氮肥高效施用在低碳農(nóng)業(yè)中的關(guān)鍵作用

熊正琴，張曉旭

（南京農(nóng)業(yè)大學(xué)資源與環(huán)境科學(xué)學(xué)院，江蘇南京 210095）

氮肥高效施用在低碳農(nóng)業(yè)中的關(guān)鍵作用

熊正琴，張曉旭

（南京農(nóng)業(yè)大學(xué)資源與環(huán)境科學(xué)學(xué)院，江蘇南京 210095）

低碳農(nóng)業(yè)是我國集約化農(nóng)業(yè)發(fā)展的必然趨勢。深入理解氮肥高效施用是實(shí)現(xiàn)低碳農(nóng)業(yè)的關(guān)鍵，可以更加明確如何集成優(yōu)化農(nóng)業(yè)管理措施增加產(chǎn)量、減少農(nóng)田生態(tài)系統(tǒng)碳排放、提高土壤固碳效應(yīng)，綜合實(shí)現(xiàn)固碳、減排、增產(chǎn)的低碳農(nóng)業(yè)發(fā)展目標(biāo)。本文概述了低碳農(nóng)業(yè)評價(jià)指標(biāo)的三個(gè)階段性研究特點(diǎn)，從田間溫室氣體排放的綜合溫室效應(yīng)拓展為涵蓋固碳效應(yīng)的凈溫室效應(yīng)，再拓展為涵蓋生命周期評價(jià)碳排放的綜合凈溫室效應(yīng)以及兼顧作物產(chǎn)量的溫室氣體強(qiáng)度。提出了如何利用當(dāng)季作物試驗(yàn)來估算農(nóng)田生態(tài)系統(tǒng)凈碳收支、結(jié)合生命周期評價(jià)當(dāng)季作物綜合凈溫室效應(yīng)和單位產(chǎn)品溫室氣體強(qiáng)度的方法。按照現(xiàn)階段低碳農(nóng)業(yè)的評價(jià)指標(biāo)，以我國稻–麥輪作生態(tài)系統(tǒng)集約化生產(chǎn)的低碳農(nóng)業(yè)模式為案例，解析氮肥施用在低碳農(nóng)業(yè)各組成包括作物產(chǎn)量、固碳效應(yīng)、CH4和N2O排放、農(nóng)業(yè)措施碳排放中的重要作用，明確氮肥高效施用在農(nóng)田生態(tài)系統(tǒng)綜合凈溫室效應(yīng)和溫室氣體強(qiáng)度中的關(guān)鍵作用，從而實(shí)現(xiàn)低碳農(nóng)業(yè)可持續(xù)發(fā)展。

低碳農(nóng)業(yè)；生態(tài)系統(tǒng)凈碳收支；土壤固碳效應(yīng)；生命周期評價(jià)；凈溫室效應(yīng)

聯(lián)合國政府間氣候變化專門委員會 (IPCC) 第五次評估報(bào)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(dǎo)致全球氣候變暖已經(jīng)成為無可爭議的事實(shí)[1]。如何緩減氣候變化對人類社會發(fā)展的影響受到世界各國政府和人民的高度重視。

1 低碳農(nóng)業(yè)評價(jià)指標(biāo)內(nèi)涵不斷擴(kuò)展的三個(gè)發(fā)展階段

1.1 低碳農(nóng)業(yè)第一階段評價(jià)指標(biāo)通常僅考慮田間溫室氣體直接排放的綜合溫室效應(yīng)

