魯顯楷，莫江明，張煒，毛慶功，劉榮臻,2，王聰,2，王森浩,2，鄭棉海，Mori Taiki，毛晉花,2，張勇群,2，王玉芳,2，黃娟

模擬大氣氮沉降對(duì)中國(guó)森林生態(tài)系統(tǒng)影響的研究進(jìn)展

魯顯楷1，莫江明1，張煒1，毛慶功1，劉榮臻1,2，王聰1,2，王森浩1,2，鄭棉海1，Mori Taiki1，毛晉花1,2，張勇群1,2，王玉芳1,2，黃娟1

(1. 中國(guó)科學(xué)院華南植物園, 退化生態(tài)系統(tǒng)植被恢復(fù)與管理重點(diǎn)實(shí)驗(yàn)室, 廣東省應(yīng)用植物學(xué)重點(diǎn)實(shí)驗(yàn)室, 廣州 510650; 2. 中國(guó)科學(xué)院大學(xué), 北京 100049)

人類(lèi)活動(dòng)加劇了活性氮的生產(chǎn)和排放，并導(dǎo)致氮沉降日益增加并全球化。目前，人類(lèi)活動(dòng)對(duì)全球氮循環(huán)的干擾已經(jīng)超出了地球系統(tǒng)安全運(yùn)行的界限。中國(guó)已成為全球氮沉降的高發(fā)區(qū)域，高氮沉降已經(jīng)威脅到生態(tài)系統(tǒng)的健康和安全，并成為生態(tài)文明建設(shè)過(guò)程中亟待理清和解決的熱點(diǎn)問(wèn)題。對(duì)國(guó)際上和中國(guó)森林生態(tài)系統(tǒng)模擬氮沉降研究的概況進(jìn)行了綜述，并從生物學(xué)和非生物學(xué)兩大過(guò)程重點(diǎn)闡述模擬氮沉降增加對(duì)中國(guó)主要森林生態(tài)系統(tǒng)影響的研究進(jìn)展。中國(guó)自2000年以后才開(kāi)始重視大氣氮沉降產(chǎn)生的生態(tài)環(huán)境問(wèn)題，中國(guó)科學(xué)院華南植物園在國(guó)內(nèi)森林生態(tài)系統(tǒng)模擬氮沉降試驗(yàn)研究上做出了開(kāi)創(chuàng)性的貢獻(xiàn)。模擬氮沉降研究表明，持續(xù)高氮輸入將會(huì)顯著改變森林生態(tài)系統(tǒng)的結(jié)構(gòu)和功能，并威脅生態(tài)系統(tǒng)的健康發(fā)展，特別是處于氮沉降熱點(diǎn)區(qū)域的中國(guó)中南部。森林生態(tài)系統(tǒng)的氮沉降效應(yīng)依賴(lài)于系統(tǒng)的氮狀態(tài)、土地利用歷史、氣候特征、林型和林齡等。最后，對(duì)未來(lái)的研究提出了一些建議，包括加強(qiáng)長(zhǎng)期跟蹤研究和不同氣候帶站點(diǎn)之間的聯(lián)網(wǎng)研究，特別是在森林生態(tài)系統(tǒng)對(duì)長(zhǎng)期氮沉降響應(yīng)與適應(yīng)的過(guò)程機(jī)制、地下碳氮吸存潛力研究、以及與其他全球變化因子的耦合研究等方面，以期為森林生態(tài)系統(tǒng)的可持續(xù)發(fā)展提供理論基礎(chǔ)和管理依據(jù)。

氮沉降; 全球變化; 森林生態(tài)系統(tǒng); 氮飽和; 氮限制; 氮素生物地球化學(xué)循環(huán); 生物多樣性; 碳吸存

1 大氣氮沉降及其趨勢(shì)

氮素是一種重要的生命元素，是蛋白質(zhì)、維生素和核酸(DNA)的重要組成部分。氮素雖然無(wú)處不在，但主要以惰性狀態(tài)(N2)存在，不能被生物直接利用，所以也是陸地生態(tài)系統(tǒng)凈初級(jí)生產(chǎn)力最為關(guān)鍵的限制性營(yíng)養(yǎng)元素[1–2]。然而人類(lèi)活動(dòng)改變了氮素的存在形態(tài)，即具有生物活性的氮化合物(活性氮)的產(chǎn)生和排放顯著增加?；钚缘饕ㄟ€原態(tài)NHx (包括NH3、RNH2和NH4+)和氧化態(tài)NOx兩種化學(xué)形式?；钚缘欧旁黾诱T發(fā)了大氣氮沉降格局的改變。工業(yè)化革命(1860年)前，活性氮主要來(lái)自雷電作用和生物固氮，大氣氮沉降量也非常低, 如陸地系統(tǒng)氮沉降量普遍在0.5~1 kg N hm–2a–1，最高不超過(guò)10 kg N hm–2a–1。然而，自20世紀(jì)中葉以來(lái)，隨著化石燃料的燃燒、化學(xué)氮肥的生產(chǎn)和使用和畜牧業(yè)的迅猛發(fā)展等人類(lèi)活動(dòng)向大氣中排放的活性氮化合物激增，大氣氮沉降也呈現(xiàn)出迅猛增加的趨勢(shì)，并達(dá)到工業(yè)化革命前的兩倍以上，并且愈來(lái)愈呈現(xiàn)出全球化趨勢(shì)[3]。目前全球氮沉降的分布中心正在發(fā)生變化：從歐美等發(fā)達(dá)國(guó)家轉(zhuǎn)向發(fā)展中國(guó)家，從溫帶區(qū)域擴(kuò)展到熱帶亞熱帶區(qū)域。

目前中國(guó)已成為世界上活性氮生產(chǎn)和排放量最大的國(guó)家[4]。近30年來(lái)(1980-2010)，中國(guó)人為源活性氮產(chǎn)生量增加了近3倍(從1.68×107t增加到4.82×107t)，其中主要增長(zhǎng)來(lái)自于化肥和工業(yè)用氮(從1.14×107t增加到3.71×107t)。如此高的活性氮排放勢(shì)必導(dǎo)致陸地和水域大氣氮沉降量的急劇增加[5–7]。Lü等[5]對(duì)中國(guó)的網(wǎng)絡(luò)監(jiān)測(cè)數(shù)據(jù)研究表明，中國(guó)中南部是氮沉降的高發(fā)區(qū)域，最高達(dá)到65 kg N hm–2a–1。如今，華南大部分地區(qū)的大氣濕沉降已超過(guò)30 kg N hm–2a–1[8],遠(yuǎn)高于嚴(yán)重威脅歐美森林生態(tài)系統(tǒng)健康和安全的氮沉降臨界負(fù)荷值[9]。Yu等[10]通過(guò)構(gòu)建1980–2015年間的中國(guó)區(qū)域大氣干沉降和濕沉降全組分動(dòng)態(tài)變化數(shù)據(jù)集，提出了中國(guó)大氣氮沉降總量和沉降模式轉(zhuǎn)型變化的三個(gè)重要特征：(1)近年來(lái)雖然中國(guó)區(qū)域的NO3–氮沉降還在持續(xù)增加，但NH4+濕沉降顯著降低，使全國(guó)氮沉降總量由快速增長(zhǎng)轉(zhuǎn)型為趨穩(wěn)狀態(tài)；(2)大氣干沉降增加導(dǎo)致了干濕沉降比的變化,由以濕沉降為主逐步轉(zhuǎn)型為濕沉降與干沉降并重; (3)大氣沉降中的NH4+/NO3–比減小，NO3–氮沉降貢獻(xiàn)在持續(xù)增加，NH4+氮沉降的貢獻(xiàn)則降低，逐漸由以NH4+沉降為主的氮沉降模式轉(zhuǎn)換為NH4+和NO3–氮沉降貢獻(xiàn)并重的新模式。

2 生態(tài)系統(tǒng)的氮沉降效應(yīng)

目前，人類(lèi)活動(dòng)干擾下的大氣氮沉降已成為全球氮素生物地球化學(xué)循環(huán)的一個(gè)重要組成部分，也是全球變化的重要驅(qū)動(dòng)因子。大氣氮沉降增加改變了生態(tài)系統(tǒng)氮循環(huán)模式和進(jìn)程，加劇了氮循環(huán)的速率，并通過(guò)耦合效應(yīng)驅(qū)動(dòng)其他元素循環(huán)(如碳、磷等)發(fā)生改變[11–13]。

作為營(yíng)養(yǎng)源和酸源，大氣氮沉降急劇增加將會(huì)威脅到生態(tài)系統(tǒng)的健康和安全，對(duì)農(nóng)田、森林、草地等陸地生態(tài)系統(tǒng)以及湖泊和海洋等水域生態(tài)系統(tǒng)都會(huì)造成影響。大氣氮沉降的負(fù)面效應(yīng)主要有:導(dǎo)致生態(tài)系統(tǒng)土壤酸化、養(yǎng)分流失，造成水體污染并富營(yíng)養(yǎng)化，改變生態(tài)系統(tǒng)元素計(jì)量化學(xué)，降低生物多樣性并改變物種組成，降低生態(tài)系統(tǒng)生產(chǎn)力和削弱生態(tài)系統(tǒng)穩(wěn)定性，加劇溫室氣體(N2O)排放等[3,9,14–19]。大氣氮沉降的持續(xù)增加，最終會(huì)導(dǎo)致生態(tài)系統(tǒng)氮飽和(nitrogen saturation)，即氮的輸入超過(guò)了生物對(duì)氮素的需求，從而加劇上述生態(tài)學(xué)效應(yīng)，并影響系統(tǒng)的健康發(fā)展[20]。此外，氮沉降還會(huì)與其他全球變化驅(qū)動(dòng)因子(如CO2濃度升高、全球變暖、O3濃度增加等)相耦合，并引發(fā)一系列的連鎖效應(yīng)(cascade effect)，相關(guān)的生態(tài)后果還有待于未來(lái)深入研究。目前，人類(lèi)活動(dòng)對(duì)全球氮循環(huán)的干擾已經(jīng)超出了地球系統(tǒng)安全運(yùn)行的界限[21]。因此，氮素在大氣-土壤-植物-水循環(huán)的生物地球化學(xué)過(guò)程成為科學(xué)家、公眾和決策者關(guān)注的主要環(huán)境問(wèn)題之一。

森林是陸地生態(tài)系統(tǒng)的主體。中國(guó)陸地系統(tǒng)的氣候復(fù)雜性和空間的變異性產(chǎn)生了豐富森林生態(tài)系統(tǒng)類(lèi)型，如各類(lèi)針葉林、針葉闊葉混交林、落葉闊葉林、常綠闊葉林和熱帶雨林等。這些森林在維護(hù)生物多樣性和生態(tài)平衡、并為社會(huì)發(fā)展提供各種供給服務(wù)中扮演著重要角色。在氮沉降全球化的背景下，本綜述主要關(guān)注中國(guó)森林生態(tài)系統(tǒng)對(duì)氮沉降增加的響應(yīng)及其機(jī)制。

