【作者】 林啸；

【导师】 刘敏； 许世远；

【作者基本信息】 华东师范大学 ， 自然地理学， 2011， 博士

【摘要】 河口区受到海陆交互作用影响,是各种生物地球化学过程相互作用最为活跃的地带,对人类活动和社会发展有着十分重要的意义。氮是组成生物有机体的主要元素。目前由于大量人为输入的氮素在许多河口海岸带已经产生了诸如赤潮爆发、底层水溶氧锐减以及温室气体排放等重大环境问题。本研究选择在全球河口海岸研究中占据重要地位的密西西比河口和长江河口区作为研究对象,通过野外实测和实验室模拟相结合的研究手段,定量研究了典型河口沉积物-水界面、上覆水柱以及沉积物-气界面氮素的生物地球化学过程,试图阐明河口区不同介质界面氮循环的主要过程和影响因素,探讨氮氧耦合动力学在河口缺氧带形成机理中所扮演的角色。主要取得了以下主要研究成果：以经典的同位素稀释法为理论基础,结合连续流培养模拟和HPLC检测技术,拓展出一种新的沉积物-水界面NH4+-N总循环通量的研究方法,并引入了可描述沉积物氮限制程度的SAD这一概念,丰富了沉积物-水界面NH4+-N循环通量研究。密西西比河口区缺氧站位的NH4+-N总再生通量(REG)与潜在吸收通量(Upot)均高于常氧站位和中间溶氧水平站位,且夏季高于冬季；缺氧季缺氧站点的沉积物潜在NH4+-N需求量(SAD)显著高于常氧站点。各站点缺氧季SAD略高于冬季常氧季对应数据(<70μmol m-2 h-1)。SAD值反映了沉积物-水界面N限制现象非常显著；常氧站点的NH4+消耗过程可能以硝化过程为主,而缺氧站点的NH4+消耗过程以厌氧过程为主。NO3-的消耗过程以反硝化作用(非耦合)为主而非DNRA。脱N2过程均以非耦合反硝化过程(DDNF)为主(>70%),耦合反硝化过程(CDNF)和厌氧氨氧化过程(PANA)的作用十分微弱。缺氧站点的DDNF(?)生脱N2过程中所占的比重更高一些,总反硝化(脱氮)过程(PDNF)和DDNF没有表现出明显的常、缺氧季差异,均主要以DDNF为主要反硝化过程。总体而言,各站点的CDNF和PANA过程均较弱,且没有明显的季节差异和空间差异。缺氧站点固氮能力较弱,缺氧季时几乎没有新氮产生而以脱氮为主要过程。相关分析表明NH4+-N实际吸收通量(Uact)与Upot(?)存在一定的互相依存关系。REG、Uact、Upot、SAD都与底层水溶解氧呈显著线性负相关关系,表明再矿化作用是REG的主要来源而非DNRA,厌氧同化过程是Uact和SAD的主要去向。未观测到各NH4+-N循环通量与叶绿素a有明显的线性相关。NH4+-N和o-PO43-浓度与所有NH4+-N循环通量呈显著正相关,显示了共同的反应底物基础。REG和Uact与NO3--N浓度存在高斯函数相关。沉积物-水界面N2循环通量中只有CDNF与底层水溶解氧呈显著线性负相关关系表明密西西比河口沉积物-水界面N2循环通量影响因素十分独特和复杂,尚需进一步分析。氮氧动力学耦合分析表明沉积物-水界面的主要耗氧过程可能是各种氮循环微生物的呼吸作用。飓风活动强烈的物理扰动削弱了几乎所有的氮循环通量。密西西比河口水柱NH4+-N循环速率具有明显的深度分异规律,存在表层>底层>中层的分布规律和距密西西比河口距离成反比的空间分异规律。缺氧季底层水的低溶氧环境对底层水N循环的影响十分明显。所有站点的Upt均高于Reg,体现了水柱中有一定的氮限制可能性。季节性差异较明显,所有站点Upt和Reg均表现出了明显的冬、夏(常、缺氧季)季节差异,且表层水的季节变异大于中、底层水体。NH4+-N再生和吸收周期也存在底层>中层>表层的深度分异规律,表层水柱的氮同化周期非常短暂,而再矿化周期普遍高于同化周期的现象也暗示了水柱中可能存在氮限制现象。缺氧站点同化和再矿化周期表现为夏季低于冬季,缺氧站点的循环周期高于常氧站点。夏季水柱体现出了DON再生周期长于NH4+-N的循环周期的“氮饥饿”现象。相关分析表明Upt和Reg循环速率都受河口陆源淡水控制,Upt明显受到温度的控制。表层水的NH4+-N的Upt和Reg速率都与盐度呈显著负相关。只有底层水的NH4+-N循环速率与水体溶解氧呈极显著负相关关系,显示了缺氧季底层水的极端低溶氧对底层水中的微生物和浮游生物产生了明显的胁迫。光照模拟实验表明表层水对NH4+-N的利用方式以光合作用为主导的自养同化为主,缺氧站位的表层海水NH4+-N的吸收过程相对于常氧站位有更多的异养微生物以及更多的硝化作用微生物参与。夏季中、底层水Reg的变化趋势和氨肽酶消耗率(AMP)的变化趋势比较一致,表明水柱NH4+-N的再生过程与微生物对ON的水解能力密切相关。“底层锁”效应致使水柱NH4+-N循环速率远低于沉积物-水界面的NH4+-N循环速率,沉积物-水界面NH4+-N循环过程对缺氧带形成影响大于水柱NH4+-N循环过程,常氧环境水柱硝化过程依然对缺氧现象起到一定的促进作用。长江口滨岸潮滩沉积物-气界面N2O排放具有明显的时空变化差异,除BLG和LC表现为微量的吸收外,其他站点N2O排放的平均值均为正值,且淡水控制区域高于咸水控制区域。夏季长江口南岸潮滩的LHK和WSK站点N2O排放的季节变异远高于其他站点。夏季N2O排放远高于其他季节,除夏季表现出从淡水控制区域向咸水控制区域减少的趋势外,其他季节没有体现出明显的沿程分布模式。总体上,长江口潮滩是N2O的排放源。夏季长江口南岸N20排放显著高于杭州湾北岸潮滩,这是由于沉积物反硝化作用或耦合硝化过程的反硝化作用可能是长江口沉积物产生N2O的主要过程。温度、沉积物沙粒含量、沉积物WFPS、夏季沉积物可交换态NH4+-N含量以及沉积物-水界面NH4+-N通量和N2O排放呈显著的正相关关系,较高的温度、较高的沉积物水分含量、较低的沉积物氧化还原电位以及沉积物NH4+-N含量是促进长江口滨岸沉积物-气界面N2O排放的环境因子；有机质对长江口滨岸沉积物-气界面N2O排放还不甚清晰；根据大部分环境因子影响分析我们推测控制长江口滨岸沉积物-气界面N2O排放的主要氮循环过程可能以反硝化作用或DNRA为主,硝化作用的贡献不显著。建立基于主成分分析的N2O排放通量半经验模型,经验证模型的计算值与实测值之间达到了极显著的线性正相关(R=0.63,P=0.0004),说明该模型具有一定的合理性和应用价值。根据该模型估算长江口潮滩湿地沉积物在采样季内的年排放N2O量约为76.6 Mg a-1,对比其他学者的总排放数据表明长江口滨岸潮滩沉积物-气界面N2O排放水平较低,但是由于未将涨潮时沉积物-水界面的排放考虑在内,可能远远低估了长江口滨岸潮滩N2O排放总体水平。更多还原

【Abstract】 Estuarine area which is strongly influenced by land-ocean interaction impact, is the most dynamic zone where kinds of biogeochemical processes occur. It also has a very important significance for human and social development. Nitrogen is the main element of organisms. Numerous anthropogenic nitrogen inputs have produced a bunch of serious environmental issues such as coastal eutrophication, hypoxia zone and nitrous oxide emissions. Therefore, this study set the Mississippi River and the Yangtze river estuarine area as research cases and focused on the nitrogen cycle biogeochemical processes occurred at the sediments-water interface (SWI), water column and