东海沉积物中氮循环的关键过程
摘要
氮元素作为海洋环境中的重要生源要素之一，通常被认为是初级生产力的限制因子，加之全球氮循环与碳循环在气候变化中的密切耦合以及人类活动对于氮元素排放的逐渐增加，海洋氮循环也就成为整个海洋生物地球化学循环中最为关键的一环。海洋中化合态氮的收支取决于游离态氮气的固定和化合态氮的丢失（N-loss），前者主要由固氮生物执行，而后者主要由反硝化细菌和厌氧铵氧化细菌所实现。由于海洋氮循环受到多种过程的控制以及在量化各种过程速率方面存在很大的不确定性，全球氮的收支是否处于平衡仍然存在很大的争议。海洋沉积物是化合态氮丢失的一个重要场所，约50~70%的化合态氮在此被移除，而其中50%以上的氮丢失又发生在陆架海沉积物中(Bohlen et al.2012; Gruber2008)，尽管该区域只占全球海洋面积的7.5%(Menard&Smith1966)。由此可见陆架海沉积物在全球海洋氮循环中所起的关键性作用。
反硝化、厌氧铵氧化和异化硝酸盐还原为铵（DNRA）是沉积物厌氧环境中氮循环的三种关键过程。前两个过程控制着沉积物中化合态氮的丢失，而DNRA则将NO_3~-转化为NH_4~+使得化合态氮在沉积物中得以保留并继续参与氮循环过程。反硝化是指在厌氧微生物作用下，从NO_3~-开始，经过一系列的异化还原反应，将NO_3~-，NO_2~-，NO和N_2O最终还原为游离态的N_2的过程，从而实现生态系统中化合态氮的移除。厌氧铵氧化是指在厌氧条件下，无机化能自养细菌以NO_2~-为电子受体，NH_4~+为电子给体的微生物氧化还原过程，此过程的最终产物同样是游离态的N_2。DNRA过程是指NO_3~-在异化硝酸盐还原细菌的作用下，经过一系列的还原过程将NO_3~-直接还原为NH_4~+的转化过程，并未实现氮丢失。三种过程在沉积物中的配比程度决定了沉积物中氮的收支，但同时对这三种过程进行的研究目前报道尚不多见。
东海作为世界上最大的陆架边缘海之一，有着较高的初级生产力。同时，西面受到世界最大河流之一—长江的输入，东南又有大洋性质的黑潮水入侵，因此使其成为最为典型的陆架海之一。由于长江流域和长江三角洲地区人类活动的加剧，大量工农业废水和生活污水随长江冲淡水输送到东海陆架，使东海近岸海区的化合态氮浓度显著升高，导致赤潮频发，且长江口外底层水体缺氧现象愈发严重。据推测，东海陆架沉积物可能是化合态氮丢失的一个“热点区域”(Seitzingeret al.2006)，但是目前对于此方面的实测研究还非常匮乏。
~(15)N同位素对技术（~(15)N Isotope Pairing Technique，简记为~(15)N IPT）已经成为研究沉积物氮循环过程的一个有效手段，并得到广泛应用。但现有的~(15)N IPT只考虑到了一种（反硝化）或两种（反硝化和厌氧铵氧化、厌氧铵氧化和DNRA）氮的转化过程(Jensen et al.2011; Nielsen1992; Risgaard-Petersen et al.2003;Thamdrup&Dalsgaard2002)，如果同时存在三种过程，且每种过程的贡献均不可忽略，则此种方法在计算每个转化过程的各自贡献量时面临着挑战。因为DNRA所产生的~(15)NH_4~+与~(15)NO_2~-结合同样可以产生30N_2，但传统上认为该产物只能由反硝化产生，这样使得现有的计算方法不能有效区分各自过程的贡献。尽管亦有方法可以解决在DNRA与厌氧铵氧化和反硝化共存条件下区分各自的贡献(Spott&Stange,2007)，但需要建立开放的稳态实验体系，在海洋沉积物研究中尚未广泛应用。同时沉积物中细胞内硝酸盐储存生物的存在对于~(15)N IPT也产生一定的影响(Sokoll et al.,2012)，多重过程的共同存在无疑增加了研究沉积物氮循环的复杂性。
有鉴于此，本文首先从理论上量化了DNRA对厌氧铵氧化和反硝化速率计算的影响，同时提出了3种方法来校正或者消除DNRA的影响，并用实测数据对该数学模型进行了验证。其次，本文利用泥浆培养考虑到DNRA的影响后深入研究了东海陆架沉积物中厌氧铵氧化、反硝化和DNRA过程以及各自在东海沉积物氮循环过程的相对贡献。然后，在对东海沉积物氮循环过程有了初步了解的基础上，结合沉积物泥浆和整柱培养技术测定了东海沉积物中的氮丢失速率。此外，因夏季长江口底层水体缺氧已被广泛报道，且底栖氮循环又与氧气水平密切相关，故利用两个航次的对比和受控实验探讨了底层水体缺氧对沉积物氮循环的影响。基于以上研究所获得的主要认识如下：
（1）基于~(15)N IPT的原理，在考虑到DNRA的影响之后，发展了一个量化的模型用于评估DNRA对于厌氧铵氧化和反硝化的影响。模型结果表明：在DNRA与厌氧铵氧化和反硝化共存的情况下，沉积物泥浆培养使用Thampdrup&Dalsgaard (2002)提出的方法所计算出的厌氧铵氧化速率被低估，且这种低估与体系中~(15)NH_4~+的摩尔分数（FA）成正比；反硝化速率被高估，高估的程度与FA和厌氧铵氧化对总的氮丢失的贡献（ra）有关。在ra相对固定时，对反硝化速率的高估同样与FA成正比；在FA相对固定时，对反硝化速率的高估随着ra的增加而增大。而DNRA对于总的氮丢失速率并没有影响，这主要是因为在泥浆培养中加入了~(15)NO_3~-使得硝酸盐不再是硝酸盐异化还原的限制因子，同时也说明DNRA的存在对于厌氧铵氧化的低估和对于反硝化的高估二者可以抵消，DNRA只是改变了~(15)N在各个过程中的配对形式并未对总的氮丢失产生影响。而对于整柱培养来说，在DNRA存在的情况下，按照Risgaard-Petersen等(2003)的计算方法计算出的氮丢失速率将会被高估，因为DNRA在硝酸盐还原层中与反硝化和厌氧铵氧化存在竞争关系，消耗了一部分硝酸盐从而降低了真实的氮丢失速率。针对泥浆培养过程中DNRA的影响，本文提出了3种方法用于校正或者消除该影响。在第一种方法中，采用分步计算方式将泥浆培养过程每个时间点所获取的FA考虑在内，得到每个时间点由厌氧铵氧化产生的30N_2的量，然后再分步计算每个时间点由厌氧铵氧化和反硝化所产生的实际的N_2量；在第二种方法中，如果FA随时间呈现线性增加，则采用平均的FA来计算由厌氧铵氧化贡献的30N_2产生速率，从而计算厌氧铵氧化和反硝化速率；在第三种方法中，泥浆培养加入~(15)NO_3~-的同时，再加入较高浓度的14NH_4~+以保证FA处于较低的水平（<5%）来消除DNRA的影响。利用黄海和东海的实测数据验证了该模型的合理性，且三种方法均可用来校正或者消除DNRA对厌氧铵氧化和反硝化的影响。同时该模型的结果表明在ra比较低（<10%）的河口与近岸沉积物中，DNRA的影响可以不予考虑，但根据现有的ra分布规律来看，随着水深的增加，ra也逐渐增加，因此在ra比较大的陆架和陆坡沉积物中，DNRA的影响则必须要谨慎对待。此外，该模型不仅仅可以用于沉积物中校正DNRA对厌氧铵氧化和反硝化的影响，也可以在大洋缺氧水体中使用。
