張亦濤,劉宏斌,王洪媛,翟麗梅,劉 申,雷秋良,任天志

1 農(nóng)業(yè)部面源污染控制重點實驗室, 中國農(nóng)業(yè)科學院農(nóng)業(yè)資源與農(nóng)業(yè)區(qū)劃研究所, 北京 100081 2 農(nóng)業(yè)部環(huán)境保護科研監(jiān)測所,天津 300191

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農(nóng)田施氮對水質(zhì)和氮素流失的影響

張亦濤1,劉宏斌1,王洪媛1,翟麗梅1,劉 申1,雷秋良1,任天志2,*

1 農(nóng)業(yè)部面源污染控制重點實驗室, 中國農(nóng)業(yè)科學院農(nóng)業(yè)資源與農(nóng)業(yè)區(qū)劃研究所, 北京 100081 2 農(nóng)業(yè)部環(huán)境保護科研監(jiān)測所,天津 300191

基于ISI Web of Science數(shù)據(jù)庫,利用文獻計量學方法分析了自1957年以來各國在農(nóng)田氮流失領(lǐng)域的研究發(fā)展態(tài)勢,綜述了農(nóng)田氮流失特征及氮流失防控措施。結(jié)果表明,目前各國對農(nóng)田氮流失的研究主要集中在施氮對水體水質(zhì)的污染和監(jiān)測方法上,涉及的關(guān)鍵詞主要有Groundwater、Water quality、Surface water、Nitrate pollution、Eutrophication、Contamination、Nonpoint source pollution、Lysimeter、Runoff、Subsurface drainage等。中國、美國和加拿大等農(nóng)業(yè)大國的研究機構(gòu)在這一領(lǐng)域的研究成果最多發(fā)文量最多的期刊主要分布在荷蘭、美國和中國。文獻分析表明,受降水、地形、土壤、施肥等諸多管理措施的影響,不同區(qū)域的農(nóng)田氮流失量差別很大,中國各類農(nóng)田的氮流失量(13.7—347 kg/hm2)明顯高于歐美國家(4—107 kg/hm2)。我國單位面積化肥(357.3 kg/hm2)和氮肥(165.1 kg/hm2)施用量均遠高于世界平均用量(87.5 kg/hm2和52.9 kg/hm2),當季氮肥利用率(17%)卻明顯低于世界平均水平(58%),表明氮肥施用過量且利用率過低是造成氮流失的關(guān)鍵因素。綜合分析農(nóng)田氮流失防控措施發(fā)現(xiàn),從源頭控制氮流失是最有效的措施,優(yōu)化農(nóng)藝管理措施和氮遷移過程攔截等分別可減少15%—92%、46%—77%的氮素流失,其中針對農(nóng)田適宜施氮量的研究最多。然而,面對糧食生產(chǎn)需求與資源短缺、水體水質(zhì)持續(xù)惡化的現(xiàn)狀,未來的研究重點應從簡單的表觀平衡向整個農(nóng)田生態(tài)系統(tǒng)的氮素循環(huán)過程轉(zhuǎn)變,更為迫切的是加快探索以水質(zhì)保護為目標的化肥氮施用閾值(造成環(huán)境污染的臨界施氮量),并推廣示范這些有效的氮流失防控措施。

農(nóng)田施氮；Web of Science；水體水質(zhì)；氮流失；防控措施；施氮閾值

A bibliometric analysis of status and trend of international research on field nitrogen application effects on nitrogen losses and water quality

氮是作物生長和產(chǎn)量形成的最重要營養(yǎng)元素,氮肥施用為滿足全球70億人口的糧食需求做出了突出貢[1],農(nóng)田氮肥施用對發(fā)達國家糧食增產(chǎn)的貢獻率在40%以上[2],對發(fā)展中國家糧食增產(chǎn)貢獻率高達55%[3]。迫于人口持續(xù)增長對糧食需求的壓力,農(nóng)田生態(tài)系統(tǒng)氮素投入越來越多,然而,農(nóng)田氮素利用率卻逐年下降,投入的過量氮素或者殘留在土壤中,或以氨、氧化亞氮的形式揮發(fā),或以徑流、淋溶等方式進入水體,即使是殘留在土壤中的氮素,最終也會以揮發(fā)或流失的形式進入環(huán)境[4],造成了嚴重的環(huán)境風險或直接的環(huán)境污染。然而,作物對氮素的利用效率受氣候、土壤、農(nóng)田管理等多因素的影響,當前全球大多數(shù)作物種植系統(tǒng)的氮盈余量大[5],氮肥利用率低[6],水溶性氮流失及氣態(tài)氮損失量高[7],農(nóng)田氮流失嚴重,并成為農(nóng)區(qū)地表水富營養(yǎng)化和地下水硝酸鹽污染的重要原因[8-9]。

