杭州稻麦菜轮作地区大气氮湿沉降

通过雨水中NH+4-N/NO-3-N比率和铵态氮自然丰度值(δ15NH+4)的变化探讨大气氮湿沉降与农作施肥活动的关联性. 2003年6月至2005年7月,采用自行设计定制的雨水收集器在浙江杭州稻-麦-蔬菜轮作地区进行了为期2a的全天候连续雨水采样分析.结果显示,杭州稻-麦-蔬菜轮作地区雨水中NH+4-N/NO-3-N比率和δ15NH+4值呈现相似的季节性变化,雨水中NH+4-N/NO-3-N的峰值出现在6月底～9月上旬,而后逐渐下降,秋冬季(10～11月份)降到最低;来春麦菜集中施肥期(2～3月份),又呈现多个小高峰;5～6月份为单季稻和蔬菜基肥和追肥集中施用期,故而峰值也达4以上;入冬后仅在麦田施肥期出现一个小高峰而后明显下降,大都降到1以下;频繁施肥期雨水中的NH+4-N/NO-3-N比率值是农闲期的2～4倍,显示雨水NH+4-N/NO-3-N比率与农田施肥活动有密切关联与同步性,但与气温无直接关联(R2=0.0129).雨水中δ15NH+4值的变化,与雨水中NH+4-N/NO-3-N比率相似,呈现明显的季节性变化:稻麦生育后期与种前空闲期为正值,麦稻蔬菜集中施肥期转为负值.雨水中NH+4-N/NO-3-N比率与δ15NH+4值对大气湿沉降中氮的来源、形态及地面NH3排放源的强度有一定的表征意义.

Atmospheric N wet deposition in Hangzhou region under rice\|wheat\|vegetable cropping system

The ratio of NH_4~+-N /NO_3~--N and natural 15N abundance of NH_4~+ (δ~(15)NH_4~+) in rainfall in Hangzhou of Zhejiang province, China was determined between June 2003 and July 2005 using customized manual rainwater sampler. Our measurements indicated a similar seasonal variation in the NH_4~+-N/NO_3~--N ratio and δ~(15)NH_4~+ value in rainfall; peaks of NH_4~+-N/NO_3~--N ratio in rainfall occurred when the farmland received fertilizers, followed by gradual decline to a lower level when the crop was harvested or the fertilizer application was terminated. In contrary, the δ~(15)NH_4~+ value stayed negative when N-substances were applied to the crop, and then turned to positive when the crop was harvested or field was in fallow. Both NH_4~+-N/NO_3~--N ratio and δ~(15)NH_4~+ value in rain precipitation were evidently correlated with the chemical nitrogen fertilizer application during various crop growth stages, the alternation of seasonal transitions, and NH_3 volatilization from other NH_3 emission sources. Therefore, they could be regarded as an indicator in determining the sources, forms and composition of N in atmospheric wet deposition as well as the intensity of various NH_3 emission sources from the land.