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喀斯特关键带水土介质中氮运移机制研究-以后寨河流域为例
其他题名Mechanism of nitrogen transport in soil and water of the karst critical zone - an example from Houzhai catchment
王忠军
学位类型博士
导师陶发祥
2019
学位授予单位中国科学院大学
学位授予地点中国科学院地球化学研究所
关键词喀斯特 关键带 氮运移 硝酸盐 同位素
摘要

氮是生态系统中重要生源要素之一,既是生命体蛋白质和遗传物质的重要组分,也是植物生长所必需的营养元素之一。人类活动导致自然界氮含量增加的同时,也使得各生态系统面临着诸多与氮相关的环境问题,比如水质恶化、水体富营养化等。喀斯特地貌在陆地上广泛分布,由于特殊的水文地质结构,喀斯特关键带中水土环境质量安全一直面临着严峻挑战。贵州省是位于世界三大喀斯特连片分布区之一的东南亚喀斯特中心,以贵州喀斯特为代表的关键带水土介质中氮的运移机制研究能够丰富对西南喀斯特地区乃至世界喀斯特关键带中氮的生物地球化学过程的认识,对脆弱的喀斯特生态系统保护具有重要意义。硝酸盐同位素和传感器技术是辨识流域内硝酸盐来源和转化过程的有效手段。前期研究发现贵州喀斯特地区农业小流域中部分水体硝酸盐含量超过WHO规定的饮用水硝态氮含量标准(10mg/L)。喀斯特地区雨季水体存在高浓度的硝酸盐,同时空间变化极大,目前对这一过程的机理还缺乏深入的认识。因此本研究选择中国西南地区管道流发育良好的喀斯特农业小流域(陈旗流域)为研究对象,同时将研究范围扩展到多种土地利用类型以及裂隙流与管道流共存的后寨河流域,来探究喀斯特关键带中氮的运移机制。获得以下认识:不同土地利用类型中土壤氮含量(有机氮、无机氮)在垂直方向呈现表层高下层低的趋势,土壤有机氮同位素变化随着土壤深度增加而逐渐增加。土壤有机氮含量和同位素值垂直方向上的变化反映了土壤中有机氮降解(矿化作用)程度的变化,上层土壤中新生有机质贡献多,随着土壤深度增加降解程度升高,残余富集重同位素的有机氮。基于氮形态和同位素多次分析,尽管喀斯特土壤层较薄,土壤中存在活跃的有机氮输入和降解过程以及无机氮的生成和消耗过程,且各种转化过程主要集中在表层土壤中。通过硝酸盐传感器在线观测方法,观测到陈旗小流域地下水出口全年NO3--N浓度变化幅度较大,较大的波动出现当地集中施肥时间段内,即旱季结束与雨季初期的过渡期。同时NO3--N浓度随流量变化而波动,表明水中NO3--N浓度受土壤中NO3--N的累积和流量变化共同影响。流域NO3--N输出通量呈现明显的季节差异,旱季输出通量仅占全年通量的9%,雨季占比高达91%。NO3--N通量与流量具有极强的正相关关系(R2=0.97),说明流域表层土壤氮素淋溶流失与流域降雨的季节性分布及农业活动密切相关,流域氮素管理需要着重考虑雨季中农业氮肥的施用与流失。对土壤水和地表径流分析发现,高强度降雨事件(地表径流产生)过程中地表径流冲刷,经较大裂隙和管道进入地下水是硝酸盐从土壤向地下水中迁移的主要途径;土壤向地下水输出的硝酸盐主要来自于洼地旱地和蔬菜地表层土壤。对不同深度的观测井降雨前后水体氮素含量及其同位素组成垂向分布特征分析发现,无机氮主要形态以NO3--N为主,降雨事件过程中不同深度地下水中NO3--N来源不同。浅层地下水中NO3--N主要来自流域洼地底部农业施肥特别是化肥和粪肥的混合,深层地下水中NO3--N可能来自喀斯特基质微孔储存的氮。NO3--N进入地下水系统后,在不同深度的地下水中浓度和同位素值受到不同程度的反硝化作用影响,浅层地下水中低,深层地下水中高。通过对陈旗流域地下水出口雨季水文变化与氮素施用关键期流量和硝态氮动态变化研究发现,降雨事件过程中NO3--N浓度随流量变化主要有三个阶段:稀释阶段、脉冲阶段和回落阶段,浓度峰值出现在流量峰值之后。采样期间NO3--N浓度、δ15N-NO3-和δ18O-NO3-值在降雨事件中的变化表明降雨事件过程中存在硝酸盐主要来源的变化,以及不同水文条件下关键带过程的差异性作用。同位素混合模型结果显示,土壤有机氮和化肥是最主要的硝酸盐来源,不同降雨强度的降雨事件中不同硝酸盐来源的贡献率不同。低强度降雨事件中土壤有机氮来源贡献高于化肥来源贡献,高强度降雨事件的流量峰值阶段化肥是最主要NO3--N来源,贡献率最高达85%。结合同位素分析发现,高强度降雨事件过程中NO3--N动态变化主要由硝酸盐主要来源变化导致的,受转化过程的影响较小。对后寨河流域两年月度采样分析表明,后寨河流域水体中NO3--N在无机氮中占绝对优势,占无机氮总量的97%。水体中NO3--N浓度和同位素值季节变化规律与当地农业活动和降雨冲刷有关。后寨河流域中硝酸盐来源在旱季和雨季略有不同,雨季中硝酸盐主要来源于化肥和粪肥的混合,旱季则主要来自于粪肥。整个流域内挥发作用,硝化反应,反硝化反应和同化吸收作用都影响硝酸盐的同位素值。同化吸收主要发生在流域地表水如中游的青山水库,上游母猪洞和地表河出口等。同时反硝化作用在包含有发育的喀斯特管网的陈旗和长冲两个小流域及汇合后的灯盏河样点中也有明显影响。通过对陈旗流域和后寨河流域两个典型喀斯特农业小流域内研究发现,农业施肥是雨季流域水体中显著的氮输出的主要来源。结合本文研究结果,建议在雨季降雨集中期及农业施肥期,合理控制施肥量、施肥方式及施肥时间。如控制喀斯特地区农田施肥总量,改变传统的撒施和表施,避免在暴雨频发时间段内大量施肥,减少向附近水体的流失。同时改变现有施肥类型以及推广部分秸秆还田,增加肥效更持久的有机肥和缓释肥施用量,进而延长有机氮向无机氮特别是硝态氮的转化周期;减少裂隙、管道密集区旱地和蔬菜地的分布,并建立植被缓冲区,从而减少氮肥从土壤向水体的快速流失,达到保护关键带水土环境质量和生态可持续保护的目的。总的来讲,喀斯特关键带水土介质中氮分布有高度的异质性,空间变化上主要受土地利用,植被覆盖,人为施肥等的影响。在时间变化上受施肥时间和降雨事件影响较大,雨水冲刷表层土壤中的硝酸盐通过快速流进入河流和地下水。深层土壤和地下水中有明显的反硝化作用,这减少了硝态氮的赋存,同时也影响枯季或慢速流主导的水文时期的硝酸盐来源。本研究利用传感器和硝酸盐氮氧同位素通过不同水土剖面氮特征研究很好地理解了喀斯特关键带水土介质中硝态氮的来源和运移机制,为类似的研究和流域水环境质量管理提供了很好的参考。

其他摘要

As one of the important nutrient elements in the ecosystem, nitrogen is not only an important component of protein and genetic material, but also one of essential nutrient element for plant growth. However, anthropology activities increase nitrogen pool in the ecosystem but also accompany many environmental issues, e.g. water quality deterioration and eutrophication. Karst landscape is widely distributed on earth. Its special hydrogeology architecture resulted into pollutants and soil erosion more severe, which will exacerbate the environmental quality of water and soil. Guizhou Province is located in the center of the karst region of Southeast Asia, which is one of three globally continuous larger karst area. Thus, the knowledge