氮肥高效施用在低碳農業中的關鍵作用

熊正琴，張曉旭

（南京農業大學資源與環境科學學院，江蘇南京 210095）

氮肥高效施用在低碳農業中的關鍵作用

熊正琴，張曉旭

（南京農業大學資源與環境科學學院，江蘇南京 210095）

低碳農業是我國集約化農業發展的必然趨勢。深入理解氮肥高效施用是實現低碳農業的關鍵，可以更加明確如何集成優化農業管理措施增加產量、減少農田生態系統碳排放、提高土壤固碳效應，綜合實現固碳、減排、增產的低碳農業發展目標。本文概述了低碳農業評價指標的三個階段性研究特點，從田間溫室氣體排放的綜合溫室效應拓展為涵蓋固碳效應的凈溫室效應，再拓展為涵蓋生命周期評價碳排放的綜合凈溫室效應以及兼顧作物產量的溫室氣體強度。提出了如何利用當季作物試驗來估算農田生態系統凈碳收支、結合生命周期評價當季作物綜合凈溫室效應和單位產品溫室氣體強度的方法。按照現階段低碳農業的評價指標，以我國稻–麥輪作生態系統集約化生產的低碳農業模式為案例，解析氮肥施用在低碳農業各組成包括作物產量、固碳效應、CH4和N2O排放、農業措施碳排放中的重要作用，明確氮肥高效施用在農田生態系統綜合凈溫室效應和溫室氣體強度中的關鍵作用，從而實現低碳農業可持續發展。

低碳農業；生態系統凈碳收支；土壤固碳效應；生命周期評價；凈溫室效應

聯合國政府間氣候變化專門委員會 (IPCC) 第五次評估報告指出，大氣二氧化碳 (CO2)、甲烷 (CH4)和氧化亞氮 (N2O) 等溫室氣體濃度增加導致全球氣候變暖已經成為無可爭議的事實[1]。如何緩減氣候變化對人類社會發展的影響受到世界各國政府和人民的高度重視。

1 低碳農業評價指標內涵不斷擴展的三個發展階段

1.1 低碳農業第一階段評價指標通常僅考慮田間溫室氣體直接排放的綜合溫室效應

農田生態系統以光合作用生產農作物為主要目的，是一種特殊的CO2交換系統，通常其凈碳交換可以近似為零，將CH4和N2O兩種溫室氣體的排放作為農田生態系統的綜合溫室效應，是低碳農業研究第一階段的評價指標。綜合溫室效應GWP (global warming potential) 指在特定時間尺度 (通常以100年時間尺度計) 內，單位質量的某一種溫室氣體相對于單位質量CO2的輻射潛力；作為一種相對指標，可以全面評價農田生態系統排放的溫室氣體對全球變暖溫室效應的貢獻[2]。在100年時間尺度上，單位質量的CH4和N2O的全球增溫潛勢分別為單位質量CO2的28倍和265倍[1]。分別表示作物生長周期內計算的季節累積排放量，根據單位質量增溫潛勢換算為CO2當量排放量。

1.2 低碳農業第二階段評價指標拓展為涵蓋固碳效應的凈溫室效應

農田生態系統固碳是當前國際社會公認的減緩大氣CO2濃度升高的重要途徑之一，如何提高農田生態系統碳儲量和固碳速率，是當前國際社會廣泛關注的焦點[3]。因此，考慮農田生態系統固碳效應，低碳農業研究第二階段的評價指標拓展為農田生態系統凈溫室效應其中，以多種不同途徑估算農田固碳效應，均存在較大的系統誤差。目前，很多研究以測定土壤呼吸的不同比例直接表征農田生態系統CO2凈交換[4]，不僅誤差較大，也沒有考慮作物系統生產力對CO2的固定，不能真實表征土壤的固碳效應。利用氣象資料、土壤基本理化性質、農業管理措施等作為基本參數的模型預測也是目前區域固碳效應的研究方法，應用比較廣泛的有DNDC[5]、DAYCENT[6]以及CENTURY[7]。估算農田固碳效應的主要方法是測量土壤有機碳 (SOC) 的變化[8]。目前比較普遍的方法是基于長期定位試驗，測定SOC含量的年際變化，再外推演繹估算土壤固碳效應[9]。通過土壤調查基礎數據庫[10]或前人研究匯總估算大尺度范圍農田有機碳變化[11]。

