Dynamic Change Mechanism Of NH₄⁺ Source-sink In The Groundwater Of Tian-e Zhou Wetland Area

As high concentration of NH₄⁺is widely detected in groundwater worldwide,the hydro-biogeochemical cycle of NH₄⁺and its source-sink problem in groundwater has become a hot spot and a difficult problem in N-cycle research.Wetland groundwater consists of seasonally flooded floodplain aquifers and aquifer groups away from the floodplain that are full of water all year round,which are NH₄⁺enrichment areas and hot zones for nitrogen biogeochemical cycling.Although a lot of useful exploratory studies have been conducted on the migration and enrichment processes of NH₄⁺in groundwater,the wetlands are rich in organic matter and microorganisms,and the hydrological cycle is strong,which makes the nitrogen biogeochemical processes in groundwater very positive and the dynamic changes are difficult to capture.The understanding of the source-sink system and dynamic change mechanism of NH₄⁺in wetland groundwater is still very limited,and a breakthrough in the key scientific issue of"source-sink identification and seasonal mechanism of NH₄⁺in groundwater"is urgently needed.The Tian-e Zhou wetland is a typical mortlake lake type wetland in the middle reaches of the Yangtze River,with a complex history of river and lake evolution,close interaction between surface water and groundwater,and the influence of agriculture,livestock breeding and other human activities,the groundwater with high NH₄⁺content is commonly developed in the area,and there are different characteristics inside and outside the Tian-e Zhou.In view of the above background,this paper focuses on the key scientific problem of"dynamic identification of NH₄⁺source-sink in groundwater and seasonal mode of action in the wet area of mortlake lake in the subtropical monsoon climate zone",takes the Tian-e Zhou wetland as a typical research area,integrates field investigation,biogeochemical seasonal monitoring and indoor simulation incubation experiments,identifies The composition and structure of groundwater of Tian-e Zhou wetland,the spatial and temporal distribution of NH₄⁺in groundwater of Tian-e Zhou wetland and its main controlling factors were analyzed,the dynamic identification criteria and methods of NH₄⁺source-sink in groundwater of Tian-e Zhou wetland were established,the characterization of NH₄⁺source-sink in groundwater was developed,and the seasonal mode of NH₄⁺source-sink in groundwater of Tian-e Zhou wetland was proposed,which provides scientific basis and theoretical support for groundwater nitrogen pollution prevention and control.This study provides scientific basis and theoretical support for the prevention and control of groundwater nitrogen pollution.The innovation of this study is to use a combination of field monitoring and indoor simulated incubation experiments to dynamically identify the source-sink pattern of NH₄⁺in groundwater and its seasonal mode of action in wet areas.The academic contribution of this study is to enrich the understanding of the source-sink dynamics of NH₄⁺in groundwater in wet areas,and the practical contribution is to provide important indications for the prevention and control of NH₄⁺pollution in water bodies.The main research progresses in this paper:1.Based on the analysis of data on NH₄⁺-N content of water bodies,major water chemistry indexes,δ¹⁵N-NH₄⁺,DOM and molecular biology,the composition and structure of groundwater of Tian-e Zhou wetland were identified,and the spatial distribution of NH₄⁺-N in groundwater and its main controlling factors of Tian-e Zhou wetland were analyzed.NH₄⁺-N in groundwater of Tian-e Zhou wetland is high and spatially heterogeneous（0.63-64.00 mg/L）,and the high NH₄⁺-N is mainly distributed in locations with thicker clay layers and outside the wetlands;correlation analysis and random forest regression models indicate that the local ultra-high NH₄⁺-N content in groundwater is geogenic（natural organic matter mineralization）,and some of the high NH₄⁺-N content is controlled by the combination of anthropogenic activities and local strong reducing conditions.The overall DOM content of groundwater shows that the shallow pore is larger than the deep pore,the variability of A-profile