過量施氮及施氮后的損失造成了嚴重的環(huán)境問題[10],氮素管理與環(huán)境質(zhì)量及人類健康的關(guān)系受到了廣泛的關(guān)注,多年來,相關(guān)研究論文發(fā)表數(shù)量持續(xù)增長。文獻計量學是對文獻進行定量分析研究的科學,被廣泛應用于分析各種學科研究的科研成果和研究趨勢[11- 14]。ISI (Institute for Scientific Information)Web of Science數(shù)據(jù)庫收錄了全球8700多種各學科領(lǐng)域的領(lǐng)先期刊的SCI(Science Citation Index)論文,覆蓋面最廣,是文獻計量分析最常用的數(shù)據(jù)庫[15]。針對特定領(lǐng)域研究的文獻計量分析可以多方面的揭示各領(lǐng)域研究發(fā)展態(tài)勢和研究熱點問題,為科研工作者探索學科新方向提供依據(jù)。

本文將從農(nóng)田施氮對水體水質(zhì)的影響、氮流失影響因素及防控措施等多個角度,探討農(nóng)田氮流失(農(nóng)田向水體遷移損失的那部分氮,不包括氣態(tài)形式的氮損失)對水體水質(zhì)的影響發(fā)展態(tài)勢,揭示影響氮素流失的主要影響因素,分析當前有效的氮流失防控措施,并明確優(yōu)化氮肥施用量是從源頭控制氮素流失的最有效措施之一,為確定基于水質(zhì)保護的化肥氮施用閾值(即水質(zhì)污染突變時的農(nóng)田施氮量)提供依據(jù)。

1 農(nóng)田施氮對水體水質(zhì)影響的文獻計量分析

農(nóng)田施用的氮肥除被作物吸收利用外,剩余氮素大部分以揮發(fā)、淋溶或徑流等形式損失進入大氣和水體[16],硝態(tài)氮和銨態(tài)氮是流失氮素的主要形態(tài),水分擾動產(chǎn)生的懸浮土壤顆粒及其溶出的養(yǎng)分可隨地表徑流水遷移流失[17],而由水分溶解的土壤氮也可垂直向下遷移出根區(qū)[18]。

早在19世紀,科學家們就注意到了氮素流失帶來的環(huán)境問題,并將用于研究水分平衡的滲濾池技術(shù)應用到了元素的淋失研究[19],此后100a間研究的主要內(nèi)容是對養(yǎng)分淋失量及其形態(tài)進行觀測,但并未深入研究淋洗的剖面特征和動態(tài)。20世紀40年代以來,氮肥大量施用造成的嚴重環(huán)境問題凸顯出來,引發(fā)了70年代以來的氮素問題研究熱潮,農(nóng)田氮素遷移逐漸受到重視,15N示蹤技術(shù)開始應用于土壤肥料效應研究,此后科學家開始關(guān)注區(qū)域尺度上的氮素流失情況,并對影響流失的因素和有效控制措施進行了深入研究和模擬,同時多學科交叉合作日益增多,并形成了諸多新興研究領(lǐng)域。1976年,美國最早提出了面源污染的概念,并進行了面源污染研究[20- 21]；20世紀80年代初,我國湖泊、水庫富營養(yǎng)化調(diào)查和河流水質(zhì)規(guī)劃是中國面源污染研究的重要開端,但該時期我國農(nóng)田肥料投入量并未顯著增加,農(nóng)田養(yǎng)分的流失監(jiān)測也未作為研究的重點,然而隨著氮肥施用量的不斷增加和農(nóng)田灌溉條件的改善,面源污染問題日益突出,相關(guān)研究迅速發(fā)展起來,自20世紀90年代起,我國的農(nóng)田面源污染實地監(jiān)測也開展起來,氮流失問題備受重視[22]。