農(nóng)田生態(tài)系統(tǒng)以光合作用生產(chǎn)農(nóng)作物為主要目的，是一種特殊的CO2交換系統(tǒng)，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(nóng)田生態(tài)系統(tǒng)的綜合溫室效應(yīng)，是低碳農(nóng)業(yè)研究第一階段的評價(jià)指標(biāo)。綜合溫室效應(yīng)GWP (global warming potential) 指在特定時(shí)間尺度 (通常以100年時(shí)間尺度計(jì)) 內(nèi)，單位質(zhì)量的某一種溫室氣體相對于單位質(zhì)量CO2的輻射潛力；作為一種相對指標(biāo)，可以全面評價(jià)農(nóng)田生態(tài)系統(tǒng)排放的溫室氣體對全球變暖溫室效應(yīng)的貢獻(xiàn)[2]。在100年時(shí)間尺度上，單位質(zhì)量的CH4和N2O的全球增溫潛勢分別為單位質(zhì)量CO2的28倍和265倍[1]。分別表示作物生長周期內(nèi)計(jì)算的季節(jié)累積排放量，根據(jù)單位質(zhì)量增溫潛勢換算為CO2當(dāng)量排放量。

1.2 低碳農(nóng)業(yè)第二階段評價(jià)指標(biāo)拓展為涵蓋固碳效應(yīng)的凈溫室效應(yīng)

農(nóng)田生態(tài)系統(tǒng)固碳是當(dāng)前國際社會公認(rèn)的減緩大氣CO2濃度升高的重要途徑之一，如何提高農(nóng)田生態(tài)系統(tǒng)碳儲量和固碳速率，是當(dāng)前國際社會廣泛關(guān)注的焦點(diǎn)[3]。因此，考慮農(nóng)田生態(tài)系統(tǒng)固碳效應(yīng)，低碳農(nóng)業(yè)研究第二階段的評價(jià)指標(biāo)拓展為農(nóng)田生態(tài)系統(tǒng)凈溫室效應(yīng)其中，以多種不同途徑估算農(nóng)田固碳效應(yīng)，均存在較大的系統(tǒng)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(nóng)田生態(tài)系統(tǒng)CO2凈交換[4]，不僅誤差較大，也沒有考慮作物系統(tǒng)生產(chǎn)力對CO2的固定，不能真實(shí)表征土壤的固碳效應(yīng)。利用氣象資料、土壤基本理化性質(zhì)、農(nóng)業(yè)管理措施等作為基本參數(shù)的模型預(yù)測也是目前區(qū)域固碳效應(yīng)的研究方法，應(yīng)用比較廣泛的有DNDC[5]、DAYCENT[6]以及CENTURY[7]。估算農(nóng)田固碳效應(yīng)的主要方法是測量土壤有機(jī)碳 (SOC) 的變化[8]。目前比較普遍的方法是基于長期定位試驗(yàn)，測定SOC含量的年際變化，再外推演繹估算土壤固碳效應(yīng)[9]。通過土壤調(diào)查基礎(chǔ)數(shù)據(jù)庫[10]或前人研究匯總估算大尺度范圍農(nóng)田有機(jī)碳變化[11]。

1.3 土壤固碳效應(yīng)由依賴長期試驗(yàn)尺度發(fā)展到當(dāng)季作物尺度

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

1.4 低碳農(nóng)業(yè)第三階段即現(xiàn)階段的評價(jià)指標(biāo)拓展為涵蓋生命周期評價(jià)碳排放的綜合凈溫室效應(yīng)以及兼顧作物產(chǎn)量的溫室氣體強(qiáng)度