（2）针对东海沉积物中所存在的硝酸盐还原过程的区分与量化，于2010年6月搭乘科学3号调查船在东海进行了5个站位的沉积物泥浆厌氧培养实验。~(15)NH_4~+、~(15)NH_4~++~(14)NO_3~-和~(15)NO_3~-分别作为~(15)N示踪剂加入培养体系中，通过检测不同的N_2同位素产物来判别不同的硝酸盐还原过程。实验结果表明加入~(15)NH_4~+的控制组，在O_2和NO_3~-均消耗殆尽的条件下，既无29N_2的生成亦无30N_2的产生，说明无有效的电子受体对~(15)NH_4~+进行厌氧氧化；而在加入~(15)NH_4~++~(14)NO_3~-的培养体系中，仅检测到了29N_2的产生，并无显著的30N_2产生，说明在所研究的沉积物中存在显著的厌氧铵氧化过程；在加入~(15)NO_3~-的培养体系中，显著的30N_2生成表明反硝化过程的存在，同时~(15)NH_4~+的产生证明DNRA过程也显著存在。通过对比~(15)NH_4~++~(14)NO_3~-和~(15)NO_3~-培养组的厌氧铵氧化速率，表明细胞内硝酸盐储存与释放过程也同时存在。在加入~(15)NO_3~-的培养体系中，在使用目前通用的Thampdrup&Dalsgaard (2002)提出的方法进行计算时，细胞内硝酸盐的释放会稀释~(15)NO_3~-，从而造成反硝化速率被低估，厌氧铵氧化速率被高估；而DNRA会造成反硝化速率被高估，而厌氧铵氧化速率被低估，从而使厌氧铵氧化在氮丢失过程中的贡献被低估。两种影响同时存在且作用相反，增加了计算过程的复杂性。将细胞内硝酸盐释放与DNRA的影响同时进行考虑后，结果表明，按照目前通用的Thampdrup&Dalsgaard (2002)提出的计算方法，在加入~(15)NO_3~-的培养体系中，如果仅仅考虑硝酸盐的释放，东海沉积物中反硝化速率将会被低估6%，而厌氧铵氧化速率将会被高估42%。考虑DNRA的影响之后，反硝化速率的低估将会被抵消，而厌氧铵氧化速率的高估程度可被降低14%。在加入~(15)NH_4~++~(14)NO_3~-的培养体系中，由于高浓度的~(15)NH_4~+加入作为背景，DNRA对厌氧铵氧化速率无显著影响。经过校正计算出的反硝化、厌氧铵氧化和DNRA速率表明，厌氧铵氧化在氮丢失过程中的贡献由较浅近岸的13%上升到较深外海的50%，说明厌氧铵氧化在东海沉积物氮丢失过程中起着重要作用。同时DNRA在硝酸盐还原过程中也起着重要的作用，其所占据的比例高达20~31%。
（3）在初步了解东海沉积物中反硝化、厌氧铵氧化和DNRA过程的基础上，利用沉积物泥浆和整柱培养相结合的~(15)N同位素对技术分别于2010年6月和10~11月测定了东海沉积物中氮丢失速率。利用Risgaard-Petersen等(2003)所建议的方法计算出的氮丢失速率处于0.13到0.85mmol N m~(-2)d~(-1)，平均0.37mmol N m~(-2)d~(-1)，其中厌氧铵氧化的贡献处于11~50%，平均24%。DNRA的速率处于0~0.05mmol N m~(-2)d~(-1)，平均0.02mmol N m~(-2)d~(-1)。利用Fick第一扩散定律的计算并结合文献数据表明，单纯使用沉积物柱培养形式的~(15)N IPT计算出的氮丢失和DNRA速率均存在一定程度的低估。在柱培养实验中，加入的~(15)NO_3~-受到扩散限制，在通常所进行的培养时间内（小于1天）~(15)NO_3~-最多渗透至表层1cm的沉积物中而不能够充分地与深层次间隙水中的~(14)NO_3~-充分混合，从而使得利用现有~(15)N IPT计算出的氮丢失速率存在低估。利用沉积物分层泥浆培养实验，计算了~(15)NO_3~-渗透深度以下硝酸盐还原层中的氮丢失速率，将其与前面计算出的氮丢失速率加和即为实际的氮丢失速率。该氮丢失速率与利用Risgaard-Petersen等(2003)方法计算出的氮丢失速率相比提高了0.8~2.2倍，平均1.6倍，推广到整个东海陆架则每年增加的氮丢失量大约有1.6TgN。校正后的DNRA平均速率为0.29mmol N m~(-2)d~(-1)，提升了1个数量级。（·4）利用2011年5月和8月在长江口及其临近海区的两个航次探讨了底层水体缺氧对于沉积物中氮循环的影响。在该实验中，沉积物间隙水中的溶解氧和硝酸盐剖面、沉积物耗氧速率、NO_3~-和NH_4~+交换通量、厌氧铵氧化速率、反硝化速率、DNRA速率、硝化速率和矿化速率均在不同溶解氧条件下进行了测定，既考虑到了不同季节所造成的底层水体天然氧气含量变化对底栖氮循环的影响，同时又探讨了在受控培养的溶解氧条件下底栖氮循环的变化。结果表明，5月份的底层水体溶解氧含量一般处于~200μmolL~(-1)，沉积物溶解氧渗透深度处于4.0~4.3mm，耗氧速率处于11.6~17.6mmol O_2m~(-2)d~(-1)。由于调查前受强台风的影响，8月份底层水体中的溶解氧未出现缺氧现象（DO<62.5μmol L~(-1)），但与5月份相比，底层水体溶解氧降低至~100μmol L~(-1)，沉积物耗氧速率降低至6.1~13.6mmol O_2m~(-2)d~(-1)，氧气渗透深度则减小到1.6~3.8mm，沉积物耗氧速率和氧气渗透深度分别降低了23%和29%。伴随着氧气浓度的降低，NO_3~-剖面所表现出的硝化层消失，NO_3~-趋向于由水体向沉积物转移，而NH_4~+则趋向于由沉积物向水体释放。厌氧铵氧化速率由0.15mmol N m~(-2)d~(-1)降低至0.06mmol N m~(-2)d~(-1)，对总的氮丢失的贡献平均由20%降低至7.4%。反硝化速率表现出轻微的上升使得总的氮丢失速率维持在0.85mmol N m~(-2)d~(-1)左右。DNRA速率则由0.02mmol N m~(-2)d~(-1)上升至0.10mmol N m~(-2)d~(-1)，平均升高了5倍。由于在8月份的航次中没有发现天然的缺氧现象，因此通过受控培养研究了在不同氧条件下（氧化状态、原位氧状态和严重缺氧状态）底栖氮循环对于水体缺氧的响应，结果表明，当底层水体溶解氧由氧化状态（DO=~200μmol L~(-1)）降低至严重缺氧状态（DO=~16μmol L~(-1)）时，沉积物耗氧速率急剧降低，平均降低了91%。NH_4~+由沉积物向水体强烈地释放，而NO_3~-则由氧化条件下的从沉积物向水体释放（0.14mmol N m~(-2)d~(-1)）转变为急剧地向沉积物转移（-0.79mmol N m~(-2)d~(-1)）。与底层水体氧化状态相比，厌氧铵氧化和反硝化在严重缺氧条件下分别降低了38%和43%，从而使总的氮丢失速率由0.92mmol N m~(-2)d~(-1)降低至0.57mmol N m~(-2)d~(-1)。DNRA速率在严重缺氧状态下增加了3倍。反硝化、厌氧铵氧化和DNRA速率的降低均可归因于严重缺氧条件下沉积物有机氮矿化速率的降低，进而导致硝化速率降低，从而使得与硝化反应相耦合的各种硝酸盐还原过程的速率均表现出不同程度的降低。
