华北平原中部地区典型深层包气带氮素迁移转化过程

Nitrogen transport and transformation processes in the typical deep vadose zone in the central North China Plain

  • 摘要: 华北平原是我国重要的粮食生产基地之一, 农业长期高强度施肥且氮肥利用率偏低, 导致大量氮素累积在包气带(尤其是根区以下的深层包气带)内, 对区域地下水环境造成严重威胁。目前, 关于华北深层包气带中氮素迁移转化研究较少且主要集中于山前平原地区, 而针对地下水埋深相对较浅、硝酸盐污染风险较高的中部平原的相关研究仍较为匮乏。本研究以华北平原中部典型冬小麦-夏玉米轮作农田为例, 通过厚包气带野外采样(深度为13 m, 3个重复)和室内理化分析, 刻画了中部典型农田厚包气带氮的赋存形态和累积特征, 结合氯离子质量平衡法和包气带环境要素, 分析了深层包气带氮素运移和主要转化过程。研究结果如下: 由于长期施肥的累积效应, 0~2.00 m根系层土壤NO3-N含量最高; 2.00~5.50 m深度剖面土壤NO3-N含量稳定在较高水平; 但是, 5.50 m深度以下土壤剖面, NO3-N含量迅速下降并趋于稳定。整体来看, 5.50 m深度以上土壤剖面中NO3-N累积量占整个剖面总累积量的86.5%。基于水文过程推断, 5.50 m深度以下剖面NO3-N含量锐减并非是高强度施用化肥未运移至该深度所致, 而与氮转化过程有关。在5.50~8.00 m深度剖面, O2含量和氧化还原电位大幅下降, 溶解性有机碳浓度相对较高, 这些环境有利于该层发生厌氧条件下的氮素转化过程, 如反硝化、硝酸盐异化还原为铵、厌氧氨氧化和有机氮厌氧矿化等。硝酸盐氮氧同位素在该层发生富集, 也证明该层存在显著的氮素转化过程。上述氮素运移及转化过程决定了深层包气带氮素分布特征及高强度农业生产对地下水环境的影响。本研究结果可为准确评估农业面源污染对地下水的长期影响并制定针对性管理策略提供重要科学依据。

     

    Abstract: The North China Plain (NCP) is a key agricultural production area in China, where long-term intensive fertilization and low nitrogen fertilizer use efficiency have resulted in significant nitrogen accumulation in the vadose zone, particularly in the deep vadose zone below the root zone. Nitrogen accumulation poses a serious threat to the regional groundwater environment. Research on nitrogen transport and transformation in the deep vadose zone of the NCP was predominantly focused on the piedmont plain. Studies on the central plain remain limited, where the groundwater table is relatively shallow and the risk of groundwater nitrate contamination is higher. This study focuses on a typical winter wheat-summer maize rotation field in the central zone of the NCP. Based on the field sampling of the thick vadose zone (13-meter depth, with three replicates) and laboratory physicochemical analyses, we characterize the nitrogen speciation and accumulation characteristics in the profile. Based on the chloride mass balance (CMB) method and environmental factor analysis of the profile, the migration and transformation processes of nitrogen in the deep vadose zone were revealed. The results demonstrate that nitrate nitrogen content is highest in the root zone (0−2 m). In the 2−5.5 m layer, nitrate nitrogen content remains stable at a relatively high level. It is worth noting that below the 5.5 m, the nitrate nitrogen content decreases rapidly and maintains at a lower value. The accumulation amount of nitrate nitrogen above the 5.5 m depth accounts for 86.5% of the total accumulation amount of the entire profile. Based on the hydrological processes, the sharp decline in nitrate nitrogen content below 5.5 m was not due to the fact that the high-intensity application of chemical fertilizers did not arrive at this depth, but rather related to the nitrogen transformation processes. Furthermore, between 5.5 and 8 m, oxygen content and redox potential significantly decline, while dissolved organic carbon concentration is relatively high. These conditions are conducive to nitrogen transformation processes under anaerobic conditions, including denitrification, nitrate reduction to ammonium, anaerobic ammonium oxidation, and organic nitrogen anaerobic mineralization. Enrichment of nitrogen and oxygen isotopes of nitrate at this depth can also explain significant nitrogen transformation processes. The nitrogen migration and transformation processes identified in this study determine the fate of nitrogen in the deep vadose zone and its impact on the regional groundwater environment, and may jointly play a barrier role in groundwater quality. The findings provide crucial scientific insights for accurately assessing the long-term impact of agricultural non-point source pollution on groundwater and for developing targeted management strategies.

     

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