丁佳瑩,董黎明,劉巖峰,孫東霞

生命周期評(píng)價(jià)在玉米種植及深加工產(chǎn)品中的應(yīng)用

丁佳瑩,董黎明*,劉巖峰,孫東霞

(北京工商大學(xué)生態(tài)環(huán)境學(xué)院,中國(guó)輕工業(yè)清潔生產(chǎn)和資源綜合利用重點(diǎn)實(shí)驗(yàn)室,國(guó)家環(huán)境保護(hù)食品鏈污染防治重點(diǎn)實(shí)驗(yàn)室,北京 100048)

對(duì)LCA在玉米種植環(huán)境影響研究中的目標(biāo)與范圍定義?清單分析?環(huán)境影響指標(biāo)與計(jì)算方法?結(jié)果解釋等方面的研究進(jìn)行了梳理分析,其中大部分文獻(xiàn)均對(duì)碳足跡進(jìn)行了計(jì)算,CML (Institute of Environmental Sciences of the University of Leiden)?ReCiPe (許多LCA領(lǐng)域研究人員共同開發(fā))和IPCC (Intergovernmental Panel on Climate Change)方法是應(yīng)用最多的3種方法,經(jīng)分析計(jì)算得到國(guó)外玉米種植單位產(chǎn)量碳足跡均值為0.50kg CO2-eq /kg,我國(guó)的均值為0.58kg CO2-eq /kg.同時(shí)本文也對(duì)LCA在玉米深加工產(chǎn)品中的研究進(jìn)展進(jìn)行了分析整理,對(duì)今后LCA應(yīng)用于玉米種植及深加工產(chǎn)品中的發(fā)展方向進(jìn)行了展望.

生命周期評(píng)價(jià)；玉米種植；玉米深加工產(chǎn)品；環(huán)境影響

玉米是全球也是中國(guó)第一大作物[1],種植面積約1.6億hm2,占全球耕地面積的11%[2].玉米不僅在保障國(guó)家糧食安全中占據(jù)重要地位,同時(shí)也是飼料、乙醇、氨基酸等許多食品和工業(yè)產(chǎn)品的重要原料.隨著世界各地食品、飼料和能源需求不斷增加,人們對(duì)玉米生產(chǎn)環(huán)境影響的關(guān)注也在不斷增加[3].不同玉米種植制度、種植過(guò)程中農(nóng)資過(guò)度投入等會(huì)帶來(lái)一系列環(huán)境問(wèn)題,同時(shí)對(duì)玉米產(chǎn)量也有影響[4].農(nóng)業(yè)系統(tǒng)目前所面臨的挑戰(zhàn)就是減少資源和能源消耗、降低環(huán)境負(fù)擔(dān)的同時(shí)生產(chǎn)更多的糧食[5],實(shí)現(xiàn)向可持續(xù)的農(nóng)業(yè)-食品系統(tǒng)轉(zhuǎn)變.因此,定量評(píng)價(jià)玉米種植過(guò)程中的環(huán)境影響對(duì)玉米農(nóng)業(yè)可持續(xù)發(fā)展至關(guān)重要.