以ISI Web of Science數(shù)據(jù)庫的全部期刊為檢索對象,限定發(fā)表時間為1957年至2014年8月,對“農(nóng)田氮肥施用”主題的SCI論文進行檢索、數(shù)據(jù)清理、文獻分類和主題分析,檢索策略：(TOPIC: ((Farmland or field or agriculture or farming) and (“nitrogen fertiliz*” or “N fertiliz*” or“fertiliz* nitrogen” or “fertiliz* N”))),共檢索出全部論文7460篇,然后從7460篇論文的關(guān)鍵詞中檢索出與“水體水質(zhì)”相關(guān)的文獻(共305篇),此類文獻主要涉及氮肥施用對地下水、地表水水質(zhì)的影響及其施用造成的硝酸鹽污染、水體富營養(yǎng)化等。

對檢索出的相關(guān)文獻利用Excel統(tǒng)計數(shù)據(jù)信息,利用高頻關(guān)鍵詞和相關(guān)關(guān)系圖對研究熱點進行研究分析,采用發(fā)文數(shù)量和被引頻次對研究機構(gòu)、出版期刊等進行分析,統(tǒng)計分析過程中均選擇第一作者、第一研究機構(gòu)。利用湯森路透公司研發(fā)的TDA(Thomson Data Analyzer)軟件,將主題關(guān)鍵詞與全部關(guān)鍵詞進行相關(guān)分析組成相互關(guān)系數(shù)矩陣,并據(jù)此繪制相互關(guān)系圖,相關(guān)關(guān)系圖揭示了各關(guān)鍵詞之間關(guān)聯(lián)性的緊密程度,圖中點的大小表示包含這一關(guān)鍵詞的文獻量多少,點之間線的粗細表示兩個關(guān)鍵詞之間的相關(guān)程度,線越粗相關(guān)程度越緊密。

1.1 研究熱點

農(nóng)田氮素施用對水體水質(zhì)的影響研究中涉及的高頻關(guān)鍵詞主要涉及到與水體類型相關(guān)的如Groundwater、Water quality、Surface water、Wastewater等,與水體污染相關(guān)的如Nitrate pollution、Eutrophication、Contamination、Nonpoint source pollution、Nitrogen pollution等,與含氮污染物相關(guān)的如Nitrate concentration、Nitrogen availability、Nitrate nitrogen、Nitrate-N等,還有與氮素流失監(jiān)測相關(guān)的Runoff、Lysimeter、Subsurface drainage等(表1)。通過繪制這些高頻詞之間的相關(guān)關(guān)系圖,發(fā)現(xiàn)各主題詞之間的關(guān)聯(lián)關(guān)系(圖1),地下水(Groundwater)、水質(zhì)(Water quality)、氮(Nitrate)、磷(Phosphorus)、農(nóng)業(yè)(Agriculture)、施肥(Fertilization)、污染(Pollution；Contamination)和環(huán)境(Environment)之間關(guān)聯(lián)密切；說明農(nóng)田化肥的施用與面源污染尤其是水體水質(zhì)是存在密切聯(lián)系的,當前的農(nóng)田施肥已經(jīng)導致了水體中養(yǎng)分含量的變化。此外,水稻田(Paddy field)的氮素損失(Nitrogen loss)以及氨(Ammonia)的關(guān)聯(lián)關(guān)系也較為集中。

1.2 研究機構(gòu)

開展農(nóng)田氮肥施用對水體水質(zhì)研究的機構(gòu)很多,但發(fā)表SCI論文最多的10個研究機構(gòu)主要分布北美和亞洲的中國,其中有5家美國研究機構(gòu)、4家中國研究機構(gòu)、1家加拿大研究機構(gòu)(表2)。中國科學院在該領(lǐng)域發(fā)表了26篇SCI研究論文,排名全球第一,美國農(nóng)業(yè)部排名第二,發(fā)表SCI論文22篇,加拿大農(nóng)業(yè)及農(nóng)業(yè)食品部和中國農(nóng)業(yè)大學排名并列第三,各發(fā)表SCI論文10篇。這些關(guān)注農(nóng)田氮肥施用與水體水質(zhì)關(guān)系的研究機構(gòu)主要分布在農(nóng)田面積大、農(nóng)業(yè)生產(chǎn)活動頻繁的國家,這些國家的農(nóng)田氮素投入均很大,而作物每年從農(nóng)田帶走的氮素卻極其有限,部分農(nóng)田氮流失進入水體并不斷累積從而導致水質(zhì)下降,直接威脅人們的飲水安全,因此備受關(guān)注。

