張志君, 秦樹(shù)平**, 袁海靜, 張玉銘, 胡春勝

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土壤氮?dú)馀欧叛芯窟M(jìn)展*

張志君1, 秦樹(shù)平1**, 袁海靜1, 張玉銘2, 胡春勝2

(1. 福建省土壤環(huán)境健康與調(diào)控重點(diǎn)實(shí)驗(yàn)室/福建農(nóng)林大學(xué)資源與環(huán)境學(xué)院 福州 350002; 2. 中國(guó)科學(xué)院遺傳與發(fā)育生物學(xué)研究所農(nóng)業(yè)資源研究中心 石家莊 050022)

自20世紀(jì)初人類(lèi)發(fā)明并掌握工業(yè)合成氨的技術(shù)以來(lái), 氮肥施用量迅速增長(zhǎng)。在一部分國(guó)家或地區(qū), 氮肥的施入量已經(jīng)超過(guò)作物對(duì)氮素的需求, 導(dǎo)致大量氮素?fù)p失到環(huán)境中, 造成氨揮發(fā)、氧化亞氮排放、地下水硝酸鹽污染等環(huán)境問(wèn)題。土壤在微生物的作用下可以通過(guò)反硝化、厭氧氨氧化等過(guò)程將活性氮素轉(zhuǎn)化為惰性氮?dú)? 達(dá)到清除過(guò)多活性氮的目的。由于大氣中氮?dú)獗尘皾舛忍? 因此很難直接準(zhǔn)確測(cè)定土壤的氮?dú)馀欧潘俾? 導(dǎo)致土壤氮?dú)馀欧磐?、過(guò)程與調(diào)控機(jī)制研究遠(yuǎn)遠(yuǎn)落后于土壤氮循環(huán)的其他方面。本文綜述了土壤氮?dú)馀欧胖饕緩?反硝化、厭氧氨氧化與共反硝化)及其對(duì)土壤氮?dú)馀欧诺呢暙I(xiàn); 測(cè)定土壤氮?dú)馀欧潘俾实姆椒?乙炔抑制法、氮同位素示蹤法、N2/Ar比率-膜進(jìn)樣質(zhì)譜法、氦環(huán)境法與N2O同位素自然豐度法)及其優(yōu)缺點(diǎn); 調(diào)控土壤氮?dú)馀欧磐康闹饕蛩?氧氣、可溶性有機(jī)碳、硝酸鹽、微生物群落結(jié)構(gòu)與功能基因表達(dá)等)及其相關(guān)作用機(jī)制。最后指出研發(fā)新的測(cè)定原位無(wú)擾動(dòng)土壤氮?dú)馔康姆椒ㄊ峭七M(jìn)本領(lǐng)域相關(guān)研究的關(guān)鍵; 定量典型生態(tài)系統(tǒng)(如旱地農(nóng)田、稻田、森林、草地與濕地)土壤氮?dú)馀欧磐? 闡明其中的微生物學(xué)機(jī)制, 模擬并預(yù)測(cè)土壤氮?dú)馀欧艑?duì)全球變化的響應(yīng)規(guī)律是本領(lǐng)域的研究熱點(diǎn)與發(fā)展方向。

土壤; 氮?dú)馀欧? 反硝化; 厭氧氨氧化; 氧化亞氮排放; 氮損失

土壤氮?dú)馀欧攀侵富钚缘?主要是指硝酸根、亞硝酸根、銨與氧化亞氮等)在微生物的作用下轉(zhuǎn)化為惰性氮(氮?dú)?的過(guò)程。這一過(guò)程在生態(tài)系統(tǒng)氮循環(huán)中具有重要作用, 它是氮循環(huán)的最后一步, 是閉合氮循環(huán)的關(guān)鍵過(guò)程, 影響生態(tài)系統(tǒng)活性氮?dú)w趨與氮素平衡。對(duì)于農(nóng)業(yè)生態(tài)系統(tǒng)來(lái)說(shuō), 土壤氮?dú)馀欧排c氮肥損失及溫室氣體氧化亞氮的排放密切相關(guān)。大氣中的氮?dú)鉂舛群芨?體積濃度約78%), 在如此高的氮?dú)獗尘皾舛认聹?zhǔn)確測(cè)定土壤氮?dú)馀欧乓恢笔菄?guó)際土壤學(xué)領(lǐng)域的方法學(xué)難題。相對(duì)于土壤氮素循環(huán)的其他環(huán)節(jié), 氮?dú)馀欧欧矫娴难芯窟€十分薄弱。本文主要從土壤氮?dú)猱a(chǎn)生與排放的主要途徑、主要測(cè)定方法學(xué)與主要的環(huán)境及微生物調(diào)控機(jī)制3個(gè)方面進(jìn)行綜述。

