摘要:土壤氮素流失對(duì)土壤生產(chǎn)力、地下水環(huán)境以及氮素整體循環(huán)造成巨大影響,這些流失的氮素形態(tài)主要包括銨態(tài)氮、硝態(tài)氮和可溶性有機(jī)氮,由于氮素形態(tài)易轉(zhuǎn)化且土壤環(huán)境因素繁雜,氮素遷移過程的研究結(jié)果存在爭議。本文總結(jié)了近10年國內(nèi)外研究中土壤氮素遷移淋失、轉(zhuǎn)化淋失的機(jī)制,以及遷移過程的影響因素。結(jié)果表明:農(nóng)業(yè)措施、作物類型、氣候是影響氮素遷移的主要宏觀因素;土壤陽離子交換量、pH、有機(jī)質(zhì)、土壤黏土礦物、鐵錳氧化物為主要微觀影響因素;土壤硝化-反硝化、同化、礦化相關(guān)微生物和酶調(diào)控氮素轉(zhuǎn)化間接影響氮素遷移。通過對(duì)不同地區(qū)、不同氮素形態(tài)和不同土壤條件中的氮素遷移特征的比較,發(fā)現(xiàn)氮素遷移的主要驅(qū)動(dòng)因素仍然模糊,因素間相互作用的效應(yīng)對(duì)氮素遷移的影響也存在研究空白。本文對(duì)土壤氮素遷移轉(zhuǎn)化的機(jī)制進(jìn)行探討、總結(jié),可為改善氮肥管理方案提供參考。
關(guān)鍵詞:土壤氮素;遷移;淋失;形態(tài);氮循環(huán)
中圖分類號(hào):X71;X52;S143.1 文獻(xiàn)標(biāo)志碼:A 文章編號(hào):2095-6819(2025)02-0277-11 doi: 10.13254/j.jare.2023.0825
氮(N)是農(nóng)作物從土壤中吸收量最大的礦質(zhì)元素,在農(nóng)業(yè)生態(tài)系統(tǒng)中,氮素來源主要包括無機(jī)化肥氮素、秸稈有機(jī)氮和畜禽糞便有機(jī)肥氮,多源氮素的投入使土壤中的總氮量通常較高,但可供作物利用的有效氮素含量卻常不足。一方面,作物氮需求與土壤氮投入之間的時(shí)空不匹配導(dǎo)致土壤氮素供需矛盾,影響作物生長和產(chǎn)量[1]。另一方面,農(nóng)業(yè)生產(chǎn)中的氮投入會(huì)通過徑流、淋溶等途徑流失。氮素的遷移流失既損害了農(nóng)業(yè)生產(chǎn),還引起地表水富營養(yǎng)化、地下水中氮超標(biāo)、水質(zhì)型缺水等一系列環(huán)境問題[2–4]。
氮素與作物產(chǎn)量密切關(guān)系,提高氮素利用效率會(huì)帶來產(chǎn)量的增加和氮污染的減少[5-6],因此提升全球氮肥利用效率(NUE)成為新的關(guān)注點(diǎn)[7-8]。但氮素的多種形態(tài):銨態(tài)氮(NH+4 -N)、硝態(tài)氮(NO-3 -N)、可溶性有機(jī)氮(DON)等易隨水體遷移流失,追蹤和管理的難度大,成為阻礙NUE提高的主要原因[8-9],農(nóng)業(yè)管理、種植作物類型、氣候和地形因素會(huì)對(duì)氮素的歸趨產(chǎn)生宏觀的影響;土壤礦物和微生物對(duì)氮的吸附和利用也會(huì)在微觀上影響氮素的遷移轉(zhuǎn)化,并且不同源氮素受這些因素影響程度存在差異[1]。這些影響氮遷移、轉(zhuǎn)化的基本機(jī)制的理解存在片面性,導(dǎo)致難以全面掌握多源氮素的遷移、轉(zhuǎn)化規(guī)律[10–12],因此需要針對(duì)土壤多源氮素遷移和循環(huán)的關(guān)鍵過程,及其主要的生物地球化學(xué)驅(qū)動(dòng)因素進(jìn)行全面總結(jié)。在我國,氮素通過地表徑流、淋溶造成面源污染,尤其在坡耕地、黃土高原等地尤其嚴(yán)峻。目前國內(nèi)對(duì)地形、土地利用方式等方面對(duì)氮素遷移問題進(jìn)行定性或定量分析的研究較為深入,但缺乏針對(duì)氮素遷移過程中阻控因子的辨析[13]。同樣地,國外重點(diǎn)研究灌溉、土壤水體運(yùn)動(dòng)對(duì)NO-3 -N遷移的控制,以及在不同生態(tài)系統(tǒng)中不同形態(tài)氮素的轉(zhuǎn)化[1],而氮淋失遷移情況在不同土壤類型中受到多種因素調(diào)控[10–12]。土壤中的生物因素,包括微生物群落、土壤氮循環(huán)相關(guān)酶活性、植物根系等,對(duì)氮素在土壤中形態(tài)轉(zhuǎn)換起重要作用,但氮遷移研究中較少將氮轉(zhuǎn)化過程結(jié)合探討,明確生物的轉(zhuǎn)化作用可以為制定從源頭減少氮的遷移量的技術(shù)措施提供理論依據(jù)。