表1 水體水質(zhì)主題前20位關(guān)鍵詞頻詞

表2 水體水質(zhì)主題發(fā)文量前10位的機構(gòu)

1.3 主流期刊和高被引論文

刊發(fā)農(nóng)田氮肥施用對水體水質(zhì)影響研究論文排名前10位的英文期刊主要在歐美國家,中國的優(yōu)秀英文期刊也對這一研究領(lǐng)域較為關(guān)注(表3),從期刊發(fā)文量來看,發(fā)文量最多的3個期刊均來自荷蘭,分別是AGRICULTURAL WATER MANAGEMENT (21篇)、AGRICULTURE ECOSYSTEMS & ENVIRONMENT(21篇)和NUTRIENT CYCLING IN AGROECOSYSTEMS(13篇),來自中國的PEDOSPHERE刊發(fā)了10篇相關(guān)文章,排在了并列第四位。歐美國家之所以重視這一領(lǐng)域研究論文的刊發(fā),一方面在于其研究水平較高,另一方面在于發(fā)達國家對于生態(tài)環(huán)境與人類健康關(guān)系的關(guān)注；歐美國家的水體水質(zhì)也曾經(jīng)歷了不斷惡化的階段,但在農(nóng)業(yè)面源污染的概念提出以來,農(nóng)田施氮與人體健康的關(guān)系也備受關(guān)注,并出臺了一系列防控措施,最具代表性的就是歐盟頒布并實施的《硝酸鹽法令》,一些國家對特定農(nóng)田、特定肥料的用量也均有限制施用標準。

該研究領(lǐng)域SCI論文被引頻次排名前五位的文章主要出自美國、新西蘭、中國和以色列(表4),被引頻次最高的文章是波士頓大學的Valiela, I于1997年在《ECOLOGICAL APPLICATIONS》雜志上發(fā)表的論文《Nitrogen loading from coastal watersheds to receiving estuaries: New method and application》,被引用了220次,該文詳述了估算入湖負荷的新方法。新西蘭林肯大學的Di, HJ綜述了各類型土地利用方式的硝態(tài)氮淋失,闡述了合理施氮減少硝態(tài)氮淋失的重要性,其論文被引用了152次。中國農(nóng)業(yè)大學的Ju, XT研究了中國山東地區(qū)作物系統(tǒng)的地下水硝態(tài)氮濃度差異,指出了氮肥過量施用可能是造成地下水硝態(tài)氮濃度超標的重要原因,其論文被引用了124次。中國農(nóng)業(yè)大學Liu, XJ教授詳述了不同施氮條件下,深層(0—300 cm)土壤硝態(tài)

圖1 水體水質(zhì)主題相關(guān)關(guān)系圖Fig.1 Affinity diagram on the topic of water quality

氮的多年分布特征,指出硝態(tài)氮淋失出100 cm是華北小麥-玉米輪作氮損失的主要途徑,其論文被引用了103次。以色列理工學院的Shaviv, A綜述了緩控釋肥的農(nóng)學效應和環(huán)境效應,闡明了控釋肥施用對作物生長的促進作用,也詳述了氮肥施用(其中包括氮淋失)對環(huán)境的不利影響,這有助于我們?nèi)嬲J識肥料效應,其論文被引用了101次。

表3 水體水質(zhì)主題發(fā)文量前10位的期刊

表4 水體水質(zhì)主題排名前5位熱點論文

2 農(nóng)田氮流失量及其影響因素

受地形地貌、土地利用、農(nóng)田管理措施等多種因素的影響,不同地區(qū)研究中的氮素流失量差別很大,同一地區(qū)不同土壤、不同種植模式之間的氮流失量也有較大差異,整體而言,世界各國氮素損失量均表現(xiàn)為隨施氮量增加而顯著增加[23- 24]。中國農(nóng)田施氮量普遍較高,由此造成的氮流失問題也很嚴重(表5),相對中國而言,英、美等發(fā)達國家的關(guān)于農(nóng)田氮素流失的研究中很少關(guān)注高量施氮,這可能是這些國家已經(jīng)采取了一系列措施從源頭上降低了過量施氮,較低的施氮條件下,其氮素流失量也相對較少。