sediment-atomosphere interface (SAI). We tried to identify and figure out the main N processes and influence factors, reveal the role of nitrogen-oxygen coupling dynamics in the hypoxia formation mechanism. The following achievements have been obtained through this research:A new approach, combining 15NH4+isotope dilution and continuous-flow techniques, provided estimates of "actual" and "net" NH4+ flux and sediment NH4+ demand (SAD) at the sediment-water interface in Mississippi river estuarine area. The SAD results indicate a rather consistent NH4+ demand at the SWI during the hypoxic season and suggest that reduced nitrogen may limit microbial dynamics in the region. The SAD concept may help us better understand the SWI NH4+-N recycling study.The gross NH4+-N regeneration fluxes (REG) and potential uptake fluxes (Upot) were both higher at hypoxic site than normoxic site and intermediate sites. All gross NH4+-N recycling fluxes were higher in hypoxic season (summer) than normoxic season (winter). The SAD values were significantly higher at hypoxic site than normoxic site when hypoxia. All SAD values were higher in summer than in winter (< 70μmol m-2 h-1). SAD values reflect the significant SWI nitrogen limitation. Nitrification could be the main NH4+-N removal process at normoxic site, but anaerobic processes such as denitrification should be the main NH4+-N removal process at hypoxic site. Direct denitrification (DDNF) but not DNRA should be the main NO3--N removal process. DDNF was the main N2 removal process. Coupled nitrification denitrification (CDNF) and potential anammox (PANA) were both insignificant at SWI. The percentage of DDNF in potential denitrificaition (PDNF) was a bit higher at hypoxic sites. There was no significantly seasonal difference for PDNF and DDNF. CDNF and PANA were generally very low and did not show any spatial or seasonal difference. Nitrogen fixation (NF) was low at hypoxic site and almost no new nitrogen was created by NF in hypoxic season.Actual uptake flux (Uact) and potential uptake flux (Upot) showed some certain interdependent relationship judged by the correlation analysis. There were significantly negative liner relationships between REG, Uact, Upot, SAD and bottom water dissolved oxygen (DO) concentrations which suggesting remineralization was the main process to replenish REG but not DNRA. Anaerobic assimilation was supposed to be the main pathway for uptake processes and SAD. There was no significant relationship between all fluxes and chlorophyll a. Bottom water NH4+-N and O-PO43- concentrations significantly correlated to all SWI NH4+-N fluxes. REG and Uact Gaussian correlated to bottom water NO3--N concentrations. Only exception of correlation analysis between SWI N2 fluxes and Bottom water hydrological characteristics was CDNF which significantly negatively correlated with bottom water DO. Further study need to be conducted since the impact factor research for SWI N cycle is still complex. The hurricane activities almost weakened all SWI N cycle fluxes.There was an obvious depth distribution pattern for water column NH4+-N recycling rate in Mississippi river estuary. Surface recycling rates were higher than bottom ones which were higher than middle ones. The site further to the Mississippi river mouth got lower recycling rates. Bottom water nitrogen recycling processes were deeply influenced by hypoxic DO condition. Potential NH4+-N uptake rates were higher than regeneration