Nitrogen plays a vital role in regulating global marine primary productivity andconsequently climate change. Its availability depends on the balance between nitrogenfixation through diazotrophs and fixed nitrogen loss via denitrification and anammox.However, whether marine nitrogen budget is in balance or not is still on debate. Thisis due to the scarcity of direct measurements for each process of marine nitrogen cycleand its complicated composition. Thus accurate rate measurement and nitrogentransformation process confirmation determine marine nitrogen budget to a largeextent. Marine sediment accounts for50~70%of global marine N-loss, and thus playsa dominant role. Among N-loss from sediment, continental shelf accounts for morethan50%of total benthic N-loss although it only comprises7.5%of the total seafloor.The East China Sea (ECS) shelf is one of the widest and most productive shelves. Itwas speculated that the ECS would be a hotspot for benthic N-loss; however, poorlystudies of benthic N-loss were conducted in this area. The aims of this thesis are toidentify the specific nitrogen transformation processes, to distinguish respectivecontribution to benthic nitrogen transformation, and to determine the extent of benthicN-loss on the ECS shelf. In this thesis, we chose the prevalent~(15)N isotope pairingmethod (~(15)N IPT) to investigate the benthic nitrogen cycle in the ECS from both fieldmeasurements and mathematical model. Furthermore, both sediment slurry and intactcore incubations were conducted in the field measurement to give a full understandingof benthic nitrogen cycle. Preliminary results were as follows:
(1) The application of isotope pairing technique (IPT) in sediments whereanammox, denitrification and DNRA coexist was discussed in detail basedon a mathematical model. The mathematical expression demonstrated thatco-occurrence of DNRA with anammox and denitrification would change~(15)N combination in anammox. Coexistence of DNRA would underestimateanammox and overestimate denitrification if their rates were calculatedaccording to the equations given by Thamdrup and Dalsgaard (2002) forslurry incubation. Therefore, ra was underestimated. The underestimation ofanammox was proportional to~(15)NH_4~+fraction in NH_4~+pools (FA); however,the overestimation of denitrification was related to both FAand ra. For intact core incubation, genuine N_2production was decreased since DNRA wouldcompete with anammox and denitrification under nitrate limiting. Threealternative procedures were proposed to correctly quantify anammox,denitrification and DNRA rates in slurry incubation. The first two methodsbased on estimating30N_2production via anammox. The third one based onconstraining FAto lower than5%in the way of elevating14NH_4~+concentration. The validity and feasibility of the proposed calculationprocedures were verified by field studies in the ECS and Yellow Sea. Ourresults implied that DNRA did not influence anammox and denitrificationsignificantly in estuarine sediment with relative low ra (<10%), however,those sediments with high ra and DNRA rate should be treated verycautiously for future studies and DNRA was strongly recommended to bemeasured.