1.3 土壤固碳效應由依賴長期試驗尺度發展到當季作物尺度

對于非長期定位試驗，很難檢測農田有機碳的變化[4]，尤其是當時間尺度縮短為一年或當季作物時，估算土壤有機碳變化的方法較少[12]。為了及時評價新研發的農田管理措施或種植技術等對農田生態系統固碳效應的潛力，本文作者提出了當季作物時間尺度上估算農田生態系統凈碳收支 (NECB) 的方法，且得到了長期定位測量土壤有機碳變化方法的有效驗證，為基于作物生長季節時間尺度的短期試驗提供了土壤固碳效應的研究方法。該方法通過測定農田異養呼吸 (Rh) 和作物生態系統凈初級生產力(NPP) 或生態系統呼吸 (Re) 和總初級生產力 (GPP)兩種途徑來計算生態系統凈生產力 (NEP)，即NEP =NPP – Rh = GPP – Re；然后，根據 NECB = NEP –H – CH4+ M計算農田生態系統凈碳收支；再根據生態系統凈碳收支與土壤有機碳之間的內在關系估算土壤有機碳 (SOC) 的變化速率[13]。上述公式中Rh與Re為靜態暗箱法測得的CO2累積排放碳量；H(Harvest) 表示因農田收獲物移出農田生態系統的總碳量，包括秸稈和籽粒碳量；CH4代表作物全生長周期內CH4累積排放碳量；M (Manure) 表示農田施入外源有機肥碳量；NPP代表作物全生長周期內作物地上、地下部分增加的總碳量[13]。

1.4 低碳農業第三階段即現階段的評價指標拓展為涵蓋生命周期評價碳排放的綜合凈溫室效應以及兼顧作物產量的溫室氣體強度

除了農田生態系統直接排放的溫室氣體CH4和N2O引發溫室效應外，在農業生產過程中化學品投入 (Ei) 和農事操作 (Eo) 也會直接或間接引起CO2排放，從而增加農田生態系統的溫室效應[14]。因此，應用生命周期評價法LCA (life cycle assessment) 評估綜合凈溫室效應時，除了前述農田生態系統CH4和N2O排放以及農田固碳效應外，還應當考慮農業措施導致的碳排放[15–16]，成為低碳農業研究第三階段的評價指標。綜合凈溫室效應計算公式為：net GWP =CH4× 28 + N2O × 265 + Eo + Ei – δSOC × 44/12 (kg CO2eq./hm2)。農業措施碳排放一方面來自化學品投入 (Ei) 如肥料、農藥等的生產、儲存、運輸、施用等過程；另一方面則來自農事操作 (Eo) 如灌溉、翻耕和收獲等消耗燃料或其他形式能源的過程。沿用國際標準化組織ISO (international organization for standardization) 對產品碳足跡的定義，低碳農業則是基于生命周期評價方法，計算農產品生產系統內各種溫室氣體排放與消納之和，并以CO2當量形式表示，評價對氣候變化的單一影響[17]。單位產品的綜合凈溫室效應即為溫室氣體強度GHGI [greenhouse gas intensity (CO2eq. kg/kg, yield)]，其計算公式為：GHGI =net GWP/作物產量 。由于溫室氣體強度兼顧作物產量和綜合凈溫室效應，是現階段低碳農業的評價指標。