3 國(guó)際上森林生態(tài)系統(tǒng)模擬氮沉降研究的歷史概況

在國(guó)際上，氮沉降對(duì)森林生態(tài)系統(tǒng)結(jié)構(gòu)和功能影響的研究始于20世紀(jì)80年代，到90年代發(fā)展為定位研究，并逐漸形成網(wǎng)絡(luò)，研究?jī)?nèi)容也不斷細(xì)化和深化。這些研究主要集中在歐洲和北美，特別是經(jīng)歷高氮沉降輸入的區(qū)域。歐共體委員會(huì)于20世紀(jì)80年代末資助了兩大研究項(xiàng)目，即氮飽和試驗(yàn)項(xiàng)目(Nitrogen saturation experiments, NITREX)和歐洲森林生態(tài)系統(tǒng)試驗(yàn)控制項(xiàng)目(Experimental manipulation of forest ecosystems in Europe, EXMA)[22]。其中NITREX項(xiàng)目在7個(gè)國(guó)家的8個(gè)試驗(yàn)點(diǎn)進(jìn)行,主要研究增加或減少大氣氮沉降對(duì)歐洲森林生態(tài)系統(tǒng)，特別是針葉林生態(tài)系統(tǒng)的影響。而EXMAN項(xiàng)目也涉及4個(gè)國(guó)家的6個(gè)試驗(yàn)站點(diǎn)，通過(guò)實(shí)驗(yàn)改變周邊大氣氮沉降的化學(xué)組成和數(shù)量進(jìn)而探討森林生態(tài)系統(tǒng)的響應(yīng)。在美國(guó)，長(zhǎng)期試驗(yàn)研究站點(diǎn)有馬薩諸塞州的哈佛森林長(zhǎng)期氮素增加試驗(yàn)(Chronic nitrogen amendment in Harvard forest)和緬因州Bear brook集水區(qū)和Mt. Ascutney森林的模擬氮沉降試驗(yàn)[20,23]。這些研究對(duì)理解溫帶和北方森林生態(tài)系統(tǒng)對(duì)大氣氮沉降的響應(yīng)及其機(jī)理有著重要貢獻(xiàn)。

截至2008年，全球范圍內(nèi)有關(guān)森林生態(tài)系統(tǒng)氮沉降研究的站點(diǎn)共計(jì)31個(gè)，絕大部分在溫帶區(qū)域，熱帶區(qū)域僅有2個(gè)：中美洲巴拿馬和中國(guó)廣東鼎湖山(圖1)[24]。

4 中國(guó)森林生態(tài)系統(tǒng)模擬氮沉降研究概況

中國(guó)氮沉降問(wèn)題起源于20世紀(jì)80年代初期的酸沉降監(jiān)測(cè)研究[25]，但自2000年后才逐步受到的重視。為了更好地理解和預(yù)測(cè)大氣氮沉降對(duì)中國(guó)森林生態(tài)系統(tǒng)的影響，中國(guó)科學(xué)院華南植物園(以下簡(jiǎn)稱(chēng)華南園)莫江明研究團(tuán)隊(duì)于2002年在廣東鼎湖山國(guó)家級(jí)自然保護(hù)區(qū)確立了我國(guó)首批森林生態(tài)系統(tǒng)長(zhǎng)期氮沉降研究樣地(圖1)；該樣地包含了熱帶亞熱帶區(qū)域代表性的3種森林類(lèi)型：季風(fēng)常綠闊葉林、針葉闊葉混交林和馬尾松針葉林[13,18,26–27]。此后, 從南到北涌現(xiàn)出更多的模擬氮沉降研究樣地。2010年北京大學(xué)牽頭建立了中國(guó)森林生態(tài)系統(tǒng)養(yǎng)分添加試驗(yàn)網(wǎng)絡(luò)(Nutrient enrichment experiments in China’s forests project, NEECF)，擁有從南(海南尖峰嶺的熱帶山地雨林)到北(內(nèi)蒙古根河的寒溫帶針葉林)分布在7個(gè)研究站點(diǎn)的10個(gè)森林類(lèi)型[28]。到目前為止，全國(guó)范圍森林生態(tài)系統(tǒng)氮沉降研究樣地共有25個(gè)，基本上涵蓋了中國(guó)的主要森林類(lèi)型(圖2)。

圖1 氮沉降試驗(yàn)研究站點(diǎn)國(guó)際分布圖[24]

模擬氮沉降研究的技術(shù)方法主要體現(xiàn)在4個(gè)方面：氮沉降的種類(lèi)、數(shù)量、施加頻率和方式。氮沉降的種類(lèi)主要有4種類(lèi)型：NH4NO3、NaNO3、(NH4)2SO4和NH4Cl，其中NH4NO3溶液是應(yīng)用最為廣泛的施加方式，硝態(tài)氮(NaNO3)和銨態(tài)氮(NH4Cl)常用來(lái)評(píng)估氧化態(tài)和還原態(tài)氮沉降帶來(lái)的影響。施氮數(shù)量通常在當(dāng)?shù)乇尘暗两祷A(chǔ)上進(jìn)行不同梯度的倍增處理，鼎湖山樣地的氮沉降處理水平分別為0、50、100和150 kg N hm–2a–1[17]。施氮頻率通常為每月1次或每?jī)稍?次，也有少數(shù)為一季度1次或者半年1次。施氮方式主要有林下和林冠施加兩種，目前絕大多數(shù)為林下施氮。2013年，華南園率先在國(guó)內(nèi)(廣東石門(mén)臺(tái)和河南雞公山)開(kāi)展了林冠模擬氮沉降試驗(yàn)，并與傳統(tǒng)的林下施氮進(jìn)行對(duì)比研究，以期更為真實(shí)地反映大氣氮沉降對(duì)森林生態(tài)系統(tǒng)的影響、以及氮沉降與林冠的交互作用[35]。

中國(guó)自2000年以后才開(kāi)始重視大氣氮沉降產(chǎn)生的生態(tài)環(huán)境問(wèn)題。截至2018年12月，森林生態(tài)系統(tǒng)依托模擬氮沉降研究樣地發(fā)表的研究論文共計(jì)720篇(圖3)。華南園在模擬氮沉降試驗(yàn)研究上做出了開(kāi)創(chuàng)性的貢獻(xiàn)，2004-2005年以華南園為第一單位發(fā)表的研究論文占國(guó)內(nèi)發(fā)表論文總數(shù)的100%。隨著國(guó)內(nèi)科學(xué)家對(duì)氮沉降問(wèn)題關(guān)注度的增加，發(fā)表的研究論文也越來(lái)越多，特別是自2010年以后，同期來(lái)自華南園的論文發(fā)表數(shù)量相對(duì)穩(wěn)定，但質(zhì)量顯著提升。如今，中國(guó)社會(huì)和經(jīng)濟(jì)高速發(fā)展誘發(fā)的高氮沉降已經(jīng)威脅到了生態(tài)系統(tǒng)的健康和安全，并成為生態(tài)文明建設(shè)過(guò)程中亟待理清和解決的熱點(diǎn)問(wèn)題。

5 模擬氮沉降增加對(duì)中國(guó)森林生態(tài)系統(tǒng)的影響

5.1 氮沉降對(duì)土壤酸化的影響

目前，隨著中國(guó)氮沉降量的增加和對(duì)硫排放的控制，人為氮源對(duì)酸沉降的貢獻(xiàn)越來(lái)越大[29,36]。據(jù)估計(jì)，在我國(guó)華南林區(qū)由氮沉降引起的土壤酸化已占沉降因素的36%以上[37]。氮沉降主要通過(guò)H+輸入、NH4+硝化和NO3–的淋溶流失、植物吸收等直接或間接過(guò)程增加土壤酸度。理論上，1 mol NH4+通過(guò)微生物的硝化作用轉(zhuǎn)化成NO3–將釋放2 mol H+[38]。由于H+與土壤膠體具有更強(qiáng)的親和性(相比于其他鹽基離子)，因此氮沉降誘導(dǎo)的H+增加會(huì)替代膠體顆粒上的鹽基離子(Ca2+和Mg2+等)，并使它們隨NO3+的淋溶而流失。當(dāng)?shù)两党掷m(xù)增加，土壤pH進(jìn)一步降低，導(dǎo)致Al3+、Fe3+或Mn2+等離子的移動(dòng)性增加，威脅生物生長(zhǎng)。

圖2 中國(guó)森林生態(tài)系統(tǒng)模擬氮沉降研究站點(diǎn)分布[13,29–34]。GH: 根河; WY: 五營(yíng); LS: 涼水; MES: 帽兒山; FS: 撫松; CBS: 長(zhǎng)白山; SHB: 塞罕壩; DLS:東靈山; TYS: 太岳山; JGS: 雞公山; GNJ: 牯牛降; TTS: 天童山; WYS: 武夷山; SX: 沙縣; QYZ: 千煙洲; SMT: 石門(mén)臺(tái); HSD: 黑石頂; DHS: 鼎湖山; HS: 鶴山; JFL: 尖峰嶺; HT: 會(huì)同; ALS: 哀牢山; TSP: 鐵山坪; HY: 洪雅; LFA: 涼風(fēng)坳。

圖3 模擬氮沉降試驗(yàn)研究中以華南植物園(SCBG)為第一單位發(fā)表的論文在全國(guó)的比重(a)及其絕對(duì)數(shù)量(b)

中國(guó)溫帶和亞熱帶森林均面臨著氮沉降升高引起的土壤酸化問(wèn)題[39]。目前僅有少數(shù)研究報(bào)道了氮沉降導(dǎo)致中國(guó)溫帶森林土壤酸化，可能是該區(qū)域土壤pH本底值較高，土壤緩沖能力較強(qiáng)[15]。然而，熱帶亞熱帶森林生態(tài)系統(tǒng)土壤緩沖能力普遍較低,且對(duì)酸源沉降更加敏感。在珠三角地區(qū)，土壤pH值沿著農(nóng)村到城市的氮沉降增加梯度而降低[40]。在南亞熱帶廣東鼎湖山自然保護(hù)區(qū)，Lu等[41]報(bào)道長(zhǎng)期氮添加顯著降低了原始森林土壤的緩沖能力并加劇土壤酸化，但是次生林和人工林響應(yīng)不明顯;不同土地利用歷史導(dǎo)致的氮狀態(tài)不同是響應(yīng)差異的重要原因, “富氮的”成熟林更容易發(fā)生氮素流失和土壤酸化。過(guò)量氮輸入將會(huì)顯著降低土壤中有效陽(yáng)離子(如Ca2+和Mg2+)含量，進(jìn)而導(dǎo)致養(yǎng)分失衡[13,42–43]?？紤]到這些熱帶亞熱帶生態(tài)系統(tǒng)大多已經(jīng)處于鋁化合物緩沖階段，在未來(lái)氮沉降增加背景下應(yīng)更關(guān)注Ca2+和Mg2+的缺乏而非鋁毒效應(yīng)[18]。