1 土壤氮?dú)猱a(chǎn)生與排放的主要途徑

土壤產(chǎn)氮?dú)獾倪^(guò)程主要有反硝化(denitrification)、厭氧氨氧化(anammox)與共反硝化(co-denitrification) 3種途徑[1]。土壤反硝化過(guò)程是指在微生物的作用下硝酸鹽經(jīng)由亞硝酸鹽、一氧化亞氮、氧化亞氮等中間產(chǎn)物最終還原為氮?dú)獾倪^(guò)程[2-5], 一般認(rèn)為土壤中反硝化過(guò)程主要由反硝化微生物驅(qū)動(dòng), 化學(xué)反硝化過(guò)程只在pH值很低的情況下發(fā)生[6-10]。厭氧氨氧化過(guò)程是指微生物在厭氧條件下利用銨鹽為電子供體, 亞硝酸鹽為電子受體, 最終產(chǎn)生氮?dú)獾倪^(guò)程[11-14]。除亞硝酸鹽外, 近年來(lái)又發(fā)現(xiàn)一些微生物能利用其他電子受體, 如Fe(Ⅲ), 進(jìn)行厭氧氨氧化產(chǎn)氮?dú)? 稱(chēng)為鐵厭氧氨氧化(feanammox)[15]。Ding等[16]將耕種年代不同序列的水稻土作為研究對(duì)象, 第一次在水稻土中驗(yàn)證了鐵厭氧氨氧化現(xiàn)象的出現(xiàn), 并提出氮?dú)馐窃撨^(guò)程的主要產(chǎn)物。共反硝化是指微生物利用反硝化的中間產(chǎn)物亞硝酸鹽為電子受體, 以銨、羥胺、疊氮化物等為電子供體, 以氮?dú)鉃樽罱K產(chǎn)物的過(guò)程[17]。在以上3種途徑中, 反硝化過(guò)程被認(rèn)為是土壤氮?dú)猱a(chǎn)生的主要途徑。厭氧氨氧化在高度厭氧的環(huán)境中, 如底泥與稻田土壤中對(duì)氮?dú)馀欧啪哂幸欢ㄘ暙I(xiàn)[18-19]。共反硝化在草地等真菌較多的生態(tài)系統(tǒng)中對(duì)氮?dú)馀欧啪哂幸欢ㄗ饔肹17]。

2 土壤氮?dú)馀欧诺闹饕獪y(cè)定方法

受到大氣中氮?dú)飧弑尘皾舛鹊挠绊? 直接測(cè)定土壤氮?dú)馀欧磐糠浅＠щy[20]。目前使用較多的主要有乙炔抑制法、氮同位素示蹤法、N2/Ar比率-膜進(jìn)樣質(zhì)譜法、氦環(huán)境法與N2O同位素自然豐度法[21]。