本文首先梳理氮素的遷移轉(zhuǎn)化特征,進(jìn)一步對(duì)影響氮素遷移的氣候、地形等宏觀因素,以及關(guān)鍵生物與非生物的微觀因素進(jìn)行了總結(jié)分析,以期為相關(guān)研究提供參考,更好地指導(dǎo)農(nóng)業(yè)生產(chǎn)。
1 氮素來源和遷移轉(zhuǎn)化
1.1 氮素來源與賦存特征
土壤中氮素含量和形態(tài)因土壤類型、氣候、植被覆蓋、人類活動(dòng)等因素而不同。一般來說,土壤中的氮分為無機(jī)氮和有機(jī)氮,無機(jī)氮包括可交換性氮和非交換性氮,其中可交換性氮是作物可直接吸收利用的速效氮,主要以游離形態(tài)即NH+4 -N、NO-3 -N、亞硝態(tài)氮(NO-2 -N)存在于土壤中,而非交換性氮主要以固定態(tài)銨固存在土壤中難以直接被作物利用。有機(jī)氮占土壤氮庫的95% 以上,主要存在于未完全分解的動(dòng)植物殘?bào)w和土壤有機(jī)質(zhì)(SOM)中,由于土壤有機(jī)質(zhì)化學(xué)形態(tài)和賦存狀況可以調(diào)控土壤微生物活動(dòng),進(jìn)而維持或提高土壤氮素供應(yīng),因此有機(jī)氮成為作物氮素吸收主要形態(tài)——礦質(zhì)氮的源和庫[14]。
如圖1所示,土壤氮素主要來源于肥料施入和大氣沉降,土壤含氮物質(zhì)轉(zhuǎn)化改變其賦存形態(tài)。無論是化肥、有機(jī)肥等多源氮素還是天然存在的氮,進(jìn)入農(nóng)田土壤后均有一部分被作物吸收或以有機(jī)態(tài)氮、固定態(tài)銨等形態(tài)貯存于土壤中[8],其余氮素容易通過氨化作用、硝態(tài)氮異化還原作用轉(zhuǎn)化為NH+4 -N進(jìn)行氨揮發(fā)損失,或通過硝化作用轉(zhuǎn)化為NO-3 -N、NO-2 -N,成為面源污染的主要氮素來源[15-16]。無機(jī)氮肥容易引發(fā)徑流和滲漏損失[17],而有機(jī)氮肥主要以SOM 或DON和轉(zhuǎn)化作用后的無機(jī)氮形態(tài)發(fā)生淋溶導(dǎo)致深層土壤高氮?dú)埩鬧18]。目前認(rèn)為土壤氮素固持能力與土壤氮轉(zhuǎn)化特點(diǎn)決定的氮素主導(dǎo)形態(tài)與調(diào)控措施、氣候條件以及所處的土壤環(huán)境條件有密切關(guān)系[19]。
1.2 土壤中不同形態(tài)氮的遷移轉(zhuǎn)化
在氮循環(huán)中控制土壤氮的形態(tài)是控制氮素遷移損失的關(guān)鍵[20-21]。不同形態(tài)氮素遷移能力不同,農(nóng)業(yè)生態(tài)系統(tǒng)中土壤氮素易遷移的形態(tài)主要是土壤中無機(jī)態(tài)氮(NH+4 -N、NO-3 -N)和DON的遷移,而土壤中氮素養(yǎng)分的淋溶損失必須同時(shí)具備兩個(gè)條件,一是土壤中有氮素養(yǎng)分積累可供淋失;二是有下滲水流,使得氮素養(yǎng)分可隨之向深層土壤剖面移動(dòng)進(jìn)而發(fā)生淋溶損失[17],其中,土壤氮素的剖面積累是造成其淋溶損失的首要條件[22]。土壤礦質(zhì)氮積累主要受氮素轉(zhuǎn)化過程調(diào)控,包括硝化-反硝化過程、微生物對(duì)有效態(tài)氮的固化過程以及含氮有機(jī)質(zhì)的礦化等[23](圖1)。