除了農(nóng)田生態(tài)系統(tǒng)直接排放的溫室氣體CH4和N2O引發(fā)溫室效應(yīng)外，在農(nóng)業(yè)生產(chǎn)過程中化學(xué)品投入 (Ei) 和農(nóng)事操作 (Eo) 也會直接或間接引起CO2排放，從而增加農(nóng)田生態(tài)系統(tǒng)的溫室效應(yīng)[14]。因此，應(yīng)用生命周期評價(jià)法LCA (life cycle assessment) 評估綜合凈溫室效應(yīng)時(shí)，除了前述農(nóng)田生態(tài)系統(tǒng)CH4和N2O排放以及農(nóng)田固碳效應(yīng)外，還應(yīng)當(dāng)考慮農(nóng)業(yè)措施導(dǎo)致的碳排放[15–16]，成為低碳農(nóng)業(yè)研究第三階段的評價(jià)指標(biāo)。綜合凈溫室效應(yīng)計(jì)算公式為：net GWP =CH4× 28 + N2O × 265 + Eo + Ei – δSOC × 44/12 (kg CO2eq./hm2)。農(nóng)業(yè)措施碳排放一方面來自化學(xué)品投入 (Ei) 如肥料、農(nóng)藥等的生產(chǎn)、儲存、運(yùn)輸、施用等過程；另一方面則來自農(nóng)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(biāo)準(zhǔn)化組織ISO (international organization for standardization) 對產(chǎn)品碳足跡的定義，低碳農(nóng)業(yè)則是基于生命周期評價(jià)方法，計(jì)算農(nóng)產(chǎn)品生產(chǎn)系統(tǒng)內(nèi)各種溫室氣體排放與消納之和，并以CO2當(dāng)量形式表示，評價(jià)對氣候變化的單一影響[17]。單位產(chǎn)品的綜合凈溫室效應(yīng)即為溫室氣體強(qiáng)度GHGI [greenhouse gas intensity (CO2eq. kg/kg, yield)]，其計(jì)算公式為：GHGI =net GWP/作物產(chǎn)量 。由于溫室氣體強(qiáng)度兼顧作物產(chǎn)量和綜合凈溫室效應(yīng)，是現(xiàn)階段低碳農(nóng)業(yè)的評價(jià)指標(biāo)。

2 高效施用氮肥是實(shí)現(xiàn)低碳農(nóng)業(yè)的關(guān)鍵

糧食安全是目前世界各國面臨的重大挑戰(zhàn)之一[18]。據(jù)FAO預(yù)測，到2030年我國糧食總產(chǎn)必須在現(xiàn)有基礎(chǔ)上提高40%以上、單產(chǎn)增加45%以上，以保障我國糧食安全[19]。目前化學(xué)氮肥利用率大多低于30%，我國氮肥用量在持續(xù)快速增長的同時(shí)，糧食產(chǎn)量增加緩慢[20]。如何同步提高作物產(chǎn)量與氮肥利用率是當(dāng)前國際社會農(nóng)業(yè)可持續(xù)發(fā)展的研究熱點(diǎn)。Tilman[21]指出必須更有效地利用農(nóng)田養(yǎng)分，以降低農(nóng)業(yè)對環(huán)境的負(fù)效應(yīng)；Swaminathan[22]提出“Evergreen Revolution”，適度增加外部投入，改善農(nóng)田生產(chǎn)效率，增強(qiáng)農(nóng)業(yè)可持續(xù)性，降低環(huán)境成本；Matson等[23]提出“集約化可持續(xù)農(nóng)業(yè)”。本文設(shè)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(tǒng)ISSM (integrated soil-crop system management)[24]，根據(jù)專家推薦結(jié)合當(dāng)?shù)貙?shí)際情況進(jìn)行氮肥水平、施用比例、種類、有機(jī)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(shí)現(xiàn)水稻高產(chǎn)、氮肥高效利用、同時(shí)降低環(huán)境影響的可持續(xù)農(nóng)業(yè)發(fā)展模式，已成功運(yùn)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(fā)展低碳農(nóng)業(yè)溫室氣體強(qiáng)度各組成要素的重要貢獻(xiàn)。

2.1 氮肥施用直接決定作物產(chǎn)量和土壤固碳效應(yīng)的增加

由表1可見，氮肥施用對作物產(chǎn)量和生態(tài)系統(tǒng)凈碳收支及固碳效應(yīng)具有明顯影響。因此，氮肥施用直接決定著低碳農(nóng)業(yè)中單位農(nóng)產(chǎn)品的綜合凈溫室效應(yīng)即溫室氣體強(qiáng)度。

