溫度和氨氮濃度對(duì)水體N2O釋放的影響

路俊玲,陳慧萍,肖 琳

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溫度和氨氮濃度對(duì)水體N2O釋放的影響

路俊玲,陳慧萍,肖 琳*

(南京大學(xué)環(huán)境學(xué)院,污染控制與資源化國(guó)家重點(diǎn)實(shí)驗(yàn)室,江蘇 南京 210023)

氧化亞氮的釋放已經(jīng)成為了一個(gè)全球性的環(huán)境問題,水體中N2O的釋放量會(huì)隨著氮含量的增加而增加.本文通過微宇宙系統(tǒng)的構(gòu)建,分析氮的轉(zhuǎn)化過程和氮轉(zhuǎn)化基因的變化,并結(jié)合結(jié)構(gòu)方程模型分析了溫度、氨氮含量對(duì)水體N2O釋放的貢獻(xiàn).研究結(jié)果發(fā)現(xiàn)氨氧化古菌和反硝化細(xì)菌豐度均與N2O釋放呈正相關(guān),表明水體中的硝化和反硝化作用都會(huì)造成N2O的釋放.氨氮濃度的升高并不直接促進(jìn)N2O的釋放,而溫度和通過硝化作用產(chǎn)生的硝態(tài)氮對(duì)N2O的釋放有促進(jìn)作用.此外,硝化速率通過促進(jìn)亞硝態(tài)氮和反硝化菌的豐度而間接地促進(jìn)N2O的釋放.

氧化亞氮；溫度；硝化；反硝化；結(jié)構(gòu)方程模型

化肥的大量使用和污水的無序排放,造成大量的氮素進(jìn)入水體,不僅加劇了水體富營(yíng)養(yǎng)化,也成為N2O的重要釋放源[1-3]. N2O作為重要的溫室氣體[4],其溫室效應(yīng)是CO2的298倍,并且能造成臭氧層的破壞,引起臭氧層空洞[5-7].已經(jīng)有研究證實(shí)在氮含量高的河道[8]、湖泊等水體都有較高的N2O釋放[9].

在缺氧和厭氧條件下,N2O的產(chǎn)生主要由反硝化過程驅(qū)動(dòng),而近年來研究發(fā)現(xiàn)氨氧化細(xì)菌(AOB)和氨氧化古菌(AOA)也能產(chǎn)生N2O[10-11]. AOB和AOA主要通過3種途徑產(chǎn)生N2O:一是通過氨單加氧酶在氧化氨的過程中N2O作為副產(chǎn)物產(chǎn)生[12];二是AOB可以通過羥胺脫氫酶直接將氨氧化過程中產(chǎn)生的羥胺(NH2OH)氧化為N2O[13];此外,AOB含有基因,還能夠在缺氧條件下通過硝化菌的反硝化作用將NO2-還原為NO和N2O[14-15].雖然目前已有大量研究表明在土壤中硝化反應(yīng)對(duì)N2O的釋放發(fā)揮重要的作用[16-19],但關(guān)于水體中硝化作用對(duì)N2O釋放的貢獻(xiàn)還不清楚.

富營(yíng)養(yǎng)化湖泊中硝化反應(yīng)速率受到氨氮濃度和溫度的影響,過高的游離氨濃度會(huì)抑制亞硝酸氧化細(xì)菌活性,導(dǎo)致N2O的積累[20].而溫度對(duì)硝化和反硝化作用均有重要影響[21],并且AOA和AOB對(duì)溫度的響應(yīng)各有不同[22].如溫度的升高可以直接促進(jìn)氨氧化菌的增殖和潛在硝化能力[23],而冬季低溫條件下氨氧化菌因?yàn)橛休^多的氧供給及較少的競(jìng)爭(zhēng)作用比夏季的豐度更高[24].溫度和氨氮如何通過影響硝化、反硝化過程,從而影響N2O的產(chǎn)生,目前尚不清晰.

