Changes in soil microbial community following the conversion of wasteland to cotton land in saline-sodic region of Northwest China

Soil microbes are essential drivers of nutrient transformation and energy flow; nevertheless, how the soil microbial communities in oasis agroecosystems respond to the conversion from wasteland to cropland remains unknown. This study selected six cotton fields that differed in the time since reclamation (0–23 years) in Xinjiang, Northwest China. Soil from the top 20 cm layer was sampled, and microbial communities were analyzed using high-throughput sequencing. We found significantly decreasing soil fungal and bacterial relative abundances during the initial nine years of conversion, which then increased before stabilizing 15–23 years post-conversion. Average abundances of fungal and bacterial taxa increased in all croplands compared with uncultivated wasteland, by 39.57% and 18.76%, respectively. Converting wasteland to cotton land significantly increased soil bacterial α-diversity (Chao1, Pielou's E, and Simpson indices) except in Good's coverage index. Increasing length of time since conversion shaped soil microbial community structure, with increases in the relative abundances of the fungal phylum Mortierellomycota and the bacterial phyla Actinobacteria, Chloroflexi, Acidobacteria, and Gemmatimonadetes; concurrent with decreases of the fungal phylum Ascomycota and the bacterial phyla Proteobacteria, Bacteroidetes, and Firmicutes. Our study reveals that short-term (9–13 years) conversion increases microbial richness and diversity due to the improvement of soil fertility; however, a longer time since conversion (15–23 years) might be detrimental to microbial community structure given the potential for secondary salinization.