5.2 氮沉降對(duì)植物化學(xué)元素的影響

大氣氮沉降會(huì)直接作用于植物，或者通過(guò)改變土壤化學(xué)元素的組成來(lái)間接影響植物元素平衡[44–45]，并最終導(dǎo)致生態(tài)系統(tǒng)水平上的元素失衡[17,29]。關(guān)于氮沉降對(duì)森林植物元素化學(xué)影響的研究主要集中于地上部分，對(duì)于植物地下部分的研究十分匱乏。

關(guān)于地上部分，國(guó)內(nèi)最先見(jiàn)于李德軍的研究[46],他總結(jié)前人研究認(rèn)為，氮沉降會(huì)造成植物體內(nèi)養(yǎng)分元素比例的失衡；對(duì)南亞熱帶3種喬木幼苗的研究表明，氮施肥增加了植物N含量，增加了N與P、K、Ca、Mg和Mn等元素的比值。除N/P比外, N/K比也可以作為植物對(duì)氮沉降響應(yīng)的敏感指標(biāo)[47]。對(duì)溫帶森林的研究表明，氮素輸入普遍增加了植物N含量[48]，進(jìn)一步驗(yàn)證了溫帶區(qū)域的氮素限制性[49]。但也有研究報(bào)道5年的N添加對(duì)溫帶落葉松()各器官N、P、K、Ca和Mg含量的影響不明顯[50]。最近的整合分析(meta-analysis)表明，在中國(guó)隨著氮沉降的增加，木本和草本植物的葉氮含量增加，但是葉磷絕對(duì)含量變化不顯著[51–52]。對(duì)元素比值的整合分析表明，氮沉降普遍降低了中國(guó)各生態(tài)系統(tǒng)植物的C/N比，增加了N/P比，但是對(duì)C/P的效應(yīng)不顯著，根本原因是氮沉降增加了植物內(nèi)氮含量[53–55]。然而，Lu等[13]在“富氮”的熱帶成熟林的研究表明，為期10年的高氮輸入并沒(méi)有顯著改變喬木植物葉片N、K、Ca、Mg和Al等元素的含量，這主要是由于該生態(tài)系統(tǒng)已經(jīng)達(dá)到氮飽和以及植物產(chǎn)生的自我適應(yīng)性調(diào)整；并由此提出了植物適應(yīng)性新假說(shuō)：即“富氮”生態(tài)系統(tǒng)植物可以通過(guò)提升自身蒸騰能力適應(yīng)高氮沉降來(lái)維持養(yǎng)分平衡。這更新了有關(guān)氮沉降增加植物葉氮含量的普遍觀(guān)點(diǎn)，因此在未來(lái)相關(guān)研究中應(yīng)當(dāng)充分考慮到生態(tài)系統(tǒng)的氮狀態(tài)和植物的適應(yīng)性。

關(guān)于地下部分，主要是針對(duì)植物細(xì)根，但研究數(shù)量遠(yuǎn)少于對(duì)植物葉片的研究，且研究結(jié)論不統(tǒng)一。Mo等[56]對(duì)熱帶次生林的研究表明，氮添加增加了細(xì)根氮素含量，但對(duì)磷含量沒(méi)有影響，進(jìn)而導(dǎo)致了N/P比降低。Zhu等[57]在南亞熱帶成熟林、混交林和馬尾松林的研究表明，為期5年的氮添加并沒(méi)有改變細(xì)根氮和磷的含量。整合分析進(jìn)一步表明，全球尺度上氮沉降顯著增加了植物總根的生物量(20.2%)，但是減少了細(xì)根的生物量(–12.8%)，顯著增加了植物細(xì)根的N含量(17.6%)，降低了細(xì)根的C/N比，但事實(shí)沒(méi)有顯著改變細(xì)根的C含量[58]。

可以看出，不同生態(tài)系統(tǒng)植物元素化學(xué)對(duì)氮沉降的響應(yīng)不同，主要原因有兩個(gè)方面：首先，植物在長(zhǎng)期生存進(jìn)化過(guò)程中形成了不同的系統(tǒng)發(fā)育特征，導(dǎo)致其對(duì)于氮沉降響應(yīng)的敏感程度因種而異, 此外，植物自我調(diào)整的適應(yīng)性(acclimation)，也會(huì)改變氮沉降對(duì)植物化學(xué)元素影響的方向[13]。其次，影響植物發(fā)育的外部環(huán)境(如氣候、土壤發(fā)育特點(diǎn)等)會(huì)進(jìn)一步?jīng)Q定植物的響應(yīng)方式。中國(guó)北方溫帶地區(qū)生態(tài)系統(tǒng)一般認(rèn)為氮限制，而南方熱帶亞熱帶生態(tài)系統(tǒng)相對(duì)更加富氮，常受磷或其他陽(yáng)離子限制，這些特點(diǎn)決定了森林植物對(duì)氮沉降響應(yīng)方式可能存在差異。

5.3 氮沉降對(duì)植物生長(zhǎng)的影響

從全球尺度來(lái)看，氮添加對(duì)不同植物物種生長(zhǎng)通常表現(xiàn)出刺激效應(yīng)[59]。不同生態(tài)系統(tǒng)和功能類(lèi)群對(duì)氮沉降響應(yīng)的敏感程度不同，次生林樹(shù)木的生長(zhǎng)一般比原始林敏感[60]，而草本植物比木本植物敏感[59,61]。同時(shí)，氮沉降對(duì)森林凈初級(jí)生產(chǎn)力(NPP)或植物生長(zhǎng)的影響也因氮沉降量而異，具有生態(tài)學(xué)中普遍的非線(xiàn)性關(guān)系[62]。中國(guó)東北地區(qū)森林中, 較低的氮添加(25 kg N hm–2a–1)導(dǎo)致最大的凈生產(chǎn)力, 隨著氮添加的增加(50 kg N hm–2a–1)正面效應(yīng)降低,氮添加量最大時(shí)(75 kg N hm–2a–1)正面效應(yīng)消失[63]。

目前，樹(shù)木徑向生長(zhǎng)對(duì)氮沉降的響應(yīng)在中國(guó)僅有少量研究報(bào)道。樹(shù)木的生長(zhǎng)響應(yīng)因個(gè)體大小(如胸徑和樹(shù)高)或生長(zhǎng)階段而不同[64–66]。劉修元等[64]研究了模擬氮沉降對(duì)落葉松原始林樹(shù)木胸徑生長(zhǎng)的影響，認(rèn)為不同高度的樹(shù)木對(duì)氮添加的響應(yīng)有很大差異, 較低樹(shù)木(樹(shù)高<16.5 m)的生長(zhǎng)對(duì)氮添加無(wú)顯著響應(yīng)，而較高(樹(shù)高>16.5 m)的樹(shù)木在中氮和高氮(50和100 kg N hm–2a–1)處理下胸徑生長(zhǎng)顯著加速，但隨著樹(shù)木高度的進(jìn)一步增加, 這種加速作用明顯下降。在亞熱帶闊葉林，Tian等[65]報(bào)道3年的氮添加(50和100 kg N hm–2a–1)使甜栲()幼樹(shù)和林下幼苗生長(zhǎng)顯著降低,但是大樹(shù)的生長(zhǎng)沒(méi)有受到影響。7年的氮添加(40 kg N hm–2a–1)導(dǎo)致重慶馬尾松()林土壤酸化和氮飽和，并使馬尾松生長(zhǎng)顯著下降[42]。從長(zhǎng)遠(yuǎn)看,氮沉降對(duì)森林群落樹(shù)木結(jié)構(gòu)的改變可能會(huì)引起物種組成和碳吸存能力的改變。

5.4 氮沉降對(duì)植物多樣性的影響

大氣氮沉降升高已成為全球多樣性喪失的第三大驅(qū)動(dòng)因素[67]。然而，中國(guó)僅有少量相關(guān)研究報(bào)道。與林下層植物相比，喬木植物對(duì)環(huán)境因素的響應(yīng)較慢，所以已有研究主要集中在林下層植物多樣性的響應(yīng)上。長(zhǎng)期氮沉降會(huì)改變林下層物種組成, 但改變的程度依賴(lài)于森林類(lèi)型、功能類(lèi)群以及氮狀態(tài)等因素。在溫帶地區(qū)，Du[68]報(bào)道3年的氮添加試驗(yàn)對(duì)北方原始森林林下層植物的物種豐富度沒(méi)有影響，但顯著增加了禾草類(lèi)植物的蓋度，并降低了矮小灌木植物的蓋度。在熱帶/亞熱帶原始林，Lu等[17]報(bào)道5年的高氮添加(>100 kg N hm–2a–1)顯著降低了林下植物多樣性(豐富度和多度)，并首次提出負(fù)面效應(yīng)主要與土壤酸化機(jī)制有關(guān)而不是傳統(tǒng)上的競(jìng)爭(zhēng)機(jī)制。Wu等[69]研究表明8年的高氮添加(>120 kg N hm–2a–1)可能通過(guò)降低土壤pH值和菌根真菌豐度來(lái)削弱亞熱帶森林林下植物豐富度。Huang等[42]通過(guò)對(duì)比NH4NO3和NaNO3處理在亞熱帶馬尾松林的效應(yīng)，認(rèn)為林下主要物種多度的降低可能是氮飽和與土壤酸化共同作用的結(jié)果。與模擬氮沉降控制試驗(yàn)的結(jié)果基本一致，Huang等[70]報(bào)道在廣州城鄉(xiāng)氮沉降梯度上(30.1~43.3 kg N hm–2a–1),成熟林林下草本層植物多樣性與氮沉降量呈負(fù)相關(guān), 與土壤中的有效Ca2+和K+濃度呈正相關(guān)。此外，氮沉降效應(yīng)也與土地利用方式有關(guān)，與原始林相比, 人工林或次生林中植物多樣性對(duì)氮沉降的響應(yīng)相對(duì)不敏感[71–72]。