相對較少,可能與這些國家較早地關(guān)注農(nóng)業(yè)面源污染,并積極采取了防控措施有關(guān),尤其是頒布的限制農(nóng)田氮素施用的法令,在減少氮流失方面效果顯著。

氮素在土壤中的殘留和累積是其流失的前提和物質(zhì)基礎(chǔ)；進入農(nóng)田的水分溶解并攜帶土壤氮向不同方向遷移,最終將其帶出農(nóng)田或根區(qū)從而導致氮素流失,因此,凡是能夠改變土壤氮素累積和水分運移的因素都會影響氮素流失的發(fā)生。農(nóng)田氮素流失發(fā)生過程受多種因素影響[41],農(nóng)田固有的特點如地形地貌、土壤質(zhì)地等影響氮素的本底流失,而種植模式、施肥、灌溉等管理措施是導致氮素流失量變化的直接原因,并且各影響因素中,只有部分農(nóng)田管理措施是可以控制調(diào)節(jié)的,固有的氣候氣象和土壤條件等是無法改變的。

降雨是產(chǎn)生氮流失的先決條件,雨強、時長等直接決定了徑流或淋溶量的大小。通常情況下,氮素流失主要發(fā)生在降雨或灌溉集中的季節(jié),強降雨和較為常見的大水漫灌會為氮素遷移提供便利條件,是導致氮流失發(fā)生的重要時期[41- 42]。地形影響地表水熱條件的重新分配,尤其坡度能直接支配地表徑流,是影響土壤氮素流失最重要的地形因素。當坡度較小時,氮主要通過隨水向下滲透而淋失,隨徑流流失量較少；當坡度較大時,表層土壤易遭到侵蝕,氮主要通過隨地表水流動遷移而發(fā)生徑流流失,向下淋失較少。在一定范圍內(nèi),坡度越大,氮素隨水土流失越嚴重[43]。此外,氮素流失量與坡度的關(guān)系還受植被覆蓋、坡長、徑流時間等多種因素影響[44- 45]。

表5 國內(nèi)外典型種植模式農(nóng)田氮流失量差異

土壤質(zhì)地決定了水分的滲透性[46],土壤大孔隙是氮素遷移的主要通道,土壤質(zhì)地越粗、孔隙越多,淋溶損失越嚴重；卵石和砂礫土中氮素的流失量較大,而粘質(zhì)和粉砂質(zhì)土壤反硝化作用較強,加之氮淋溶速度很慢,因而淋失量較小[46]。偶爾在粘質(zhì)土壤中觀測到的較大的硝態(tài)氮淋失可能也是由于大孔隙的存在造成的[46]。不同的土地利用方式導致不同程度的水土和養(yǎng)分流失,相同土壤類型下,糧田的氮負荷要高于果園和林地[46],而種植蔬菜的農(nóng)田養(yǎng)分流失量明顯高于作物輪作種植模式[47]。

施氮量、氮肥種類、施用時期等都與氮流失量密切相關(guān),施氮量與氮流失量之間存在顯著的正相關(guān)關(guān)系[48],對氮素徑流的發(fā)生起主導作用,正常水文年,施氮量的增加會顯著增加徑流水的氮含量[25]。適當?shù)挠袡C肥處理與化肥處理相比可顯著減少氮素流失,緩控釋肥可以控制氮素釋放,提高氮素利用率,進一步降低氮素流失風險[49]。氮素流失多發(fā)生在多雨季節(jié),施肥時間與產(chǎn)流時間越近,徑流硝態(tài)氮濃度越高,因此避開雨期施肥可顯著降低氮素流失量[50],同時水肥耦合也能明顯減少氮素流失[51],優(yōu)化施肥的同時優(yōu)化灌溉更能實現(xiàn)經(jīng)濟和環(huán)境效益雙贏[52]。