of Karst Critical Zone (KCZ) from this study can improve understanding of nitrogen cycle in karst area. Furthermore, it will be significantly for the protection of fragile ecosystem, e.g. karst aquifer system.The nitrate isotopes and online nitrate sensor technology are useful approaches to identify the fate of nitrate among different components within critical zone system. Previous studies have found that the nitrate nitrogen concentration in typical agricultural catchment in Southwest of China can exceeded the standard of drinking water WHO threshold (10mg/L). The high nitrate concentrations were observed in wet season and showed spatial variation. However, the mechanism of transformation and driven factors are still required scientific understanding. Therefore, this study choose two typical catchment, Houzhai catchment and Chenqi sub-catchment, with different karst aquifer systems and land use to explore the mechanism of nitrogen fate in the karst critical zone. The obtained results and conclusions are as follows:The content of soil nitrogen (including organic (SON) and inorganic nitrogen) in different land use types was decreased with depth, while the δ15N-SON values increased with depths. The vertical variation of SON content and their isotope values indicated more new organic matter produced in the top of soil layer and residual SON was enriched in 15N as increase the degradation degree with depth. The results of nitrogen species and δ15N-SON suggested that nitrogen transformations, e.g. N production and consumption processes between organic nitrogen and inorganic nitrogen in the soil in top layer, are active. The NO3--N sensor online monitoring at Chenqi sub-catchment generated that large the annual variation of NO3--N concentration with more pulses and large fluctuations during intensive fertilization period, especially the transition period between the later of dry season and early of wet season. The pulses of NO3--N concentration were more respond to discharge, suggesting that the NO3--N concentration were interacted by the discharge and accumulation of NO3--N in the soil. The NO3--N loading from the catchment showed obvious seasonal differences. The NO3--N export showed strong seasonal with contribution from wet season up to 91%. The significant positive relationship (R2=0.97) between NO3--N loading and discharge were observed. Basing on sensor technology, NO3--N leaching from top of soil layer was more depended on the seasonal rainfall and agricultural activities. The strong seasonal exporting should be note which should be considered the fertilization application time and guideline in the wet season.On the view of different runoff during rainfall events, waters from valley agricultural area and vegetable land leached most of nitrogen to underground stream by fractures or conduits. The vertical variation of nitrate concentration and isotopic composition in observation wells during rainfall events suggested high heterogeneity, e.g. nitrate concentration and isotopic composition. Nitrate sources in the shallow groundwater derived from applied fertilizers in valley farmland, such as the mixing of chemical fertilizer and manure, whereas nitrate isotopic composition in the deep groundwater suggested denitrification occurrence in karst matrix where nitrate stored. The nitrate characteristics in the observation well suggested denitrification influenced its isotopic composition, particularly in deep groundwater.The high resolution of