5.5 氮沉降對(duì)土壤微生物和酶活性的影響

土壤微生物群落在森林生態(tài)系統(tǒng)中扮演著核心角色，其生長(zhǎng)代謝能調(diào)控森林生態(tài)系統(tǒng)的物質(zhì)循環(huán)和能量流動(dòng)，影響土壤養(yǎng)分狀態(tài)、有機(jī)質(zhì)的數(shù)量及穩(wěn)定性和土壤溫室氣體的排放等。氮沉降加劇會(huì)對(duì)土壤微生物群落的數(shù)量、組成和功能活性產(chǎn)生影響。

5.5.1 土壤微生物群落

野外試驗(yàn)表明，氮添加通常會(huì)減少微生物的生物量[30,51]。氮添加引起的土壤pH降低、可利用碳降低、土壤溶液毒害等是導(dǎo)致微生物量減少的主要原因[73–75]。但也有研究報(bào)道氮添加對(duì)土壤微生物量沒(méi)有顯著影響[76]。

氮添加對(duì)微生物量的影響還與添加量、氮添加類(lèi)型、季節(jié)、微生物種類(lèi)及林型有關(guān)。低氮添加(50 kg N hm–2a–1)沒(méi)有顯著改變溫帶油松()林土壤微生物生物量，但中氮和高氮添加(>100 kg N hm–2a–1)則顯著降低了微生物生物量[77]。氨態(tài)氮添加(20~80 kg N hm–2a–1)增加了亞熱帶的冷杉()種植園細(xì)菌生物量，但硝態(tài)氮添加(20~80 kg N hm–2a–1)則降低了細(xì)菌生物量[78]。Wang等[79]報(bào)道氨態(tài)氮添加(120 kg N hm–2a–1)在非生長(zhǎng)季顯著降低了亞熱帶冷杉和松樹(shù)種植園真菌生物量,在生長(zhǎng)季則對(duì)真菌生物量影響不顯著，但在兩個(gè)季節(jié)對(duì)細(xì)菌生物量都沒(méi)有顯著影響。對(duì)鼎湖山3個(gè)南亞熱帶森林的研究表明，氮添加降低了季風(fēng)常綠闊葉林的土壤微生物量，但對(duì)針闊混交林和針葉林的土壤微生物量則沒(méi)有顯著影響[80]。

氮添加可以改變土壤微生物的群落組成。首先，氮添加會(huì)改變真菌群落(真菌、叢枝菌根真菌和外生菌根真菌)的組成, 氮添加(70 kg N hm–2a–1)在春季會(huì)增加北方落葉松森林擔(dān)子菌門(mén)的相對(duì)豐度, 但在夏季則會(huì)減少擔(dān)子菌門(mén)的相對(duì)豐度[81]。氮添加(50~ 100 kg N hm–2a–1)會(huì)降低武夷山的亞熱帶常綠闊葉林叢枝菌根真菌的比例，但氮添加(150 kg N hm–2a–1)增加了鼎湖山季風(fēng)常綠闊葉林叢枝菌根真菌的相對(duì)豐度[74,82]。氮添加(50~300 kg N hm–2a–1)能使亞熱帶濕地松()林對(duì)氮敏感的外生菌根真菌缺失[83]。其次，氮添加會(huì)影響細(xì)菌群落的組成, 基于磷脂脂肪酸分析技術(shù)，氮添加會(huì)降低革蘭氏陰性細(xì)菌的相對(duì)豐度，增加革蘭氏陽(yáng)性細(xì)菌/革蘭氏陰性細(xì)菌比[74]。基于高通量焦磷酸測(cè)序的研究也表明，氮添加會(huì)影響細(xì)菌群落的組成[81,84–85]。Nie等[85]報(bào)道，氮添加(105 kg N hm–2a–1)在干季會(huì)降低鼎湖山南亞熱帶常綠闊葉林酸桿菌門(mén)的相對(duì)豐度，但會(huì)增加變形菌門(mén)及放線(xiàn)菌門(mén)的相對(duì)豐度。第三，氮添加會(huì)影響真菌與細(xì)菌的比例。如氮添加增加了中國(guó)南部亞熱帶森林真菌的相對(duì)豐度，減少細(xì)菌的相對(duì)豐度，從而提高真菌/細(xì)菌比[74,86]。然而，氮添加降低了千煙洲的亞熱帶森林真菌/細(xì)菌比[73,79,87]。還有研究表明氮添加對(duì)真菌/細(xì)菌比沒(méi)有顯著影響[69,88]。

5.5.2 土壤酶活性

根據(jù)資源分配模型[89]，氮添加可通過(guò)影響土壤養(yǎng)分的有效性來(lái)改變土壤酶活性。通常，氮添加會(huì)增加土壤有效氮的含量從而降低與微生物氮獲取相關(guān)的酶活性(乙酰氨基葡糖酶、亮氨酸氨基肽酶、蛋白酶、脲酶等)，但會(huì)增加與微生物碳(纖維素二糖水解酶、葡萄糖苷酶、木糖苷酶等)和磷(堿性磷酸酶、酸性磷酸單酯酶、酸性磷酸二酯酶等)獲取相關(guān)的酶活性。然而，針對(duì)中國(guó)森林的研究表明，氮添加對(duì)酶活性的影響并不完全符合該理論框架[74,79,90]。如氮添加(100 kg N hm–2a–1)降低了北方落葉松森林土壤蛋白酶和脫氫酶活性，但對(duì)葡萄糖苷酶與酸性磷酸單酯酶的活性沒(méi)有顯著影響[76]。氮添加(50和100 kg N hm–2a–1)增加了千煙洲的亞熱帶冷杉種植園土壤乙酰氨基轉(zhuǎn)移酶、葡萄糖苷酶和酸性磷酸單酯酶活性[91]，但氨態(tài)氮和硝態(tài)氮添加(40 kg N hm–2a–1)降低了當(dāng)?shù)貪竦厮闪滞寥琅c微生物碳、氮、磷獲取相關(guān)酶活性[73]。Fan等[76]報(bào)道氮添加(40和80 kg N hm–2a–1)增加了亞熱帶米櫧()林表層土壤的乙酰氨基葡糖酶和酸性磷酸單酯酶活性，但降低了下層土壤氮乙酰氨基轉(zhuǎn)移酶的活性。還有研究表明，氮添加對(duì)各類(lèi)土壤酶活性都沒(méi)有顯著影響[92–93]。因此，氮添加對(duì)土壤酶活性的影響取決于氮添加的量、氮添加的類(lèi)型、林型、酶的種類(lèi)和土層等因素。

5.6 氮沉降對(duì)土壤動(dòng)物的影響

土壤動(dòng)物是指土壤中和落葉下生存的各種動(dòng)物的總稱(chēng)。土壤動(dòng)物作為生態(tài)系統(tǒng)中重要的分解者和消費(fèi)者，對(duì)凋落物的分解、養(yǎng)分周轉(zhuǎn)、微生物群落調(diào)控、以及生態(tài)系統(tǒng)結(jié)構(gòu)和功能的維持均有重要作用。氮沉降的增加會(huì)對(duì)土壤動(dòng)物的生物量、多樣性和組成產(chǎn)生影響。

在中國(guó)，氮沉降對(duì)土壤動(dòng)物的研究最早開(kāi)展于鼎湖山自然保護(hù)區(qū)[94–95]。氮沉降對(duì)森林土壤動(dòng)物影響的結(jié)論并不一致，施氮量和林型都是重要的影響因素。首先，施氮量可能存在閾值效應(yīng)。Xu等[94]對(duì)南亞熱帶3種典型森林(季風(fēng)常綠林、針闊混交林和針葉林)的研究表明，1年的氮處理并未對(duì)土壤動(dòng)物生物量產(chǎn)生顯著影響，但低氮處理(50 kg N hm–2a–1)各林分生物量都有不同程度的上升，而高氮處理(100 kg N hm–2a–1)均出現(xiàn)下降。Xu等[96]對(duì)鼎湖山森林苗圃地的研究進(jìn)一步表明，氮沉降對(duì)土壤動(dòng)物群落多樣性的影響存在閾值效應(yīng)(100 kg N hm–2a–1)。在北亞熱帶楊樹(shù)人工林進(jìn)行2年氮添加試驗(yàn)表明,中等濃度的氮添加對(duì)土壤動(dòng)物群落有促進(jìn)作用，高濃度則有抑制作用[97]；氮添加4年后，低氮和高氮水平分別顯著增加和降低了土壤動(dòng)物總密度和植食性土壤動(dòng)物密度，均表明氮添加對(duì)土壤動(dòng)物的影響存在閾值作用[98]。其次，不同林型的氮沉降響應(yīng)可能不同。南亞熱帶成熟林土壤動(dòng)物密度、類(lèi)群數(shù)等多樣性指數(shù)隨氮輸入增加而降低，針葉林則相反，而針葉闊葉混交林則無(wú)明顯響應(yīng)[95]。亞熱帶人工林土壤線(xiàn)蟲(chóng)對(duì)氮輸入的響應(yīng)均不顯著[99–101]。氮添加增加了溫帶針葉林土壤動(dòng)物(跳蟲(chóng)和螨蟲(chóng))的豐度[102]。

此外，不同類(lèi)型的土壤動(dòng)物對(duì)氮沉降的響應(yīng)也可能不一致。對(duì)溫帶人工林[落葉松和水曲柳()]的研究表明，施肥改變了兩林分不同食性土壤動(dòng)物的密度，導(dǎo)致腐食性土壤動(dòng)物數(shù)量降低，植食性土壤動(dòng)物數(shù)量增加，但捕食性土壤動(dòng)物數(shù)量變化不明顯；這表明不同食性土壤動(dòng)物對(duì)氮沉降的響應(yīng)也不一致[103]。此外, 也有研究表明加氮對(duì)植食性線(xiàn)蟲(chóng)密度無(wú)顯著影響，但可以改變外來(lái)生物(如蚯蚓)和植食性線(xiàn)蟲(chóng)之間的相互作用關(guān)系，從而潛在影響生態(tài)系統(tǒng)的功能[104]。

5.7 氮沉降對(duì)溫室氣體排放的影響

5.7.1 氮沉降對(duì)CO2排放的影響

森林土壤呼吸是CO2進(jìn)入大氣的重要過(guò)程，了解氮沉降對(duì)森林土壤呼吸及其組分(自養(yǎng)呼吸和異氧呼吸)的影響，對(duì)于理解森林碳源/匯功能和穩(wěn)定性碳庫(kù)改變、以及預(yù)測(cè)未來(lái)氣候變化均非常重要。由于森林類(lèi)型、環(huán)境條件和施氮持續(xù)時(shí)間不同，森林土壤呼吸對(duì)氮沉降的響應(yīng)包括促進(jìn)作用[74,105–106]、抑制作用[29,107]和無(wú)影響[108–110]。