本文通過構(gòu)建微宇宙系統(tǒng),研究氨氮濃度和溫度對(duì)水體中氮的轉(zhuǎn)化、N2O釋放過程的影響;通過構(gòu)建結(jié)構(gòu)方程模型,區(qū)分硝化、反硝化過程對(duì)N2O釋放的貢獻(xiàn)及其對(duì)環(huán)境因子的響應(yīng).這對(duì)更全面地認(rèn)識(shí)和指導(dǎo)富營(yíng)養(yǎng)化水體修復(fù)措施的實(shí)施,減少溫室氣體N2O的排放具有重要意義.

1 材料與方法

1.1 試驗(yàn)設(shè)計(jì)

微宇宙試驗(yàn),通過在實(shí)驗(yàn)室模擬不同的生態(tài)系統(tǒng)進(jìn)行研究,既能夠保持生態(tài)系統(tǒng)的復(fù)雜性,又使結(jié)果具有可重復(fù)性,在生態(tài)學(xué)研究中被廣泛應(yīng)用[25].本次研究于2017年6月在太湖(30°58′14.37′′N, 120°8′16.40′′E)采集水樣,冰袋低溫保存立即運(yùn)回實(shí)驗(yàn)室.取3L水樣置于5L的加塞廣口玻璃缸中,構(gòu)建微宇宙系統(tǒng)[26].微宇宙系統(tǒng)設(shè)2個(gè)氨氮濃度,分別為不添加和添加NH4Cl至氨氮終濃度為30mg/L.分別于5,15及25℃下黑暗靜置培養(yǎng),防止藻類的生長(zhǎng).在試驗(yàn)開始的第1,3,5,10,15,20和25d定期采集水樣,用于理化性質(zhì)和定量PCR分析.每組試驗(yàn)3個(gè)平行.

1.2 水樣理化性質(zhì)分析

總氮(TN),氨氮(NH4+),硝態(tài)氮(NO3-)和亞硝態(tài)氮(NO2-)、總有機(jī)碳(TOC)測(cè)定均采用文獻(xiàn)中的方法[27].

NH4+凈轉(zhuǎn)化速率(NATR)和NO3-凈轉(zhuǎn)化速率(NNR)的計(jì)算分別采用公式(1)和公式(2)進(jìn)行:

式中:C為(n+x) d的NH4+或NO3-濃度(mg/L),C為第() d的NH4+或NO3-濃度(mg/L),為天數(shù)(d).

1.3 N2O的采集和測(cè)定

每次采樣時(shí)用注射器采集5mL頂空氣體,轉(zhuǎn)移到3mL的密閉玻璃瓶中用于分析N2O的產(chǎn)生量.N2O的濃度采用Agliet7890B氣相色譜儀測(cè)定,檢測(cè)器為電子捕獲器(ECD).檢測(cè)條件為:柱溫55℃、后檢測(cè)器(ECD)溫度300℃,載氣為高純氮?dú)?流速為30mL/min,燃?xì)鉃闅錃?流速為45mL/min[28].

1.4 DNA的提取和實(shí)時(shí)熒光定量PCR(qPCR)

DNA的提取和qPCR參照文獻(xiàn)的方法進(jìn)行[29]. qPCR采用20μL反應(yīng)體系進(jìn)行:10μL SYBR?Premix Ex Taq?(Takara), 100nM引物, 1.5μL模板DNA.細(xì)菌和古菌的基因分別采用amoA1F/amoA2R, Arch–amoAF/Arch–amoAR引物進(jìn)行擴(kuò)增,反應(yīng)條件為:94℃ 2min;94℃ 20s,57℃ 30s (AOB) 或55℃ 30s (AOA),40個(gè)循環(huán).反硝化基因的擴(kuò)增采用引物cd3af/r3cd,反應(yīng)條件為:94℃ 2min;94℃ 30s, 57 ℃ 45s,72℃ 45s,40個(gè)循環(huán).

