灌溉水盐度对滴灌棉田土壤氨挥发的影响

【目的】氨挥发是农田氮素损失的重要途径之一,咸水灌溉直接或间接影响土壤的理化性质,进而影响土壤氨挥发,但目前对于咸水灌溉下氨挥发的报道还较少。因此通过田间试验研究尿素滴灌施肥条件下,淡水和咸水灌溉对棉田土壤氨挥发的影响。【方法】试验设置淡水和咸水两种灌溉水,其电导率(EC)分别为0.35和8.04d S/m(分别用CK和SW表示),氮肥(N)用量为240 kg/hm2。氨挥发的收集采用密闭室法,用稀硫酸作为氨的吸收液,测定用靛酚蓝比色法。【结果】1)灌溉施肥后,咸水滴灌棉田土壤盐分、脲酶活性和铵态氮含量均显著高于淡水滴灌。SW处理土壤电导率(EC1∶5)较CK平均高出4.53倍。灌溉施肥后SW处理土壤脲酶活性迅速增加,第4天达到最大,随后降低,SW处理脲酶活性较CK处理平均增加了20.6%。SW处理土壤铵态氮含量明显高于CK处理,尤其是灌溉施肥后第2天,SW处理铵态氮含量比CK处理增加了66.1%。2)SW处理棉田土壤p H值低于CK处理,但在灌溉施肥周期内都呈先增加后降低趋势,p H的变化在7.6~8.0之间。3)SW处理抑制了硝化作用,SW处理土壤硝态氮含量较CK处理显著降低。SW处理土壤硝态氮含量平均较CK低7.68%。4)3个灌溉施肥周期的平均温度分别为24.6℃、26.05℃和24.9℃,因此在第2个和第3个灌溉施肥周期氨挥发高,第1个灌溉施肥周期的总降水量最大,分别比第2和3个灌溉施肥周期高3.7 mm和10.2 mm,但降水量远远小于灌溉量,因此对于氨挥发影响不大。5)总体上,土壤氨挥发损失量在灌溉施肥后1~2天最大,占氨挥发总量的45.7%~79.3%,随后呈降低趋势;灌溉施肥后第1天土壤氨挥发最大,在3个灌溉施肥周期,SW处理第1天的氨挥发较CK分别增加70.7%、69.43%和60.8%。SW处理棉田土壤氨挥发显著高于CK处理。在三个连续灌溉施肥周期内,SW处理棉田土壤氨挥发累积总量为10.98 kg/hm2,CK处理为7.57 kg/hm2,SW处理较CK处理增加了45.1%。【结论】咸水灌溉促进了脲酶活性,但抑制了土壤的硝化作用,导致铵态氮含量增加,加剧了氨的挥发。温度升高促进土壤氨挥发,少量降雨对氨挥发影响不大。因此,滴灌施肥条件下,咸水灌溉会增加氨挥发损失。

Effects of the salinity of irrigation water on soil ammonia volatilization in drip-irrigated cotton fields

【Objectives】 Ammonia volatilization is one of the main pathways of nitrogen loss from farmland soil. Saline water irrigation can directly or indirectly affect the physical and chemical properties of soil, which influence soil NH₃ volatilization. The objective of this study was to investigate the effect of irrigation water salinity on soil NH₃ volatilization in cotton fields after drip fertigation with urea. 【Methods】 The experiment consisted of two levels of irrigation water salinity: 0.35 dS/m (freshwater, CK) and 8.04 dS/m (saline water, SW), the N application rate was 240 kg/ha. Volatilized NH₃ was collected using the closed chamber method, the NH₃ was trapped using dilute sulfuric acid. The amount of NH₃ in the traps was measured using theindophenol blue colorimetric method. 【Results】 1) The soil salinity, urease activity, andNH⁺₄-N content in the saline water treatment were significantly higher than the freshwater treatment. The soil salinity treated with saline water was averaged 4.53 times higher than that with the freshwater. Soil urease activity increased rapidly in the saline water treatment after fertigation. Urease activities reached a maximum four days after fertigation and then declined. Urease activity was averaged 20.6% higher in the SW treatment than in the freshwater treatment. The NH⁺₄-N content in the saline water treatment was significantly higher than in the freshwater treatment. The greatest difference between the two treatments was observed two days after fertigation with theNH⁺₄-N content 66.1% greater in the saline water treatment than in the freshwater treatment. 2) Soil pH was lower in the saline water treatment than in the freshwater treatment. However, soil pH in the saline water treatment increased after fertigation and then decreased with a range from 7.6 to 8.0. 3)Saline water irrigation inhibited nitrification, soil NO^-₃-N concentrations were significantly lower by an average of 7.68% in the saline water treatment than in the fresh water treatment.4) The average temperatures during the three fertigation cycles were 24.6℃, 26.05℃, and 24.9℃, respectively. NH₃ volatilization reached a higher level during the second and third cycles than the first one. The greatest amount of rainfall was received during the first fertigation cycle, which was 3.7 mm and 10.2 mm higher than the second and third fertigation cycles. However, rainfall had little effect on ammonia volatilization due to the rainfall amount was far less than the irrigation.5) Generally, soil NH₃ volatilization reached a maximum amount one or two days after fertigation with urea and then declined. Soil NH₃ volatilization accounted for 45.7%-79.3% of the total NH₃ volatilization during the first two days after fertigation. In the first day of the three fertigations, soil NH₃ volatilization was 70.7%, 69.3% and 60.8% higher in the saline water treatment than in the freshwater treatment correspondingly. Soil NH₃ volatilization was significantly higher in the saline water treatment than in the fresh water irrigation. Total NH₃ volatilization during three consecutive fertigation cycles was 10.98 kg/ha in the saline water treatment, and 7.57 kg/ha in the freshwater treatment. Total NH₃ volatilization in the saline water treatment was 45.1% higher than in the freshwater treatment. 【Conclusions】 Saline water irrigation promoted urease activity. However, saline irrigation water significantly inhibited nitrification, depressed the transformation of NH⁺₄-N to NO^-₃-N. This led to an increase in soil NH⁺₄-N content and NH₃ volatilization. High temperature increased NH₃ volatilization. The small amount of rainfall during the experiment had little effect on NH₃ volatilization. The results indicated that irrigation with saline water increased N loss due to NH₃ volatilization after drip fertigation with urea.