生命周期評(píng)價(jià)(LCA)方法是對(duì)產(chǎn)品或系統(tǒng)整個(gè)生命周期環(huán)境影響及其潛在影響進(jìn)行研究的工具.該方法貫穿產(chǎn)品生命周期全過(guò)程,即從原材料獲取、生產(chǎn)、使用、直至最終處置.國(guó)際標(biāo)準(zhǔn)化組織于1997年將LCA結(jié)構(gòu)確定為目的和范圍的確定、清單分析、影響評(píng)價(jià)及結(jié)果解釋4部分[6].明確的研究目標(biāo)是確定研究范圍即系統(tǒng)邊界的基礎(chǔ)[7].隨著研究目標(biāo)的確定,功能單位也隨之確定.精準(zhǔn)定義功能單位是LCA計(jì)算結(jié)果準(zhǔn)確性?實(shí)用性的前提[8].尤其在比較研究中,不同功能單位對(duì)結(jié)果的影響尤為顯著[9]. LCA研究需要使用原始數(shù)據(jù)(實(shí)驗(yàn)測(cè)量?實(shí)地調(diào)研等)[10]進(jìn)行計(jì)算,當(dāng)原始數(shù)據(jù)不可得或不可用時(shí),也可以通過(guò)文獻(xiàn)調(diào)研、模型模擬或采用數(shù)據(jù)庫(kù)中的數(shù)據(jù)等進(jìn)行計(jì)算[11-12].生命周期環(huán)境影響評(píng)估(LCIA)中的歸一化和加權(quán)評(píng)估不是環(huán)境影響評(píng)估中必須的步驟,但當(dāng)需要比較幾種不同方案時(shí),歸一化和加權(quán)評(píng)估則是重要的步驟[13].生命周期解釋則是將其他部分與LCA結(jié)論和建議聯(lián)系起來(lái)的“樞軸”[14].相比于其他環(huán)境影響評(píng)價(jià)方法,LCA具有系統(tǒng)、客觀等優(yōu)勢(shì).并且已經(jīng)成為一種廣泛應(yīng)用的產(chǎn)品環(huán)境特征分析和決策支持工具[15].LCA最初為評(píng)估工業(yè)系統(tǒng)的環(huán)境影響而開發(fā)[16],隨著消費(fèi)者對(duì)綠色食品的關(guān)注,LCA在農(nóng)業(yè)-食品系統(tǒng)中迅速發(fā)展[17].目前在與食品有關(guān)的農(nóng)作物種植[18-20]、畜牧業(yè)[21-22]、漁業(yè)[23-25]、食品制造業(yè)[26-29]中都得到了廣泛應(yīng)用,已經(jīng)成為指導(dǎo)農(nóng)業(yè)生產(chǎn)向可持續(xù)系統(tǒng)轉(zhuǎn)變的主要工具.目前,針對(duì)LCA在玉米中的應(yīng)用研究雖然很多,但各個(gè)研究所收集的排放數(shù)據(jù)內(nèi)容?數(shù)據(jù)獲取方法不盡相同.在量化玉米種植過(guò)程中施肥帶來(lái)的環(huán)境影響時(shí),Wang等[30]考慮了農(nóng)田重金屬污染,但Li等[31]和Bacenetti等[32]則未進(jìn)行考慮.在探究玉米種植模式環(huán)境影響時(shí),Boone等[33]考慮了農(nóng)藥揮發(fā),但Zhang等[34]、Feng等[35]和Fantin等[36]均未考慮.清單數(shù)據(jù)收集的完整性與準(zhǔn)確性會(huì)影響環(huán)境影響結(jié)果的可信度.另外,我國(guó)缺乏玉米種植環(huán)境影響數(shù)據(jù)庫(kù),在進(jìn)行以玉米為原料的產(chǎn)品LCA研究時(shí),通常選用國(guó)外數(shù)據(jù)庫(kù)中的數(shù)值,Yang等[37]在對(duì)味精生產(chǎn)過(guò)程清潔生產(chǎn)措施進(jìn)行生命周期評(píng)估時(shí),原材料玉米就選擇了Ecoinvent數(shù)據(jù)庫(kù)數(shù)據(jù).那么國(guó)內(nèi)外玉米種植環(huán)境影響數(shù)值是否存在差異?這關(guān)乎到玉米深加工產(chǎn)品環(huán)境影響計(jì)算結(jié)果的準(zhǔn)確性.

鑒于此,本文梳理了國(guó)內(nèi)外LCA在評(píng)估玉米種植環(huán)境影響中的應(yīng)用,分析了目標(biāo)與范圍定義、清單分析、環(huán)境影響指標(biāo)與計(jì)算方法選擇、結(jié)果不確定性來(lái)源等方面的差別與聯(lián)系.另外,農(nóng)業(yè)是溫室氣體的重要排放源,占人類活動(dòng)溫室氣體排放總量的23%～30%[38],結(jié)合我國(guó)作為能源消費(fèi)和碳排放大國(guó),在《氣候變化聯(lián)合聲明》中承諾2030年左右實(shí)現(xiàn)碳排放達(dá)峰[39],本文還得到了國(guó)內(nèi)外玉米種植碳足跡結(jié)果范圍.同時(shí)也對(duì)LCA在玉米深加工產(chǎn)品中的有限應(yīng)用進(jìn)行了概述,展望了LCA在玉米種植及深加工產(chǎn)品中應(yīng)用的發(fā)展趨勢(shì),旨在為促進(jìn)玉米種植可持續(xù)發(fā)展及玉米深加工產(chǎn)品綠色供應(yīng)鏈建設(shè)提供科學(xué)依據(jù).