2 高效施用氮肥是實現低碳農業的關鍵

糧食安全是目前世界各國面臨的重大挑戰之一[18]。據FAO預測，到2030年我國糧食總產必須在現有基礎上提高40%以上、單產增加45%以上，以保障我國糧食安全[19]。目前化學氮肥利用率大多低于30%，我國氮肥用量在持續快速增長的同時，糧食產量增加緩慢[20]。如何同步提高作物產量與氮肥利用率是當前國際社會農業可持續發展的研究熱點。Tilman[21]指出必須更有效地利用農田養分，以降低農業對環境的負效應；Swaminathan[22]提出“Evergreen Revolution”，適度增加外部投入，改善農田生產效率，增強農業可持續性，降低環境成本；Matson等[23]提出“集約化可持續農業”。本文設定的集約化栽培模式依托于稻–麥輪作體系土壤–作物綜合管理系統ISSM (integrated soil-crop system management)[24]，根據專家推薦結合當地實際情況進行氮肥水平、施用比例、種類、有機肥配施、種植密度以及土壤水分管理等措施的不同整合，旨在實現水稻高產、氮肥高效利用、同時降低環境影響的可持續農業發展模式，已成功運行[16,25–26]。因此本文解析上述集約化栽培模式中氮肥施用對發展低碳農業溫室氣體強度各組成要素的重要貢獻。

2.1 氮肥施用直接決定作物產量和土壤固碳效應的增加

由表1可見，氮肥施用對作物產量和生態系統凈碳收支及固碳效應具有明顯影響。因此，氮肥施用直接決定著低碳農業中單位農產品的綜合凈溫室效應即溫室氣體強度。

2.2 氮肥施用影響稻田生態系統溫室氣體CH4和N2O的田間直接排放

氮肥對稻田生態系統CH4排放量的影響極其復雜，可能增加，可能減少，也可能沒有影響，具體情況與土壤性質、水稻品種、肥料種類、施用時間、施用方式以及施用量有關[27]。施用氮肥促進植株生長，提高植株CH4傳輸速率，同時抑制土壤CH4氧化[28]，從而促進CH4排放。然而，銨態氮和CH4的共同存在也可能促進甲烷氧化菌的生長、促進CH4氧化，從而降低CH4排放量。施用有機肥則是促進稻田生態系統CH4排放的重要因素[26,29–30]，其促進程度取決于有機肥的成分、性質以及施用方法。有機肥和化肥結合施用或者避免在淹水條件下直接施用有機肥，均可有效減少稻田CH4排放。

表 1 2011～2014年稻–麥輪作周期中氮肥用量、作物產量、生態系統凈碳收支、固碳效應及溫室氣體強度Table 1 Mean nitrogen fertilizer application rate, grain yield, net ecosystem carbon budget, SOC sequestration rate and greenhouse gas intensity over rice-wheat annual cycles from 2011 to 2014

氮肥施用也直接影響稻田生態系統N2O的排放量。施用化學氮肥能夠顯著增加土壤中NH4+-N與NO3–-N的含量，繼而增強硝化作用和反硝化作用的強度，從而促進土壤N2O的產生與排放。通常認為，隨著化學氮肥用量增加，土壤N2O排放量呈線性增加[1]。減少氮肥施用量或應用硝化抑制劑均可減少土壤硝化和反硝化過程產生的N2O[31]。當作物地上部氮盈余量等于或小于作物最佳需氮量時，土壤N2O排放變化較??；當施氮量超出作物地上部最大需求量時，N2O排放量急劇增加。因此，越來越多的研究表明，N2O排放與施氮量之間呈指數關系[32–33]。依據作物需肥特征優化施肥時間與方式，調整氮、磷、鉀施用比例，選用長效緩釋氮肥[34]，提高氮肥利用率，可有效減少N2O排放[35]。有機肥施用對土壤N2O是正效應還是負效應影響比較復雜[36]，主要取決于不同種類有機肥的C/N[37]及施用方法。

2.3 氮肥施用對于農業措施引起的碳排放貢獻突出

本文系統邊界為水稻和小麥田間生產階段，從播種到作物收獲生命周期全過程，包括農用化學品投入 (Ei) 和農事操作 (Eo) 等農業措施引起的碳排放；各不同栽培模式化學品投入與農事操作通過生命周期評價方法估算結果見表2[16]。農用化學品投入(Ei) 包括水稻和小麥種植過程中施用的肥料包括氮肥、磷肥和鉀肥和農藥 (除草劑、殺蟲劑與殺菌劑)在生產、儲存和運輸過程中產生的碳排放；農事操作 (Eo) 主要包括灌溉、翻耕與收獲等過程中農業機械消耗燃料或其他形式能源所引起的碳排放。在稻-麥輪作生態系統中，來自化學品投入 (Ei) 引起的GWP變化范圍為CO2eq.734～4362 kg/hm2；來自農事操作 (Eo) 引起的GWP變化范圍為CO2eq.1296～1708 kg/hm2。