當(dāng)無機(jī)氮肥輸入土壤時(shí),將NH+4 -N氧化成NO-3 -N的硝化作用是生態(tài)系統(tǒng)氮素轉(zhuǎn)化的重要途徑,作用過程中產(chǎn)生的NO-3 -N 是氮素淋溶損失的主要氮形態(tài)[24]。NO-3 -N的形態(tài)轉(zhuǎn)化可減少易淋失的氮形態(tài)。這種轉(zhuǎn)化主要有三種還原的途徑:第一,NO-3 -N異化還原作用[25];第二,土壤氮同化作用[26];第三,反硝化作用[27](圖1)。
當(dāng)有機(jī)氮肥輸入土壤中時(shí),土壤氮素礦化即微生物作用下土壤有機(jī)氮產(chǎn)生NH+4 -N。這一過程是在一系列土壤胞外酶(水解酶、氧化酶、裂解酶和脫氫酶等)的作用下進(jìn)行的[28]。大量研究發(fā)現(xiàn)DON能夠從需氮量高的生態(tài)系統(tǒng)中流失,并且DON是生態(tài)系統(tǒng)中持續(xù)恒定的微小氮素?fù)p失的主要來源,是限制生態(tài)體系生產(chǎn)與氮素貯存的重要因素[29]。同樣,稻田研究發(fā)現(xiàn)DON的淋溶和徑流是稻田最為顯著的氮素淋失途徑[30]。
2 宏觀因素對(duì)氮素遷移的影響
2.1 農(nóng)業(yè)管理措施對(duì)氮素遷移的影響
2.1.1 施肥
目前農(nóng)業(yè)管理措施通過肥料管理、水氮耦合、耕作方式等方面控制氮素?fù)p失。首先,肥料管理被證實(shí)與農(nóng)田氮遷移流失直接關(guān)聯(lián)[31](圖1)。根據(jù)15N同位素監(jiān)測(cè)長江流域稻田發(fā)現(xiàn),無機(jī)氮肥施用量超過170kg·hm-2時(shí),氮素?fù)p失會(huì)急劇增加[32-33],超過45%的氮素從水稻-土壤體系向周圍環(huán)境擴(kuò)散[34–36]。在太湖地區(qū)長期輪作試驗(yàn)也發(fā)現(xiàn)徑流氮素?fù)p失與無機(jī)氮肥施用量呈線性正相關(guān)[37]。雖然不同地區(qū)的響應(yīng)存在差異但結(jié)論一致:無機(jī)氮肥施用量的增加會(huì)加劇土壤氮素向環(huán)境擴(kuò)散的風(fēng)險(xiǎn)[2,38-41],進(jìn)一步發(fā)現(xiàn)這是由于施用大量無機(jī)氮肥后破壞了土壤氮素的平衡,超出土壤固氮能力范圍導(dǎo)致農(nóng)田的氮素遷移增加[8,32–34]。而氮素用量相似情況下,由于有機(jī)氮肥以有機(jī)形態(tài)存在,土壤無機(jī)氮含量短期沒有發(fā)生巨大變化,因此土壤氮素遷移量波動(dòng)相應(yīng)較小[2,42],理論上有機(jī)氮肥替代無機(jī)氮可以減少氮素遷移,但研究發(fā)現(xiàn)以動(dòng)物糞便形態(tài)施用的有機(jī)肥存在增加氮素淋失的現(xiàn)象,而長期實(shí)地試驗(yàn)中,施用綠肥的有機(jī)系統(tǒng)氮素淋失風(fēng)險(xiǎn)高于常規(guī)系統(tǒng)[43]。因此有學(xué)者指出當(dāng)土壤本身含氮量高時(shí),施用有機(jī)肥會(huì)升高農(nóng)田DON含量,形成以DON為主導(dǎo)的氮素淋溶[44]。
此外,不同肥料施用時(shí)期也會(huì)顯著影響氮素去向。例如,在水稻分蘗期和穗分化期施充足氮量會(huì)增加作物吸收肥料氮素比例[35],同時(shí)降低相應(yīng)的氮?dú)埩艉偷獡p失,因?yàn)樵摃r(shí)期施氮促進(jìn)了無機(jī)氮肥供應(yīng)與作物氮需求之間的同步性[34,41],但根據(jù)長期、高頻次的田間觀測(cè)發(fā)現(xiàn),即使在水稻生長時(shí)期及時(shí)施肥,稻田水體中的氮濃度也會(huì)在施肥當(dāng)天達(dá)到峰值,隨后在15 d內(nèi)經(jīng)歷快速下降后趨于穩(wěn)定的變化過程,說明氮肥施用初期均屬于氮遷移淋溶高風(fēng)險(xiǎn)期[39,41,45]。綜上所述,在施肥時(shí)期需要考慮作物的生長發(fā)育階段及其對(duì)氮素需求,盡量匹配各時(shí)期需氮量,減少氮素遷移淋溶風(fēng)險(xiǎn)。