(2) Benthic nitrogen transformation pathways were investigated in the sedimentof the East China Sea (ECS) in June of2010using the~(15)N isotope pairingtechnique. Slurry incubations indicated that denitrification, anammox anddissimilatory nitrate reduction to ammonium (DNRA) as well as intracellularnitrate release occurred in the ECS sediments. These four processes did notexist independently, nitrate release therefore diluted the~(15)N labeling fractionof NO_3~-–, and a part of the~(15)NH_4~+derived from DNRA also formed30N_2viaanammox. Hence current methods of rate calculations led to over andunderestimations of anammox and denitrification respectively. Following theprocedure outlined in Thamdrup and Dalsgaard (2002), denitrification rateswere slightly underestimated by an average6%without regard to the effectof nitrate release, while this underestimation could be counteracted by thepresence of DNRA. On the contrary, anammox rates calculated from~(15)NO_3~-–experiment were significantly overestimated by42%without consideringnitrate release. In our study, this overestimation could only be compensated14%by taking DNRA into consideration. In a parallel experiment amendedwith~(15)NH_4~++~(14)NO_3~-–, anammox rates were not significantly influenced byDNRA due to the high background of~(15)NH_4~+addition. The significantcorrelation between potential denitrification rate and sediment organic mattercontent (r=0.68, p<0.001, Pearson) indicated that denitrification wasregulated by organic matter, while, no such correlations were found for anammox and DNRA. The relative contribution of anammox to the totalN-loss increased from13%at the shallowest site near the Changjiang estuaryto50%at the deepest site on the outer shelf, implying the significant role ofanammox in benthic nitrogen cycling in the ECS sediments, especially on theouter shelf. N-loss as N_2was the main pathway, while DNRA was also animportant pathway accounting for20~31%of benthic nitrate reduction in theECS. Our study demonstrates the complicated interactions among differentbenthic nitrogen transformations and the importance of consideringdenitrification, DNRA, anammox and nitrate release together when designingand interpreting future studies.
(3) Benthic N-loss and DNRA rates on the ECS continental shelf were directlymeasured in two cruises in June and October-November2010usingprevalent~(15)N isotope pairing technique (IPT) through sediment intact coreand slurry incubations. Measured benthic N-loss rate ranged from0.13to0.85mmol N m~(-2)d~(-1)with an average of0.37mmol N m~(-2)d~(-1)using revisedIPT suggested by Risgaard-Petersen (2003), where anammox accounts for11~49%with an average of24%. DNRA ranged from0to0.05mmol N m~(-2)d~(-1)with an average of0.02mmol N m~(-2)d~(-1). According to the Fick’s first law,we speculated that just a pseudo-steady state was established for~(15)NO_3~-when IPT was employed.~(15)NO_3~-could not diffuse more than1cm insediment, which caused a substantial part of benthic N-loss missingcontributed by NO_3~-below~(15)NO_3~-penetration zone. Furthermore, severalevidence from re-examination based on published literature also supportedthat benthic N-loss was underestimated when IPT was applied. After addingthe missing benthic N-loss based on sediment slurry incubation, total benthicN-loss increased by a factor of0.8~2.2with an average of1.6and DNRAincreased by1order of magnitude to an average of0.29mmol N m~(-2)d~(-1)when the missing benthic N-loss based on sediment slurry incubation wasadded. The additional benthic N-loss was calculated with an average of1.6Tg N a-1in the East China Sea. If this additional N-loss could be extrapolatedto the global continental shelves, then an excess of80Tg N a-1can then beadded to the global marine nitrogen loss budget, accounting for52±23%ofthe global marine benthic nitrogen loss, which further aggravates theunbalance between nitrogen fixation and loss in marine environment.
(4) The response of benthic nitrogen cycle to bottom water hypoxia wassynthetically investigated in the Changjiang estuary and adjacent area in twocruises in May and August2011. In this study benthic oxygen uptake,sediment oxygen profile, net flux of nitrate and ammonium, anammox,denitrification, DNRA, nitrification and mineralization were studied using~(15)N isotope pairing and O_2microelectrode techniques. Sediment oxygenuptake and oxygen penetration depth averagely decreased23%and29%witha natural50%decline of bottom water oxygen from~200μM in May to~100μM in August. NH_4~+inclined to release from sediment to overlyingwater, while NO_3~-was prone to shifting from efflux to influx to sediment. rasignificantly decreased from20%to7.4%, leading to anammox ratedecreased by a factor of2.5from0.15to0.06mmol N m~(-2)d~(-1). However,denitrification showed a slight increase causing total benthic N-loss stablizedat0.85mmol N m~(-2)d~(-1). DNRA showed a significant increase by a factor of5from0.02to0.10mmol N m~(-2)d~(-1). These changes were consistent with theresults derived from artificially bottom water oxygen condition controlledexperiments (oxic, ambient and severe hypoxia): sediment oxygen uptakedecreased by as much as91%when bottom water oxygen dropped92%fromnormal oxic condition (~200μM) to severe hypoxia (~16μM). NH_4~+showedan enhanced release to overlying water from~0to0.60mmol N m~(-2)d~(-1)andNO_3~-shifted from an efflux of0.14mmol N m~(-2)d~(-1)to an influx of0.79mmol N m~(-2)d~(-1). Denitrification and anammox showed an average decreaseof38%and43%under severe hypoxia, leading to total benthic N-lossdecline of38%from0.92to0.57mmol N m~(-2)d~(-1). DNRA showed anelevation by a factor of3although it only accounted for less than10%oftotal nitrate reduction.
