【作者】 王丹；

【导师】 李培月；

【作者基本信息】 长安大学 ， 水利工程， 2024， 博士

【摘要】 农业地区地下水硝酸盐（NO₃^-）污染因其污染面积大、持续周期长而成为全球面临的严重问题。NO₃^-在水环境中具有易溶性和高迁移性,并且涉及复杂的生物地球化学循环过程,地下水一旦受到污染很难治理。明确农业活动影响下土壤-地下水系统中硝酸盐生物地球化学作用机制,是削减地下水硝酸盐污染负荷的基础与关键。然而,目前有关农业活动下硝酸盐生物地球化学作用机制尚不明晰。本文选取关中平原两个典型农业区:泾惠渠灌区和周至-眉县区域作为研究区。前者位于渭河北岸,以玉米-小麦为主要作物,后者位于渭河南岸,以猕猴桃为主要作物。在系统开展野外调查的基础上,综合利用水化学、稳定同位素(δ¹⁸O-H₂O、δ¹⁵N-NO₃^-和δ¹⁸O-NO₃^-)、16S r RNA测序、宏基因测序、贝叶斯稳定同位素混合模型（Mix SIAR）、随机森林模型等多种研究手段,结合在不同溶解有机碳（DOC）和水力梯度条件下的室内实验与模拟,深入探讨了农业活动影响下地下水硝酸盐生物地球化学作用机制,取得的主要结论如下:（1）查明了关中平原典型农业区土壤-地下水系统中硝酸盐的时空分布特征及主控因素。研究发现,周至-眉县区域猕猴桃园土壤硝态氮累积量高于耕地,主要因耕地转为果园后化肥用量增加,导致包气带硝酸盐累积加剧。泾惠渠灌区耕地土壤NO₃^-浓度高于周至-眉县区域,归因于该区域具有更长的农业活动及生活排污历史。周至-眉县区域雨季地下水硝酸盐浓度高于旱季,而泾惠渠灌区则在旱季更高。居民区和蔬菜地为两区域中硝酸盐浓度最高的土地利用类型。对周至-眉县区域地下水硝酸盐影响最大的因素依次为地下水埋深（16.8%）、土壤硝酸盐（10.7%）和溶解氧（10.3%）;泾惠渠灌区则依次为土壤硝酸盐（14.0%）、DOC（11.0%）和地下水埋深（10.6%）。（2）量化了不同土地利用类型下地下水硝酸盐来源的季节性差异。研究发现,在周至-眉县区域,旱季和雨季不同土地利用类型的地下水硝酸盐来源均为:土壤氮>化肥>粪肥&污水>大气沉降。尽管总体趋势一致,但不同土地利用类型下各来源的具体贡献存在差异。其中,猕猴桃园地下水硝酸盐受化肥的季节性影响尤为突出,雨季占19.5%,旱季增至27.9%。泾惠渠灌区不同土地类型下地下水硝酸盐来源季节性差异更为显著。居民用地的主要来源为粪肥&污水,雨季占74.9%,旱季降至67.6%。蔬菜地的主要硝酸盐来源为化肥,而耕地则在旱季以化肥贡献最高（38.7%）,雨季则以土壤氮为主（42.4%）。（3）识别了地下水硝酸盐迁移转化的生物地球化学作用过程。研究区地下水主要发生硝化作用,而反硝化作用和硝酸盐异化还原为铵（DNRA）作用则较弱。硝酸盐浓度升高会抑制硝化作用并促进反硝化作用,伴随硝化菌丰度下降、反硝化菌丰度上升。计算了两个地区的土壤和地下水氮循环潜力,表明泾惠渠灌区土壤和地下水的硝化潜力（4.14%～10.93%）低于周至-眉县区域（10.85%～16.48%）,而反硝化潜力（10.16%～22.24%）则高于周至-眉县区域（6.99%～15.92%）。泾惠渠灌区较高的反硝化潜力表明该区域相较于周至-眉县更具有效的自然氮污染缓解机制。（4）构建了硝酸盐水动力-生物地球化学模型,并利用该模型揭示了地下水硝酸盐生物地球化学作用机制。本研究构建了以对流-弥散地下水动力学为基础,耦合由功能基因驱动的生物化学反应模块的水动力-生物地球化学模型。室内实验数据验证表明,该模型在模拟地下水中NO₃^-、NO₂^-和NH₄⁺浓度变化方面具备高适应性和可信度。通过验证后的模型进行模拟并发现,水力梯度和DOC浓度协同调控着硝化、反硝化和DNRA过程中的关键功能基因表达,决定了NO₃^-、NO₂^-和NH₄⁺的迁移和转化路径。总体来看,高水力梯度下硝化作用占主导,而低水力梯度与高DOC条件则更有利于反硝化和DNRA过程。（5）揭示了硝酸盐生物地球化学作用对碳循环的影响。硝化作用释放的H⁺显著增强了地下水系统中碳酸盐和硅酸盐的风化作用,并导致CO₂消耗速率减慢,碳汇程度降低,这可能是地下水由碳汇转变为碳源的关键因素。在旱季,泾惠渠灌区硝化作用对碳酸盐和硅酸盐风化的贡献较高,分别为7.56%和4.76%;而在雨季,周至-眉县区域的硝化作用对碳酸盐风化的贡献显著提高,分别为34.18%和18.18%。此外,硝化作用与卡尔文循环（CBB）和还原性三羧酸（rTCA）途径之间存在显著相关性,表明硝化反应不仅在氮循环中起关键作用,还通过影响碳固定途径间接影响着碳循环。更多还原

【Abstract】 Groundwater nitrate（NO₃^-）pollution in agricultural areas has become a critical global issue due to its widespread occurrence and persistent nature.NO₃^-is highly soluble and mobile in aquatic environments,participating in complex biogeochemical cycling processes that hinder effective remediation once pollution occurs.A comprehensive understanding of the biogeochemical processes governing nitrate dynamics in soil-groundwater systems influenced by agricultural activities is crucial for mitigating groundwater nitrate pollution.However,these mechanisms remain unclear.This study investigates two representative agricultural regions in the Guanzhong Basin:the Jinghuiqu Irrigation District on the northern bank of the Wei River,dominated by maize-wheat cropping systems,and the Zhouzhi-Mei Region on the southern bank,primarily cultivating kiwifruit.A range of analytical approaches was employed,including