Microbial mechanisms of cotton straw return to regulate soil N cycling in saline water drip irrigation cotton fields in arid zones

Long-term saline irrigation exacerbates soil salinity , inhibits nitrogen cycling efficiency, and threatens crop yield. Straw returning can mitigate these adverse effects, but the mechanism is unclear. This study conducted field experiments and set up treatments fresh water (FWCK), fresh water with straw (FWST), saline water (SWCK), and saline water with straw (SWST) to elucidate the driving mechanism of straw returning to the field on microbial nitrogen cycling. The results showed that whether straw is returned to the field or not, compared to fresh water irrigation, saline water irrigation increased soil salinity and water content, decreased NO 3 − -N and NH 4 + -N content, potential nitrification rate, cotton nitrogen uptake , and yield. Returning straw to the field (SWST vs SWCK) increased the availability of NH 4 + -N by reducing soil salinity, adjusting pH and water content, up-regulated the expression of nitrogen fixation and nitrate reduction genes, down-regulated the expression of nitrification and denitrification genes, and ultimately promoting cotton nitrogen uptake and yield. This study constructs a structural equation model of soil properties, bacterial communities , N transformation functional genes, and enzymes to reveal the mechanism of cotton straw improving saline soil , and transforms the ecological risks of saline irrigation into opportunities for nitrogen efficiency. On this basis, combined with the characteristics of regional saline alkali, optimize the straw saline water collaborative management mode, promote the improvement of soil fertility and effective regulation of nitrogen, provide theoretical support for the rational utilization of saline and straw resources, and achieve green and sustainable development of agriculture . • Saline water irrigation promotes nitrogen fixation and denitrification in cotton. • Saline water irrigation inhibits dissimilatory nitrate reduction and nitrification. • Straw return to the field promotes dissimilatory nitrate reduction and N fixation. • Straw return inhibits nitrification and denitrification. • Soil salinity, water, and pH control N transformation enzymes and functional genes.