Our work in the ECS provided preliminary understanding of benthic nitrogentransformations, but there is still a long way to go to exhibit a comprehensive andintensive picture of nitrogen cycle due to its inherent complexity. With the boomingdiscoveries in the field of marine nitrogen cycle, studies will surely face numerousopportunities and challenges.
反硝化、厌氧铵氧化和异化硝酸盐还原为铵（DNRA）是沉积物厌氧环境中氮循环的三种关键过程。前两个过程控制着沉积物中化合态氮的丢失，而DNRA则将NO_3~-转化为NH_4~+使得化合态氮在沉积物中得以保留并继续参与氮循环过程。反硝化是指在厌氧微生物作用下，从NO_3~-开始，经过一系列的异化还原反应，将NO_3~-，NO_2~-，NO和N_2O最终还原为游离态的N_2的过程，从而实现生态系统中化合态氮的移除。厌氧铵氧化是指在厌氧条件下，无机化能自养细菌以NO_2~-为电子受体，NH_4~+为电子给体的微生物氧化还原过程，此过程的最终产物同样是游离态的N_2。DNRA过程是指NO_3~-在异化硝酸盐还原细菌的作用下，经过一系列的还原过程将NO_3~-直接还原为NH_4~+的转化过程，并未实现氮丢失。三种过程在沉积物中的配比程度决定了沉积物中氮的收支，但同时对这三种过程进行的研究目前报道尚不多见。
东海作为世界上最大的陆架边缘海之一，有着较高的初级生产力。同时，西面受到世界最大河流之一—长江的输入，东南又有大洋性质的黑潮水入侵，因此使其成为最为典型的陆架海之一。由于长江流域和长江三角洲地区人类活动的加剧，大量工农业废水和生活污水随长江冲淡水输送到东海陆架，使东海近岸海区的化合态氮浓度显著升高，导致赤潮频发，且长江口外底层水体缺氧现象愈发严重。据推测，东海陆架沉积物可能是化合态氮丢失的一个“热点区域”(Seitzingeret al.2006)，但是目前对于此方面的实测研究还非常匮乏。
~(15)N同位素对技术（~(15)N Isotope Pairing Technique，简记为~(15)N IPT）已经成为研究沉积物氮循环过程的一个有效手段，并得到广泛应用。但现有的~(15)N IPT只考虑到了一种（反硝化）或两种（反硝化和厌氧铵氧化、厌氧铵氧化和DNRA）氮的转化过程(Jensen et al.2011; Nielsen1992; Risgaard-Petersen et al.2003;Thamdrup&Dalsgaard2002)，如果同时存在三种过程，且每种过程的贡献均不可忽略，则此种方法在计算每个转化过程的各自贡献量时面临着挑战。因为DNRA所产生的~(15)NH_4~+与~(15)NO_2~-结合同样可以产生30N_2，但传统上认为该产物只能由反硝化产生，这样使得现有的计算方法不能有效区分各自过程的贡献。尽管亦有方法可以解决在DNRA与厌氧铵氧化和反硝化共存条件下区分各自的贡献(Spott&Stange,2007)，但需要建立开放的稳态实验体系，在海洋沉积物研究中尚未广泛应用。同时沉积物中细胞内硝酸盐储存生物的存在对于~(15)N IPT也产生一定的影响(Sokoll et al.,2012)，多重过程的共同存在无疑增加了研究沉积物氮循环的复杂性。
有鉴于此，本文首先从理论上量化了DNRA对厌氧铵氧化和反硝化速率计算的影响，同时提出了3种方法来校正或者消除DNRA的影响，并用实测数据对该数学模型进行了验证。其次，本文利用泥浆培养考虑到DNRA的影响后深入研究了东海陆架沉积物中厌氧铵氧化、反硝化和DNRA过程以及各自在东海沉积物氮循环过程的相对贡献。然后，在对东海沉积物氮循环过程有了初步了解的基础上，结合沉积物泥浆和整柱培养技术测定了东海沉积物中的氮丢失速率。此外，因夏季长江口底层水体缺氧已被广泛报道，且底栖氮循环又与氧气水平密切相关，故利用两个航次的对比和受控实验探讨了底层水体缺氧对沉积物氮循环的影响。基于以上研究所获得的主要认识如下：
（1）基于~(15)N IPT的原理，在考虑到DNRA的影响之后，发展了一个量化的模型用于评估DNRA对于厌氧铵氧化和反硝化的影响。模型结果表明：在DNRA与厌氧铵氧化和反硝化共存的情况下，沉积物泥浆培养使用Thampdrup&Dalsgaard (2002)提出的方法所计算出的厌氧铵氧化速率被低估，且这种低估与体系中~(15)NH_4~+的摩尔分数（FA）成正比；反硝化速率被高估，高估的程度与FA和厌氧铵氧化对总的氮丢失的贡献（ra）有关。在ra相对固定时，对反硝化速率的高估同样与FA成正比；在FA相对固定时，对反硝化速率的高估随着ra的增加而增大。而DNRA对于总的氮丢失速率并没有影响，这主要是因为在泥浆培养中加入了~(15)NO_3~-使得硝酸盐不再是硝酸盐异化还原的限制因子，同时也说明DNRA的存在对于厌氧铵氧化的低估和对于反硝化的高估二者可以抵消，DNRA只是改变了~(15)N在各个过程中的配对形式并未对总的氮丢失产生影响。而对于整柱培养来说，在DNRA存在的情况下，按照Risgaard-Petersen等(2003)的计算方法计算出的氮丢失速率将会被高估，因为DNRA在硝酸盐还原层中与反硝化和厌氧铵氧化存在竞争关系，消耗了一部分硝酸盐从而降低了真实的氮丢失速率。针对泥浆培养过程中DNRA的影响，本文提出了3种方法用于校正或者消除该影响。在第一种方法中，采用分步计算方式将泥浆培养过程每个时间点所获取的FA考虑在内，得到每个时间点由厌氧铵氧化产生的30N_2的量，然后再分步计算每个时间点由厌氧铵氧化和反硝化所产生的实际的N_2量；在第二种方法中，如果FA随时间呈现线性增加，则采用平均的FA来计算由厌氧铵氧化贡献的30N_2产生速率，从而计算厌氧铵氧化和反硝化速率；在第三种方法中，泥浆培养加入~(15)NO_3~-的同时，再加入较高浓度的14NH_4~+以保证FA处于较低的水平（<5%）来消除DNRA的影响。利用黄海和东海的实测数据验证了该模型的合理性，且三种方法均可用来校正或者消除DNRA对厌氧铵氧化和反硝化的影响。同时该模型的结果表明在ra比较低（<10%）的河口与近岸沉积物中，DNRA的影响可以不予考虑，但根据现有的ra分布规律来看，随着水深的增加，ra也逐渐增加，因此在ra比较大的陆架和陆坡沉积物中，DNRA的影响则必须要谨慎对待。此外，该模型不仅仅可以用于沉积物中校正DNRA对厌氧铵氧化和反硝化的影响，也可以在大洋缺氧水体中使用。