2.1 乙炔抑制法

乙炔抑制法的原理是利用高濃度乙炔(體積濃度>1%, 通常為10%)抑制土壤中氧化亞氮還原酶的活性, 進(jìn)而阻止氧化亞氮還原為氮?dú)? 最后通過(guò)測(cè)定氧化亞氮的累積量來(lái)間接表征土壤氮?dú)馀欧磐縖22-23]。該方法具有簡(jiǎn)單易操作等優(yōu)點(diǎn)。因此該方法在過(guò)去的30年中被廣泛用于土壤反硝化速率及氮?dú)馀欧磐康臏y(cè)定, 積累了大量田間試驗(yàn)數(shù)據(jù)[24-25]。但是乙炔抑制法具有難以克服的缺點(diǎn): 第一, 高濃度乙炔在抑制土壤氧化亞氮還原酶活性的同時(shí)會(huì)對(duì)土壤硝化過(guò)程產(chǎn)生抑制作用, 導(dǎo)致反硝化的底物——硝酸鹽的供給減少[26-27]。第二, 一部分反硝化微生物對(duì)乙炔不敏感, 這些微生物在碳含量較高而硝酸鹽濃度較低(<10 μmol?L-1)的環(huán)境下能夠促進(jìn)氧化亞氮還原為氮?dú)鈁28-29]。2010年, Zhong等[30]研究中國(guó)太湖梅梁灣表層沉積物潛在的反硝化速率的季節(jié)性變化, 發(fā)現(xiàn)沉積物反硝化速率在春季最高, 夏季和秋季最低, 主要是由于硝酸鹽濃度和水溫的季節(jié)差異,并且限制反硝化速率的關(guān)鍵因素是硝酸鹽。夏季和秋季, 水體中的硝酸鹽已被耗盡, 大大抑制了太湖反硝化作用對(duì)氮的有效去除。第三, 乙炔很難均勻地?cái)U(kuò)散到土壤中, 如果有一部分土壤空隙中沒(méi)有乙炔, 則會(huì)導(dǎo)致該處氧化亞氮被還原為氮?dú)鈁31]。2008年, Woodward等[32]通過(guò)研究河岸地帶的土壤, 發(fā)現(xiàn)反硝化作用不是硝酸鹽損失的主要途徑, 僅占地下水中硝酸鹽去除率的3%, 所以使用乙炔抑制法測(cè)定河岸地帶的土壤反硝化速率會(huì)造成一定的低估, 主要因?yàn)楹影锻寥垒^高的含水量阻礙了乙炔的擴(kuò)散。第四, 一些微生物會(huì)將乙炔當(dāng)作碳源而將其分解利用, 導(dǎo)致土壤中的乙炔濃度降低, 影響其對(duì)氧化亞氮還原的抑制作用[28]。最后, 在好氧條件下, 高濃度乙炔(>0.1 kPa)會(huì)促使反硝化的中間產(chǎn)物一氧化氮分解或者催化其氧化為二氧化氮, 導(dǎo)致氮?dú)馀欧磐康凸繹33]。1997年, Bollmann等[34]采用乙炔抑制技術(shù)測(cè)定了29種土壤的反硝化速率、一氧化氮的生成量和氧化速率以及氧化亞氮的凈釋放速率。他們?cè)?0 Pa乙炔條件下測(cè)定了一氧化氮和氧化亞氮排放速率, 發(fā)現(xiàn)乙炔抑制了硝化作用而不是反硝化作用, 并且在乙炔濃度大于0.1 kPa時(shí)證明了乙炔能夠增強(qiáng)對(duì)一氧化氮氧化的催化作用, 產(chǎn)生的二氧化氮被土壤吸收, 不能再進(jìn)一步轉(zhuǎn)換為氧化亞氮, 從而造成對(duì)反硝化速率的低估。對(duì)乙炔抑制法的上述潛在缺點(diǎn), 很多文獻(xiàn)進(jìn)行了理論上的定性分析[35-37]。近年來(lái), 一些學(xué)者進(jìn)一步用直接定量氮?dú)獾姆椒▽?duì)乙炔抑制法的系統(tǒng)誤差進(jìn)行了定量研究。2013年, Qin等[38]發(fā)現(xiàn)在厭氧條件下不同土壤基本理化性質(zhì)之間的差異對(duì)乙炔抑制法的系統(tǒng)誤差也會(huì)造成影響,高濃度乙炔(>0.1 kPa)無(wú)法完全抑制土壤氧化亞氮還原酶活性, 導(dǎo)致乙炔抑制法測(cè)定的土壤反硝化潛勢(shì)比直接定量氮?dú)夥ㄆ?%~98%, 同時(shí)乙炔抑制法的系統(tǒng)誤差與土壤有機(jī)質(zhì)與養(yǎng)分含量呈顯著負(fù)相關(guān)關(guān)系。這些結(jié)果表明: 對(duì)于有機(jī)質(zhì)與養(yǎng)分含量較高的土壤, 如農(nóng)田土壤, 乙炔抑制法能夠較準(zhǔn)確地表征土壤反硝化潛勢(shì); 而對(duì)于有機(jī)質(zhì)含量低的貧瘠土壤, 如荒漠土壤, 使用乙炔抑制法測(cè)定土壤反硝化潛勢(shì)會(huì)帶來(lái)較大的誤差?另外一些學(xué)者將乙炔抑制法與其他氮同位素示蹤法進(jìn)行了比較研究, 也發(fā)現(xiàn)乙炔抑制法存在負(fù)偏差[35]。上述研究表明: 乙炔抑制法雖然具有操作簡(jiǎn)單, 費(fèi)用低廉的優(yōu)點(diǎn), 但具有難以克服的內(nèi)在缺陷。在使用乙炔抑制法測(cè)定氮?dú)馔繒r(shí)要慎重考慮其系統(tǒng)誤差[39]。