各施氮模式中氮肥施用對Ei的貢獻率高達66%～75%，是Ei中最主要的碳排放來源 (圖1)。一方面是因為氮肥本身在生產和運輸過程中需要消耗大量的化石燃料，導致氮肥施用引起的碳排放高；另一方面，集約化農業生產中糧食產量的增加主要依靠氮肥的投入。氮肥不僅是Ei的主要組成部分，也是農業措施碳排放Ei+Eo的主要組成部分[16]。如圖1所示，與CH4與N2O排放引起的溫室效應相比，農業措施引起的碳排放對溫室效應的貢獻不容忽視。農業措施碳排放在CH4、N2O排放與農業措施碳排放引起的總溫室效應中占25%～38%。各模式中Ei引起的溫室效應占總溫室效應的10%～25%；Eo則為6%～24%[38]。申建波等[39]研究發現，常規管理措施中農業管理的潛在溫室效應占總溫室效應的29%。梁龍等[40]研究表明，河北平原推薦管理措施中農業管理的潛在溫室效應在總溫室效應中的比重為31%。因此，合理施用氮肥，提高氮肥利用率，不僅可以降低氮肥施用后流失到環境中造成的環境污染，減緩稻田生態系統CH4和N2O的直接排放，還可以降低因氮肥施用造成的間接碳排放，同時增加作物產量和土壤固碳效應，降低單位產品的綜合凈溫室效應，實現集約化生產下的低碳農業目標。

3 低碳農業研究展望

3.1 低碳農業旨在實現集約化農業生產方式下的低碳目標

隨著農業現代化與集約化的進展，碳耗總量增加是必然的。農業提倡“低碳”不等于減少碳耗總量的所謂“低碳農業”，而是要努力追求以較低的單位產品耗碳率換取較高的固碳率。為此，需要集成優化農業管理措施、提高氮肥利用率，兼顧實現固碳、減排、增產的低碳農業發展目標，提高單位產品的碳效率、促進農業可持續發展[41]。雖然作物生產與溫室效應之間存在復雜的交互作用和區域特征[42–43]，但隨著國內外對資源利用效率和環境保護意識的逐漸增強和管理水平的逐步提高，我國農業集約化生產中單位產品的溫室效應體現出逐漸減緩的趨勢[44–45]。還應該在追求實現單位產品低碳農業的同時，獲取更高的單位產品經濟效益[44,46–47]。綜合考慮低碳農業發展的評價指標和驅動因素，增強科普宣傳，影響政府決策，已成為國內外低碳農業的研究趨勢[46,48–49]。