3 氮肥施用對氮流失的影響

化肥尤其是氮肥為我國糧食持續(xù)增產(chǎn)做出了重要貢獻(圖2)。我國糧食產(chǎn)量由1979年的3.32億t增加到2012年的5.90億t,單產(chǎn)也從2,237 kg/hm2增加到3,608 kg/hm2。2012年,全球化肥和氮肥消費量分別為1.70億t和1.03億t,單位播種面積用量分別為87.5 kg/hm2和52.9 kg/hm2,而我國化肥和氮肥用量分別為5839萬t、2698萬t,單位播種面積化肥(357.3 kg/hm2)和氮肥(165.1 kg/hm2)施用量均遠高于世界平均用量。全球主要糧食作物(水稻、小麥、玉米)產(chǎn)量25.5億t,其中中國生產(chǎn)了5.41億t[53],我國以世界26%的氮肥消費量生產(chǎn)了世界21%的糧食,較低的氮肥利用率,勢必造成資源的浪費和潛在的環(huán)境風險。

圖2 單位農(nóng)田種植面積肥料用量和糧食產(chǎn)量(1979—2012)(中國統(tǒng)計年鑒,2013)Fig.2 Fertilizer rate and food production of per hectare farmland (1979—2102) (China Statistical Yearbook, 2013)

目前全球氮肥用量相比20世紀增加了100倍,其中60%的氮肥被用于主要糧食作物的生產(chǎn)。據(jù)估計,到2050年世界人口將達到93億,對糧食的需求將增加50%—70%,如果氮肥利用率不能明顯提高,氮肥總投入量需要在目前的基礎(chǔ)上再增加1倍才能滿足人類對糧食的需求[54]。中國糧食作物氮肥利用率遠低于世界平均水平,有研究指出,我國小麥和玉米氮肥利用率分別為28%和26%[55],而世界平均水平為54%和63%[18,54,56]。相應地,我國土壤氮素殘留率較高,但下茬作物對殘留的氮素利用有限[57],大多氮素最終流失出農(nóng)田,進入環(huán)境導致生態(tài)系統(tǒng)功能退化。目前,農(nóng)業(yè)已成為陸地生態(tài)系統(tǒng)中重要的氮素排放源,也是大多數(shù)河流湖泊水體污染主因之一,因此,農(nóng)田氮素流失規(guī)律及防控措施研究應受到重視[4,58]。

4 農(nóng)田氮流失防控措施

大量研究表明農(nóng)田氮素流失是目前諸多水體水質(zhì)惡化的重要原因,源頭控制和氮遷移過程攔截是控制農(nóng)田氮素流失的主要策略。優(yōu)化農(nóng)藝管理措施,在源頭上避免土壤擾動造成的水土遷移,減少不合理灌溉造成的農(nóng)田排水,也可顯著減少氮素流失；在氮素出田至進入水體遷移過程中,利用人工濕地、緩沖帶等工程措施,可以進一步減少進入水體的氮量。研究表明,合理的農(nóng)田管理措施如優(yōu)化施肥和種植結(jié)構(gòu)、保護耕地、節(jié)水灌溉等可減少15%—92%的氮素流失,而緩沖帶、人工濕地、徑流攔截等遷移過程控制措施可減少50%—77%的氮素損失(表6)。對于何種措施防控效果更好,并沒有嚴格的評價標準。氮素遷移過程控制措施大都是工程技術(shù)措施,建設難度較高,而我國農(nóng)田面積大,地形復雜,緩沖帶、人工濕地等氮素遷移過程阻斷工程建設普及難度較大[59]。因地制宜地采取合理的農(nóng)田管理措施,尤其是優(yōu)化施肥、優(yōu)化種植結(jié)構(gòu)、保護性耕作及節(jié)水灌溉[60- 62]等措施更易推廣普及。

