亚热带小流域浅层地下水不同形态氮含量的时空变异特征

为了定量研究流域尺度上氮素(N)形态的时空变异特征,以湖南省长沙县亚热带湘江源头小流域(134.4 km~2)为研究对象,2011年(1—12月)定位观测了小流域菜地、茶园、旱地、林地、两季稻田和一季稻田6种土地利用类型下浅层地下水总氮(TN)、硝态氮(NO_3~--N)、铵态氮(NH_4~+-N)浓度的动态变化,运用空间分析技术分析了各观测指标的时空变异特征。结果表明:研究区浅层地下水NH_4~+-N、NO_3~--N和TN均具有强烈的空间自相关性(块金系数分别为0.76%、8.50%、4.41%),结构变异占主导地位,变程分别为540、580、570 m。小流域浅层地下水TN、NH_4~+-N和NO_3~--N月均浓度变化趋势不尽相同,TN和NO_3~--N月均浓度的动态变化相对比较平缓,而NH_4~+-N的变幅较大,TN和NH_4~+-N的峰值出现在2011年7月,NO_3~--N无明显高峰;TN、NO_3~--N和NH_4~+-N的平均浓度分别为2.97、1.12 mg N·L~(-1)和1.32 mg N·L~(-1)。研究区浅层地下水N的浓度分布特征与土地利用类型关系密切,茶园、稻田为浅层地下水N分布高浓度区,且茶园地下水N浓度最高,林地为N分布低浓度区。

Spatiotemporal variability of different nitrogen forms in shallow groundwater of a small watershed in the subtropical region of China

This study aimed at evaluating the spatial and temporal variability of different nitrogen(N) forms at the watershed scale. Shallow groundwater samples were taken at 200 locations under six different land use types(vegetable production, tea plantation, dry land, forest, double-rice cropping field, single-rice cropping field) across the headstream of the Xiangjiang River watershed in Changsha County, China. The concentrations of total nitrogen(TN), nitrate(NO₃^--N), and ammonium nitrogen(NH₄⁺-N) were determined through continuous observation for one year(1/2011-12/2011) in shallow groundwater(80~130 cm) in the small watershed. The spatiotemporal variability of TN, NO₃^--N, and NH₄⁺-N concentrations were analyzed using spatial analysis and geostatistical methods. The contents of TN, NO₃^--N, and NH₄⁺-N in the shallow groundwater had strong spatial auto-correlation with nugget-to-sill ratios of 0.76%, 8.50%, and 4.41%, respectively. This means that structural spatial variance for TN, NO₃^--N, and NH₄⁺-N played a dominant role across the studied watershed. Spatially, the TN, NO₃^--N, and NH₄⁺-N ranged from 540~580 m. The monthly average concentrations of TN, NO₃^--N, and NH₄⁺-N differed from each other in the shallow groundwater of the study area. The average concentrations of TN, NO₃^--N, and NH₄⁺-N were 2.97, 1.12 mg·L^-1, and 1.32 mg·L^-1, respectively. The TN and NO₃^--N concentrations did not vary much; in contrast, the NH₄⁺-N concentrations varied dramatically. The peak concentrations of TN and NH₄⁺-N occurred in July 2011, but the NO₃^--N concentrations varied little and were stable over time. The spatial distributions of N concentrations in the shallow groundwater had close relationships with land use type. The concentrations of TN, NH₄⁺-N, and NO₃^--N were highest under tea plantation, of medium value under rice cropping field, and lowest under forest land use types.