| 【Object】 Fresh water scarcity has become a fundamental and chronic problem for sustainable agricultural development in arid regions. The quality of irrigation water has deteriorated in many arid regions during recent years, increasing the value of brackish water and saline water as alternative water sources for irrigation. However, the use of either brackish or saline water increases the risk of soil salinization. This study, which included both laboratory and long-term, fixed-position experiments, focused on the utilization of saline water resources. Specifically, this paper examined the effects of drip-irrigation water salinity and N rate on 1) temporal changes in the distribution of water and salt in the soil profile; 2) the transformation, distribution, and leaching of N in the cotton root zone; 3) the composition, structure and functional diversity of the soil microbial community; 4) cotton growth, yield, water use efficiency and N use efficiency; 5) the mechanisms of N movement, transformation, and crop uptake. The objectives of this paper were to provide a scientific basis for efficient utilization of water and nutrient resources in arid areas. 【Method】The laboratory experiment focused on the effect of irrigation water salinity and N rate on urea-N transformation and enzyme activity in grey desert soil. The experiment included five soil salinity levels: 0.44, 0.84, 1.40, 1.90, and 3.46 ds?m-1. There were also three N application rates: 0, 1, and 2 g?kg-1.The fixed-position field experiment focused on the effects of drip-irrigation with saline water under plastic film mulch on(1) the distribution and movement of soil water, soil salt, and NO3-N;(2) soil microbial populations, microbial activity, community structure diversity, functional diversity; and 3) cotton growth, water-use efficiency, and N-use efficiency. The experiment, which was conducted from 2011 to 2013, used a complete randomized block design with three levels of irrigation water salinity(0.35, 4.61, and 8.04 d S·m-1, respectively referred to as fresh water, brackish water, and saline water) and four N application rates(0, 240, 360 or 480 kg N hm-2, respectively referred to as N0, N240, N360, and N480). Irrigation with either brackish or saline 【Result】(1) Irrigation with either brackish or saline water increased soil salinity in the root zone. Salt mainly accumulated in the 60-80 cm depth, especially in the saline water treatment. Excessive N(≥360 kg N hm-2) application increased salt accumulation in the root zone. The final salt content and the net salt accumulation increased as irrigation water salinity and N application rate increased. The total salt content at the end of the three-year field study was 4 times higher in the brackish water treatment and 7 times higher in the saline water treatment than in the the fresh water treatment. The net salt accumulation at the end of the three-year field study was 11 times higher in the brackish water treatment and 18 times higher in the saline water treatment than in the the fresh water treatment. The total salt contents were 3, 7, and 9% higher in the N240, N360, and N480 treatments, respectively, than in the N0 treatment. Similarly, net amount of the cumulative salt was 4, 8, and 11% higher in the N240, N360, and N480 treatments, respectively, than in the N0 treatment. The amount of salt leached out of the root zone during both the growing- and non-growing seasons increased as irrigation water salinity increased. The amount of salt leached during the growing season in the fresh water treatment was 16 times less than that in the brackish water treatment and 33 times less than that in the saline water treatment. The amount of salt leaching decreased significantly as N application rate increased during the growing season. In general, irrigation with brackish or saline water significantly increased soil water content in the root zone. In contrast, soil water content decreased as the N application rate increased. Soil water mainly accumulated in the 60-80 cm depth. Soil water storage and water drainage increased sharply as irrigation water salinity increased, but decreased as N application rate increased. Evapotranspiration decreased sharply as irrigation water salinity increased, but increased as N application rate increased.(2) Soil salinity and N application rate had significant effects on N transformations in the soil. Soil NH4-N concentrations increased as the nitrogen application rate and soil salinity both increased. Nitrogen application significantly increased soil p H and urease activity, accelerating the hydrolysis of urea. Soil urease activity decreased significantly as soil salinity increased when no urea was applied. In the fertilized treatments, low soil salinity had no significant effect on urease activity, whereas high soil salinity increased urease activity. Soil salinity and urea application rate had significant interactive effects on nitrification. The effect of soil salinity on nitrification increased as the nitrogen application rate increased. Soil salinity had relatively little effect on nitrate reductase activity. However, soil salinity significantly inhibited nitrite reductase activity. High salinity and nitrogen application increased soil p H. This might lead to an increase in the loss of soil N due to ammonia volatilization.