國(guó)內(nèi)最早見(jiàn)于鼎湖山的研究報(bào)道，氮沉降顯著抑制了我國(guó)南亞熱帶成熟森林土壤呼吸能力[27], 但對(duì)該區(qū)域內(nèi)混交林和馬尾松林土壤呼吸速率沒(méi)有影響[108]。氮添加引起的細(xì)根生物量減少是抑制森林土壤自養(yǎng)呼吸(Ra)的直接原因，而氮添加導(dǎo)致的土壤酸化將減少微生物量及降低其活性是引起異氧呼吸(Rh)能力下降的根本原因[27,75]。一般而言，氮沉降引起的微生物生物量、凋落物量和土壤有機(jī)碳庫(kù)的變化更易引起土壤異氧呼吸速率的改變[106]。整合分析表明，氮沉降總體降低了森林土壤呼吸速率的1.44%[106]，其響應(yīng)程度與氮添加的持續(xù)時(shí)間有關(guān)。試驗(yàn)早期，外源氮通過(guò)改變參與凋落物分解的微生物基因表達(dá)、改變其群落結(jié)構(gòu)和活性，顯著改變土壤異氧呼吸[79,111–112]。氮沉降對(duì)土壤自養(yǎng)呼吸的影響程度取決于其對(duì)細(xì)根生物量的影響，在“富氮”森林中，外源氮素輸入可顯著降低細(xì)根生物量，從而顯著抑制森林土壤自養(yǎng)呼吸能力[27]；而在相對(duì)“貧氮”系統(tǒng)中，氮添加刺激植物光合作用，增加細(xì)根生物量以獲取更多的營(yíng)養(yǎng)元素(如磷等)和水分，從而顯著提高了該森林土壤自養(yǎng)呼吸能力[74]。此外，氮輸入形式不同對(duì)森林土壤呼吸的影響也不相同，通常情況下，NH4NO3為氮源的模擬試驗(yàn)響應(yīng)程度比較顯著[113–114]。氮添加減少根際碳的輸入，同時(shí)降低微生物量及活性，減少土壤有機(jī)質(zhì)的分解速率[115]，根系可能通過(guò)改變形態(tài)和分泌物來(lái)減輕氮輸入對(duì)根際的影響。氮沉降可通過(guò)對(duì)細(xì)根生長(zhǎng)和微生物結(jié)構(gòu)及活性的影響來(lái)改變土壤呼吸的溫度敏感性(Q10值)[27,116]。

5.7.2 氮沉降對(duì)N2O排放的影響

森林土壤是N2O的源，含氮底物(NH4+-N和NO3–-N)通過(guò)硝化及反硝化細(xì)菌等，將NH4+-N和NO3–-N在不同條件下(好氧或厭氧)發(fā)生硝化或反硝化作用[117]，產(chǎn)生N2O并排放至大氣中，其排放過(guò)程主要受土壤溫度和濕度的調(diào)節(jié)。氮沉降背景下，森林土壤N2O排放量增加，尤其在熱帶/亞熱帶森林中(約增加739%)[118]。近年來(lái)國(guó)內(nèi)雖相繼開(kāi)展了一系列研究，由于區(qū)域和林型不同，仍沒(méi)有統(tǒng)一的響應(yīng)結(jié)論報(bào)道。

國(guó)內(nèi)最早的控制試驗(yàn)報(bào)道見(jiàn)于在我國(guó)鼎湖山(南亞熱帶)森林的研究[16]，氮添加顯著增加了南亞熱帶成熟林土壤無(wú)機(jī)氮(DON)含量，進(jìn)而刺激了N2O排放[16,119–120]，但對(duì)該區(qū)域的針闊混交林和馬尾松林土壤N2O排放沒(méi)有影響，人為干擾程度導(dǎo)致森林本身“氮狀態(tài)”的不同引起響應(yīng)差異性[16,120]。同一樣地土壤進(jìn)行室內(nèi)培養(yǎng)試驗(yàn)也表明，外源氮輸入顯著降低了成熟林土壤15N自然豐度，增加其N(xiāo)2O排放，而對(duì)混交林和馬尾松林土壤的N2O排放速率影響不明顯[121]。南亞熱帶豆科固氮樹(shù)種[大葉相思()]人工林土壤N2O排放速率在氮添加處理后顯著增加，而非固氮樹(shù)種[桉樹(shù)(sp.)林]的響應(yīng)不明顯[122]。對(duì)亞熱帶千煙洲人工杉木林的研究表明，氮沉降顯著促進(jìn)了N2O排放速率, 提高了4~7倍[123]，而且NH4+-N添加對(duì)土壤N2O排放的促進(jìn)作用高于NO3–-N[124]，可能是氮添加改變了氨氧化細(xì)菌的豐度(增加AOA、減少AOB)引起的[125]。來(lái)自重慶鐵山坪(西南地區(qū))森林的研究(15N示蹤)表明，NH4NO3添加顯著增加土壤N2O排放量，NH4+-N和NO3–-N對(duì)N2O的貢獻(xiàn)率沒(méi)有差異[126]；然而對(duì)同一地區(qū)森林土壤15N示蹤室內(nèi)培養(yǎng)6 d，土壤N2O排放的71%~100%來(lái)源于NO3–-15N[127]。對(duì)海南尖峰嶺長(zhǎng)期氮添加森林土壤進(jìn)行室內(nèi)培養(yǎng)，結(jié)果表明N2O、N2排放及N2O/N2比例均沒(méi)有明顯改變(與對(duì)照比較)。在厭氧條件下，隨著氮素輸入，熱帶次生林土壤N2O排放量減少、N2排放量增加，隨氮添加N2O/N2比例降低[128]，進(jìn)一步驗(yàn)證了反硝化過(guò)程可能是調(diào)控我國(guó)熱帶/亞熱帶森林土壤N2O產(chǎn)生的主要過(guò)程。

對(duì)我國(guó)北方溫帶森林的研究表明，氮添加引起長(zhǎng)白山森林地表可溶性無(wú)機(jī)氮(DON)增加，加快土壤氮素循環(huán)速率，并增加土壤N2O的排放[129–130]，這些促進(jìn)作用主要集中在春季(凍融)和夏季(生長(zhǎng)季節(jié))[110]。氮沉降可明顯增加反硝化速率，進(jìn)而增加土壤N2O排放量[131]；尿素添加增加了凋落物層和礦質(zhì)土壤NO3–-N含量，從而引起N2O排放量呈指數(shù)增加[132]。與紅松()林相比，北方闊葉林土壤中的有機(jī)質(zhì)在氮添加下更容易被微生物分解、有機(jī)氮被礦化成無(wú)機(jī)氮，增加了硝化和反硝化作用底物，從而有更多的N2O排出[133]。對(duì)黑龍江涼水國(guó)家級(jí)自然保護(hù)區(qū)森林的研究也表明，隨著氮處理水平的增加，早期高氮添加顯著增加N2O排放能力[134]，且對(duì)N2O排放促進(jìn)作用主要發(fā)生在生長(zhǎng)季節(jié)[135]。在北京西山林場(chǎng)的試驗(yàn)表明，模擬添加NH4NO3、(NH4)2SO4、NaNO3對(duì)N2O排放速率分別增加了356%、266%和188%[136]。這表明氮沉降對(duì)我國(guó)森林土壤N2O排放的影響取決于系統(tǒng)本身的氮狀態(tài)、施氮量、施氮形態(tài)和實(shí)驗(yàn)周期等因素。

5.7.3 氮沉降對(duì)CH4吸收的影響

森林土壤一般認(rèn)為是大氣CH4的匯(CH4氧化與產(chǎn)生過(guò)程的綜合)。已有研究表明，氮沉降對(duì)我國(guó)森林土壤CH4氧化及產(chǎn)生過(guò)程均有不同程度的影響。

國(guó)內(nèi)有關(guān)森林土壤CH4吸收通量對(duì)氮沉降的響應(yīng)研究最早見(jiàn)于南亞熱帶森林(鼎湖山)的報(bào)道[137],氮添加引起的土壤NH4+與CH4氧化菌的競(jìng)爭(zhēng)以及酸化環(huán)境下Al3+的毒害作用可能是引起CH4吸收通量減少的主要原因[118,137]。不同N素形態(tài)(NH4+-N和NO3–-N)對(duì)CH4吸收能力的影響程度不同[125,138]。早期氮添加顯著降低了我國(guó)南亞熱帶固氮樹(shù)種(大葉相思)人工林土壤CH4吸收能力，而對(duì)非固氮樹(shù)種(桉樹(shù)林)人工林沒(méi)有影響[139]；氮添加對(duì)鼎湖山成熟林和次生林(針闊葉混交林)土壤CH4吸收通量顯著抑制，但對(duì)該區(qū)域馬尾松林沒(méi)有顯著影響[140], 氮沉降抑制森林土壤CH4吸收速率的程度與系統(tǒng)本身的“氮狀態(tài)”呈正相關(guān)關(guān)系[139]。另外，氮沉降對(duì)土壤CH4吸收的影響與森林類(lèi)型密切相關(guān)[132,141]，闊葉林比針葉林更為敏感，原因是闊葉林中凋落物分解速率較快、土壤中留存了更多的有效氮[142]。

截止目前，氮沉降對(duì)我國(guó)森林土壤主要溫室氣體通量影響的微生物機(jī)制仍不清晰，亟需借助于基因(DNA)芯片技術(shù)及高通量測(cè)序等手段，解析調(diào)控不同溫室氣體通量的微生物豐度、群落結(jié)構(gòu)及活性對(duì)氮沉降的響應(yīng)，并綜合考慮CO2、N2O及CH4溫室效應(yīng)潛能，才能更好地理解未來(lái)氮沉降增加背景下，森林生態(tài)系統(tǒng)對(duì)區(qū)域乃至全球氣候變化的調(diào)控作用。

5.8 氮沉降對(duì)生態(tài)系統(tǒng)固氮能力的影響

生物固氮是自然生態(tài)系統(tǒng)重要的氮素來(lái)源之一。在工業(yè)化革命以前，生物固氮作為生態(tài)系統(tǒng)主要的氮素來(lái)源，在生態(tài)系統(tǒng)養(yǎng)分循環(huán)和生產(chǎn)力方面發(fā)揮不可替代的作用[143–144]。然而，隨著工業(yè)的發(fā)展、人口增加和化石燃料的過(guò)度使用，大氣氮沉降逐年加劇[145]。氮沉降的加劇已經(jīng)對(duì)自然生態(tài)系統(tǒng)的固氮能力產(chǎn)生了影響。據(jù)估計(jì)，在過(guò)去15年間，陸地自然生態(tài)系統(tǒng)固氮速率已從100~290 Tg N a–1降低為44 Tg N a–1[145]，而導(dǎo)致該現(xiàn)象的主要原因是人為活動(dòng)排放的活性氮沉降[146]。