2.1.2 灌溉
漫灌和滴灌是研究較多的兩種灌溉方式。漫灌方式易形成水層,對(duì)土壤產(chǎn)生較大的水勢(shì)壓,進(jìn)而增大滲漏量,氮素淋溶量隨著滲漏量增加而增加[46]。滴灌方式將氮肥溶于溶液滴灌至土壤,該條件下NO-3 -N淋洗量均低于漫灌處理[47-48],灌溉方式由漫灌改為滴灌后總氮損失量減少的報(bào)道也較多[45,49-50]。改善灌溉方式已經(jīng)確定可以減少氮素淋失,但灌溉施肥技術(shù)與氮素循環(huán)、氮素?fù)p失的關(guān)系仍有爭議。寧夏回族自治區(qū)的氮污染分析中指出,肥料的施用是灌溉區(qū)水體氮污染的主要原因[51],而我國華北平原小麥-玉米種植制度研究獲得相反結(jié)論:灌溉量而非施肥量是氮素淋溶的主要影響因素,在同等施氮量的情況下,滴灌比漫灌使土層總氮淋失量減少了17.5%[47],節(jié)省18%~36%的灌溉用水會(huì)使氮淋失降低15%~37%[52]。除了研究區(qū)域的差異,結(jié)論的爭議可能是由于其他因素的影響,在50年的水文模擬中,雖然漫灌到滴灌的轉(zhuǎn)變降低15% 的氮遷移淋失率,但漫灌和滴灌的長期表現(xiàn)受到降水變化控制的重大年際差異的影響,滴灌方式下氮淋失對(duì)年度氣象條件的變化更敏感,并且在肥料投入相似的情況下,豐水年兩種灌溉方式對(duì)氮淋失的影響結(jié)果相似[53]。作為重要的農(nóng)業(yè)措施,改善灌溉技術(shù)可被列為氮淋失緩解策略,此外,灌溉技術(shù)帶來的田間氮素潛在變化及其他因素的強(qiáng)烈干擾也需要更多研究與實(shí)踐。
2.1.3 種植模式
在農(nóng)業(yè)管理措施中,種植模式的選擇直接影響施肥后氮素在土壤中的遷移。不同作物搭配生長在協(xié)同提高作物產(chǎn)量、降低無機(jī)氮徑流污染方面具有很大潛力[54]。目前研究較多的是套種作物和填閑作物對(duì)氮素淋失的調(diào)控。作物的合理搭配種植對(duì)土壤無機(jī)氮淋洗的降低作用主要通過淺根系和深根系對(duì)不同土壤層次氮素的全面吸收,同時(shí)交叉的根系網(wǎng)能夠充分利用土壤空間的水分,起到降低水分淋溶和土壤無機(jī)氮含量的作用[55]。根據(jù)此理論,基于丹麥長期作物輪作試驗(yàn)發(fā)現(xiàn),無論在有機(jī)還是常規(guī)化肥管理系統(tǒng)中套種作物均能減少60% 的氮素淋失,但豆科和非豆科套種作物減少氮素淋失風(fēng)險(xiǎn)的能力相當(dāng)[43],無論有機(jī)還是常規(guī)系統(tǒng)中填閑作物均能減少氮素淋失且有機(jī)系統(tǒng)中氮的淋失風(fēng)險(xiǎn)高于其他系統(tǒng)[43]。同樣,耕作方式的改變也影響氮素遷移。國外學(xué)者發(fā)現(xiàn)免耕的耕作方式下土壤無機(jī)氮淋洗率低于翻耕[56],而國內(nèi)學(xué)者發(fā)現(xiàn)玉米收獲后免耕處理的土壤硝酸鹽積累量高,氮淋失增加[57]。這說明免耕對(duì)氮淋失風(fēng)險(xiǎn)的阻控效應(yīng)較低且呈現(xiàn)較大不確定性,原因在于耕作通過影響土壤水分和溫度從而影響氮遷移,但自然條件和作物類型的差異會(huì)直接影響耕作措施對(duì)氮淋失的影響效果[58]。因此在選擇合適的耕作方式和種植模式時(shí),需要綜合多方面因素,以達(dá)到提高農(nóng)業(yè)生產(chǎn)力和減少環(huán)境問題的目標(biāo)。
2.2 氣候和地形對(duì)氮素遷移的影響
2.2.1 降雨