hydrochemical analyses,stable isotope techniques(δ¹⁸O-H₂O,δ¹⁵N-NO₃^-,andδ¹⁸O-NO₃^-),16S r RNA sequencing,metagenomic sequencing,the Bayesian stable isotope mixing model（Mix SIAR）,random forest modeling,and laboratory experiments and simulations under varying dissolved organic carbon（DOC）and hydraulic gradient conditions.These methods collectively elucidate the biogeochemical mechanisms of groundwater nitrate under agricultural influence.The main conclusions are as follows:（1）This study investigates the temporal and spatial distribution of nitrate in the soil-groundwater system in typical agricultural areas of the Guanzhong Basin.It also identifies the controlling factors.The results show that nitrate accumulation in the soil of kiwifruit orchards in the Zhouzhi-Mei Region is higher than in cultivated land,primarily due to the increased use of fertilizers following the conversion of farmland to orchards,which has intensified nitrate buildup in the unsaturated zone.In contrast,the NO₃^-concentration in cultivated land soil in the Jinghuiqu Irrigation District exceeds that in the Zhouzhi-Mei Region,owing to the district’s longer agricultural history and domestic wastewater discharge.Groundwater nitrate levels in the Zhouzhi-Mei Region are higher during the rainy season,whereas in the Jinghuiqu Irrigation District,they peak during the dry season.In both regions,residential areas and vegetable fields exhibit the highest nitrate concentrations.The key factors influencing groundwater nitrate in the Zhouzhi-Mei Region are groundwater depth（16.8%）,soil nitrate（10.7%）,and dissolved oxygen（10.3%）.In the Jinghuiqu Irrigation District,the main factors are soil nitrate（14.0%）,DOC（11.0%）,and groundwater depth（10.6%）.（2）This study quantifies the seasonal differences in nitrate sources in groundwater under different land use types.The sources of groundwater nitrate in the Zhouzhi-Mei Region during the dry and rainy seasons under different land use types are ranked as follows:soil nitrogen>chemical fertilizer>manure&sewage>atmospheric deposition.Although the overall trend is consistent,the specific contributions of each source vary across different land use types.Notably,the seasonal influence of chemical fertilizers on groundwater nitrate in kiwi orchards was particularly pronounced,with contributions increasing from 19.5%during the rainy season to 27.9%in the dry season.In the Jinghuiqu Irrigation District,the seasonal variability in nitrate sources was more pronounced across land use types.In residential areas,manure and sewage were the dominant sources,accounting for 74.9%in the rainy season and decreasing to 67.6%in the dry season.For vegetable fields,chemical fertilizers were the primary nitrate source,while in farmlands,chemical fertilizers contributed the most during the dry season（38.7%）,and soil nitrogen was the dominant source in the rainy season（42.4%）.