表6 氮素流失防控措施

優(yōu)化農(nóng)田施氮量可從源頭上減少氮隨水流失的發(fā)生機率[64,69],是眾多防控措施之中最節(jié)約成本、節(jié)省勞力、最易被農(nóng)民接受的方法[70]我國農(nóng)田面積大而單戶農(nóng)田面積較小,受氣候、地形、種植模式、政策等諸多因素的影響,基于不同目的確定田塊尺度適宜施氮量的方法多種多樣(表7)。以滿足作物生長發(fā)育過程中氮素需求為目的方法,包括利用葉綠素儀監(jiān)測作物長勢、根據(jù)土壤無機氮含量進行氮素實時監(jiān)測調(diào)控、借助長期定位監(jiān)測建立經(jīng)驗施肥模型等,此類方法的特點是可以指導何時施氮,確定經(jīng)驗施氮量,間接緩解了環(huán)境污染風險,但是這一方法是以滿足作物養(yǎng)分需求為主,并不直接考慮環(huán)境效應。以獲得最高產(chǎn)量或經(jīng)濟效益為目的的方法,包括測土配方施肥、建立氮肥-產(chǎn)量或氮肥-經(jīng)濟效益效應曲線等,其特點是可以通過明顯的突變點確定基于作物最高產(chǎn)量或經(jīng)濟效益的施氮量。以氮肥環(huán)境風險估算為目的的方法,包括氮淋失潛力估算、各種氮素損失風險估算模型等,其中采取表觀平衡法核算農(nóng)田氮素投入與支出比例,減少土壤氮素盈余是目前最常用的確定適宜施氮量的方法[23,71],其特點是沒有明顯的突變點,僅局限于風險評估,也就是環(huán)境風險之間的相互比較,不能直接確定造成環(huán)境污染突變的施氮臨界值。以執(zhí)行相關(guān)規(guī)章制度為目的方法,包括執(zhí)行相關(guān)環(huán)境保護規(guī)章、根據(jù)環(huán)保法規(guī)限定施氮量及施氮時期[72- 73]等,但這一方法是建立在對某一環(huán)境指標有明確的限制性規(guī)定基礎(chǔ)上的,如歐盟實施已久的《硝酸鹽法案》中規(guī)定有機肥氮用量不能高于170 kg/hm2,這也是農(nóng)田施氮與環(huán)境目標聯(lián)系最為密切的確定施氮臨界值方法,但這一方法多用于后期監(jiān)測調(diào)整管理措施。以上各種方法均是在田塊尺度確定的適宜施氮量,然而在區(qū)域尺度上仍然缺乏較統(tǒng)一的施氮量,中國科學院南京土壤研究所的朱兆良院士首先提出了區(qū)域平均適宜施氮量的概念[74],并實地驗證了區(qū)域平均適宜施氮量的合理性和可行性[75],這為區(qū)域尺度氮肥管理提供了參考。

表7 農(nóng)田適宜施氮量的確定方法

5 問題與展望

現(xiàn)有研究中所闡述的農(nóng)田氮素流失量多數(shù)為出田氮量,而對于流失出田的氮素如何遷移進入水體以及這一遷移過程中的氮形態(tài)變化特征仍不清楚；此外,淋失出根區(qū)或徑流出農(nóng)田的氮素雖然并未直接進入環(huán)境,但這些氮素難以再被利用而成為嚴重的污染源,因此,今后的研究除了要精確量化農(nóng)田氮素出田外,還要進一步闡明氮素在出田至目標水體這一階段內(nèi)的變化特征,并積極探索消除農(nóng)田氮流失環(huán)境風險的有效措施。

農(nóng)田氮流失的發(fā)生及其流失量受多種因素影響,分析當前氮肥施用現(xiàn)狀,揭示氮素向水體的遷移規(guī)律,有助于制定合理、有效的針對性氮流失防控措施。諸多農(nóng)田管理措施中,施氮量是最直接、最易控制的關(guān)鍵管理措施,在保證一定糧食產(chǎn)量的基礎(chǔ)上,確定基于環(huán)境安全尤其是水質(zhì)保護的合理農(nóng)田施氮量是可行的,也是平衡產(chǎn)量需求與環(huán)境污染之間矛盾的有效措施。然而,當前種植業(yè)面源污染愈加嚴重,為防治由此造成的水體水質(zhì)持續(xù)惡化,迫切需要制定以水質(zhì)保護為目標的農(nóng)田氮肥施用閾值(造成水質(zhì)污染的施氮臨界值),全國各大高校及科研院所也都為實現(xiàn)這一目標不斷努力,但是目前在這一研究方向上并無突破。