2.2 氮同位素示蹤法

氮同位素示蹤法的原理是將15N高豐度的硝酸鹽施入土壤, 利用同位素質(zhì)譜儀測(cè)定經(jīng)由反硝化產(chǎn)生的氮?dú)馔凰刎S度推算土壤氮?dú)馀欧磐縖40-41]。氮同位素示蹤法始于20世紀(jì)50年代末, 已被廣泛應(yīng)用于各種土壤類(lèi)型的反硝化速率及氮?dú)馀欧磐康臏y(cè)定[42-46]。氮同位素示蹤法也存在一些缺點(diǎn): 第一, 標(biāo)記的硝酸鹽很難均勻擴(kuò)散到土壤中, 因此無(wú)法保證新加入的標(biāo)記硝酸鹽與土壤本底硝酸鹽是否混合均勻[20]。但Steingruber等[47]發(fā)現(xiàn)標(biāo)記硝酸鹽和本底硝酸鹽的非均勻混合對(duì)反硝化速率造成的誤差不超過(guò)10%, 遠(yuǎn)遠(yuǎn)小于試驗(yàn)系統(tǒng)誤差。其他研究也發(fā)現(xiàn), 即使土壤中硝酸鹽的標(biāo)記是不均勻的, 仍可以對(duì)反硝化產(chǎn)生的氮?dú)庾龀鰷?zhǔn)確的估計(jì)[48]。第二, 土壤微生物優(yōu)先利用輕質(zhì)同位素, 這些因素都可能給土壤氮?dú)馀欧磐康臏y(cè)定帶來(lái)不確定性[49]。第三, 新加入的標(biāo)記硝酸鹽增加了土壤原有反硝化反應(yīng)的底物濃度[48], 這對(duì)一些氮源缺乏的生態(tài)系統(tǒng)來(lái)說(shuō)會(huì)顯著影響該系統(tǒng)的氮?dú)馀欧潘俾? 進(jìn)而高估土壤氮?dú)馀欧潘俾?。因此氮同位素示蹤法主要運(yùn)用于氮素含量較高的生態(tài)系統(tǒng), 如農(nóng)田生態(tài)系統(tǒng)[48]。

2.3 N2/Ar比率-膜進(jìn)樣質(zhì)譜法

N2/Ar比率-膜進(jìn)樣質(zhì)譜法的原理是利用膜進(jìn)口質(zhì)譜儀測(cè)定水中的N2/Ar比率來(lái)推算覆水土壤中氮?dú)馀欧潘俾? 如稻田與底泥中的氮?dú)馀欧潘俾?。水中的Ar濃度相對(duì)穩(wěn)定, 而水中的N2濃度主要受反硝化等產(chǎn)氮?dú)膺^(guò)程的影響[50-51]。該方法的優(yōu)點(diǎn)是可以同時(shí)測(cè)定通過(guò)反硝化、厭氧氨氧化、共反硝化等多種途徑產(chǎn)生的氮?dú)饪偱欧潘俾蔥51]。缺點(diǎn)是該方法難以運(yùn)用于非覆水土壤中氮?dú)馀欧潘俾实臏y(cè)定, 另外該方法只能運(yùn)用于室內(nèi)培養(yǎng)的原狀土柱, 難以在田間原位土壤中實(shí)施[52]。Li等[53]利用N2/Ar比率-膜質(zhì)譜法對(duì)水稻土施肥后0~21 d的原位氮?dú)馀欧胚M(jìn)行了測(cè)定, 發(fā)現(xiàn)反硝化產(chǎn)生的氮?dú)鈸p失約占氮肥投入量的4.7%。

2.4 氦環(huán)境法

該方法的原理是在密閉空間中利用惰性氣體氦氣置換土壤孔隙中氮?dú)? 人為降低氮?dú)獗尘皾舛? 進(jìn)而使用氣相色譜直接測(cè)定土壤排出的微量氮?dú)鈁54-59]。該方法的優(yōu)點(diǎn)是可以直接測(cè)定土壤排出的氮?dú)馔? 避免了人為添加抑制劑/氮源而對(duì)土壤氮?dú)馀欧磐吭斐蓾撛谟绊慬26]。另外該方法可以廣泛運(yùn)用于濕地與旱地原狀土柱經(jīng)由多種途徑(反硝化、厭氧氨氧化、共反硝化)產(chǎn)生的氮?dú)馔?。該方法的缺點(diǎn)是需要建造高密閉性的培養(yǎng)裝置[54], 該裝置的氣密性直接決定該方法的測(cè)定精度。

