咸水滴灌对棉田土壤N2O排放和反硝化细菌群落结构的影响

淡水资源短缺是干旱区农业可持续发展所面临的严峻问题,合理利用咸水灌溉是缓解淡水资源不足的重要手段.长期咸水灌溉会导致土壤盐分积累,进而影响氮素的转化和N_2O的排放.本研究通过10 a咸水灌溉试验,探究咸水灌溉对棉田土壤N_2O排放、反硝化细菌丰度和群落结构组成的影响.试验采用灌溉水盐度和施氮量两因子2×2随机区组设计,其中灌溉水盐度(以电导率表示)设置2个水平:0.35 dS·m~(-1)和8.04 dS·m~(-1),施氮量设2个水平:0 kg·hm~(-2)和360 kg·hm~(-2)(分别用SFN0、SHN0、SFN360和SHN360表示).结果表明,长期咸水滴灌棉田土壤盐分、含水量和NH~+_4-N含量显著增加,pH值、NO~-_3-N、有机质和全氮含量显著降低.咸水灌溉处理显著抑制N_2O排放,不施氮肥和施氮肥处理下分别较淡水灌溉降低45.19%和43.50%.氮肥施用显著增加N_2O排放,施肥处理N_2O排放较不施肥处理增加161%.不施肥条件下,咸水灌溉显著降低反硝化酶活性、nirK、nirS和nosZ基因丰度,α多样性.施肥条件下,咸水灌溉对nosZ型反硝化细菌的丰度无显著影响,但显著降低反硝化酶活性和nirK、nirS基因丰度.咸水灌溉和氮肥施用共同改变nirK、nirS和nosZ型反硝化细菌群落结构,灌溉水盐度对于反硝化细菌群落结构的影响要大于施肥.Lefse分析显示nirK、nirS和nsoZ型反硝化细菌差异物种随着灌溉水盐度的增加而增加,咸水灌溉显著改变反硝化细菌群落结构,导致优势种群数量增加.上述结果表明,长期咸水灌溉降低土壤N_2O排放,但会导致土壤盐分的持续上升,nosZ、nirK和nirS丰度的增加会促进N_2O排放.

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

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.