（3）This study identifies the biogeochemical processes of nitrate in groundwater.In the study area,nitrification is the dominant process in groundwater,while denitrification and dissimilatory nitrate reduction to ammonium（DNRA）are relatively weak.An increase in nitrate concentration inhibits nitrification and promotes denitrification,accompanied by a decrease in the abundance of nitrifying bacteria and an increase in denitrifying bacteria.The nitrification potential of soil and groundwater in the two regions was calculated,showing that the Jinghuiqu Irrigation District has a lower nitrification potential（4.14%-10.93%）compared to the Zhouzhi-Mei Region（10.85%-16.48%）,while the denitrification potential（10.16%-22.24%）in the Jinghuiqu Irrigation District is higher than in the Zhouzhi-Mei Region（6.99%-15.92%）.The higher denitrification potential in the Jinghuiqu Irrigation District suggests that this region has a more effective natural nitrogen pollution mitigation mechanism compared to the Zhouzhi-Mei Region.（4）This study develops a hydrodynamic-biogeochemical model and reveals the biogeochemical mechanisms of nitrate in groundwater.This model integrates convection-dispersion groundwater dynamics with a biogeochemical reaction module driven by functional genes.Laboratory data validation confirmed the model’s accuracy in simulating NO₃^-,NO₂^-,and NH₄⁺dynamics in groundwater.Simulations showed that hydraulic gradient and DOC concentration jointly influence the expression of key functional genes driving nitrification,denitrification,and DNRA,thereby shaping the migration and transformation pathways of NO₃^-,NO₂^-,and NH₄⁺.Overall,nitrification dominates under high hydraulic gradient conditions,while low hydraulic gradient and high DOC conditions are more favorable for denitrification and DNRA processes.（5）This study reveals the impact of nitrate biogeochemical processes on the carbon cycle.The H⁺released during nitrification significantly enhances the weathering of carbonates and silicates in the groundwater system,leading to a slower CO₂consumption rate and a reduced carbon sink capacity.This may be a key factor in the transformation of groundwater from a carbon sink to a carbon source.During the dry season,nitrification in the Jinghuiqu Irrigation District contributed 7.56%and 4.76%to carbonate and silicate weathering,respectively.In contrast,during the rainy season,nitrification in the Zhouzhi-Mei Region contributed significantly more to carbonate weathering,at 34.18%and 18.18%.Additionally,a strong correlation between nitrification and the Calvin-Benson-Bassham（CBB）and reductive tricarboxylic acid（r TCA）pathways was observed,underscoring the dual role of nitrification in regulating both the nitrogen cycle and the carbon cycle by influencing carbon fixation pathways.更多还原