2.5 田間原位無(wú)擾動(dòng)土壤氮?dú)馔繙y(cè)定方法

目前大部分土壤氮?dú)馀欧诺臏y(cè)定方法, 如乙炔抑制法只局限在室內(nèi)測(cè)定[24]; 氮同位素示蹤法適合本底氮素含量較高的土壤[48], 如農(nóng)田土壤; N2/Ar比率法只適用于具有覆水的條件, 無(wú)法測(cè)定無(wú)覆水條件的濕地或旱地土壤的原位氮?dú)馀欧潘俾蔥53,60]。田間原位土樣采集后如何避免溶解態(tài)的氮?dú)獠会尫诺綒庀嘀幸约皻庀嘀械牡獨(dú)獠晃廴舅嘀械娜芙鈶B(tài)氮?dú)馐切枰⒁獾膯?wèn)題。另外該方法無(wú)法測(cè)定通過(guò)非溶解途徑(如氣泡)排出土體的氮?dú)? 可能低估氮?dú)馀欧磐?。傳統(tǒng)的原狀土柱-直接定量氮?dú)夥? 由于裝置氣密性要求較高, 難以測(cè)定大直徑原狀土柱原位反硝化通量[54]。最近秦樹(shù)平等[61]將原狀土柱-直接定量氮?dú)夥右愿倪M(jìn), 利用兩層密封罐體之間的無(wú)氮?dú)鈯A層來(lái)抵消大氣中的高濃度氮?dú)獾男孤? 將罐內(nèi)土柱中的氣體置換成無(wú)氮?dú)獾娜斯ず铣蓺?79%氦氣, 21%氧氣), 最后利用Robot系統(tǒng)測(cè)定罐內(nèi)氮?dú)?、氧化亞氮與一氧化氮的濃度變化, 使大直徑原狀土柱原位氮?dú)馔繙y(cè)定成為可能(雙密閉原狀土柱-直接定量氮?dú)夥?。然而, 雙密閉原狀土柱-直接定量氮?dú)夥ㄒ灿胁豢煽朔娜秉c(diǎn): 首先需要將土柱中的氣體置換成低濃度無(wú)氮?dú)獾娜斯ず铣蓺怏w(如氦/氧混合氣), 這一操作過(guò)程可能會(huì)對(duì)土壤團(tuán)聚體結(jié)構(gòu)、含水量等因素造成一定潛在影響; 另外密閉環(huán)境下會(huì)造成培養(yǎng)體系不能與外界進(jìn)行物質(zhì)交換, 如: 土柱周?chē)跛猁}無(wú)法擴(kuò)散進(jìn)入密閉土柱, 密閉土柱產(chǎn)生的氣體無(wú)法擴(kuò)散到大氣中, 造成產(chǎn)物的累積, 進(jìn)而影響土壤氮?dú)馀欧潘俾蕼y(cè)定的準(zhǔn)確度。對(duì)于硝酸鹽含量較高、厭氧程度較高的濕地土壤, 一種基于氧化亞氮15N同位素自然豐度的方法已經(jīng)被證明能夠用于原位無(wú)擾動(dòng)濕地生態(tài)系統(tǒng)的氮?dú)馀欧磐康臏y(cè)定。Qin等[62]利用自然豐度15N同位素分餾理論與He/N2置換技術(shù), 集成創(chuàng)新了土壤反硝化測(cè)定方法體系。該方法的原理是利用在氧化亞氮還原為氮?dú)膺^(guò)程中的自然豐度同位素分餾原理, 將15N同位素豐度值與N2O/N2排放比率之間建立經(jīng)驗(yàn)函數(shù)關(guān)系, 利用氧化亞氮通量與N2O同位素的值來(lái)推算田間原位無(wú)擾動(dòng)氮?dú)馀欧磐?。該方法的?yōu)點(diǎn)是測(cè)定過(guò)程對(duì)被測(cè)定系統(tǒng)的擾動(dòng)很小, 測(cè)定值反映田間實(shí)際氮?dú)馀欧徘闆r。缺點(diǎn)是無(wú)法測(cè)定除反硝化途徑以外其他途徑產(chǎn)生的氮?dú)?15N同位素豐度值與N2O/N2排放比率建立的經(jīng)驗(yàn)函數(shù)隨不同土壤類(lèi)型及環(huán)境條件有一定變異性, 會(huì)對(duì)試驗(yàn)結(jié)果造成一定的誤差; 最后, 氧化亞氮穩(wěn)定同位素自然豐度法目前只證明適用于受氮素面源污染的濕地與河流底泥、稻田等厭氧程度較高, 氮素含量較高的土壤類(lèi)型, 而對(duì)于旱地土壤硝化—反硝化過(guò)程來(lái)說(shuō), 這兩種途徑產(chǎn)生的氧化亞氮具有不同的氮同位素值范圍, 當(dāng)這兩種途徑對(duì)氧化亞氮產(chǎn)生量的貢獻(xiàn)發(fā)生變化時(shí), 理論上會(huì)導(dǎo)致氧化亞氮15N同位素豐度值產(chǎn)生變化, 進(jìn)而影響15N同位素豐度值與N2O/N2排放比率建立的經(jīng)驗(yàn)函數(shù)的變化, 從而導(dǎo)致試驗(yàn)誤差的出現(xiàn)。