森林是固氮微生物分布比較廣泛的生態(tài)系統(tǒng),森林土壤、凋落物層、附生植物(苔蘚和地衣)、豆科植物根瘤和植物葉片等都具有固氮微生物的分布[147]。美洲和歐洲等地區(qū)已開(kāi)展了氮沉降(或氮添加處理)對(duì)森林固氮的影響研究，多數(shù)結(jié)果表明外源氮輸入對(duì)森林固氮產(chǎn)生抑制作用[148–151]。由于經(jīng)濟(jì)的快速發(fā)展，中國(guó)已成為大氣氮沉降較為嚴(yán)重的地區(qū)之一[152]，但當(dāng)前有關(guān)氮沉降對(duì)我國(guó)森林固氮影響的研究相對(duì)較少。Zheng等[153]的研究表明，氮添加降低了我國(guó)南亞熱帶鶴山大葉相思林土壤、凋落物和根瘤的固氮速率，但僅降低尾葉桉()林土壤的固氮速率。氮沉降對(duì)鶴山人工林固氮速率的抑制效應(yīng)可以通過(guò)人為管理方式(如施加磷肥)得以緩解[90]。在我國(guó)南亞熱帶鼎湖山森林中，Zheng等[154]比較了不同演替階段森林，表明氮沉降抑制了干擾林的固氮速率，但對(duì)恢復(fù)林的沒(méi)有顯著影響，這暗示加強(qiáng)森林保護(hù)可以有效增加森林的固氮潛力。此外，Zheng等[32]的研究表明，在鼎湖山土壤“氮飽和”成熟林中長(zhǎng)期氮添加處理對(duì)森林總固氮速率的影響很小，這一定程度上驗(yàn)證了Hedin等[155]提出的熱帶森林“氮悖論”假設(shè)。Wang等[156]報(bào)道氮添加顯著降低了江西杉木人工林土壤固氮微生物豐度和固氮速率。Tang等[157]報(bào)道鼎湖山和長(zhǎng)白山森林土壤固氮菌對(duì)氮添加的響應(yīng)存在差異，即長(zhǎng)白山森林土壤固氮菌豐度對(duì)氮添加較為敏感(固氮菌豐度顯著降低)，而鼎湖山森林土壤固氮菌對(duì)氮添加的敏感性較低(固氮菌豐度沒(méi)有顯著變化)。因此，氮添加降低森林固氮的潛在機(jī)理是：(1)通過(guò)增加森林土壤有效氮的含量而減弱固氮菌的競(jìng)爭(zhēng)優(yōu)勢(shì)[147]; (2)通過(guò)加劇土壤其他養(yǎng)分(如磷等)的流失進(jìn)而制約生物固氮的過(guò)程[90]; (3)通過(guò)改變固氮附生植物和結(jié)瘤植物的氮素獲取方式，從而減少對(duì)固氮菌的能量分配，進(jìn)一步降低森林的固氮量[158]。

5.9 氮沉降對(duì)氮循環(huán)的影響

氮循環(huán)直接影響著土壤氮狀況和土壤中氮的去向，是森林生態(tài)系統(tǒng)最重要的功能之一。森林生態(tài)系統(tǒng)氮循環(huán)可以分成外循環(huán)與內(nèi)循環(huán)。外循環(huán)包括氮的輸入(生物固氮、氮沉降)和氮的輸出(淋溶、徑流流失、反硝化與氨的揮發(fā))，內(nèi)循環(huán)則是指氮在生態(tài)系統(tǒng)內(nèi)部各氮庫(kù)之間的周轉(zhuǎn)，包括氮的礦化、硝化、植物和微生物對(duì)氮的固持等。氮沉降加劇會(huì)顯著影響森林生態(tài)系統(tǒng)氮的輸入和輸出，還會(huì)改變氮在生態(tài)系統(tǒng)內(nèi)部的周轉(zhuǎn)[159]。

氮沉降加劇會(huì)增加中國(guó)森林生態(tài)系統(tǒng)氮的輸入。在中國(guó)南部亞熱帶森林中，隨著氮沉降量的增加，森林穿透雨中的氮含量也隨之增加[40]。由于目前大部分氮沉降模擬實(shí)驗(yàn)都是在林下施加氮肥，因而氮添加對(duì)森林穿透雨中氮含量的影響并不能模擬氮沉降對(duì)穿透雨氮輸入的影響。華南園在廣東石門(mén)臺(tái)和河南雞公山開(kāi)展的林冠模擬氮沉降的試驗(yàn)可能讓我們更準(zhǔn)確地了解氮沉降對(duì)森林穿透雨氮?jiǎng)討B(tài)的影響[35]。

氮輸入的增加還會(huì)加劇森林土壤氮素的淋失。氮是陸地生態(tài)系統(tǒng)最普遍的限制性元素之一。氮添加使土壤總氮在熱帶、亞熱帶、溫帶和北方森林分別增加26.9%、5.4%、4.5%和17.3%[51]。但是，過(guò)量的氮輸入會(huì)改變森林生態(tài)系統(tǒng)的氮狀況，使森林生態(tài)系統(tǒng)由“氮限制”狀態(tài)變成“氮飽和”狀態(tài)，導(dǎo)致氮的淋溶[20]。在重慶鐵山坪亞熱帶混交林中，氨態(tài)氮(43 kg N hm–2a–1)和硝態(tài)氮(40 kg N hm–2a–1)添加都顯著增加了土壤硝態(tài)氮的淋溶[160]。在鼎湖山南亞熱帶常綠闊葉林、針闊葉混交林、針葉林中，氮添加顯著加劇了土壤無(wú)機(jī)氮和有機(jī)氮的淋溶[18,41,161]。最近的原位氮同位素示蹤研究則表明，即使在“氮飽和”的森林土壤，新輸入土壤的氮也不是直接從土壤中流失,而是先在生態(tài)系統(tǒng)內(nèi)部進(jìn)行重新分配和周轉(zhuǎn)[162]。

氮輸入增加可以改變土壤氮素轉(zhuǎn)化過(guò)程，包括氮的礦化、硝化和氮固持。氮添加對(duì)我國(guó)溫帶和北方森林土壤氮的礦化和硝化過(guò)程通常表現(xiàn)為促進(jìn)或不影響[88,163–165]。Gao等[164]報(bào)道氮添加(40 kg N hm–2a–1)增加了溫帶混交林土壤氮的凈礦化速率，但對(duì)凈硝化速率沒(méi)有顯著影響。氮添加則對(duì)賽罕烏拉的白樺()次生林同時(shí)增加了氮的凈礦化速率和凈硝化速率[163]。氮添加促進(jìn)了長(zhǎng)白山溫帶混交林總氮礦化速率，但對(duì)總硝化速率沒(méi)有顯著影響[166]。對(duì)中國(guó)熱帶和亞熱帶森林，氮添加對(duì)氮的礦化及硝化有促進(jìn)和不影響[122]、或抑制作用[167]。氮添加降低了鼎湖山自然保護(hù)區(qū)和查灣自然保護(hù)區(qū)的常綠闊葉林土壤氮的凈礦化和凈硝化速率[167–168], 也降低了千煙洲亞熱帶混交林和松林土壤氮的總礦化速率[169–170]?？傊砑訉?duì)中國(guó)森林土壤氮素轉(zhuǎn)化過(guò)程的影響取決于氮添加的數(shù)量、種類(lèi)和持續(xù)時(shí)間以及林型等因素。

目前，關(guān)于氮沉降對(duì)土壤氮素轉(zhuǎn)化過(guò)程的了解大多基于凈氮素轉(zhuǎn)化速率，還鮮有研究揭示氮沉降背景下，土壤氮素轉(zhuǎn)化速率的變化與相關(guān)的微生物群落變化之間的聯(lián)系。未來(lái)需要更多地將總氮素轉(zhuǎn)化速率與土壤微生物群落及酶活性綜合考慮，以更加準(zhǔn)確地揭示氮沉降影響土壤氮素轉(zhuǎn)化速率機(jī)理。

5.10 氮沉降對(duì)磷循環(huán)的影響

森林磷循環(huán)包括磷素的輸入(巖石風(fēng)化或沉降輸入)和輸出(淋溶)、在生態(tài)系統(tǒng)各磷庫(kù)中的固持(土壤磷的有效性、植物體的固持和微生物的固持)及在各磷庫(kù)間的流轉(zhuǎn)(如凋落物的分解和回收利用)等。目前國(guó)內(nèi)有關(guān)氮沉降對(duì)對(duì)磷循環(huán)的研究極少。Zhou等[171]在鼎湖山長(zhǎng)期高氮沉降背景下的淋溶試驗(yàn)表明，氮沉降顯著降低了冠層穿透雨中的磷素，但對(duì)地表徑流及土壤溶液的磷素淋溶沒(méi)有顯著影響，這可能與植物磷吸收、凋落物分解的抑制作用、土壤有效磷低和較強(qiáng)的生物固持作用有關(guān)。

作為磷素循環(huán)的一部分，土壤有效磷對(duì)氮沉降響應(yīng)的報(bào)道最常見(jiàn)，但相關(guān)結(jié)果并不一致。有研究表明，氮沉降降低了土壤磷的有效性，如中國(guó)東北帽兒山落葉林[172]。近年來(lái)的一些研究卻表明，氮添加并沒(méi)有降低甚至增加了土壤磷的有效性[173–175], Wang等[174]對(duì)桉樹(shù)進(jìn)行短期氮磷添加處理，高氮添加降低了亞熱帶森林土壤可溶性有機(jī)磷，同時(shí)有效磷含量上升，這些差異可能與地域間氮沉降量、土壤磷素含量、土壤鈣鐵鋁離子含量、土壤pH、森林物種組成和土地利用方式不同有關(guān)[175]。

對(duì)生態(tài)系統(tǒng)而言，氮添加導(dǎo)致生態(tài)系統(tǒng)氮磷計(jì)量化學(xué)失衡(增加植物氮/磷比)，并可能導(dǎo)致系統(tǒng)磷限制的發(fā)生[176–177]。然而，植物在磷限制壓力下可以通過(guò)一系列的生理生態(tài)過(guò)程來(lái)緩解磷素的相對(duì)不足。首先，氮沉降促進(jìn)植物對(duì)磷的吸收[49,178]。Deng等對(duì)華北落葉松的研究表明，氮添加通過(guò)改變叢枝菌根真菌與外生菌根的比例來(lái)促進(jìn)對(duì)磷素的吸收,以保持體內(nèi)氮磷比的穩(wěn)定[49]。第二，氮輸入會(huì)促進(jìn)植物體對(duì)磷的重吸收[179–180]，See等[181]對(duì)英國(guó)不同養(yǎng)分狀態(tài)森林氮磷重吸收的研究表明，隨著土壤總氮含量的上升，磷素的重吸收率隨之升高。氮沉降還可以通過(guò)改變微生物學(xué)過(guò)程(如磷素的固持、酶的分泌等)來(lái)影響土壤磷素周轉(zhuǎn)[54,74]。鄭棉海等的研究表明，施加氮肥顯著提高了鼎湖山季風(fēng)林土壤的酸性磷酸酶活性[182]。此外，在氮/磷比較高的熱帶地區(qū)，森林生態(tài)系統(tǒng)傾向于閉合型循環(huán)，降低磷素的淋溶[171,183]。