土壤氮素遷移主要隨水體流動(dòng),造成土壤水體變化的主要因素包括降雨和灌溉兩種情況。在灌溉系統(tǒng)管理良好地區(qū),降雨特征(包括降雨量、持續(xù)降雨時(shí)間和強(qiáng)度)成為總氮流失量的關(guān)鍵驅(qū)動(dòng)因子[45]。施肥后立即發(fā)生的早稻季降雨會(huì)導(dǎo)致氮素養(yǎng)分的流失,其中降雨量與土壤總氮流失量呈極顯著正相關(guān),這表明降雨-徑流過程是區(qū)域內(nèi)養(yǎng)分流失的主要驅(qū)動(dòng)因素[45],施肥后的徑流氮損失受降雨事件影響較大,無論如何耕作管理,降雨期間暴露于強(qiáng)降雨的表層土壤中可提取的土壤硝酸鹽含量平均高32%,耕作系統(tǒng)中降雨的加劇將增加氮向地下水的淋失[59]。在美國中西部,降雨改變了種植制度上的水分布局,增加了深層滲漏,而這些改變會(huì)影響氮的運(yùn)轉(zhuǎn)[60]。在農(nóng)作物的耕作過程中,肥料的施用和土壤無機(jī)氮的大量擾動(dòng)可能使耕作系統(tǒng)更容易受到降雨強(qiáng)度的影響,特別是在施用肥料后不久發(fā)生極端事件的情況下,例如在我國西南紫色土區(qū)域,暴雨期間降雨強(qiáng)度的增加會(huì)增大氮的徑流濃度使其更易淋失[61]。而在干旱和半干旱生態(tài)系統(tǒng)中,長時(shí)間的干旱使得無機(jī)氮肥和原位無機(jī)氮在土壤中積累,快速的再濕潤往往會(huì)產(chǎn)生分解和凈氮礦化[59],降雨會(huì)進(jìn)一步擴(kuò)大氮供應(yīng)和需求之間的不同步,導(dǎo)致氮的淋溶損失。
2.2.2 溫度、濕度
溫度、濕度影響土壤氮素的總氮礦化,一般認(rèn)為氮礦物速率隨溫度的升高和濕度的增大而增加,且二者存在明顯的正交互作。而土壤的溫度、濕度達(dá)到一定范圍時(shí),氮礦化反而迅速下降,這可能由于溫度的升高增強(qiáng)了土壤微生物的活動(dòng),從而使得氮轉(zhuǎn)化過程加劇[62]。例如同一地區(qū)早稻季化肥用量少于晚稻季,但早稻的氮肥利用效率相對(duì)較低,可能由于早稻期溫度較低影響稻田氮利用和氮損失[41]。而在不同氣候區(qū)如地中海[63]和亞熱帶地區(qū)[64],氣候直接影響濕度,間接影響氮素隨水體的遷移[45],氣候還可以通過影響覆蓋作物從而減少氮素淋溶[65–67]。在對(duì)氮負(fù)荷的預(yù)測(cè)中,氣候變化也被認(rèn)為是影響小流域氮徑流和氮污染的重要因素[68]。
2.2.3 地形
鑒于土壤水過程對(duì)氮遷移和轉(zhuǎn)化的重要性,影響土壤水過程的因素可以解釋氮?jiǎng)討B(tài)的顯著變化,氣候[69]和地形[70]等因素在區(qū)域尺度上影響了水分的空間分布,同樣也影響土壤氮的運(yùn)輸和轉(zhuǎn)化。坡耕地水土流失嚴(yán)重,氮淋失現(xiàn)象頻發(fā)[71],黃土高原丘陵溝壑地區(qū)土壤水氮難以循環(huán),氮素與其他土壤養(yǎng)分流失嚴(yán)重[72]。此外,喀斯特山地中特殊的地形也會(huì)導(dǎo)致土壤氮的空間異質(zhì)性,氮的有效態(tài)方面,石縫gt;石洞gt;土面gt;石坑gt;石溝,說明石縫微地貌的土壤活性有機(jī)碳、堿解氮及速效磷空間異質(zhì)性較低[73]。土壤作為氮等營養(yǎng)物質(zhì)和有機(jī)碳的主要陸地儲(chǔ)存庫,氮添加后山坡地形比山谷地形在蛋白質(zhì)解聚、硝化速率方面更高,導(dǎo)致氮淋失較多[74]。地形還可通過影響土壤溫度和濕度[75],進(jìn)而影響土壤速效氮去向[76]。目前地形位置調(diào)節(jié)土壤轉(zhuǎn)化對(duì)氮添加的響應(yīng)從而影響氮淋溶損失的觀點(diǎn)缺少相應(yīng)探討[74]。
農(nóng)業(yè)管理、氣候和地形對(duì)土壤氮素的影響主要通過水氮耦合過程實(shí)現(xiàn),即土壤中的水控制氮素的遷移、固持和轉(zhuǎn)化[77]。水分運(yùn)移是氮素運(yùn)移的重要驅(qū)動(dòng)力,而地形影響水的流向,同樣也影響了氮素的遷移。除了宏觀因素,土壤中氮的含量和化學(xué)組成也受微觀土壤水分過程的影響。在水分作用下,氮素遷移受其在土壤礦物和有機(jī)質(zhì)上的吸附解吸控制。由于不同形態(tài)氮素遷移能力存在差異,因此調(diào)控氮素形態(tài)轉(zhuǎn)化的微生物也對(duì)氮遷移產(chǎn)生了貢獻(xiàn)。