3 土壤氮?dú)馀欧诺沫h(huán)境和微生物調(diào)控機(jī)制

3.1 氧氣

在反硝化、厭氧氨氧化與共反硝化這3種途徑中, 反硝化是最早被確認(rèn)的氮?dú)馀欧磐緩? 也是研究最多的一條途徑。由于氧化亞氮還原酶需要在厭氧條件下才能保持活性, 因此厭氧環(huán)境被認(rèn)為是反硝化發(fā)生的必要條件。傳統(tǒng)土壤學(xué)理論認(rèn)為反硝化過(guò)程是一個(gè)嚴(yán)格的厭氧過(guò)程[63], 但是相關(guān)研究表明: 在土壤中存在氧氣/硝酸鹽共呼吸(co-respiration)現(xiàn)象[64], 陸續(xù)從土壤中分離到了一些能在好氧條件下產(chǎn)生氮?dú)獾姆聪趸⑸锞闧65-67]。1984年, Robertson等[68]在反硝化處理系統(tǒng)中首次分離出好氧反硝化菌, 并發(fā)現(xiàn)了好氧反硝化酶系的存在, 同時(shí)證實(shí)了在脫氮副球菌()生長(zhǎng)過(guò)程中, 如果氧氣和硝酸鹽共同存在, 其生長(zhǎng)速率會(huì)比二者單獨(dú)存在時(shí)高[68-70]。目前, 越來(lái)越多的證據(jù)表明: 土壤微生物可能在好氧條件下進(jìn)行反硝化脫氮[65,71-75]。由于土壤中存在團(tuán)聚體等厭氧微域, 土壤中的好氧反硝化作用還一直沒(méi)有被證實(shí)。Qin等[76]最近利用低溫低速離心, 將土壤微生物與土壤顆粒相分離, 排除潛在的厭氧微域后, 利用前期改進(jìn)的反硝化研究新方法證實(shí): 土壤微生物能通過(guò)好氧反硝化作用產(chǎn)生氮?dú)?。上述結(jié)果表明: 厭氧環(huán)境可能不是土壤反硝化脫氮的必要條件, 反硝化脫氮可能廣泛存在于好氧與厭氧環(huán)境中。具體的好氧反硝化氮?dú)馀欧艡C(jī)制還有待于進(jìn)一步研究。

厭氧氨氧化被認(rèn)為在高度厭氧的土壤中對(duì)氮?dú)馀欧啪哂幸欢ㄗ饔? 最高可占到氮?dú)馀欧帕康?0%[77-78]。Shan等[79]綜合運(yùn)用氮同位素示蹤法與N2/Ar比率法測(cè)定了中國(guó)11種典型水稻土的厭氧氨氧化速率, 發(fā)現(xiàn)厭氧氨氧化占總氮?dú)鈸p失比率的4.48%~9.23%。厭氧氨氧化氮?dú)馀欧潘俾逝c土壤可溶性有機(jī)碳和硝酸鹽含量呈顯著正相關(guān)關(guān)系, 而在旱地土壤中, 厭氧氨氧化對(duì)氮?dú)馀欧诺呢暙I(xiàn)普遍認(rèn)為比在濕地土壤中低(約0.3%~37%)。值得注意的是, 以往的研究只是測(cè)定了土壤通過(guò)厭氧氨氧化排放氮?dú)獾臐摿? 由于培養(yǎng)條件與田間實(shí)際條件差異巨大, 因此上述厭氧氨氧化在氮?dú)饪偱欧帕恐兴嫉谋嚷士赡芘c田間實(shí)際存在較大差異。