（2）针对东海沉积物中所存在的硝酸盐还原过程的区分与量化，于2010年6月搭乘科学3号调查船在东海进行了5个站位的沉积物泥浆厌氧培养实验。~(15)NH_4~+、~(15)NH_4~++~(14)NO_3~-和~(15)NO_3~-分别作为~(15)N示踪剂加入培养体系中，通过检测不同的N_2同位素产物来判别不同的硝酸盐还原过程。实验结果表明加入~(15)NH_4~+的控制组，在O_2和NO_3~-均消耗殆尽的条件下，既无29N_2的生成亦无30N_2的产生，说明无有效的电子受体对~(15)NH_4~+进行厌氧氧化；而在加入~(15)NH_4~++~(14)NO_3~-的培养体系中，仅检测到了29N_2的产生，并无显著的30N_2产生，说明在所研究的沉积物中存在显著的厌氧铵氧化过程；在加入~(15)NO_3~-的培养体系中，显著的30N_2生成表明反硝化过程的存在，同时~(15)NH_4~+的产生证明DNRA过程也显著存在。通过对比~(15)NH_4~++~(14)NO_3~-和~(15)NO_3~-培养组的厌氧铵氧化速率，表明细胞内硝酸盐储存与释放过程也同时存在。在加入~(15)NO_3~-的培养体系中，在使用目前通用的Thampdrup&Dalsgaard (2002)提出的方法进行计算时，细胞内硝酸盐的释放会稀释~(15)NO_3~-，从而造成反硝化速率被低估，厌氧铵氧化速率被高估；而DNRA会造成反硝化速率被高估，而厌氧铵氧化速率被低估，从而使厌氧铵氧化在氮丢失过程中的贡献被低估。两种影响同时存在且作用相反，增加了计算过程的复杂性。将细胞内硝酸盐释放与DNRA的影响同时进行考虑后，结果表明，按照目前通用的Thampdrup&Dalsgaard (2002)提出的计算方法，在加入~(15)NO_3~-的培养体系中，如果仅仅考虑硝酸盐的释放，东海沉积物中反硝化速率将会被低估6%，而厌氧铵氧化速率将会被高估42%。考虑DNRA的影响之后，反硝化速率的低估将会被抵消，而厌氧铵氧化速率的高估程度可被降低14%。在加入~(15)NH_4~++~(14)NO_3~-的培养体系中，由于高浓度的~(15)NH_4~+加入作为背景，DNRA对厌氧铵氧化速率无显著影响。经过校正计算出的反硝化、厌氧铵氧化和DNRA速率表明，厌氧铵氧化在氮丢失过程中的贡献由较浅近岸的13%上升到较深外海的50%，说明厌氧铵氧化在东海沉积物氮丢失过程中起着重要作用。同时DNRA在硝酸盐还原过程中也起着重要的作用，其所占据的比例高达20~31%。
（3）在初步了解东海沉积物中反硝化、厌氧铵氧化和DNRA过程的基础上，利用沉积物泥浆和整柱培养相结合的~(15)N同位素对技术分别于2010年6月和10~11月测定了东海沉积物中氮丢失速率。利用Risgaard-Petersen等(2003)所建议的方法计算出的氮丢失速率处于0.13到0.85mmol N m~(-2)d~(-1)，平均0.37mmol N m~(-2)d~(-1)，其中厌氧铵氧化的贡献处于11~50%，平均24%。DNRA的速率处于0~0.05mmol N m~(-2)d~(-1)，平均0.02mmol N m~(-2)d~(-1)。利用Fick第一扩散定律的计算并结合文献数据表明，单纯使用沉积物柱培养形式的~(15)N IPT计算出的氮丢失和DNRA速率均存在一定程度的低估。在柱培养实验中，加入的~(15)NO_3~-受到扩散限制，在通常所进行的培养时间内（小于1天）~(15)NO_3~-最多渗透至表层1cm的沉积物中而不能够充分地与深层次间隙水中的~(14)NO_3~-充分混合，从而使得利用现有~(15)N IPT计算出的氮丢失速率存在低估。利用沉积物分层泥浆培养实验，计算了~(15)NO_3~-渗透深度以下硝酸盐还原层中的氮丢失速率，将其与前面计算出的氮丢失速率加和即为实际的氮丢失速率。该氮丢失速率与利用Risgaard-Petersen等(2003)方法计算出的氮丢失速率相比提高了0.8~2.2倍，平均1.6倍，推广到整个东海陆架则每年增加的氮丢失量大约有1.6TgN。校正后的DNRA平均速率为0.29mmol N m~(-2)d~(-1)，提升了1个数量级。（·4）利用2011年5月和8月在长江口及其临近海区的两个航次探讨了底层水体缺氧对于沉积物中氮循环的影响。在该实验中，沉积物间隙水中的溶解氧和硝酸盐剖面、沉积物耗氧速率、NO_3~-和NH_4~+交换通量、厌氧铵氧化速率、反硝化速率、DNRA速率、硝化速率和矿化速率均在不同溶解氧条件下进行了测定，既考虑到了不同季节所造成的底层水体天然氧气含量变化对底栖氮循环的影响，同时又探讨了在受控培养的溶解氧条件下底栖氮循环的变化。结果表明，5月份的底层水体溶解氧含量一般处于~200μmolL~(-1)，沉积物溶解氧渗透深度处于4.0~4.3mm，耗氧速率处于11.6~17.6mmol O_2m~(-2)d~(-1)。由于调查前受强台风的影响，8月份底层水体中的溶解氧未出现缺氧现象（DO<62.5μmol L~(-1)），但与5月份相比，底层水体溶解氧降低至~100μmol L~(-1)，沉积物耗氧速率降低至6.1~13.6mmol O_2m~(-2)d~(-1)，氧气渗透深度则减小到1.6~3.8mm，沉积物耗氧速率和氧气渗透深度分别降低了23%和29%。伴随着氧气浓度的降低，NO_3~-剖面所表现出的硝化层消失，NO_3~-趋向于由水体向沉积物转移，而NH_4~+则趋向于由沉积物向水体释放。厌氧铵氧化速率由0.15mmol N m~(-2)d~(-1)降低至0.06mmol N m~(-2)d~(-1)，对总的氮丢失的贡献平均由20%降低至7.4%。反硝化速率表现出轻微的上升使得总的氮丢失速率维持在0.85mmol N m~(-2)d~(-1)左右。DNRA速率则由0.02mmol N m~(-2)d~(-1)上升至0.10mmol N m~(-2)d~(-1)，平均升高了5倍。由于在8月份的航次中没有发现天然的缺氧现象，因此通过受控培养研究了在不同氧条件下（氧化状态、原位氧状态和严重缺氧状态）底栖氮循环对于水体缺氧的响应，结果表明，当底层水体溶解氧由氧化状态（DO=~200μmol L~(-1)）降低至严重缺氧状态（DO=~16μmol L~(-1)）时，沉积物耗氧速率急剧降低，平均降低了91%。NH_4~+由沉积物向水体强烈地释放，而NO_3~-则由氧化条件下的从沉积物向水体释放（0.14mmol N m~(-2)d~(-1)）转变为急剧地向沉积物转移（-0.79mmol N m~(-2)d~(-1)）。与底层水体氧化状态相比，厌氧铵氧化和反硝化在严重缺氧条件下分别降低了38%和43%，从而使总的氮丢失速率由0.92mmol N m~(-2)d~(-1)降低至0.57mmol N m~(-2)d~(-1)。DNRA速率在严重缺氧状态下增加了3倍。反硝化、厌氧铵氧化和DNRA速率的降低均可归因于严重缺氧条件下沉积物有机氮矿化速率的降低，进而导致硝化速率降低，从而使得与硝化反应相耦合的各种硝酸盐还原过程的速率均表现出不同程度的降低。