is significant and the content in the outer part of the wetland is larger than the inner part;δ¹⁵NH₄⁺shows that the source of NH₄⁺-N is spatially heterogeneous and mixed with multiple sources（-20.72‰～1.21‰）,the NH₄⁺-N in groundwater of the deep pore is of natural origin（organic matter burial）,the middle part of the shallow pore is divided into anthropogenic activity source（mineral fertilizer）,partly natural sources（atmospheric precipitation/surface water）.The relative abundance of microbial communities related to the NH₄⁺-N source-sink process of groundwater from largest to smallest:DNRA,Feammox,Mineralization,Nitrification and Anammox,with the first two accounting for about 95.66%.The relative abundance of nitrogen functional genes ranged from large to small:ure C,Acidimicrobiaceae A6,amo A-AOB,nrf A,amo A-AOA and hzs A,with DOC,Fe²⁺and Eh being the key factors affecting their abundance distribution.Natural organic matter mineralization brings a certain degree of native NH₄⁺-N background values to groundwater,while high levels of NH₄⁺-N are controlled by a combination of human activities and local redox conditions.Locally active agricultural activities lead to enrichment of NH₄⁺-N in shallow groundwater,but it is not easily imported into deep aquifers with overlying clay layers;while in shallow pores without overlying clay layers or near surface water bodies,NH₄⁺-N is imported through surface runoff or the hydrologic cycle,while local redox environmental changes due to the hydrologic cycle stimulate in situ organic matter mineralization,ultimately leading to enrichment of NH₄⁺-N in groundwater.Redox conditions and deep/shallow sediment types act on the NH₄⁺-N source-sink process by influencing the abundance of nitrogen-functioning microorganisms and functional genes,which in turn control the spatial distribution of NH₄⁺-N in groundwater.2.Based on the seasonal monitoring of NH₄⁺-N content in water bodies,major water chemistry indexes,δ¹⁵N-NH₄⁺,DOM and molecular biology data,the temporal variability of NH₄⁺-N source-sink in groundwater and its main controlling factors were analyzed,the criteria and methods for dynamic identification of NH₄⁺-N source-sink in groundwater in the Tian-e Zhou wetland area were established,and the characterization of NH₄⁺-N source-sink in groundwater was developed.There is a natural hydrological cycle in the study area,with surface water recharging groundwater during the abundant period and groundwater draining to surface water during the dry period;the NH₄⁺-N content of groundwater has a spatial and temporal variability distribution（0.30-73.50 mg/L）,and the NH₄⁺-N content in the shallow pore is higher than that in the deep pore and the seasonal variation is more significant;the A1 deep pore has the highest value of NH₄⁺-N content（41.25-73.5mg/L）,while the NH₄⁺-N content in its shallow pore is not high（1.63-6.15 mg/L）.The deep borehole of profile A is dominated by low molecular terrestrial humus;the shallow borehole of profile A is influenced by agricultural cultivation and poultry farming,and is dominated by microbial humus;the deep borehole of profile C（especially the inner part of Swansea）is dominated by microbial humus.The organic matter content in the deep pore of profile C（especially the inner side of the Swan Island）is very low,which is a result of the rapid erosion and accumulation of the Yangtze River over the years resulting in insufficient time for organic matter deposition;the shallow pore of profile C is dominated by the newly deposited low-molecular terrestrial-derived organic matter.profile C generally shows the characteristics of the shallow organic matter being richer than the deeper layer and the outer side being richer than the inner side,and the heterogeneous distribution of DOM is the reason for the seasonal NH₄⁺-N in the groundwater of the shallow pore with a thicker clay layer The seasonal peak of NH₄⁺-N in groundwater of shallow boreholes with thicker clay layers is significantly higher than that of other boreholes.The abundance of nitrogen functional genes showed a gradually increasing trend from the dry period to the wet period in profile A,and the groundwater containing higher content of microbial-derived humus showed more