3.2 可溶性有機(jī)碳與硝酸鹽

可溶性有機(jī)碳被認(rèn)為是異養(yǎng)反硝化微生物提供自身增值所需的有機(jī)碳源, 同時(shí)為微生物提供電子供體進(jìn)行反硝化作用[80]。Qin等[81-82]通過(guò)研究硝酸鹽、可溶性有機(jī)碳與氧氣含量等關(guān)鍵環(huán)境因素對(duì)土壤反硝化速率及產(chǎn)物構(gòu)成的調(diào)控機(jī)制, 發(fā)現(xiàn)碳源增加顯著提升土壤反硝化速率, 同時(shí)降低了反硝化產(chǎn)物中N2O/N2的比率。在深層土壤, 特別是植物根層以下土壤中, 可溶性有機(jī)碳含量普遍較低, 可溶性有機(jī)碳匱乏而不是反硝化微生物數(shù)量是限制深層土壤反硝化氮?dú)馀欧潘俾实年P(guān)鍵因素。最近又有研究表明: 除可溶性有機(jī)碳以外的其他反硝化電子供體, 比如通過(guò)電極直接提供電子, 可以強(qiáng)化深層缺碳土壤反硝化氮?dú)馀欧潘俾蔥83]。

硝酸鹽是反硝化作用的底物, 大部分文獻(xiàn)認(rèn)為反硝化速率與土壤硝酸鹽濃度呈正相關(guān)關(guān)系。但是高濃度硝酸鹽會(huì)抑制氧化亞氮還原酶活性[84-86], 導(dǎo)致反硝化產(chǎn)物中N2O/N2比率升高。

3.3 微生物群落結(jié)構(gòu)與功能基因

反硝化其實(shí)是由硝酸還原酶基因(、)、亞硝酸還原酶基因(、)、一氧化氮還原酶基因(、)和氧化亞氮還原酶基因()驅(qū)動(dòng)將硝酸鹽轉(zhuǎn)換為亞硝酸鹽、一氧化氮、氧化亞氮和氮?dú)獾倪^(guò)程, 長(zhǎng)期以來(lái)一直以為氧化亞氮還原酶由典型反硝化細(xì)菌具有的基因編碼[87]。然而, 近幾年卻發(fā)現(xiàn)土壤中存在大量未知的非典型反硝化細(xì)菌, 這些細(xì)菌不具有完整的反硝化能力, 只具有氧化亞氮還原能力, 是全世界巨大的氧化亞氮匯[88-89]。2014年, Jones等[89]發(fā)現(xiàn)歐洲土壤中的非典型反硝化菌的數(shù)量和系統(tǒng)發(fā)育多樣性決定了歐洲陸地生態(tài)系統(tǒng)的氧化亞氮匯的容量。非典型反硝化細(xì)菌的基因(CladeⅡ)與典型反硝化細(xì)菌的基因(CladeⅠ)在系統(tǒng)發(fā)育方面距離比較遠(yuǎn), 同時(shí)在基因結(jié)構(gòu)和調(diào)控方面差異也較大。目前還沒(méi)有系統(tǒng)地開(kāi)展土壤氧化亞氮還原細(xì)菌生態(tài)學(xué)的研究, 對(duì)典型農(nóng)田土壤基因CladeⅠ和CladeⅡ群落組成和豐度知之甚少, 土壤氧化亞氮還原菌與氧化亞氮匯效應(yīng)的關(guān)系尚不明確。在草地生態(tài)系統(tǒng)中, 土壤真菌可以通過(guò)共反硝化作用產(chǎn)生氮?dú)鈁1,17], 但是目前農(nóng)業(yè)生態(tài)系統(tǒng)中真菌共反硝化研究的認(rèn)知還未完全清晰, 需要更深層次的研究。

4 結(jié)論

目前對(duì)典型生態(tài)系統(tǒng)(如旱地農(nóng)田、稻田、森林、草地與濕地)的土壤氮?dú)馀欧磐咳匀蝗狈μ镩g原位觀測(cè)數(shù)據(jù), 基于室內(nèi)間接測(cè)定的氮?dú)馀欧磐烤哂休^大不確定性, 與原位通量存在較大差異。土壤氮?dú)馀欧诺难芯咳匀皇苤朴谘芯糠椒ǖ南拗? 雖然近年來(lái)發(fā)展了一些新的技術(shù), 仍然缺乏廣譜、可靠與簡(jiǎn)便的土壤原位氮?dú)馀欧艤y(cè)定方法。研發(fā)新的田間原位無(wú)擾動(dòng)測(cè)定技術(shù)仍然是土壤氮?dú)馀欧叛芯康氖滓獑?wèn)題。就目前的技術(shù)水平來(lái)說(shuō), 多種方法聯(lián)合運(yùn)用是提高土壤氮?dú)馀欧磐繙y(cè)定精度的一種有效策略。近年來(lái)發(fā)現(xiàn)的土壤氮?dú)馀欧判峦緩?如厭氧氨氧化、共反硝化與好氧氮?dú)馀欧胚^(guò)程)對(duì)土壤氮?dú)馀欧诺南鄬?duì)貢獻(xiàn)及相關(guān)的微生物學(xué)機(jī)制是目前研究的熱點(diǎn)。典型生態(tài)系統(tǒng)土壤氮?dú)馀欧哦颗c模擬、未來(lái)環(huán)境變化(如氮沉降增加、土壤酸化、溫度升高等)對(duì)土壤氮?dú)馀欧诺挠绊憴C(jī)制是本領(lǐng)域的重要研究方向。