5.11 氮沉降對(duì)凋落物分解的影響

凋落物是植物產(chǎn)生的枯落物或有機(jī)碎屑，植物體以產(chǎn)生凋落物的形式將營(yíng)養(yǎng)返還到土壤表面，而凋落物在微生物分解的作用下，向環(huán)境中釋放植物可吸收利用的營(yíng)養(yǎng)。陸地生態(tài)系統(tǒng)中約90%的凈初級(jí)生產(chǎn)量以凋落物的形式歸還給土壤[184]，養(yǎng)分再循環(huán)供給植物生長(zhǎng)發(fā)育。因而，凋落物分解過(guò)程受環(huán)境因子、凋落物質(zhì)量和生物因子等多因素共同調(diào)控。華南園率先開(kāi)展了氮沉降增加對(duì)森林凋落物分解方面的研究，國(guó)內(nèi)代表性的長(zhǎng)期氮沉降樣地有鼎湖山、長(zhǎng)白山、大小興安嶺等，研究?jī)?nèi)容涵蓋了凋落物的產(chǎn)量動(dòng)態(tài)、分布、分解過(guò)程和影響因素等。

目前，氮沉降對(duì)凋落物分解速率的研究結(jié)果尚不一致，有促進(jìn)[185–187]、抑制[188–189]和無(wú)顯著影響[190–191]。同時(shí)也有學(xué)者提出，氮沉降對(duì)凋落物分解的影響具有階段性[192–193]，在初期促進(jìn)其分解, 而后期反而受到高氮量的抑制作用。此外，凋落物分解過(guò)程還受到演替進(jìn)程的影響。莫江明等[194]的研究表明，氮沉降對(duì)凋落物分解的影響隨亞熱帶森林演替從正效應(yīng)轉(zhuǎn)向負(fù)效應(yīng)。

中國(guó)學(xué)者對(duì)氮沉降與凋落物分解的研究多采用人工施加硝酸銨氮肥，其他無(wú)機(jī)氮或有機(jī)氮使用頻次相對(duì)降低，且施氮研究時(shí)間都比較短，大都關(guān)注1、2年的凋落物動(dòng)態(tài)，鮮少關(guān)注長(zhǎng)期凋落物動(dòng)態(tài)的研究。氮沉降對(duì)凋落物分解影響的研究結(jié)果雖然存在很大差異，但中國(guó)西南和南方等的亞熱帶森林，大多都表現(xiàn)出抑制凋落物分解的趨勢(shì)[133,188–189,195]。然而北方溫帶闊葉林和針葉林等對(duì)氮添加的響應(yīng)更為多樣，氮添加對(duì)凋落物分解多具正負(fù)效應(yīng)，且很多表現(xiàn)出階段性的效應(yīng)[185,187,193]。

5.12 氮沉降對(duì)植物源有機(jī)揮發(fā)物的影響

植物源揮發(fā)性有機(jī)物(biogenic volatile organic compounds, BVOCs)是指植物直接排放的沸點(diǎn)為50℃~260℃，室溫下飽和蒸汽壓超過(guò)133.322 Pa的易揮發(fā)性碳?xì)浠衔?，主要包括非甲烷烴類(lèi)(烷烴、烯烴、芳香烴)和含氧原子的醛、酮、酚、醇、醚、酯等揮發(fā)性有機(jī)物。BVOCs來(lái)源于植物的光合碳同化物，是碳(C)素循環(huán)的一個(gè)重要組成部分，其排放與環(huán)境因子息息相關(guān)，并對(duì)脅迫環(huán)境有顯著響應(yīng)[196–197]。

國(guó)內(nèi)氮沉降對(duì)森林植物BVOCs排放影響的報(bào)道極少，僅見(jiàn)于鼎湖山。黃娟等[198]提出了氮沉降對(duì)植物BVOCs影響的假說(shuō)模型：在氮限制系統(tǒng)中，氮沉降的增加補(bǔ)充了系統(tǒng)所需的氮素，有利于植物的生長(zhǎng)，大量BVOCs的排放會(huì)受到抑制；在氮素豐富或飽和的系統(tǒng)中，氮沉降導(dǎo)致系統(tǒng)氮素過(guò)飽和或富營(yíng)養(yǎng)化，不利于植物的生長(zhǎng), 刺激BVOCs的排放增加。該假說(shuō)模型在相對(duì)“氮限制”的鼎湖山苗圃地得到了驗(yàn)證，即為期一年半的氮添加(100 kg N hm–2a–1)顯著抑制了海南紅豆()和陰香()幼苗甲醛和乙醛的排放[199–200]。

5.13 氮沉降對(duì)生態(tài)系統(tǒng)碳吸存的影響

氮沉降通過(guò)影響森林生態(tài)系統(tǒng)主要碳庫(kù)(植物生物量和土壤有機(jī)碳庫(kù))對(duì)生態(tài)系統(tǒng)碳吸存產(chǎn)生影響(圖4)。氮沉降對(duì)生態(tài)系統(tǒng)碳吸存的影響包括對(duì)植被碳庫(kù)(vegetation carbon pool)和土壤碳庫(kù)(soil carbon pool)的影響。氮沉降通過(guò)改變植物的光合作用和呼吸作用的強(qiáng)弱來(lái)調(diào)節(jié)植物體作為生物碳庫(kù)的凈碳固存，同時(shí)又通過(guò)影響植物體產(chǎn)生凋落物的生物量大小及凋落物質(zhì)量(包括地上部分凋落物及根系凋落物)來(lái)調(diào)節(jié)進(jìn)入土壤碳庫(kù)的碳輸入量，此外，氮沉降還影響微生物群落與活性，從而控制著土壤碳庫(kù)中各種碳組分間的轉(zhuǎn)化(如有機(jī)碳的分解礦化與土壤呼吸)，并與淋溶等其他生態(tài)過(guò)程共同調(diào)節(jié)著土壤碳庫(kù)的碳儲(chǔ)量。陸地生態(tài)系統(tǒng)通常受到氮素的限制[5]，所以外源性氮添加會(huì)促進(jìn)生態(tài)系統(tǒng)初級(jí)生產(chǎn)力[1–2,53]。Yue等[53]報(bào)道，氮添加后全球尺度上的陸地生態(tài)系統(tǒng)地上部分與地下部分的碳含量分別增加了25.65%和15.93%。Chen等[201]通過(guò)收集中國(guó)陸地生態(tài)系統(tǒng)數(shù)據(jù)并進(jìn)行整合分析，義為中國(guó)的模擬氮沉降試驗(yàn)增加了地上植物碳庫(kù)，但地下植物碳庫(kù)顯著減少。這種地上生物量的增加趨勢(shì)與全球尺度上的變化趨勢(shì)相似[2,53]，而地下生物量的響應(yīng)與全球尺度上的變化模式不同。Lu等[71]通過(guò)全球陸地生態(tài)系統(tǒng)土壤碳儲(chǔ)存對(duì)氮沉降響應(yīng)的Meta分析表明，盡管不同地區(qū)的地上植物碳庫(kù)對(duì)氮沉降的響應(yīng)強(qiáng)度不同，但總體上呈現(xiàn)正響應(yīng)，氮添加處理下地上植物碳庫(kù)增加了35.7%[71]。中國(guó)地上與地下生物量對(duì)氮沉降的響應(yīng)與全球模式不同的原因可能在于中國(guó)氮沉降量非常高，因此氮沉降對(duì)根系的負(fù)效應(yīng)可能更為強(qiáng)烈。模擬氮沉降處理降低了鼎湖山季風(fēng)林的根系生物量，同時(shí)增加了死亡根系的生物量[57]；這可能是氮添加通過(guò)影響土壤酸化而對(duì)根系產(chǎn)生了毒副作用。此外，過(guò)多的氮可以通過(guò)影響初級(jí)生產(chǎn)力來(lái)抑制碳的吸收。氮添加對(duì)森林植被，尤其是林下植被有顯著的負(fù)效應(yīng)，而對(duì)大樹(shù)的影響還需要長(zhǎng)期研究[17,65,202]。通常認(rèn)為，“富氮”森林生態(tài)系統(tǒng)中的植物生長(zhǎng)和生產(chǎn)力對(duì)長(zhǎng)期氮添加沒(méi)有明顯響應(yīng)[13]。

圖4 森林生態(tài)系統(tǒng)碳吸存過(guò)程框架圖

土壤碳庫(kù)是陸地生態(tài)系統(tǒng)中重要的碳庫(kù)。土壤碳庫(kù)是碳通過(guò)枯葉(如葉，根和枝)輸入和通過(guò)分解、淋溶輸出的凈儲(chǔ)量。因此，土壤碳的分解過(guò)程是控制森林生態(tài)系統(tǒng)中土壤碳庫(kù)量的關(guān)鍵，而包括底物質(zhì)量、氣候、水分及土壤化學(xué)性質(zhì)等各種因素控制著底物的分解過(guò)程[203]。氮是控制有機(jī)物分解的重要因素。近幾十年來(lái)由于人類(lèi)活動(dòng)的影響, 氮沉降量急劇增加，預(yù)計(jì)在熱帶亞熱帶地區(qū)將進(jìn)一步增加[3]。因此，了解土壤有機(jī)質(zhì)分解對(duì)高氮輸入的響應(yīng)是預(yù)測(cè)未來(lái)全球碳動(dòng)態(tài)變化的關(guān)鍵。

許多研究表明，除分解的早期階段外氮添加會(huì)降低有機(jī)物的分解，所以氮沉降可以提高森林生態(tài)系統(tǒng)中的土壤碳庫(kù)[204]。在“氮飽和”的南亞熱帶原始森林，氮添加抑制了凋落物分解[1,26]。同時(shí)氮添加早期促進(jìn)了人工林中的凋落物分解，這與氮相對(duì)較貧乏時(shí)的原因相同[26]。隨后進(jìn)行的凋落物分解研究與這一趨勢(shì)相反，隨著時(shí)間的推移，更多的氮輸入生態(tài)系統(tǒng)中，導(dǎo)致凋落物分解受到抑制[205]。相應(yīng)地，對(duì)鼎湖山成熟林的研究也表明，氮肥施加抑制了土壤呼吸[27]。因此，未來(lái)持續(xù)高氮沉降輸入可能會(huì)通過(guò)抑制有機(jī)質(zhì)分解來(lái)促進(jìn)土壤碳吸存。

