitrogen (SNN), and a negative correlation with soil bulk density (SBD) and pH. Using a random forest model, we predicted that the rTCA cycle would significantly influence SOC. In conclusion, this study explored the link between soil organic carbon and microbial composition and metabolic processes in different vegetation restoration types, which provides a new reference for soil carbon uptake mechanisms.

Keywords: Loess Plateau, planted forest, Vegetation restoration, microbial carbon sequestration pathways