Effects Of Saline Water Irrigation On Key Nitrifying Microorganims In A Drip-irrigated Cotton Field

[Objective] Nitrification, which is carried out mainly by microoroganisms, is the central process in the N cycle. The first and rate limiting step of nitrification is ammonia oxidation. Ammonia-oxidizing archaea(AOA) and ammonia-oxidizing bacteria(AOB) are the main microorganisms involved in this step. The effect of saline water irrigation on soil moisture, soil salinity, inorganic N dynamics, and cotton N uptake were investigated in this field experiment. Nitrifying microbial populations, denitrifying microbial populations, and enzyme activities were also evaluated in this study. The objectives of this experiment were(i) to learn more about the effects of saline irrigation water and N rate on the abundance and composition of AOA and AOB,(ii) to determine the relationship between nitrification and ammonia oxidizers, and(iii) to provide a theoretical foundation for future studies about the combined use of saline water irrigation and N fertilizer.[Method] A field experiment about the effects drip-irrigated, saline irrigation water and N rate was begun at the Shihezi University Agriculture Experiment Station in 2009. The experiment consisted of a 3 × 2factorial design with three levels of irrigation water salinity and two N rates. The water salinities were 0.35,4.61 and 8.04 d S m-1, representing fresh water(FW), brackish water(BW), and saline water(SW),respectively. The two N rates were 0 and 360 kg ha-1(referred to as N0 and N360, respectively). The treatments were maintained on the same plots each year. The results presented in this paper were determined using samples collected in 2013 and 2014, which were the fifth and sixth years of the study.[Result](1) Soil moisture, salinity and NH4+concentrations increased and NO3-concentrations decreased as irrigation water salinity increased. Nitrogen application significantly increased soil NH4+and NO3-concentrations. Among the soil depths in this study, soil moisture, salinity and NO3-concentration were all greatest in the 60 to 80 cm soil depth. Salt concentrations were second highest in the 0 to 20 cm depth. At cotton harvest, the highest soil NH4+concentrations were observed in the 0 to 40 cm depth. Nitrogen fertilizer application significantly increased cotton N uptake and yield. In the unfertilized(N0) plots, cotton N uptake and yield was significantly greater in the BW and SW treatments than in the FW treatment. In the fertilized(N360) plots, there was no significant different in cotton yield between the BW and FW treatments; however the SW treatment significantly reduced cotton yield. Nitrogen use efficiency decreased significantly as irrigation water salinity increased.(2) Nitrogen application significantly increased the populations of culturable nitrifying and denitrifying bacteria. Nitrogen fertilizer also increased enzyme activities. The population of nitrite oxidizing bacteria was significantly decreased by BW irrigation, whereas the population of ammonium oxidizing bacteria and denitrifying bacteria were not affected by BW. In comparison, the populations of ammonium oxidizing bacteria, nitrite oxidizing bacteria, and denitrifying bacteria were significantly reduced by SW irrigation.The activity of hydroxylamine reductase, nitrite reductase and nitrate reductase decreased significantly as water salinity increased.(3) Potential nitrification rates increased after N application. Potential nitrification rates were significantly greater in the BW treatment than in the FW treatment. In contrast, the SW treatment significantly reduced potential nitrification rates compared with the FW treatment. Nitrogen application had no significant effect on AOA abundance, but significantly increased AOB abundance. The BW and SW treatments increased AOA abundance, but there were no significant difference between the BW and SW treatments. In contrast, the BW treatment increased AOB abundance but the SW treatment significantly reduced AOB abundance.(4) Nitrogen fertilizer application significantly increased AOB diversity, but not AOA diversity. The BW treatment significantly increased AOB diversity compared with the FW treatment but had no effect on AOA diversity. In comparison, The SW treatment significantly increased AOA diversity but decreased AOB diversity significantly.(5) Statistical analyses showed that potential nitrification rates were positively correlated with AOB abundance. Nitrate concentrations were also significantly and positively correlated with AOB abundance.However, neither potential nitrification rate nor NO3-concentrations were significantly correlated with AOA abundance. The diversity of AOA was negatively correlated with soil organic C, total N, and microbial biomass C. The diversity of AOB was positively correlated with soil NO3- concentrations, soil organic C, AOB abundance, and potential nitrification rates.(6) Phylogenetic analyses of the amo A gene sequences showed that all AOA sequences fell within cluster S(soil origin) and cluster M(marine and sediment origin). All AOB sequences were affiliated with Nitrosospira-like and Nitrosomonas-like species. Regardless of irrigation water salinity and N rate, the soil was dominated by Nitrosospira cluster 3a.[Conclusion]Long-term Saline water irrigation decreased AOB abundance and diversity, but increased AOA abundance and diversity. Nitrogen application had no effect on the abundance and diversity of AOA, but significantly increased the abundance and diversity of AOB. Nitification was significantly and positively correlated with AOB abundance and diversity. This indicates that AOB has greater effect than AOA on nitrification in drip-irrigated cotton field with saline water irrigation.