1.5 統(tǒng)計(jì)分析和結(jié)構(gòu)方程模型分析

水體理化指標(biāo)和qPCR數(shù)據(jù)進(jìn)行One–Way ANOVA分析,平均數(shù)的比較采用Fisher’s LSD檢測(cè).生物和環(huán)境因子間的關(guān)系進(jìn)行Pearson相關(guān)性分析,并通過結(jié)構(gòu)方程模型分析環(huán)境因子對(duì)氮轉(zhuǎn)化過程、相關(guān)功能微生物和N2O釋放的直接和間接影響[30].以上統(tǒng)計(jì)分析均在IBM SPSS 22.0(SPSS Inc.,美國(guó))中進(jìn)行.

2 結(jié)果與分析

2.1 不同氨氮濃度和溫度條件下氮轉(zhuǎn)化及N2O的釋放

在無外源氮添加的情況下,體系中的氨氮主要來源于有機(jī)氮降解,在3個(gè)溫度下氨氮濃度都很低.不同溫度下對(duì)照組氨氮的凈轉(zhuǎn)化速率在0.06~ 0.11mg NH4+-N/(L×d)范圍內(nèi)波動(dòng),組間沒有顯著性差異(圖1,2),表明在低氨氮情況下,溫度對(duì)氨氮的轉(zhuǎn)化沒有顯著性影響.

圖1 NH4+-N(a), TN(b), NO3--N(c)在不同氨氮濃度和溫度條件下的變化

圖2 氨氮凈轉(zhuǎn)化速率(NATR) (a)和硝態(tài)氮凈轉(zhuǎn)化速率(NNR) (b)

在低氨氮情況下,在5℃時(shí),最終有2.72mg/L的硝態(tài)氮產(chǎn)生,其凈產(chǎn)生速率與氨氮的轉(zhuǎn)化速率基本相同(0.1mg NO3--N/(L×d),說明硝化作用在氮的轉(zhuǎn)化中占據(jù)優(yōu)勢(shì).在15℃和25℃時(shí),分別只有7%和3%的氮氮減少量被轉(zhuǎn)化為硝態(tài)氮,同時(shí)硝態(tài)氮的凈產(chǎn)生速率也只占氨氮轉(zhuǎn)化的18%和15%(圖2),表明溫度的升高促進(jìn)了反硝化作用的進(jìn)行并使體系中總氮降低.

圖3 不同氨氮濃度和溫度下N2O的產(chǎn)生

在有高氨氮添加時(shí),氨氮濃度在所有處理組中都持續(xù)下降,表明即使在低溫下氨氮也能夠被快速轉(zhuǎn)化(圖1).同時(shí),氨氮的轉(zhuǎn)化速率在25℃時(shí)達(dá)到最高,表明高氨氮情況下氨氮的轉(zhuǎn)化速率與溫度呈正相關(guān).在15℃時(shí),NO3-的產(chǎn)生速率約為氨氮轉(zhuǎn)化速率的60%,在25d時(shí),超過90%的氨氮被轉(zhuǎn)化為硝態(tài)氮,表明此時(shí)硝化作用仍占主要作用.在25℃時(shí),體系中氨氮濃度的減少(28.86mg/L)與總氮的減少(20.30mg/L)和硝態(tài)氮的生成量(8.45mg/L)之和呈化學(xué)劑量平衡,說明耦合了硝化作用的反硝化過程在氮的轉(zhuǎn)化過程中占主要作用.

N2O的產(chǎn)生在培養(yǎng)開始時(shí)即可以在所有體系中檢測(cè)到.N2O 的產(chǎn)生量在未添加氨氮的體系中較低.在添加了高濃度氨氮的體系中,5℃條件下N2O的產(chǎn)生量也顯著低于15和25℃的處理.在15和25℃條件下,從第5d起N2O的產(chǎn)生量即明顯高于對(duì)照,其中25℃時(shí)N2O的產(chǎn)生速率和產(chǎn)生量顯著高于15℃.