obvious seasonal changes and higher abundance;while in profile C,it showed a trend of rapid increase and then decrease,and overall showed the characteristics of higher abundance in the abundant period than in the dry period.Unlike the unilateral waterfront in profile A,both the Yangtze River and Tian-E Lake flow through profile C.The hydrological cycle results in stronger material and energy exchange,and the boreholes adjacent to surface water have higher contents of low-molecular terrestrial-derived humic substances and NH₄⁺-N.Adequate microbial source/land-based organic matter,active redox environment,rich microbial community and nitrogen functional genes are the keys to the active seasonal variation of NH₄⁺-N source-sink in groundwater.3.Based on the above two field studies,in situ incubation,single-factor control experiments and two-factor simulation experiments of fluctuation of redox conditions and input of exogenous organic matter were conducted indoors using short-term flooding incubation method and groundwater simulation solution for C3 powder sand,C3 fine sand,C6 powder sand and C6 fine sand for the main control factors（sediment type,redox conditions and exogenous organic matter）to reveal the dynamic mechanism of NH₄⁺source-sink in groundwater and propose the seasonal mode of action of NH₄⁺source-sink in groundwater in wet area of ox-yoke lake in subtropical monsoon climate zone.Under the conditions of stable redox environment and no exogenous organic matter input,the peak NH₄⁺release from different sediments under in situ aerobic/anaerobic conditions did not differ significantly（0.19 mg/L;0.20 mg/L）.Exogenous glucose addition significantly promoted the release of NH₄⁺,and the aerobic conditions were more favorable for microbial metabolism using glucose to produce more NH₄⁺（peak ammonium nitrogen release under aerobic/anaerobic plus glucose:1.46 mg/L and 1.33mg/L）;sediment NH₄⁺release under aerobic conditions was jointly controlled by DOC and TN,and the main processes were mineralization,anammox,DNRA and nitrification Under the anaerobic condition,the main processes were DNRA,feammox and nitrification due to the abundance of native organic matter and exogenous organic matter in the sediment.The exogenous tryptophan promoted the release of NH₄⁺more than glucose,and the aerobic conditions were more favorable for microorganisms to use tryptophan for metabolism,which made the release of NH₄⁺much higher than other experiments（NH₄⁺release from aerobic/anaerobic plus tryptophan:22.91 mg/L and2.92 mg/L）.Under aerobic conditions,as the carbon source far exceeded the amount of nitrogen source,the release limit of NH₄⁺was controlled by TN,and the main processes were mineralization and DNRA;under anaerobic conditions,although organic matter was sufficient,anoxic conditions restricted the utilization of organic matter by microorganisms,making the release of NH₄⁺much lower than the aerobic condition,and the main processes were mineralization,feammox and anammox;Under the condition of sufficient organic matter and electron donor,DNRA has a significant enrichment effect on NH₄⁺;tryptophan as a marker of domestic wastewater,its contribution to the high NH₄⁺content cannot be ignored in the shallow groundwater that is vulnerable to human activities and oxygen input due to hydrological cycle.Due to the limited amount of organic matter in the experimental system,NH₄⁺was generally more consumed than enriched,and the seasonal variation was favorable to the"sink"of NH₄⁺,and the process of DNRA in the groundwater environment with fluctuating redox conditions might be the key to NH₄⁺enrichment.The seasonal variation is favorable to the"sink"of NH₄⁺.In the two-factor simulation experiment with fluctuating redox conditions and exogenous organic matter input,the DNRA was very strong（peak NH₄⁺release 2.92 mg/L）,which significantly contributed to the enrichment of NH₄⁺compared with the fluctuating redox conditions（peak NH₄⁺release 0.16 mg/L）.Although anammox also responded significantly to the removal of NH₄⁺,the exogenous organic matter input from seasonal changes would intensify the mineralization process while producing a portion of NH₄⁺,making the groundwater still maintain a high concentration of NH₄⁺.