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張志君, 秦樹(shù)平, 袁海靜, 張玉銘, 胡春勝. 土壤氮?dú)馀欧叛芯窟M(jìn)展[J]. 中國(guó)生態(tài)農(nóng)業(yè)學(xué)報(bào), 2018, 26(2): 182-189

ZHANG Z J, QIN S P, YUAN H J, ZHANG Y M, HU C S. Advance in soil dinitrogen emission[J]. Chinese Journal of Eco-Agriculture, 2018, 26(2): 182-189

Advance in soil dinitrogen emission*

ZHANG Zhijun1, QIN Shuping1**, YUAN Haijing1, ZHANG Yuming2, HU Chunsheng2

(1. Key Laboratory of Soil Environment Health and Regulation in Fujian Province / College of Resources and Environment, Fujian Agricultural and Forestry University, Fuzhou 350002, China; 2. Center for Agricultural Resources Research, Institute of Genetics and Developmental Biology, Chinese Academy of Sciences, Shijiazhuang 050022, China)

The amount of applied nitrogen fertilizer has increased dramatically since the invention of the industrial ammonia synthesis in the early 20th century. In some countries or regions, the amount of nitrogen fertilizer input has exceeded crop nitrogen demand. This has led to a large amount of nitrogen losses to the environment, causing environmental pollution such as ammonia volatilization, nitrous oxide emission and groundwater contamination. Soil microbes can transform active nitrogen into inert dinitrogen and consequently remove superfluous nitrogen from soil via denitrification and anammox. Direct and precise measurement of soil denitrification has been a continuous challenge due to high background concentration of atmospheric dinitrogen, which has hindered progress in research on soil dinitrogen emissions. This paper reviewed the main pathways of soil dinitrogen emission [denitrification, dissimilatory nitrate reduction to ammonium (DNRA) and co-denitrification] and their contributions to soil dinitrogen emission. It also covered the methods of soil dinitrogen flux determination (acetylene inhibition technique,15N tracing method, N2/Ar membrane-inlet mass spectrometry, helium environment method and natural abundance15N2O isotopic method) and their advantages, disadvantages. The key factors regulating soil dinitrogen emission (oxygen, dissolved organic carbon, nitrate, microbial community structure and functional gene expression) and the related mechanisms were also summarized. In conclusion, it was essential to develop new methods fordinitrogen flux determination in undisturbed soils. More studies were needed to quantify soil dinitrogen flux in typical ecosystems (such as dryland, farmland, forest, grassland and wetland), clarify microbial mechanism involved, and simulate and predict the responses of soil dinitrogen emission to global change.

Soil;Dinitrogen emission; Denitrification; Anaerobic ammonia oxidation; Nitrous oxide emission; Nitrogen loss

, E-mail: qinshuping@sjziam.ac.cn

Nov. 21, 2017;

Dec. 4, 2017

10.13930/j.cnki.cjea.171070

X51; S154.1

A

1671-3990(2018)02-0182-08

秦樹(shù)平, 主要從事土壤反硝化方法學(xué)及反硝化脫氮機(jī)理研究。E-mail: qinshuping@sjziam.ac.cn 張志君, 主要研究方向?yàn)楹醚鯒l件下土壤性質(zhì)對(duì)乙炔抑制法測(cè)定誤差的影響。E-mail: 15895217170@163.com

2017-11-21

2017-12-04

* This study was supported by the National Natural Science Foundation of China (41530859, 41771331, 41571291).

* 國(guó)家自然科學(xué)基金重點(diǎn)項(xiàng)目(41530859)與國(guó)家自然科學(xué)基金面上項(xiàng)目(41771331, 41571291)資助