Nitrogen plays a vital role in regulating global marine primary productivity andconsequently climate change. Its availability depends on the balance between nitrogenfixation through diazotrophs and fixed nitrogen loss via denitrification and anammox.However, whether marine nitrogen budget is in balance or not is still on debate. Thisis due to the scarcity of direct measurements for each process of marine nitrogen cycleand its complicated composition. Thus accurate rate measurement and nitrogentransformation process confirmation determine marine nitrogen budget to a largeextent. Marine sediment accounts for50~70%of global marine N-loss, and thus playsa dominant role. Among N-loss from sediment, continental shelf accounts for morethan50%of total benthic N-loss although it only comprises7.5%of the total seafloor.The East China Sea (ECS) shelf is one of the widest and most productive shelves. Itwas speculated that the ECS would be a hotspot for benthic N-loss; however, poorlystudies of benthic N-loss were conducted in this area. The aims of this thesis are toidentify the specific nitrogen transformation processes, to distinguish respectivecontribution to benthic nitrogen transformation, and to determine the extent of benthicN-loss on the ECS shelf. In this thesis, we chose the prevalent~(15)N isotope pairingmethod (~(15)N IPT) to investigate the benthic nitrogen cycle in the ECS from both fieldmeasurements and mathematical model. Furthermore, both sediment slurry and intactcore incubations were conducted in the field measurement to give a full understandingof benthic nitrogen cycle. Preliminary results were as follows:
(1) The application of isotope pairing technique (IPT) in sediments whereanammox, denitrification and DNRA coexist was discussed in detail basedon a mathematical model. The mathematical expression demonstrated thatco-occurrence of DNRA with anammox and denitrification would change~(15)N combination in anammox. Coexistence of DNRA would underestimateanammox and overestimate denitrification if their rates were calculatedaccording to the equations given by Thamdrup and Dalsgaard (2002) forslurry incubation. Therefore, ra was underestimated. The underestimation ofanammox was proportional to~(15)NH_4~+fraction in NH_4~+pools (FA); however,the overestimation of denitrification was related to both FAand ra. For intact core incubation, genuine N_2production was decreased since DNRA wouldcompete with anammox and denitrification under nitrate limiting. Threealternative procedures were proposed to correctly quantify anammox,denitrification and DNRA rates in slurry incubation. The first two methodsbased on estimating30N_2production via anammox. The third one based onconstraining FAto lower than5%in the way of elevating14NH_4~+concentration. The validity and feasibility of the proposed calculationprocedures were verified by field studies in the ECS and Yellow Sea. Ourresults implied that DNRA did not influence anammox and denitrificationsignificantly in estuarine sediment with relative low ra (<10%), however,those sediments with high ra and DNRA rate should be treated verycautiously for future studies and DNRA was strongly recommended to bemeasured.