2.2 氮肥施用影響稻田生態(tài)系統(tǒng)溫室氣體CH4和N2O的田間直接排放

氮肥對稻田生態(tài)系統(tǒng)CH4排放量的影響極其復(fù)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(zhì)、水稻品種、肥料種類、施用時(shí)間、施用方式以及施用量有關(guān)[27]。施用氮肥促進(jìn)植株生長，提高植株CH4傳輸速率，同時(shí)抑制土壤CH4氧化[28]，從而促進(jìn)CH4排放。然而，銨態(tài)氮和CH4的共同存在也可能促進(jìn)甲烷氧化菌的生長、促進(jìn)CH4氧化，從而降低CH4排放量。施用有機(jī)肥則是促進(jìn)稻田生態(tài)系統(tǒng)CH4排放的重要因素[26,29–30]，其促進(jìn)程度取決于有機(jī)肥的成分、性質(zhì)以及施用方法。有機(jī)肥和化肥結(jié)合施用或者避免在淹水條件下直接施用有機(jī)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(chǎn)量、生態(tài)系統(tǒng)凈碳收支、固碳效應(yīng)及溫室氣體強(qiáng)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(tài)系統(tǒng)N2O的排放量。施用化學(xué)氮肥能夠顯著增加土壤中NH4+-N與NO3–-N的含量，繼而增強(qiáng)硝化作用和反硝化作用的強(qiáng)度，從而促進(jìn)土壤N2O的產(chǎn)生與排放。通常認(rèn)為，隨著化學(xué)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(yīng)用硝化抑制劑均可減少土壤硝化和反硝化過程產(chǎn)生的N2O[31]。當(dāng)作物地上部氮盈余量等于或小于作物最佳需氮量時(shí)，土壤N2O排放變化較??；當(dāng)施氮量超出作物地上部最大需求量時(shí)，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(shù)關(guān)系[32–33]。依據(jù)作物需肥特征優(yōu)化施肥時(shí)間與方式，調(diào)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(jī)肥施用對土壤N2O是正效應(yīng)還是負(fù)效應(yīng)影響比較復(fù)雜[36]，主要取決于不同種類有機(jī)肥的C/N[37]及施用方法。

2.3 氮肥施用對于農(nóng)業(yè)措施引起的碳排放貢獻(xiàn)突出

本文系統(tǒng)邊界為水稻和小麥田間生產(chǎn)階段，從播種到作物收獲生命周期全過程，包括農(nóng)用化學(xué)品投入 (Ei) 和農(nóng)事操作 (Eo) 等農(nóng)業(yè)措施引起的碳排放；各不同栽培模式化學(xué)品投入與農(nóng)事操作通過生命周期評價(jià)方法估算結(jié)果見表2[16]。農(nóng)用化學(xué)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(nóng)藥 (除草劑、殺蟲劑與殺菌劑)在生產(chǎn)、儲存和運(yùn)輸過程中產(chǎn)生的碳排放；農(nóng)事操作 (Eo) 主要包括灌溉、翻耕與收獲等過程中農(nóng)業(yè)機(jī)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(tài)系統(tǒng)中，來自化學(xué)品投入 (Ei) 引起的GWP變化范圍為CO2eq.734～4362 kg/hm2；來自農(nóng)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(xiàn)率高達(dá)66%～75%，是Ei中最主要的碳排放來源 (圖1)。一方面是因?yàn)榈时旧碓谏a(chǎn)和運(yùn)輸過程中需要消耗大量的化石燃料，導(dǎo)致氮肥施用引起的碳排放高；另一方面，集約化農(nóng)業(yè)生產(chǎn)中糧食產(chǎn)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(nóng)業(yè)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(yīng)相比，農(nóng)業(yè)措施引起的碳排放對溫室效應(yīng)的貢獻(xiàn)不容忽視。農(nóng)業(yè)措施碳排放在CH4、N2O排放與農(nóng)業(yè)措施碳排放引起的總溫室效應(yīng)中占25%～38%。各模式中Ei引起的溫室效應(yīng)占總溫室效應(yīng)的10%～25%；Eo則為6%～24%[38]。申建波等[39]研究發(fā)現(xiàn)，常規(guī)管理措施中農(nóng)業(yè)管理的潛在溫室效應(yīng)占總溫室效應(yīng)的29%。梁龍等[40]研究表明，河北平原推薦管理措施中農(nóng)業(yè)管理的潛在溫室效應(yīng)在總溫室效應(yīng)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(huán)境中造成的環(huán)境污染，減緩稻田生態(tài)系統(tǒng)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(shí)增加作物產(chǎn)量和土壤固碳效應(yīng)，降低單位產(chǎn)品的綜合凈溫室效應(yīng)，實(shí)現(xiàn)集約化生產(chǎn)下的低碳農(nóng)業(yè)目標(biāo)。

