Effect Of Drip Irrigation With Saline Water On Soil N₂O Emission In Cotton Field

[Object] Nitrous oxide（N₂O） is one of the most important greenhouse gases, participating in many photochemical reactions, destroying the ozone layer, and increasing surface radiation. At the same time,N₂ O also contributes to the greenhouse effect. Therefore, scholars around the world are concerned about N₂ O. The objectives of this experiment were（i） to learn more about the relative contributions of nitrification and denitrification to N₂ O emission as affected by irrigation water salinity and N rate;（ii） to explore the effects of saline irrigation water and N rate on soil N₂ O emission; and（iii） to increase understanding about the link between nitrifier/denitrifier ratio and N₂ O emission. The results of this study provide theoretical basis for the rational utilization of water and N resources in arid areas so that N₂ O emission can be reduced.[Method] The field experiment focused on the effect of irrigation water salinity and N rate on（1） soil physiochemical properties and N₂ O emission under plastic film mulch and（2） nitrifying/denitrifying enzyme activity and abundance. The experiment, which was conducted from 2014 to 2015, used a completely randomized block design with three levels of irrigation water salinity [0.35, 4.61, and 8.04 d S m-1, referred to as fresh water（FW）, brackish water（BW）, and saline water（SW）, respectively] and two N rates（0 and 360 kg N ha-1, respectively referred to as N0 and N360）.The laboratory experiment focused on the relative contribution of nitrification/denitrification to N₂ O emission under different irrigation water salinity and N rates. The experiment included three irrigation water salinity treatments（FW, BW, and SW） and three N fertilizers: no fertilizer（CK）, 15NH4NO3, and NH415NO3.[Result]（1） The laboratory experiment showed that soil NH4+-N decreased rapidly until 4 d after fertilization and then remained stable. Soil NO3--N increased until d 4 and then decreased slightly. Soil N₂ O emission rates reached a maximum on d 9 and decreased in the order BW, SW > FW. Soil nitrification and denitrification occurred simultaneously under aerobic conditions, but the contribution of nitrification to soil N₂ O emission decreased as irrigation water salinity increased. In contrast, the contribution of denitrification to soil N₂ O emission increased as irrigation water salinity increased.（2） Soil moisture, salinity, and NH4+-N content increased whereas soil pH, NO3--N, total N, organic matter, and available P decreased as irrigation water salinity increased. Nitrogen application significantly increased mineral N, total N, and soil organic matter.（3） We observed of the total N₂ O emission during the 6 d period, 13.5-23.2% was emitted during the first day, 19.8²9.8% was emitted during the second days, and 16.4²0.3% was emitted during the third days.Soil N₂ O emission rates were significantly（180%） greater in N360 than in N0. The cumulative N₂ O emission during the observation period in different treatments ranged from 0.13 to 0.41 kg ha-1. Of the total N₂ O emission in N360, 19.5-26.3% was emitted during the first day, 42.0⁴8.6% was emitted during the second days, and 10.1²1.6%was emitted during the third days. Nitrogen application significantly increased soil cumulative N₂ O emissions. Soil cumulative N₂ O emissions were 173% higher in N360 than in N0. Irrigation water salinity significantly affected soil N₂ O emission rate and cumulative N₂ O emission.In the N0 plots, soil N₂ O emission rate and cumulative N₂ O emission increased significantly as irrigation water salinity increased. In the N360 plots, soil cumulative N₂ O emission was greatest in BW and least in FW.（4） Correlation analysis showed that soil moisture, NH4+-N, NO3--N, and available P were positively correlated with N₂ O emission rate. Soil p H, salinity and available K had no significant correlation with N₂ O emission rate.（5） Application of N fertilizer can significantly increase soil microbial biomass C, microbial biomass N,enzyme activity related to nitrification and denitrification, the abundance of AOA, AOB, and the abundance of nir K and nir S. However, with the exception of AOA and AOB abuncance, all of these variables decreased as irrigation water salinity increased. The AOA and AOB abundance was significantly affected by interaction between irrigation water salinity and N rate. In the N0 plots, AOA and AOB abundance increased as irrigation water salinity increased. In the N360 plots, AOA abundance was highest in BW and lowest in FW. In contrast, AOB abundance decreased as irrigation water salinity increased.（6） Correlation analysis showed that soil salinity was negatively correlated with MBC, MBN, HYR, NIR,NR, nir K and nir S abundance. Soil N₂ O emission was significantly positively correlated with soil MBN,HYR, NR, AOA, AOB, nir K and nir S abundance.[Conclusion]Long-term irrigation with saline water changed the soil physical and chemical properties, leading to significant increases in soil N₂ O emission. Brackish water and saline water significantly increased AOA abundance and significant decreased nir K, and nir S denitrifier abundance. There was significant positive correlation between soil N₂ O emission and AOA, AOB, nir K, and nir S denitrifier abundance, which indicated that N₂ O was produced by nitrification and denitrification in cotton fields that were drip-irrigated with saline water. Saline water increased the contribution of denitrification to soil N₂ O emission, but decreased the contribution of nitrification to soil N₂ O emission.