3.2 氮肥高效施用是實現低碳農業的關鍵

reshing(kWh/hm2)0.94 horu 5 5.1粒79775為sp脫6.3、1413 Th 17 131727363937474773數utandfarms,n ho eat 07放5、系ual rice-wh .0 0.15、0排estimated carboal h)forthreshing獲harvest iesel,kg/hm2)C emission from farm practice (Eo) (CO2 eq.kg/hm2)fertilizer, p碳的en th g/kW 11 Crop 11收11111111373737373737 0.2、收Eoreferto theagrochemical inp n,and en nn 和nitrog practiceintensity翻獲tsin thea或planting liedptio(D播st[C eq.kg/(cm·hm2)] as referred to L, as種vent)別C cost (C eq.k m2·a)]2為erapp consum移Crop(E ivalen Farm 22222232323 1.3、耕232323分) p栽數g/kg reandco 6 C 0.0725 g/(h 系i and xideequ 度 放O2eq.k 強 排eq. kg/(cm·hm2)]；as5.1 g and作碳ufactu操的Lal[14]。E an n w lantin io 事耕3737373737377 127 127 127 127 127 12劑5.16[C考atio p p[C 農 翻Plough iesel,kg/hm2)菌為參殺數數.9 C cost (C eq.k放(D 和系系排放s to carbon d劑放放and 3 t)forcro Ei) 碳2013 8080 Irrigation 55555555 1514排1514 0 1041 0 1041 0 1041 0 1041蟲排排.1、的施.3, 5殺碳碳hina’sfertilizer m g/even o、(cm)2012 656565 80 tion 65 80 1514 1514劑溉措n coefficientforirrig(E施ntribu 溉草灌業075,6灌75 20117550505050 1419 1419 946123 946123 946123 946123除、 各措co h)。5,0.0 C cost (C eq.k m chemical input(Ei) (CO2 eq.kg/hm2)碳理投er ation and 式ent 餅dataspecific to C肥模.2栽Treatm 12管34123-N 4菜籽.2和培-N -N -N -N -N -N -N 、, 0.3hecarbon emissio入NN FP SM IS SM IS SM IS SM IS NN FP SM IS SM IS SM IS SM IS 肥0.1 (C eq.kg/kg)；農e collected ere 1 e ly.T品op鉀和、25(C eq.kg/kW, 0.1劑icid ha rvesting, 3 e業farm 學s from 2011 to 2014肥0.07 04).W化tion 菌44.4 Fung 6 4.49 4.45359595979129磷殺、取0.07肥數為ing and農nd al(20系uta rota e Insecticid 氮系數統劑855568。放系放排放fertilizer, respectiv態蟲18202020274133殺3737375076排碳排Zn生碳，碳作m2)作后的操據粒輪Siand 2事數脫iesel)fortillage, rak 2麥e草22246除Herb ely,as referred to L 4646–icalinputlevel(kg/h 46 icalinput 劑icid 2 4646農的t)；hecarbon emission coefficients w稻和費年入消ns. T 2014餅manure Chem g/even racticesforchemical inp erapp肥投和50品產50 e,respectivlied) p入00000000學生2 2011～t p 籽22226262化集icid ng g/kg st(C eq.kg/kg, d en 投 菜品Farm em Ch 的收3.2 (C eq.k中，fu agem 學4 C化期藝為肥Zn 平0水鋅0001515000066周工數st(C eq.kco表an 入 作產系holerice-wheat rotatio co ere 0.9 l m 肥Si 投000000005858輪生放ra 硅225 225 C emission fro麥的排稻肥碳0.1 C gricultu 000000放555588個鋅的鉀肥K 303030303645排碳161616161924整和種uring thew 7 and.0肥P 00 0 18 18062222285硅或nsd erbicide, insecticideand Table 2 A 磷18182125131313131518Eo 指肥播i 和根移據栽ere 0肥N 000200)[14].氮483643486002288 1716 2059 2288 2860：E)；anure,h n emission coefficients w式ent ote）, diesel)；f carbon emissio al(2004栽Treatm 模12341234m,farm m hecarbo toL培-N -N -N -N -N -N -N -N （N. kg/kg NN 注FP SM IS SM IS SM IS SM IS NN FP SM IS SM IS SM IS SM IS 3.9 (C eq.kg/kg和(C eq operation o tassiu po em(2 issions coefficients w 004). T referred

我國農田總施氮量世界第一，氮肥生產工藝比較落后，為了降低氮肥生產過程引起的間接碳排放，升級改造我國氮肥生產工藝勢在必行[50]。據IPCC統計，農業措施所引起的CO2排放占全球CO2總排放量的20%[1]。隨著農業現代化過程中化肥、農藥的投入以及大型機械的運用，農業措施引起的碳排放對生態系統凈溫室效應的貢獻將會越來越大。近來不同研究者針對我國主要農作物[42,45]、蔬菜[51–52]種植等開展的碳足跡研究，都體現出氮肥施用這一單因子在農業碳排放中的重要地位；即使不考慮田間N2O排放的溫室效應，僅肥料施用占農業碳排放的比例就高達48%[53]。氮肥高效施用直接決定著作物產量、生態系統凈碳收支、土壤固碳效應以及CH4和N2O排放，是農業措施碳排放的首要貢獻者，也是實現集約化生產方式下低碳農業的關鍵驅動因子。