3 低碳農(nóng)業(yè)研究展望

3.1 低碳農(nóng)業(yè)旨在實(shí)現(xiàn)集約化農(nóng)業(yè)生產(chǎn)方式下的低碳目標(biāo)

隨著農(nóng)業(yè)現(xiàn)代化與集約化的進(jìn)展，碳耗總量增加是必然的。農(nóng)業(yè)提倡“低碳”不等于減少碳耗總量的所謂“低碳農(nóng)業(yè)”，而是要努力追求以較低的單位產(chǎn)品耗碳率換取較高的固碳率。為此，需要集成優(yōu)化農(nóng)業(yè)管理措施、提高氮肥利用率，兼顧實(shí)現(xiàn)固碳、減排、增產(chǎn)的低碳農(nóng)業(yè)發(fā)展目標(biāo)，提高單位產(chǎn)品的碳效率、促進(jìn)農(nóng)業(yè)可持續(xù)發(fā)展[41]。雖然作物生產(chǎn)與溫室效應(yīng)之間存在復(fù)雜的交互作用和區(qū)域特征[42–43]，但隨著國內(nèi)外對資源利用效率和環(huán)境保護(hù)意識的逐漸增強(qiáng)和管理水平的逐步提高，我國農(nóng)業(yè)集約化生產(chǎn)中單位產(chǎn)品的溫室效應(yīng)體現(xiàn)出逐漸減緩的趨勢[44–45]。還應(yīng)該在追求實(shí)現(xiàn)單位產(chǎn)品低碳農(nóng)業(yè)的同時(shí)，獲取更高的單位產(chǎn)品經(jīng)濟(jì)效益[44,46–47]。綜合考慮低碳農(nóng)業(yè)發(fā)展的評價(jià)指標(biāo)和驅(qū)動因素，增強(qiáng)科普宣傳，影響政府決策，已成為國內(nèi)外低碳農(nóng)業(yè)的研究趨勢[46,48–49]。