3 土壤微觀因素對(duì)氮素遷移的影響
3.1 控制土壤中氮素遷移轉(zhuǎn)化的非生物因素
3.1.1 土壤組分
土壤氮素遷移的非生物控制因素主要是土壤黏土礦物、鐵錳氧化物以及土壤有機(jī)物,這些土壤組成通過吸附、固存調(diào)節(jié)氮素的遷移[1](圖2)。田間原位15N示蹤試驗(yàn)證明黑土農(nóng)田土可通過黏礦物對(duì)銨固定[78],從而提高無機(jī)氮素在土壤的保持和利用;2∶1型的黏土礦物(水云母+蒙脫石+蛭石)通常具有較高的凈負(fù)電荷和更大的比面積[11],增加了NH+4 -N在其表面的吸附,在調(diào)節(jié)氮素遷移方面發(fā)揮更多作用。此外,土壤礦物還可以與有機(jī)物形成有機(jī)無機(jī)復(fù)合體[79],進(jìn)而將土壤中游離的DON固定在復(fù)合體中阻礙有機(jī)氮的遷移[80]。對(duì)不同地區(qū)的亞熱帶土壤研究發(fā)現(xiàn),DON的吸附特性主要受土壤內(nèi)部成分影響,這些成分包括有機(jī)膠體、無機(jī)膠體以及有機(jī)無機(jī)復(fù)合體的數(shù)量及其存在形態(tài)[11]。
另一方面,不同形態(tài)氮素的轉(zhuǎn)化特征與過程決定土壤對(duì)氮素的截留進(jìn)而影響氮素遷移。在以往的研究中,土壤黏土礦物和礦物相關(guān)有機(jī)物在氮轉(zhuǎn)化的礦化作用中常常被忽視[1],但對(duì)澳大利亞亞熱帶土壤的氮礦化研究發(fā)現(xiàn),土壤物理化學(xué)性質(zhì)(尤其是黏土礦物)對(duì)氮礦化的影響比微生物的影響更重要[12]。最新的土壤氮礦化概念理論強(qiáng)調(diào)土壤黏土礦物和礦物相關(guān)有機(jī)質(zhì)是氮礦化的關(guān)鍵介質(zhì)[1]。相似地,土壤鐵錳氧化物對(duì)NH+4 -N含量的增加有促進(jìn)作用[3],這是由于這些礦物對(duì)有機(jī)質(zhì)的吸附和解吸促進(jìn)了土壤可利用氮的吸附和轉(zhuǎn)化[81-82]。
3.1.2 土壤理化性質(zhì)
如圖2所示,土壤中交換性陽離子(K+、Ca2+、Mg2+和Na+)通過置換該形態(tài)氮使其固存在土壤礦物上[83],成為影響氮素總硝化速率的重要指標(biāo),CEC含量高,銨離子易被吸附固定而不易被硝化流失[65]。土壤pH也是影響土壤氮轉(zhuǎn)化的重要因素[84]。首先,pH 與土壤硝化速率呈正相關(guān)[20,85],而硝化速率提升時(shí),pH間接提高NH+4 -N同化和NO-3 -N同化速率[26]。在NO-3 -N異化還原過程中,硝酸鹽異化還原過程會(huì)受高pH影響加強(qiáng)對(duì)NO-3 -N的競(jìng)爭[20],與此同時(shí)反硝化強(qiáng)度被削弱。而當(dāng)有機(jī)氮肥輸入時(shí),pH通過調(diào)控微生物生物量及微生物群落結(jié)構(gòu)作用于土壤氮的礦化。在一項(xiàng)基于全球統(tǒng)計(jì)研究中,氮循環(huán)中礦質(zhì)氮在土壤中的積累量受到土壤酸化的抑制[86],而pH與SOM、鐵氧化物等因素存在復(fù)雜的相互作用[20,87]。因此,pH影響了包括無機(jī)氮和有機(jī)氮在氮素淋失和固化的各個(gè)重要步驟,可能是氮素遷移轉(zhuǎn)化的重要控制因素。目前對(duì)氮素遷移轉(zhuǎn)化存在較顯著影響的非生物因素主要是上述5種土壤組分和基本理化性質(zhì),然而對(duì)于這些因素在影響氮遷移及相關(guān)轉(zhuǎn)化過程之間的協(xié)同效應(yīng)和相對(duì)貢獻(xiàn)尚不明確。
3.2 控制土壤中氮素遷移轉(zhuǎn)化的生物因素