rates which indicate the water column nitrogen limitation. Upt and Reg showed significant seasonal differences. Larger seasonal variability was observed at surface than at middle and bottom. The NH4+-N regeneration and uptake turnover times also showed the same depth independent distribution pattern as recycling rates. The turnover time for surface uptake was very shot (< 0.5 d). The observation of longer regeneration turnover times than uptake also suggested the water column limitation phenomenon. The recycling turn overtimes at hypoxic site were lower in summer than in winter, and were higher than the ones at normoxic site. DON regeneration turnover times were longer than NH4+-N recycling rates in summer. This observation showed the typical water column nitrogen starvation.Upt and Reg were all deeply influenced by the freshwater from Mississippi river judged by the correlation analysis. Surface Upt and Reg significantly negatively correlated with salinity. Bottom water recycling rates signifcatnly negatively correlated with DO, suggesting low oxygen stress. Light-dark comparison study indicated that the main process for surface water NH4+-N uptake should be assimilation supported by photosynthesis. More heterotrophic microbes participated in uptake process at hypoxic site than normoxic site. Ammonia peptide enzyme consumption rate (AMP) showed the same pattern as Reg in middle and bottom water in summer indicating that ammoniation process correlated a lot to organic nitrogen hydrolysis ability. Water column NH4+-N recycling rates were far below SWI recycling fluxes which was caused by bottom seal effect. SWI nitrogen processes should play a more important role in hypoxia formation mechanism, but water column nitrification still contribute to oxygen depleting phenomenon at normoxic site or in normoxic season.There was a obvious spatial and temporal pattern in Yangtze river estuarine SAI N2O emission. All sites showed a positive average N2O flux value except at BLG. The sites influenced by fresh water got higher values than the ones influenced by sea water. Emissions at LHK and WSK in summer were far above than other sites in summer. Summer values were higher than ones in other season. The spatial distribution pattern of values depleted from fresh water control area to sea water control area only showed in summer. The Yangtze river estuarine intertidal area generally is the source of N2O emission. Denitrification or coupled denitrification were supposed to be the main generation process of N2O.Significantly correlations were observed between temperature, sediment sand percentage, sediment WFPS, summer sediment exchangeable NH4+-N concentrations, SWI NH4+-N fluxes and N2O emission respectively. It was not clear the relationship between ON and N2O emission. The main generation processes of N2O emission were supposed to be denitrification or DNRA. There was no obvious sign for nitrification contribution to N2O emission.We built a semi-empirical N2O emission model based on principle component analysis which was significantly correlated to the field measured values (R=0.63, P=0.0004). The annual N2O emission was 76.6 Mg in the sampling year based on the model we built. That number was insignificant compared to the total emission in that year from other researches. But this number might be far underestimated since we did not take the SWI N2O emission into consideration while rising tide water logging period.更多还原