2.2 氨氧化(amoA)和亞硝酸鹽還原酶(nirS)基因豐度的動(dòng)態(tài)變化

如圖4所示,在全部微宇宙體系中,氨氧化菌的和反硝化菌的基因豐度都隨著時(shí)間而增加.AOA的基因豐度大約比AOB的基因高一個(gè)數(shù)量級(jí)(圖4),表明AOA是水體中主要的氨氧化菌.相比于對(duì)照,在5和25℃時(shí)AOA的基因拷貝數(shù)隨著氮的添加而降低,在15℃時(shí)添加銨鹽促進(jìn)了AOA的生長(zhǎng).

與氨氧化菌相比,低溫對(duì)反硝化菌增殖的抑制作用更為顯著.在5和15℃條件下,25d時(shí),未添加氨氮的對(duì)照組中的拷貝數(shù)僅較起始拷貝數(shù)增加了20%和30%.但在25℃條件下,添加氨氮的處理組中的拷貝數(shù)則迅速增加,表明此時(shí)反硝化菌具有較高的代謝活性.

圖4 amoA基因(a、b)及nirS(c)基因豐度變化

2.3 溫度、氨氮濃度對(duì)氮轉(zhuǎn)化過程及N2O釋放的影響

通過person相關(guān)性分析發(fā)現(xiàn),N2O的釋放與溫度、NO2-和NO3-濃度、PNR以及AOA-、AOB-和(表1)呈顯著正相關(guān).但氨氮和總氮濃度都與N2O的釋放無顯著相關(guān).

表1 生物和環(huán)境因子與N2O釋放的相關(guān)性分析

圖5 影響N2O產(chǎn)生因子的SEM分析

圖中所標(biāo)均具有顯著性作用,實(shí)線代表正向促進(jìn)作用,虛線代表抑制作用.線的粗細(xì)代表作用強(qiáng)度的大小

進(jìn)一步通過SEM分析了溫度和氨氮濃度對(duì)氮轉(zhuǎn)化過程、N2O釋放的直接和間接影響,所構(gòu)建的結(jié)構(gòu)方程模型能夠解釋90%的N2O釋放的影響(2=0.90).SEM結(jié)果表明NO3-(=0.767,<0.001)和溫度(=0.136,<0.05)顯著提高N2O的釋放,但NH4+對(duì)N2O的釋放沒有直接作用(圖5).NH4+顯著促進(jìn)(=0.251,<0.05)和NO2-(=0.277,<0.01)的量,而(=0.493,<0.001)和NO2-(=0.433,<0.001)與NO3-呈顯著正相關(guān).此外,AOA對(duì)(=0.651,<0.001)和PNR(=0.748,<0.001)有強(qiáng)烈的促進(jìn)作 用.

3 討論

氮的生物可利用性及溫度是影響氮轉(zhuǎn)化的兩大重要因素.結(jié)構(gòu)方程模型分析的結(jié)果表明,溫度的升高能直接促進(jìn)N2O的釋放,而高氨氮主要是通過加快硝化速率,縮短硝化反應(yīng)時(shí)間,從而加速N2O的釋放.高氨氮體系中,N2O的釋放在體系中很快開始,并且即使在以硝化作用為主的體系中,N2O依然能夠大量產(chǎn)生,這表明在水體中硝化作用可以是產(chǎn)生N2O的主要途徑.在未添加氨氮的處理組中也檢測(cè)到了N2O的釋放,說明有機(jī)氮經(jīng)礦化作用后所產(chǎn)生的NH4+也能通過硝化作用增加N2O的釋放.在25℃和高氨氮情況下,20d以后體系中的氮以硝態(tài)氮形式存在,并且氮轉(zhuǎn)化過程以反硝化作用為主,表明反硝化作用也在N2O的產(chǎn)生中發(fā)揮重要作用.溫度影響著氮轉(zhuǎn)化細(xì)菌的生長(zhǎng)以及酶的活性,同時(shí)也會(huì)影響N2O從水體中釋放的傳質(zhì)阻力,這些因素造成了在低溫(5℃)條件下N2O的產(chǎn)生量較低.在本研究的微宇宙試驗(yàn)中,反硝化過程并不占優(yōu)勢(shì).在低溫條件下,的豐度也增長(zhǎng)緩慢,表明低溫抑制了反硝化細(xì)菌的活性和生長(zhǎng).但與前人報(bào)道[24]類似,5℃的低溫并未對(duì)AOA和AOB的增長(zhǎng)造成顯著性抑制.N2O的釋放與NO2-和NO3-濃度呈現(xiàn)顯著的正相關(guān)關(guān)系,這也表明在NO2-和NO3-生成的過程中伴隨著N2O的釋放[16].但隨著氨氧化的進(jìn)行,體系中的DO逐漸被消耗造成缺氧或厭氧環(huán)境,這時(shí)反硝化作用的發(fā)生也將導(dǎo)致大量N2O的釋放.

