中亚热带典型农业小流域氮素输出特征及监测采样频率研究

以湖南省长沙县的脱甲小流域为例,研究了中亚热带典型农业小流域中氮素输出的规律,并采用统计学方法确定合理的氮素输出监测频率.研究结果表明,该地区农业小流域氮素输出过程受降雨-径流过程和农业管理活动的影响明显,除颗粒态氮(PN)外,流域出口水体中铵态氮(NH+4-N)、硝态氮(NO-3-N)、可溶性氮(DN)和全氮(TN)的浓度均与沟渠径流量显著相关(p0.01).降雨-径流过程和农业管理活动也会影响河道水体中氮素化学形态,但其对氮素化学形态的影响往往是短期的,因为在整个观测期间内,仅NH+4-N/TN比例与径流量有显著相关关系(p0.01).对于不同形态氮素采样频率计算结果表明,水体中氮素浓度变异系数越大,在允许的误差范围内需要的监测频率越高.按照平均值误差的20%控制,NH+4-N和PN的采样频率要达到每天2次才能满足要求,采样频率显著高于NO-3-N、DN和TN(1天1次).如果将典型小流域中的氮素输出监测频率定为每天1次,NH+4-N和PN的相对误差将达到30%.该研究为阐明中亚热带农业小流域氮素输出规律和制定氮素监测策略和规范提供了样本.

Nitrogen export characteristics and sampling frequency for nitrogen monitoring from a typical small agricultural catchment in central subtropics

Tuojia catchment located in Changsha County,Hunan Province was selected for studying the rules of nitrogen (N) export from a typical small agricultural catchment in central subtropics.Sampling frequency for N monitoring was determined with a statistic method.The results suggested that N export from the catchment were significantly influenced by the rainfall-runoff processes and agricultural management practices.Except the particle nitrogen (PN),ammonium nitrogen (NH⁺₄-N),nitrate nitrogen (NO^-₃-N),soluble nitrogen (DN),and total nitrogen (TN) in stream water were significantly correlated with the stream discharge during the observation period (p <0.01).The rainfall-runoff processes and agricultural management practices could influence the chemical compositions of N in stream water.However,these influences seemed to be short-term,because only the proportion of NH⁺₄-N/TN was significantly cor-related with the stream discharge during the observation period.In addition,the results for sampling frequency in different N forms showed that the higher coefficient of variation of N concentrations in stream water was,the higher sampling frequency for accurately estimating N export within permissible range of error.Given the permissible range of error of 20%,a sampling frequency of twice per day was required for NH⁺₄-N and PN monitoring,significantly higher than the once per day for NO^-₃-N,DN and TN monito-ring.If sampling frequency for N export was set to be once per day,the relative error would be up to 30%.Therefore,our results provided impo-rtant information for characterizing the N export process and developing the strategy and norms of the N observation in small agricultural catchments in central subtropics.