(3) Irrigation water salinity had significant effects on soil NO3-N concentrations. Both the brackish water and the saline water treatments significantly inhibited nitrification, resulting in a decline in soil NO3-N in the root zone. Soil NO3-N concentrations in the fresh water treatment were 17% higher than that in the brackish water treatment and 29% higher than that in the saline water treatment. However, brackish and saline water both significantly increased soil NH4-N concentrations. Soil NO3-N concentrations were also affected by N application rate. Soil NO3-N concentrations declined with time in both the N0 and the N240 treatments. In comparison, soil NO3-N concentrations remained nearly the same in the N360 and N480 treatments. Irrigation with either brackish or saline water significantly increased soil NO3-N leaching. The amount of NO3-N leaching increased as N application increased. The amounts of NO3-N leaching were 281, 340, and 570% greater, respectively, in N240, N360, and N240 than in N0. The brackish and saline water treatments both significantly inhibited the activities of soil nitrate reductase and nitrite reductase, but increased the activity of soil urease. Nitrogen application increased the activities of soil urease, nitrate reductase, and nitrite reductase.(4) Irrigation with either brackish or saline water reduced bacterial populations, but increased fungal populations. Soil microbial biomass C and N, basal respiration, and the microbial quotient declined significantly as irrigation water salinity increased; however, the metabolic quotient increased. In the unfertilized plot(N0), soil total phosopholipid fatty acids(PLFAs) decreased as irrigation water salinity increased. In the fertilized plots, soil total PLFAs were lowest in the saline water treatment. There was no significant difference in total PLFAs between the fresh water and brackish water treatments. On average, soil total PLFAs in the fresh water treatment were 9% greater than in the brackish water treatment and 34% greater than in the saline water treatment. Soil bacteria, gram-positive bacteria, gram-negative bacteria, fungal PLFAs, and the fungi/bacteria ratio also declined significantly as irrigation water salinity increased. In the unfertilized plot(N0), irrigation with brackish water increased soil microbial activity. However, in the fertilized plot, the fresh water treatment always had the highest AWCD values. Nitrogen application tended to increase AWCD values. Compared with N0, the AWCD values were 22% higher in N240, 17% higher in N360, and 21% higher in N480. Irrigation water salinity had significant effects on C source utilization. In both the fresh water and the brackish water treatments, the most utilized compounds were carbohydrates, amines, amino acids, and polymers. The least used compounds were phenolic acids and carboxylic acid. In the saline water treatment, the most utilized compounds were carbohydrates, amino acids, and polymers. The least used compounds were amines, carboxylic acid, and phenolic acid. The Shannon indices declined significantly as irrigation water salinity increased. The Simpson index increased significantly as irrigation water salinity increased. Nitrogen application reduced the Simpson index.(5) Between 85 and 90% of the root mass was in the 0 to 20 cm soil depth. Root mass was greater in the fresh water treatment than in the saline water treatment. However, there was no significant difference in root mass between the saline water and brackish water treatments. Most(83%-90%) of the root length density was observed in the 0 to 60 cm soil depth. Root mass and average root length density both decreased as soil depth decreased. In the unfertilized plot(N0), average root length density, root surface area, and root volume were all higher in the fresh water treatment than in either the brackish water or the saline water treatments. In the fertilized plot(N360), average root length density, root surface area and root volume were significantly higher in both the brackish water and the saline water treatments than in the fresh water treatment. Nitrogen application reduced root length density, root surface area, root volume, and root diameter. The root/shoot ratios increased significantly as irrigation water salinity increased. The root/shoot ratios were 18% higher in the brackish water treatment and 38% higher in the saline water treatment than in the fresh water treatment. Nitrogen application had no significant effects on the root/shoot ratio. Short-term application of brackish water had no serious negative effect on cotton biomass, N uptake, cotton yield, and water and N use efficiency. Irrigation with saline water significantly decreased cotton biomass, N uptake, cotton yield, and water and N use efficiency. Nitrogen application significantly increased cotton biomass, N uptake, and cotton yield in all the three water salinity treatments. However, the positive effects of N application were reduced in the saline water treatment. Long-term application of either brackish water or saline water had serious negative effects on cotton growth, water and N use efficiency. Application of N fertilizer(0–360 kg N?hm-2) can alleviate salt damage, promote growth, and increase both cotton yield and water use efficiency. 【Conclusion】Irrigation with saline water significantly inhibited soil microbial activity, changed soil microbial structural diversity, reduced soil microbial functional diversity, and significantly inhibited nitrification. Irrigation with either brackish or saline water increased soil salinity in the root zone, increased NO3-N leaching, and reduced N use efficiency. Short-term application of brackish water did not have a serious negative effect on either the water use efficiency or the N use efficiency of cotton. However, saline water irrigation significantly reduced root growth, shoot growth, water use efficiency, and N use efficiency. |
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