目前,確定田塊尺度適宜施氮量的方法較多,但在區(qū)域尺度上,隨著農(nóng)田面源氮污染問題日益突出,控制區(qū)域氮肥用量越來越重要,而要解決這一問題,應當建立區(qū)域乃至全國范圍內(nèi)的農(nóng)業(yè)面源污染監(jiān)測網(wǎng)絡,在長期定位監(jiān)測基礎(chǔ)上,創(chuàng)建合理的面源氮污染核算方法,測算出種植業(yè)源污染物產(chǎn)生與排放系數(shù),并明確農(nóng)田氮肥施用與水體水質(zhì)的關(guān)系,最終確定導致水體污染的氮肥施用臨界值。

致謝：中國科學院文獻情報中心在文獻檢索和數(shù)據(jù)分析過程中給予幫助,彭皓老師幫助寫作，特此致謝。

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ZHANG Yitao1, LIU Hongbin1, WANG Hongyuan1, ZHAI Limei1, LIU Shen1, LEI Qiuliang1, REN Tianzhi2,*

1MinistryofAgricultureKeyLaboratoryofNonpointSourcePollutionControl/InstituteofAgriculturalResourcesandRegionalPlanning,ChineseAcademyofAgriculturalSciences,Beijing100081,China

2Agro-EnvironmentalProtectionInstitute,MinistryofAgriculture,Tianjin300191,China

A bibliometric analysis method was used to identify the developmental trend of international research on nitrogen losses in farmlands as affected by nitrogen applications, based on peer-reviewed studies available in the ISI Web of Science since 1957. We also reviewed control and prevention measures for farmland nitrogen loss, with the same method. The bibliometric analysis showed that the current focus of research regarding nitrogen losses worldwide involved assessing and monitoring of the effects of nitrogen fertilization on water pollution and quality. Keywords included Groundwater, Water quality, Surface water, Nitrate pollution, Eutrophication, Contamination, Nonpoint source pollution, Lysimeter, Runoff, and Subsurface drainage, etc. Research institutions that contributed to a large number of research findings are mainly resided in large agricultural countries, including China, United States, and Canada, and the mainstream journals that included most relevant papers are published in the Netherlands, United States, and China. Literature analysis showed that nitrogen losses are site-specific because it can be influenced by precipitation, topography, soil properties, and forms, rates, timing and placement of fertilizers, as well as other management practices. Nitrogen losses in China (13.7—347 kg/hm2) was significantly higher than that of countries in Europe and North America (4—107 kg/hm2). On average, the fertilizer use is 357.3 kg/ hm2and nitrogen rate is 165.1 kg/hm2in China, which was much higher than the world average application rates (fertilizer rate: 87.5 kg/hm2; nitrogen rate: 52.9 kg/hm2). As a result, nitrogen use efficiency during one crop growth period in China (17%) was significantly lower than the world average (58%). These results indicated that excessive fertilizer application and underutilization of nitrogen were main reasons for nitrogen losses to surface and ground water. By comprehensively analyzing prevention and control measures for farmland nitrogen losses, we found that reducing nitrogen losses from the source was the most effective measure. Optimizing agronomic management practices and intercepting nitrogen migration could reduce nitrogen losses by 15% to 82%. Among a large diversity of studies, the largest group of studies focused on optimizing nitrogen applications. However, faced with resource shortage, water quality deterioration, and food production pressure, future research focus should shift from nitrogen balance to study of the nitrogen cycle in the entire farmland ecosystem. Further, there is an urgent need to determine nitrogen fertilization thresholds (critical nitrogen levels resulting in risk of pollution) for water quality protection. Effective prevention and control measures for nitrogen losses will provide theoretical and technical supports to balance the conflicts among food demand, resource conservation, and environmental protection.

farmland nitrogen; Web of Science; water quality; N loss; prevent and control measures; critical nitrogen rate

公益性行業(yè)(農(nóng)業(yè))科研專項(201303089, 201003014)

2015- 04- 14;

日期:2016- 01- 22

10.5846/stxb201504140764

*通訊作者Corresponding author.E-mail: rentianzhi@caas.cn

張亦濤,劉宏斌,王洪媛,翟麗梅,劉申,雷秋良,任天志.農(nóng)田施氮對水質(zhì)和氮素流失的影響.生態(tài)學報,2016,36(20):6664- 6676.

Zhang Y T, Liu H B, Wang H Y, Zhai L M, Liu S, Lei Q L, Ren T Z.A bibliometric analysis of status and trend of international research on field nitrogen application effects on nitrogen losses and water quality.Acta Ecologica Sinica,2016,36(20):6664- 6676.