3.2.1 硝化和反硝化作用
土壤中的微生物主要通過其功能基因表達(dá)和相關(guān)酶活性從而調(diào)控土壤氮素轉(zhuǎn)化[88],同時(shí)這些生物因素受外界環(huán)境變化對(duì)轉(zhuǎn)化各過程速率造成影響,最終改變土壤氮素的遷移能力[89]。本文總結(jié)了對(duì)氮素遷移影響較大的硝化、反硝化、同化和礦化過程中主要的調(diào)控微生物。微生物控制硝化作用分為兩步:氨氧化過程和亞硝酸鹽氧化過程[90]。氨氧化過程主要由氨氧化古菌(AOA)和氨氧化細(xì)菌(AOB)催化完成,AOA 和AOB 通過調(diào)控氨單加氧酶(AMO)催化NH+4氧化成NH2OH[91],以及羥胺氧化還原酶(HAO)催化NH2OH 氧化成NO-2 -N。亞硝酸鹽氧化過程主要由亞硝酸鹽氧化菌(NOB)催化完成[92]。NOB 通過亞硝酸鹽氧化還原酶(NXR)將NO-2 -N進(jìn)一步氧化為NO-3 -N[93–96]。在整個(gè)硝化作用中的限速步驟是氨氧化過程[97],通過測(cè)定氨氧化過程的功能基因豐度發(fā)現(xiàn),AOA控制著農(nóng)業(yè)酸性土壤的硝化過程[92]。而在石灰性土壤上,尤其土壤pH值為6.5~7.0可以觀察到最高的硝化速率[98-99],此時(shí)AOB主導(dǎo)氨氧化過程調(diào)控土壤中積累硝酸鹽的速率[100],同時(shí)也影響了土壤中氮素的循環(huán)和遷移。
參與反硝化作用的微生物種類繁多,涉及細(xì)菌、古菌、真菌和放線菌,因此通常以nirK、nosZ等為標(biāo)記基因,對(duì)環(huán)境中的反硝化微生物進(jìn)行研究。這些基因通過膜結(jié)合硝酸鹽還原酶(NAR)、亞硝酸鹽還原酶(NIR)參與NO-3 -N的消耗[101],以及調(diào)控土壤中有機(jī)氮素的轉(zhuǎn)化[102]。目前通過檢測(cè)基因豐度以及上述酶活性發(fā)現(xiàn),水稻土中反硝化速率與nosZ 豐度呈正相關(guān)關(guān)系[103-104],而反硝化作用可能會(huì)明顯導(dǎo)致土壤中無機(jī)氮的損失[104]。但反硝化微生物是否能通過上述一系列反應(yīng)與氮素遷移直接或間接聯(lián)系,以及主要起作用的菌群尚不清楚。
硝酸鹽的另一種還原方式是硝酸鹽異化還原。部分細(xì)菌和真菌在厭氧條件下可以將NO-3 -N異化還原為NH+4 -N,該過程有利于氮素在土壤中的蓄持[90]。與此相對(duì),厭氧氨氧化作為另一種重要的氮素轉(zhuǎn)化過程,在厭氧條件下將NH+4 -N和NO-2 -N轉(zhuǎn)化為N2,減少水體氮素流失。用作追蹤的hzsB基因豐度與厭氧氨氧化速率的關(guān)系也存在爭議,如在我國南方水稻土研究中,hzsB基因豐度和厭氧氨氧化速率、土壤硝酸鹽濃度呈正相關(guān),但也有hzsB基因豐度與厭氧氨氧化速率無關(guān)的報(bào)道[104],因此在土壤氮素遷移的研究中提及該過程相對(duì)較少。
3.2.2 同化和礦化作用
同化作用指土壤NH+4 -N和NO-3 -N被同化為微生物生物量氮(MBN),該作用中MBN短期存儲(chǔ)后可經(jīng)過礦化再利用,從而降低氮的遷移損失。而礦化作用又稱氨化作用,是指土壤有機(jī)組分被轉(zhuǎn)化為無機(jī)氮,由于所有微生物生長都需要氮源,以往研究認(rèn)為大部分土壤微生物類群都會(huì)參與氮素同化和礦化[105],其中細(xì)菌和真菌在其中為主導(dǎo)者[106],真菌可能比細(xì)菌更快地降解有機(jī)氮[107]。因此研究多以土壤微生物生物量、細(xì)菌和真菌的基因豐度、豐富度指數(shù)、香農(nóng)指數(shù)為指標(biāo)對(duì)比氮同化、礦化微生物對(duì)外界環(huán)境變動(dòng)的響應(yīng)。全球尺度研究發(fā)現(xiàn),土壤微生物生物量的增加能夠直接提高氮同化速率[86],但土壤氮素的硝化速率、氨化速率也會(huì)隨之增加[20,24,26],微生物介導(dǎo)的氮循環(huán)由合成代謝過程主導(dǎo)轉(zhuǎn)變?yōu)榉纸獯x過程,會(huì)增加氮素淋溶風(fēng)險(xiǎn)。