1 玉米種植的LCA研究

1.1 目的和范圍的確定

1.1.1 研究目標(biāo) 玉米種植LCA研究目標(biāo)包括:量化種植過(guò)程中施用化肥所帶來(lái)的環(huán)境影響;識(shí)別玉米種植生命周期內(nèi)各階段重要環(huán)境因素;進(jìn)行種植模式的對(duì)比、改進(jìn).表1為當(dāng)前LCA研究的研究目的與定義的功能單位.數(shù)據(jù)來(lái)源有實(shí)地調(diào)研、官方統(tǒng)計(jì)數(shù)據(jù)、實(shí)測(cè)、模型模擬和相關(guān)文獻(xiàn)5種渠道.

表1 玉米種植LCA研究中的研究目標(biāo)、研究對(duì)象及區(qū)域、功能單位和其他排放數(shù)據(jù)

續(xù)表1

1.1.2 系統(tǒng)邊界 定義系統(tǒng)邊界對(duì)LCA模型至關(guān)重要[50].系統(tǒng)邊界的相同與否決定了LCA研究結(jié)果是否具有可比性[8].玉米種植包括播種、噴灑農(nóng)藥、施用化肥、灌溉等過(guò)程.系統(tǒng)邊界如圖1所示,玉米作為原料類產(chǎn)品,成熟收割后作為食品和工業(yè)原材料.意大利、美國(guó)、比利時(shí)等國(guó)家關(guān)于玉米種植LCA研究較多,我國(guó)關(guān)于玉米種植LCA研究多在河北、山東、山西等省份.不同國(guó)家和地區(qū)的玉米種植LCA研究都是“從搖籃到大門”的研究,即從原材料生產(chǎn)開始,到成熟收割為止.有文獻(xiàn)將秸稈破碎[31]、玉米收獲后干燥[35]、儲(chǔ)存[32]過(guò)程也納入研究范圍內(nèi).

1.1.3 功能單位 如表1所示,在本文所分析的相關(guān)研究中,功能單位可分為兩種,一是基于每單位質(zhì)量產(chǎn)品產(chǎn)量(質(zhì)量);二是基于每單位種植面積(hm2).也有文獻(xiàn)將單位產(chǎn)值(元)做為功能單位[45].

功能單位的選擇取決于研究目標(biāo)和研究對(duì)象類型[8].如果將研究目標(biāo)聚焦在種植環(huán)節(jié),例如比較同一種植區(qū)域選擇何種作物進(jìn)行種植可以減少對(duì)該區(qū)域的環(huán)境影響,由于不同作物產(chǎn)量往往不同,這時(shí)選擇單位種植面積作為功能單位比選擇單位質(zhì)量農(nóng)產(chǎn)品更為合適.如果研究目標(biāo)聚焦在產(chǎn)品上,而不是進(jìn)行農(nóng)作物種類的挑選,則可以選擇單位質(zhì)量或者同時(shí)選擇面積和質(zhì)量作為功能單位.例如有研究[41]對(duì)玉米集約化生產(chǎn)方式進(jìn)行評(píng)價(jià),結(jié)果表明玉米集約化生產(chǎn)每公頃溫室氣體排放量增加40%,但單位質(zhì)量玉米碳足跡可降低37%.進(jìn)行玉米種植的目的是生產(chǎn)玉米供人類消費(fèi),按單位質(zhì)量玉米產(chǎn)品進(jìn)行環(huán)境影響計(jì)算便于不同地區(qū)生產(chǎn)的玉米進(jìn)行對(duì)比.

圖1 玉米種植LCA研究系統(tǒng)邊界

1.2 清單分析

玉米種植數(shù)據(jù)收集分為3部分,一是背景數(shù)據(jù)收集,即種子、化肥、農(nóng)藥等生產(chǎn)數(shù)據(jù);二是實(shí)景數(shù)據(jù)收集,即田間作業(yè)部分,包括播種、灌溉、施肥、收割等過(guò)程的資源、能源投入;三是向環(huán)境中的排放,包括溫室氣體(土壤CO2和N2O)的排放、土壤有機(jī)碳的變化、氮磷的淋溶流失、氨揮發(fā)、農(nóng)藥揮發(fā)及肥料、灌溉水等帶入農(nóng)田的重金屬污染.清單數(shù)據(jù)來(lái)源需要根據(jù)研究區(qū)域特點(diǎn)確定,不同數(shù)據(jù)來(lái)源會(huì)影響計(jì)算結(jié)果的準(zhǔn)確性.表1中所列其他排放數(shù)據(jù)就是除溫室氣體排放外不同文獻(xiàn)所收集的排放清單.

