Changes in soil microbial carbon fixation pathways along the oasification process in arid desert region: A confirmation based on metagenome analysis

Microbial carbon fixation has been proved to play a key role in soil carbon accumulation in arid desert oasis, but the fixation mechanism remains unclear. To solve it, we used metagenomic sequencing technology to measure the changes of soil microbial community structure, carbon fixation-related genes and carbon fixation pathway, as well as their relationships with soil properties between natural desert shrubbery (CK) and four oasis farmlands with different cultivation years (i.e., 3, 18 and 30 years) in the arid desert region of northwest China. The results showed that the contents of four soil carbon fraction (soil organic carbon/SOC, soil recalcitrant organic carbon/ROC, soil water solution carbon /WSC, and microbial biomass carbon/MBC) all significantly increased with the cultivation years (30 years >= 18 years > 6 years >= 3 years >= CK) (P < 0.05). Microbial composition was significant difference among CK, 3-, 6-, 18- and 30-years cultivated farmlands. Bacteria were the soil dominant taxa, and its relative abundance gradually decreased with the cultivation years. Proteobacteria, Actinobacteria, Bacteroidetes and Acidobacteria were the dominant phylum of soil microorganisms in desert shrubbery and oasis farmlands. Their relative abundance changed obviously with cultivation years. Six microbial carbon fixation pathways. i.e., the reductive tricarboxylic acid cycle (rTCA), dicarboxylate/4-hydroxybutyrate cycle (DC/4-HB cycle), 3-hydroxypropionate bicycle (3-HP bi-cycle), 3-hydroxypropionate/4-hydroxybutyrate cycle (3-HP/4-HB cycle), Calvin cycle and the Wood-Ljungdahl pathway (WL pathway), were found in all soils, but only the rTCA, DC/4-HB, 3-HP/4-HB cycles and WL pathway significantly changed with the cultivation years. The rTCA cycle was the most common fixation pathway, followed by DC/4-HB cycle and 3-HP/4-HB cycle, while WL pathway was least detected. Soil properties, especially pH, recalcitrant organic carbon, bulk density, total salinity and soil water contents, affected microbial community composition and carbon fixation-related genes in the oasification process. Our results suggested that oasification affected microbial carbon fixation by changing the soil environment due to human agricultural activities. Limited by energy supply in the nutrient-poor soils, rTCA and DC/4-HB cycles were mainly used by microorganisms to fix carbon in oasis farmlands.