沉積物中通過反硝化作用產(chǎn)生N2O已經(jīng)引起了重視[2].相比于沉積物而言,湖泊、河流等水體中上覆水的體量更大,直接接受外源的氮輸入,并且在N2O的釋放過程中受到的傳質(zhì)阻力更小而更容易釋放.本文的研究結(jié)果表明,除了反硝化作用,水體中的硝化作用也會(huì)產(chǎn)生N2O,并且隨著溫度和氨氮濃度的升高,N2O的釋放量也隨之增加.溫度的升高可以直接促進(jìn)N2O的釋放.氨氮濃度的升高雖然并不直接作用于N2O的釋放,但其一方面可以通過促進(jìn)硝化作用的進(jìn)行而增加N2O的釋放;同時(shí),硝化作用所產(chǎn)生的NO2-和NO3-又可以通過耦合反硝化過程間接促進(jìn)N2O的產(chǎn)生.結(jié)構(gòu)方程模型分析結(jié)果表明,溫度和氨氮濃度的升高均能促進(jìn)N2O的釋放,并且進(jìn)一步區(qū)分了硝化、反硝化在N2O釋放過程中的貢獻(xiàn),這使得我們對(duì)溫度及不同形態(tài)的氮素對(duì)N2O釋放風(fēng)險(xiǎn)的影響有了更全面的認(rèn)識(shí).因此,富營(yíng)養(yǎng)化水體治理過程中,不僅要控制以硝酸鹽為主的反硝化過程,更要加強(qiáng)對(duì)氨氮等其它各種形式氮素的控制,這對(duì)于有效降低N2O的產(chǎn)生,減少溫室氣體的排放具有重要意義.

4 結(jié)論

4.1 水體中硝化、反硝化作用的增加均能促進(jìn)N2O的釋放.

4.2 溫度的升高可以直接促進(jìn)水體中N2O的釋放,而氨氮濃度的升高主要通過提高硝化反應(yīng)速率,加速N2O的產(chǎn)生.

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Coupling effect of temperature and ammonia on N2O emission in surface water.

LU Jun-ling, CHEN Hui-ping, XIAO Lin*

(State Key Laboratory of Pollution Control and Resource Reuse, School of the Environment, Nanjing University, Nanjing 210023, China)., 2019,39(1)：330~335

The increase of nitrous oxide emission has become a worldwide problem. The emission of nitrous oxide from water body increased with the increase of N inputs. Our study analyzed the roles of temperature and ammonia in N2O emission through quantification of nitrogen transformation and related functional genes. The results showed that the abundance of ammonia oxidizing archaea and denitrifiers positively correlated to N2O emission in water. Structural equation model revealed ammonia content had no direct effect on N2O emission, however, high temperature and nitrification directly accelerated the release of N2O. In addition, nitrification process also increased the release of N2O indirectly through promoting abundance of denitrifiers and nitrite content.

nitrous oxide；temperature；nitrification；denitrification；structural equation mode

X524

A

1000-6923(2019)01-0330-06

路俊玲(1994-),女,山東青島人,南京大學(xué)碩士研究生,主要研究方向?yàn)榄h(huán)境微生物學(xué).發(fā)表論文2篇.

2018-06-19

國(guó)家水體污染控制與治理科技重大專項(xiàng)(2017ZX07204002)

* 責(zé)任作者, 教授, xiaolin@nju.edu.cn