此外,目前研究已經(jīng)確定氨化作用是一系列酶催化反應(yīng)的結(jié)果,包括蛋白酶、脲酶、亮氨酸氨基肽酶、β-1,4-葡萄糖苷酶[86]。酶驅(qū)動(dòng)-地球生態(tài)系統(tǒng)模型中認(rèn)為胞外酶是微生物介導(dǎo)土壤氮循環(huán)的主導(dǎo)因素[108]。相似地,例如Wang等[101]研究表明脲酶活性與土壤中礦質(zhì)氮含量呈顯著正相關(guān),與此同時(shí)酶是微生物相關(guān)功能基因的表達(dá)。因此,雖然明確了氮素遷移過程中涉及的氮素形態(tài)和含量受各種酶催化,但是具體影響遷移中氮形態(tài)和含量的關(guān)鍵酶,微生物功能基因變化與酶調(diào)節(jié)的氮?jiǎng)討B(tài)循環(huán)是否存在直接聯(lián)系等問題一直存在爭議[109-110](表1)。
4 總結(jié)與展望
與氮素遷移存在關(guān)聯(lián)的因素包括:降雨、氣溫等環(huán)境變化,土壤黏土礦物、有機(jī)質(zhì)、鐵錳氧化物等非生物因素,以及土壤生物方面的硝化-反硝化、同化、礦化相關(guān)微生物。但這些因素間的交互作用對(duì)氮素遷移的影響報(bào)道較少,現(xiàn)有的研究多數(shù)在室內(nèi)或只考慮個(gè)別因素影響的條件下進(jìn)行,并且缺乏長期觀測(cè)。氮素遷移、淋溶驅(qū)動(dòng)因素的研究多集中在單一土壤物理、生物因素或它們的簡單集合,這種過度簡化模型的研究忽略了多個(gè)因素的復(fù)雜相互作用,難以預(yù)測(cè)和評(píng)估不同管理措施對(duì)土壤氮素的綜合影響,因此土壤氮素遷移和轉(zhuǎn)化研究需要綜合考慮生態(tài)系統(tǒng)受到的干擾因素。
為了提升氮素利用效率,防控氮素流失引起的面源污染,促進(jìn)農(nóng)業(yè)生產(chǎn)的同時(shí)保障生態(tài)環(huán)境安全,今后氮素遷移的研究可以從以下幾個(gè)方面入手:
(1)多尺度關(guān)聯(lián)分析:深化多尺度研究,不僅關(guān)注微觀尺度的土壤理化性質(zhì)與微生物作用,還要拓展到宏觀尺度的區(qū)域乃至全球尺度的氮素遷移規(guī)律。綜合考慮氣候、地形、植被覆蓋等多種環(huán)境因素,建立多指標(biāo)評(píng)價(jià)體系,全面評(píng)估氮素遷移能力的影響因素。
(2)生態(tài)系統(tǒng)耦合關(guān)系:針對(duì)不同類型的生態(tài)系統(tǒng)(如稻田、林地等),研究氮素轉(zhuǎn)化過程與遷移的耦合關(guān)系,揭示不同生態(tài)系統(tǒng)中的氮素循環(huán)特點(diǎn)。
(3)微生物時(shí)空演變:在全球范圍內(nèi)開展長期、連續(xù)的監(jiān)測(cè),掌握土壤氮素遷移、轉(zhuǎn)化相關(guān)微生物的時(shí)空演變特征,深入剖析微生物群落的演替規(guī)律及其氮素遷移轉(zhuǎn)化的相關(guān)作用機(jī)制,揭示驅(qū)動(dòng)因子及其影響途徑。
(4)模型與技術(shù)的整合:將多尺度關(guān)聯(lián)分析、生態(tài)系統(tǒng)耦合關(guān)系以及微生物時(shí)空演變的研究成果整合到氮素遷移循環(huán)模型中,提高模型的預(yù)測(cè)精度和實(shí)用性,并將模型與遙感、GIS等現(xiàn)代技術(shù)結(jié)合,實(shí)現(xiàn)氮素遷移轉(zhuǎn)化的實(shí)時(shí)監(jiān)測(cè)和動(dòng)態(tài)管理,為農(nóng)業(yè)可持續(xù)發(fā)展提供有力支撐。
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