目前玉米種植LCA研究輸入清單完整,均考慮了能源(柴油?電力)?灌溉水和農(nóng)資投入,其中,農(nóng)資投入包括化肥(礦質(zhì)肥或有機(jī)肥)、種子、柴油、農(nóng)藥等.排放清單包括玉米產(chǎn)品和向環(huán)境中的排放.目前玉米種植LCA研究清單數(shù)據(jù)收集差異主要在排放數(shù)據(jù).CO2和N2O是玉米種植系統(tǒng)釋放的兩種重要溫室氣體,有研究表明,大氣中的CO2、N2O來(lái)源于農(nóng)業(yè)活動(dòng)的比例分別為20%和90%[5].根據(jù)IPCC(Intergovernmental Panel on Climate Change)第五次報(bào)告,N2O的折算系數(shù)為298 CO2當(dāng)量[51]. CO2與N2O釋放量會(huì)因氣候、土壤條件和作物管理差異而發(fā)生很大變化[52].目前研究均對(duì)這兩種溫室氣體排放量進(jìn)行了收集,但存在多種計(jì)算方法.其他排放數(shù)據(jù)并不是所有的研究都進(jìn)行了考慮,且收集方式不一.

1.2.1 溫室氣體 對(duì)于溫室氣體的計(jì)算方法有以下3種,一是通過(guò)實(shí)驗(yàn)測(cè)得,主要方法有箱法?微氣象學(xué)法?超大箱長(zhǎng)光程紅外色譜法和同位素法,其中微氣象學(xué)法是觀測(cè)氣體擴(kuò)散最佳測(cè)定方法[53].Glenn等人在加拿大某地進(jìn)行的一項(xiàng)野外實(shí)驗(yàn)就是采用微氣象技術(shù)測(cè)量土壤CO2和N2O的排放量[52].

二是通過(guò)機(jī)理模型進(jìn)行模擬.Abdalla等[54]對(duì)DNDC (Denitrification-Decomposition)模型進(jìn)行校正后估算了河北省不同施氮量下夏玉米-冬小麥兩熟制N2O的排放和作物產(chǎn)量.DNDC模型即反硝化-分解作用模型,用于模擬農(nóng)業(yè)生態(tài)系統(tǒng)中碳?氮生物地球化學(xué)過(guò)程,由Li等[55]于1992年根據(jù)美國(guó)農(nóng)田管理方式為基礎(chǔ)提出,在中國(guó)?加拿大等多個(gè)國(guó)家得到了廣泛應(yīng)用.Bacenetti通過(guò)EFE-SO(Estimation of Fertilisers Emissions-Software)軟件模擬了施用肥料后氨揮發(fā)量、N2O排放量以及硝酸鹽和磷酸鹽的排放量[32].EFE-SO模型由英國(guó)曼徹斯特大學(xué)研究小組開發(fā),用來(lái)估算與肥料相關(guān)的氮排放.Lee等人采用EPIC (Environmental Policy- Integrated Climate)模型模擬碳、氮、磷循環(huán)及現(xiàn)場(chǎng)環(huán)境釋放[3].EPIC模型是定量評(píng)價(jià)“氣候-土壤-作物-管理”系統(tǒng)的綜合動(dòng)力學(xué)模型,于20世紀(jì)80年代初由美國(guó)德克薩斯農(nóng)工大學(xué)黑土地研究中心和美國(guó)農(nóng)業(yè)部草地土壤水分研究所共同開發(fā)[56].