nitrate isotopes during rainy season and fertilization periods at the groundwater outlet of Chenqi catchment suggested that rainfall events showed three periods: dilution period, pulse period and falling period. The NO3--N peak lagged the hydrograph peak. It also indicated that nitrate sources in different periods can be varied. The stable isotopic mixing model calculation showed that soil organic nitrogen and chemical fertilizer were the main nitrate sources during the high discharge fluctuation periods but it can also depend on rainfall intensify with high contribution from chemical fertilization (up to 85% during high rainfall intensify). This high-resolution data also suggested that transformation was less impacted on isotopic composition as short retention time in KCZ. The two years monthly sampling data in large catchment (Houzhai) suggested that NO3--N is the main inorganic nitrogen form, accounting for 97% of the total inorganic nitrogen. The seasonal variation of NO3--N characteristics are related to local agricultural activities and rainfall flushing effect. Nitrate sources also showed seasonal variation. The nitrate was mainly derived from the mixture of chemical fertilizer and manure in the wet season, while mainly derived from manure in the dry season. Nitrate transformations, such as volatilization, nitrification, denitrification and assimilation were all affected the nitrate fate in Houzhai catchment. For example, assimilation was mainly observed in surface water, like Qingshan reservoir, Muzhudong reservoir and Surface river outlet during the wet season. The obvious denitrification was also observed in the two well-developed sub-catchments (Chenqi and Changchong) and their mixed surface river, Dengzhan River.Overall, agricultural fertilization was the major contributor to the strong seasonal nitrogen export in Houzhai catchment. It is worth to note that effective fertilization guideline may useful to reduce N concentration and improve the water quality, such as decrease the intensity of fertilizers, improvement fertilization method and fertilization times in the wet season. To avoid the directly export to underground system by conduit, the surround of karst conduits should avoid farmland. The organic fertilizer and slow-release fertilizer with longer fertilizer effect change and spread appropriate amount of straw returning to the field, which can prolong the conversion period of available nitrogen for plants uptake should encourage to promote in karst area.Overall, the distribution of nitrogen in the water and soil of karst critical zone was highly heterogeneous and more depend on land use, vegetation cover and fertilization. Its temporal variation is more depend on fertilization time and rainfall events. The flushing effect from top soil layer can fully leach nitrate to the groundwater and river through rapid flow. In spite of the fast hydrological response in karst area, significant denitrification in deep soils and groundwater can be observed, which decreased the nitrate concentration. The sensor approach and isotopic utilization in this study supply new insight on nitrate fate in karst critical zone, which will provide fundamental theory to manage catchment water quality.

页数142
语种中文
文献类型学位论文
条目标识符http://ir.gyig.ac.cn/handle/42920512-1/10741
专题研究生
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王忠军. 喀斯特关键带水土介质中氮运移机制研究-以后寨河流域为例[D]. 中国科学院地球化学研究所. 中国科学院大学,2019.
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