圖1 不同集約化栽培模式下稻麥輪作生態系統CH4、N2O與農業措施碳排放 (Ei 和Eo) 溫室效應百分比Fig. 1 Contribution percentages of CH4, N2O emissions, Ei and Eo from farm management among different intensively managed cultivation patterns of rice-wheat annual rotations

3.3 低碳農業需要從根本上注重提高農田土壤固碳效應、提高土壤生產力

近些年來，越來越多的研究開始考慮農田固碳效應[4,54]，許多研究表明稻田具有很強的固碳效應[55–56]。土壤有機碳含量的高低是農作物高產穩產的基礎。國內外研究一致表明，農業管理措施如施肥、種植制度、灌溉、耕作等直接影響土壤有機碳的變化[57]；肥料、氮沉降和氣候變化等也間接影響土壤有機碳庫的變化[54,58]。據報道，近20年來我國大陸53%～59%的農田SOC含量呈增長趨勢，30%～31%下降，4%～6%基本持平[56]。我國大陸農田表土有機碳貯量總體增加311.3～401.4 Tg，這主要歸因于秸稈還田、有機肥施用和化肥投入的增加，合理的養分配比以及少 (免) 耕技術的推廣。通過合理有效的農田管理措施，例如有機肥與化肥的合理配施[57]，可以調節農田土壤由碳源轉變為碳匯，增強土壤固碳效應，同時提高土壤生產力[59]。然而，土壤固碳效應與氮肥施用之間的關系還存在很大的不確定性，需要針對特定的生態系統和生態環境開展長期研究[60]。生物質炭與氮肥的配合應用[61–62]則是提高農田土壤固碳效應、實現低碳農業的新趨勢。

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Key role of efficient nitrogen application in low carbon agriculture

XIONG Zheng-qin, ZHANG Xiao-xu
( College of Resources of Environmental Sciences, Nanjing Agricultural University, Nanjing 210095, China )

Low carbon agriculture is an inevitable trend for sustainable intensive agriculture in China. Efficient nitrogen fertilization is the key driving factor for achieving low carbon agriculture, understanding that will help the integration and optimization of agricultural management measures, achieving the goals of soil carbon sequestration, greenhouse gas mitigations and yield improvement, and thus to sustain intensive low carbon agriculture. Low carbon agriculture has experienced three development stages from the points of connotation and research methods. The initial stage was developed from total global warming potentials of greenhouse gas emissions from croplands, then the concept was changed to net global warming potentials covering greenhouse gas emissions and soil carbon sequestration, now is focused on the net total global warming potentials with additional carbon emissions derived from field management and chemical inputs and then to yield scaled greenhouse gas intensity associated with life cycle assessment. Moreover, net ecosystem carbon budget and soil carbon sequestration were developed from conventional long term field experiment to the current crop seasonal scale short term field experiment. Based on the crop seasonal scale soil carbon sequestration and life cycle assessment, the net total global warming potential and yield-scaled greenhouse gas intensity were fully developed as well. As a case study of life cycle assessment and net ecosystem carbon budget, we analyzed the contributions of nitrogen fertilization to grain yield, soil carbon sequestration, methane (CH4) and nitrous oxide (N2O) emissions and agricultural managements associated carbon emissions under intensive rice-wheat annual rotation system with different scenarios, and thus highlighted the key driving role of efficient nitrogen fertilization in sustainably achieving low carbon agriculture in terms of net total global warming potential and yield scaled greenhouse gas intensity.

low carbon agriculture; net ecosystem carbon budget; soil carbon sequestration;life cycle assessment; net global warming potential

2017–07–24 接受日期：2017–10–20

公益性行業（農業）科研專項（201503106）；國家自然科學基金項目（41471192）資助。

熊正琴（1973—），女，重慶涪陵人，博士，教授，主要從事碳氮循環與生態環境研究。E-mail：zqxiong@njau.edu.cn