Nitrous Oxide Emission and Denitrifying Bacterial Communities as Affected by Drip Irrigation with Saline Water in Cotton Fields

Abstract

A shortage of freshwater resources has become a fundamental and chronic problem for sustainable agriculture development in arid regions. Use of saline water irrigation has become an important means for alleviating freshwater scarcity. However, long-term irrigation with saline water may cause salt accumulation in the soil, and further affect nitrogen transformation and N2O emission. To investigate this, we conducted a ten-year field experiment to evaluate the effect of irrigation water salinity and N amount on N2O emission and denitrifying bacterial communities. The experimental design was a 2×2 factorial with two irrigation water salinity levels (salinity levels are expressed as electrical conductivity), 0.35 dS·m-1 and 8.04 dS·m-1, and two N amounts, 0 kg·hm-2 and 360 kg·hm-2, representing SFN0, SHN0, SFN360, and SHN360, respectively. The results indicated that long-term saline water irrigation significantly increased soil salinity, moisture, and NH4+-N content, whereas it decreased soil pH, NO3--N, organic matter, and total nitrogen content. Irrigation with saline water significantly inhibited N2O emission, being associated with a decreased in level of 45.19% (unfertilized plots) and 43.50% (fertilized plots) compared with irrigation with fresh water. N2O emission increased as the N amount increased; the N2O emission was 161% higher in the fertilized plots than in the unfertilized plots. In the unfertilized plots, saline water irrigation significantly reduced the activity of denitrifying enzymes, the abundance of nirK, nirS, and nosZ, and the diversity of denitrifying bacterial communities. In the fertilized plots, saline water irrigation did not significantly affect the abundance of nosZ, whereas it significantly reduced the abundance of nirK and nirS. Saline water irrigation and nitrogen application altered the community structures of denitrifying bacteria with nirK, nirS, and nosZ; the irrigation water salinity seemed to have a greater impact on the denitrifying bacterial community in comparison with fertilization. Linear discriminant analysis (LDA) effect size (LEfSe) analysis demonstrated that denitrifying bacterial potential biomarkers increased as the water salinity increased, meaning that saline water irrigation could alter the community structures of denitrifying bacteria, and promote the growth of dominant species. Our findings indicate that increased abundance of nosZ, nirK, and nirS promoted N2O emission, and although long-term saline water reduced soil N2O emission, it resulted in a continuous increase of soil salinity. The emission of N2O had extremely positive correlation with soil NO3--N, organic matter, total nitrogen, denitrifying bacteria abundance, and denitrifying enzyme activities, and was negatively correlated with soil moisture. The soil physiochemical properties and the community structure of denitrifying bacteria had a significant influence on soil N2O emission in cotton fields, and nirS bacteria showed the highest association with N2O emission, thus it might be a dominant microflora in the process of denitrification. This information will aid in reducing atmospheric N2O emissions in agriculturally productive alluvial grey desert soils.