3.2 氮肥高效施用是實(shí)現(xiàn)低碳農(nóng)業(yè)的關(guān)鍵

reshing(kWh/hm2)0.94 horu 5 5.1粒79775為sp脫6.3、1413 Th 17 131727363937474773數(shù)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(shù)g/kg reandco 6 C 0.0725 g/(h 系i and xideequ 度 放O2eq.k 強(qiáng) 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(nóng) 翻Plough iesel,kg/hm2)菌為參殺數(shù)數(shù).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(yè)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(nóng)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(yè)farm 學(xué)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(shù)為ing and農(nóng)nd al(20系uta rota e Insecticid 氮系數(shù)統(tǒng)劑855568。放系放排放fertilizer, respectiv態(tài)蟲18202020274133殺3737375076排碳排Zn生碳，碳作m2)作后的操據(jù)粒輪Siand 2事數(shù)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(nóng)的t)；hecarbon emission coefficients w稻和費(fèi)年入消ns. T 2014餅manure Chem g/even racticesforchemical inp erapp肥投和50品產(chǎn)50 e,respectivlied) p入00000000學(xué)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(xué)4 C化期藝為肥Zn 平0水鋅0001515000066周工數(shù)st(C eq.kco表an 入 作產(chǎn)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(gè)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(jù)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(nóng)田總施氮量世界第一，氮肥生產(chǎn)工藝比較落后，為了降低氮肥生產(chǎn)過程引起的間接碳排放，升級改造我國氮肥生產(chǎn)工藝勢在必行[50]。據(jù)IPCC統(tǒng)計(jì)，農(nóng)業(yè)措施所引起的CO2排放占全球CO2總排放量的20%[1]。隨著農(nóng)業(yè)現(xiàn)代化過程中化肥、農(nóng)藥的投入以及大型機(jī)械的運(yùn)用，農(nóng)業(yè)措施引起的碳排放對生態(tài)系統(tǒng)凈溫室效應(yīng)的貢獻(xiàn)將會越來越大。近來不同研究者針對我國主要農(nóng)作物[42,45]、蔬菜[51–52]種植等開展的碳足跡研究，都體現(xiàn)出氮肥施用這一單因子在農(nóng)業(yè)碳排放中的重要地位；即使不考慮田間N2O排放的溫室效應(yīng)，僅肥料施用占農(nóng)業(yè)碳排放的比例就高達(dá)48%[53]。氮肥高效施用直接決定著作物產(chǎn)量、生態(tài)系統(tǒng)凈碳收支、土壤固碳效應(yīng)以及CH4和N2O排放，是農(nóng)業(yè)措施碳排放的首要貢獻(xiàn)者，也是實(shí)現(xiàn)集約化生產(chǎn)方式下低碳農(nóng)業(yè)的關(guān)鍵驅(qū)動因子。

圖1 不同集約化栽培模式下稻麥輪作生態(tài)系統(tǒng)CH4、N2O與農(nóng)業(yè)措施碳排放 (Ei 和Eo) 溫室效應(yīng)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(nóng)業(yè)需要從根本上注重提高農(nóng)田土壤固碳效應(yīng)、提高土壤生產(chǎn)力

近些年來，越來越多的研究開始考慮農(nóng)田固碳效應(yīng)[4,54]，許多研究表明稻田具有很強(qiáng)的固碳效應(yīng)[55–56]。土壤有機(jī)碳含量的高低是農(nóng)作物高產(chǎn)穩(wěn)產(chǎn)的基礎(chǔ)。國內(nèi)外研究一致表明，農(nóng)業(yè)管理措施如施肥、種植制度、灌溉、耕作等直接影響土壤有機(jī)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(jī)碳庫的變化[54,58]。據(jù)報(bào)道，近20年來我國大陸53%～59%的農(nóng)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(nóng)田表土有機(jī)碳貯量總體增加311.3～401.4 Tg，這主要?dú)w因于秸稈還田、有機(jī)肥施用和化肥投入的增加，合理的養(yǎng)分配比以及少 (免) 耕技術(shù)的推廣。通過合理有效的農(nóng)田管理措施，例如有機(jī)肥與化肥的合理配施[57]，可以調(diào)節(jié)農(nóng)田土壤由碳源轉(zhuǎn)變?yōu)樘紖R，增強(qiáng)土壤固碳效應(yīng)，同時(shí)提高土壤生產(chǎn)力[59]。然而，土壤固碳效應(yīng)與氮肥施用之間的關(guān)系還存在很大的不確定性，需要針對特定的生態(tài)系統(tǒng)和生態(tài)環(huán)境開展長期研究[60]。生物質(zhì)炭與氮肥的配合應(yīng)用[61–62]則是提高農(nóng)田土壤固碳效應(yīng)、實(shí)現(xiàn)低碳農(nóng)業(yè)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(yè)（農(nóng)業(yè)）科研專項(xiàng)（201503106）；國家自然科學(xué)基金項(xiàng)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(huán)與生態(tài)環(huán)境研究。E-mail：zqxiong@njau.edu.cn