5.14 森林生態(tài)系統(tǒng)氮沉降去向研究

沉降到生態(tài)系統(tǒng)中的氮經(jīng)過(guò)一系列的循環(huán)周轉(zhuǎn)一般有4個(gè)去向：存留到植物和土壤的各個(gè)組分中，以氣體形式(含N化合物的排放)或液體形式(淋溶流失)離開(kāi)生態(tài)系統(tǒng)。15N示蹤技術(shù)被認(rèn)為是氮沉降背景下研究氮循環(huán)的有力工具，Templer等[206]對(duì)溫帶和北方森林生態(tài)系統(tǒng)的研究表明，15N添加后短期內(nèi)(約1年)，森林生態(tài)系統(tǒng)總的15N回收率維持在75%左右，且土壤(有機(jī)質(zhì)層和表層礦質(zhì))是最大的氮匯，其次是植物。

近年來(lái)，我國(guó)陸續(xù)開(kāi)展了基于15N示蹤手段研究生態(tài)系統(tǒng)氮去向的相關(guān)試驗(yàn)。在熱帶(亞熱帶)區(qū)域的鼎湖山成熟林、重慶鐵山坪的亞熱帶森林、海南熱帶山地原始林的研究(15NH415NO3、15NH4NO3或NH415NO3)表明，15N添加1年后生態(tài)系統(tǒng)15N回收率高達(dá)50%~70%以上，而土壤溶液中15N回收率為14%~33%[162,207–208]。因此，以NH4NO3為主要氮沉降形式的土壤和植物是最大的氮匯。然而,在鐵山坪亞熱帶森林進(jìn)行的Na15NO3示蹤研究表明, 18個(gè)月內(nèi)土壤溶液中15N回收率高達(dá)74%，這反映了生態(tài)系統(tǒng)在氮飽和情境下難以保持過(guò)量硝態(tài)氮的輸入[160]。在長(zhǎng)白山溫帶森林進(jìn)行的15NH4NO3和NH415NO3示蹤研究表明，1年后仍有80%15N存留在植物和土壤中，高于熱帶森林[207]。在亞熱帶常綠闊葉林和北方針葉林，Sheng等[209]利用(15NH4)2SO4和K15NO3兩種15N進(jìn)行示蹤研究,15NH4+的回收率在兩個(gè)森林中相差不大，15NO3–的回收率在針葉林高于亞熱帶闊葉林；和NH4+相比, NO3–更容易從生態(tài)系統(tǒng)中流失。這些表明，熱帶亞熱帶森林仍具有不可忽視的氮匯，而且沉降氮的形態(tài)將進(jìn)一步?jīng)Q定氮素在生態(tài)系統(tǒng)中的命運(yùn)。

6 結(jié)論和展望

通過(guò)上面綜述，我們可以得出兩個(gè)重要結(jié)論:首先，中國(guó)近三十年來(lái)大氣氮沉降總量顯著增加, 特別是NO3–氮沉降；其次，模擬氮沉降研究表明,持續(xù)高氮輸入將會(huì)顯著改變森林生態(tài)系統(tǒng)的結(jié)構(gòu)和功能，特別是處于氮沉降熱點(diǎn)區(qū)域的中國(guó)中南部。主要表現(xiàn)如下：(1)北方和溫帶森林仍存在氮素施肥效應(yīng)，熱帶森林則趨向氮飽和、無(wú)明顯的正生長(zhǎng)效應(yīng)；(2)過(guò)量氮沉降將會(huì)導(dǎo)致土壤酸化和養(yǎng)分失衡；(3)氮沉降增加促進(jìn)了氮循環(huán)速率及其轉(zhuǎn)化過(guò)程，但抑制了森林的固氮速率，并影響到了碳、磷等其他元素循環(huán)、凋落物分解進(jìn)程和溫室氣體排放；(4)氮沉降降低了林下層植物，并改變了土壤微生物群落結(jié)構(gòu)；(5)高氮沉降背景下熱帶亞熱帶森林仍具有不可忽視的氮匯；(6)森林生態(tài)系統(tǒng)的氮沉降效應(yīng)依賴(lài)于系統(tǒng)的氮狀態(tài)、土地利用歷史、氣候特征、林型和林齡等。

盡管目前的研究對(duì)深入理解和評(píng)估大氣氮沉降增加對(duì)中國(guó)森林生態(tài)系統(tǒng)健康的影響做出了積極貢獻(xiàn)，但是在全球變化背景下未來(lái)仍有以下幾個(gè)方面值得深入探討。

首先，在研究方式上應(yīng)兼顧到以下3點(diǎn)：(1)需要追蹤研究森林生態(tài)系統(tǒng)的長(zhǎng)期氮沉降效應(yīng)。目前的大部分氮沉降試驗(yàn)研究期限較短(<10年)，研究結(jié)果的有效性和可靠性還有待于長(zhǎng)期驗(yàn)證。(2)要加強(qiáng)各站點(diǎn)之間的聯(lián)網(wǎng)研究，以更好地探究生態(tài)系統(tǒng)現(xiàn)象背后運(yùn)轉(zhuǎn)的規(guī)律，以期為森林生態(tài)系統(tǒng)的可持續(xù)發(fā)展提供理論基礎(chǔ)和管理依據(jù)。(3)加強(qiáng)對(duì)我國(guó)不同氣候和植被類(lèi)型生態(tài)系統(tǒng)響應(yīng)研究的評(píng)估，這主要是考慮到不同氣候帶的水、土、氣、生等各方面均存在差異。

其次，在研究?jī)?nèi)容上有以下3個(gè)方面需要強(qiáng)化：(1)森林生態(tài)系統(tǒng)對(duì)長(zhǎng)期氮沉降響應(yīng)與適應(yīng)的過(guò)程機(jī)制有待于深入研究，因?yàn)槟壳暗难芯扛赜诟窬帧?2)長(zhǎng)期氮沉降對(duì)中國(guó)森林碳吸存與固氮潛力的影響，主要是考慮到地下碳氮循環(huán)過(guò)程仍存在巨大的不確定性。(3)需要加強(qiáng)與其他全球變化因子(如全球變暖、CO2濃度升高、降雨格局改變等)的耦合研究，以更好的評(píng)估生態(tài)系統(tǒng)未來(lái)的發(fā)展趨勢(shì)。

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Effects of Simulated Atmospheric Nitrogen Deposition on Forest Ecosystems in China: An Overview

LU Xian-kai1, MO Jiang-ming1, ZHANG Wei1, MAO Qing-gong1, LIU Rong-zhen1,2, WANG Cong1,2, ZHENG Mian-hai1, WANG Sen-hao1,2, MORI Taiki1, MAO Jin-hua1,2, ZHANG Yong-qun1,2, WANG Yu-fang1,2, HUANG Juan1

(1. Key Laboratory of Vegetation Restoration and Management of Degraded Ecosystem, Guangdong Provincial Key Laboratory of Applied Botany, South China Botanical Garden, Chinese Academy of Sciences, Guangzhou 510650, China; 2. University of Chinese Academy of Sciences, Beijing 100049, China)

Human activities, such as combustion of fossil fuel, production and application of nitrogenous fertilizer, and intensive livestock production, have been accelerating the production and emission of reactive nitrogen (e.g., NH4+, NO3–), leading to elevated nitrogen (N) deposition at regional and global scales. Human interference with N cycle has gone beyond the safe operating space for humanity. China is one of the three regions with the highest N deposition in the world. High N deposition has threatened the health and safety of terrestrial ecosystems, which should be addressed urgently during the process of ecological civilization construction. The research history on simulated N deposition in China and world was reviewed, focused on how simulated N deposition affects forest ecosystems in China, including soil acidification, plant element chemistry, plant growth and diversity, soil microbial community and enzyme activities, soil fauna, greenhouse gas emissions, ecosystem N and phosphorus cycles, soil N transformation, ecosystem N fixation, litter decomposition, and ecosystem carbon sequestration. The atmospheric N deposition has been concerned since 2000s. In 2002, the first long-term forest ecosystem N manipulative experiments were established by South China Botanical Garden (SCBG) of the Chinese Academy of Sciences, which is playing a leading role in the field of nitrogen deposition and forest ecosystems in China. In 2013, SCBG, for the first time, designed a novel experiment with canopy addition of N (CAN) vs. understory addition of N (UAN) in China. Results from N manipulative experiments across China showed that continuing high N deposition greatly altered forest structure and functioning, threatening ecosystem health, especially in the south-central China. The main results are as follows: (1) There is a fertilization effect of N deposition in temperate and boreal forests, but there seem no positive effects on plant growth in N-rich tropical forests because of N saturation. (2) Excess N deposition can lead to soil acidification and nutrient imbalance. (3) Elevated N deposition has accelerated N cycling rate and its transformation process, but depressed ecosystem N fixation rate, and altered ecosystem P availability and cycling, litter decomposition process and greenhouse gas emissions. (4) High N deposition reduced understory plants diversity and changed the structure of soil microbial community. (5) Nitrogen deposition generally simulates aboveground vegetation C sequestration across China, but there remains uncertain on belowground soil C sequestration. (6) Tropical and subtropical forest ecosystems are non-ignorable N sinks, depending on the forms and fates of added N. (7) The effects of N deposition on forest ecosystems are variable, depending on ecosystem N status, land-use history, climate, and forest types and ages. Considering that there remain uncertainties on the long-term effects of N deposition in China, it is suggested that it is necessary to continue the present studies in a longer term, and to expand ajointly consider multiple global change factors (e.g., climate warming, CO2enrichment, changes in precipitation patterns), all of which are important for forest management and sustainable development in the future.

Nitrogen deposition; Global change; Forest ecosystem; Nitrogen saturation; Nitrogen limitation; Nitrogen biogeochemical cycle; Biodiversity; Carbon sequestration

10.11926/jtsb.4113

2019–06–21

2019–08–20

國(guó)家自然科學(xué)基金項(xiàng)目(41731176, 31700422); 中國(guó)科學(xué)院青年創(chuàng)新促會(huì)基金項(xiàng)目(2015287)資助

This work was supported by the National Natural Science Foundation of China (Grant No. 41731176, 31700422); and the Project for Youth Innovation Promotion of Chinese Academy of Science (Grant No. 2015287).

魯顯楷, 研究員, 主要研究方向?yàn)槿蜃兓鷳B(tài)學(xué)，氮素生物地球化學(xué)。E-mail: luxiankai@scbg.ac.cn