白洋淀流域厚包气带硝态氮迁移转化过程及其主控因素

Migration and transformation of nitrate at the typical geomorphic types of deep vadose zone in the plain area of Baiyangdian Watershed

  • 摘要: 白洋淀流域位于华北平原西部, 受地下水过量开采和农业过量施肥的双重影响, 平原区包气带厚度增大、氮储量增加, 目前关于流域尺度受地貌类型控制的厚包气带土壤剖面氮素迁移转化过程仍不明确。本文选择白洋淀流域平原农区两个典型的冲洪积扇不同地貌类型的9个深层(0~20 m)土壤剖面, 结合水化学和氯离子平衡分析的方法, 研究不同地貌沉积类型农田厚包气带硝态氮(NO3-N)累积和淋失特征、转化过程及其影响因素。结果表明:1)由地貌类型影响的土壤质地决定着包气带土壤NO3-N累积和淋失量的空间差异性分布, 20 m土壤剖面NO3-N累积量变化范围为352.7 ~ 3276.7 kg·hm−2, 根系层以下土壤NO3-N淋失通量变化范围为9.8 ~ 252.0 kg·hm−21·a−1, 且从冲洪积扇上游至下游湖泊洼地区NO3-N累积量增加、氮淋失量降低; 2)0~20 m土壤剖面上均存在硝化反应, 以砂土为主的冲洪积扇深层土壤因硝化和反硝化共同作用引起硝态氮峰值变化, 而在以粘土为主的泛滥平原反硝化反应主要发生10 m以上包气带中。该研究可为白洋淀流域及雄安新区面源污染阻控和水环境保护提供科学支撑。

     

    Abstract: Excess nitrogen is accumulated in the vadose zone resulting in the gradual increase of NO3-N accumulation due to the overuse of nitrogen fertilizer. The Baiyangdian Basin is located in the western part of the North China Plain, has been impacted by over-exploitation groundwater resulting in decrease of groundwater levels and increase of vadose zone thickness. However, at a large spatial scale, the understanding of mechanism and controlling factors of nitrogen leaching and transformation affected by the difference of geomorphic types and soil texture is still unclear until now. In this study, we selected 9 deep soil profiles (20 m depth) through two typical alluvial-proluvial fan systems (including the loessal terrace, alluvial-proluvial fan, flood plain, and lake depressions) in the Baiyangdian Lake Watershed to investigate the accumulation, leaching, and transformation processes of NO3-N by the hydrochemical analysis and the chloride ion balance method. Results shows: 1) Soil texture of soil profile, influenced by geomorphic types, has a spatial difference with sand-texture of alluvial-proluvial fan and clay-texture of flood plain to determining the spatial distribution of NO3-N accumulation and leaching in the vadose zone. The NO3-N accumulation ranges from 352.7 to 3276.7 kg·hm−2 in the soil profiles of 20 m depth, with the maximum accumulation occurring in the flood plain and lake depressions. Conversely, NO3-N leaching rate below the root zone ranges from 9.8 to 252.0 kg·hm−2·a−1, with the maximum leaching occurring in alluvial-proluvial fan. The NO3-N accumulation shows increasing trend while NO3--N leaching shows decreasing trend from upstream to downstream in soil profiles controlled by the same geomorphic types. The nitrogen accumulation of orchards (1544.0~3133.8 kg·hm−2) is higher than that of vegetables (2641.6 kg·hm−2) and wheat-maize (352.7~3276.7 kg·hm−2) among different land use types. 2) The transformation of nitrogen in the deep vadose zone of Baiyangdian Lake Watershed is mainly influenced by crop absorption, nitrification and denitrification. Nitrification occurs in the whole soil profiles of 20 m depth, particularly in alluvial-proluvial fan with sand-texture layers. The variation of peak value in deep sandy loam soil of alluvial-proluvial fan area is caused by the joint effect of nitrification and denitrification. However, the denitrification occurred mainly in the vadose zone above 10 m depth in flood plain or lake depressions of clay-texture layers due to the decreasing of soil organic carbon from the surface to the deep vadose zone. This research not only advances current research about nitrate migration and transformation from point-scale to watershed scale but also provides valuable insights for future investigations in the physical and geochemical processes of nitrogen leaching at a watershed scale. Additionally, this study also provides scientific support for the control of agricultural non-point source pollution, protection of water environment, and integration of land use management in the Baiyangdian Watershed.

     

/

返回文章
返回