(2) Benthic nitrogen transformation pathways were investigated in the sedimentof the East China Sea (ECS) in June of2010using the~(15)N isotope pairingtechnique. Slurry incubations indicated that denitrification, anammox anddissimilatory nitrate reduction to ammonium (DNRA) as well as intracellularnitrate release occurred in the ECS sediments. These four processes did notexist independently, nitrate release therefore diluted the~(15)N labeling fractionof NO_3~-–, and a part of the~(15)NH_4~+derived from DNRA also formed30N_2viaanammox. Hence current methods of rate calculations led to over andunderestimations of anammox and denitrification respectively. Following theprocedure outlined in Thamdrup and Dalsgaard (2002), denitrification rateswere slightly underestimated by an average6%without regard to the effectof nitrate release, while this underestimation could be counteracted by thepresence of DNRA. On the contrary, anammox rates calculated from~(15)NO_3~-–experiment were significantly overestimated by42%without consideringnitrate release. In our study, this overestimation could only be compensated14%by taking DNRA into consideration. In a parallel experiment amendedwith~(15)NH_4~++~(14)NO_3~-–, anammox rates were not significantly influenced byDNRA due to the high background of~(15)NH_4~+addition. The significantcorrelation between potential denitrification rate and sediment organic mattercontent (r=0.68, p<0.001, Pearson) indicated that denitrification wasregulated by organic matter, while, no such correlations were found for anammox and DNRA. The relative contribution of anammox to the totalN-loss increased from13%at the shallowest site near the Changjiang estuaryto50%at the deepest site on the outer shelf, implying the significant role ofanammox in benthic nitrogen cycling in the ECS sediments, especially on theouter shelf. N-loss as N_2was the main pathway, while DNRA was also animportant pathway accounting for20~31%of benthic nitrate reduction in theECS. Our study demonstrates the complicated interactions among differentbenthic nitrogen transformations and the importance of consideringdenitrification, DNRA, anammox and nitrate release together when designingand interpreting future studies.
(3) Benthic N-loss and DNRA rates on the ECS continental shelf were directlymeasured in two cruises in June and October-November2010usingprevalent~(15)N isotope pairing technique (IPT) through sediment intact coreand slurry incubations. Measured benthic N-loss rate ranged from0.13to0.85mmol N m~(-2)d~(-1)with an average of0.37mmol N m~(-2)d~(-1)using revisedIPT suggested by Risgaard-Petersen (2003), where anammox accounts for11~49%with an average of24%. DNRA ranged from0to0.05mmol N m~(-2)d~(-1)with an average of0.02mmol N m~(-2)d~(-1). According to the Fick’s first law,we speculated that just a pseudo-steady state was established for~(15)NO_3~-when IPT was employed.~(15)NO_3~-could not diffuse more than1cm insediment, which caused a substantial part of benthic N-loss missingcontributed by NO_3~-below~(15)NO_3~-penetration zone. Furthermore, severalevidence from re-examination based on published literature also supportedthat benthic N-loss was underestimated when IPT was applied. After addingthe missing benthic N-loss based on sediment slurry incubation, total benthicN-loss increased by a factor of0.8~2.2with an average of1.6and DNRAincreased by1order of magnitude to an average of0.29mmol N m~(-2)d~(-1)when the missing benthic N-loss based on sediment slurry incubation wasadded. The additional benthic N-loss was calculated with an average of1.6Tg N a-1in the East China Sea. If this additional N-loss could be extrapolatedto the global continental shelves, then an excess of80Tg N a-1can then beadded to the global marine nitrogen loss budget, accounting for52±23%ofthe global marine benthic nitrogen loss, which further aggravates theunbalance between nitrogen fixation and loss in marine environment.
(4) The response of benthic nitrogen cycle to bottom water hypoxia wassynthetically investigated in the Changjiang estuary and adjacent area in twocruises in May and August2011. In this study benthic oxygen uptake,sediment oxygen profile, net flux of nitrate and ammonium, anammox,denitrification, DNRA, nitrification and mineralization were studied using~(15)N isotope pairing and O_2microelectrode techniques. Sediment oxygenuptake and oxygen penetration depth averagely decreased23%and29%witha natural50%decline of bottom water oxygen from~200μM in May to~100μM in August. NH_4~+inclined to release from sediment to overlyingwater, while NO_3~-was prone to shifting from efflux to influx to sediment. rasignificantly decreased from20%to7.4%, leading to anammox ratedecreased by a factor of2.5from0.15to0.06mmol N m~(-2)d~(-1). However,denitrification showed a slight increase causing total benthic N-loss stablizedat0.85mmol N m~(-2)d~(-1). DNRA showed a significant increase by a factor of5from0.02to0.10mmol N m~(-2)d~(-1). These changes were consistent with theresults derived from artificially bottom water oxygen condition controlledexperiments (oxic, ambient and severe hypoxia): sediment oxygen uptakedecreased by as much as91%when bottom water oxygen dropped92%fromnormal oxic condition (~200μM) to severe hypoxia (~16μM). NH_4~+showedan enhanced release to overlying water from~0to0.60mmol N m~(-2)d~(-1)andNO_3~-shifted from an efflux of0.14mmol N m~(-2)d~(-1)to an influx of0.79mmol N m~(-2)d~(-1). Denitrification and anammox showed an average decreaseof38%and43%under severe hypoxia, leading to total benthic N-lossdecline of38%from0.92to0.57mmol N m~(-2)d~(-1). DNRA showed anelevation by a factor of3although it only accounted for less than10%oftotal nitrate reduction.
Our work in the ECS provided preliminary understanding of benthic nitrogentransformations, but there is still a long way to go to exhibit a comprehensive andintensive picture of nitrogen cycle due to its inherent complexity. With the boomingdiscoveries in the field of marine nitrogen cycle, studies will surely face numerousopportunities and challenges.
引文
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