三是對(duì)于N2O排放量還可以根據(jù)已有研究中給出的排放因子進(jìn)行計(jì)算.IPCC建議N2O排放因子為1%,即以N2O形式損失的氮占所施肥料中氮含量的1%[57].我國(guó)學(xué)者通過(guò)多年田間實(shí)驗(yàn)也確定了不同氣候?土壤特性等條件下的N2O排放因子.Ju等[58]確定了我國(guó)夏玉米、冬小麥-夏玉米輪作的N2O排放因子為施用氮肥含氮量的0.44%～0.59%和0.10%～0.59%.有學(xué)者[59]建議在有機(jī)碳含量為4.5～ 15.6g/kg的石灰性土壤旱地施氮的年直接排放因子為(0.54±0.09)%.Gao等[60]建議華北平原玉米N2O排放因子為0.53%.

農(nóng)田溫室氣體排放機(jī)理復(fù)雜且受到多種因素影響,需要根據(jù)具體區(qū)域現(xiàn)場(chǎng)條件來(lái)確定.田間測(cè)定排放量雖準(zhǔn)確度高,但對(duì)于區(qū)域性研究測(cè)量難度較大.我國(guó)幅員遼闊,不同地區(qū)土壤特性?耕作制度等差異較大,采用已知的排放因子進(jìn)行計(jì)算,與實(shí)際排放量會(huì)存在一定差異[50].而在使用基于國(guó)外情況建立的模型對(duì)我國(guó)農(nóng)田溫室氣體排放進(jìn)行估計(jì)時(shí),應(yīng)結(jié)合我國(guó)的土壤、農(nóng)田管理方式等條件對(duì)模型進(jìn)行修正,加強(qiáng)模型在中國(guó)的本地化研究.

1.2.2 土壤有機(jī)碳 土壤有機(jī)碳變化可以通過(guò)實(shí)驗(yàn)法、經(jīng)驗(yàn)公式和模型進(jìn)行測(cè)量和估算.Zhang等[44]通過(guò)實(shí)地取樣測(cè)得土壤容重和實(shí)驗(yàn)室重鉻酸鉀加熱法測(cè)定土壤有機(jī)碳濃度,計(jì)算土壤有機(jī)碳儲(chǔ)量.韓冰等[61]搜集整理了全國(guó)典型農(nóng)業(yè)長(zhǎng)期試驗(yàn)站數(shù)據(jù),通過(guò)自建經(jīng)驗(yàn)公式和情景分析,發(fā)現(xiàn)提高化肥施用量、秸稈還田、增施有機(jī)肥和推廣免耕可以使我國(guó)農(nóng)田土壤的固碳量分別提高至94.91,42.23, 41.38和2.58Tg/a(百萬(wàn)t/a).Lu等[62]根據(jù)我國(guó)農(nóng)業(yè)土壤有機(jī)碳實(shí)驗(yàn)數(shù)據(jù)庫(kù)建立了土壤固碳經(jīng)驗(yàn)?zāi)Ｐ?能夠?qū)ξ覈?guó)玉米種植區(qū)的氮肥施用量、秸稈還田量、是否免耕等引起的土壤固碳量變化進(jìn)行模擬.

1.2.3 氮磷的淋溶流失、氨揮發(fā)、農(nóng)藥揮發(fā)和重金屬 氮磷淋溶流失和氨揮發(fā)均取自文獻(xiàn)數(shù)據(jù)[44].Ju等[58]通過(guò)田間實(shí)驗(yàn),確定玉米氨揮發(fā)量為施氮量的19.4%,華北平原玉米氮淋溶量為施氮量的12.1%.陳新平等人確定夏玉米氨揮發(fā)量為施入氮素量的16%[63].Gaynor等[64]確定磷流失量為無(wú)機(jī)肥和有機(jī)肥投入總量的1%.

農(nóng)藥揮發(fā)到大氣中以及排放到地下水和地表水中的量可以通過(guò)Pest LCI 2.0進(jìn)行估算[33],該模型能夠估計(jì)農(nóng)藥對(duì)空氣、地表水和地下水的排放,用于生命周期清單數(shù)據(jù)(LCI)建模.也有學(xué)者確定農(nóng)藥排放到空氣、淡水和土壤中的標(biāo)準(zhǔn)殘留率分別為每單位重量農(nóng)藥的10%、1%和43%[65].有研究在計(jì)算人類生態(tài)毒性和水生生態(tài)毒性這兩種環(huán)境影響指標(biāo)時(shí)還考慮了化肥、灌溉帶入農(nóng)田的重金屬,參數(shù)來(lái)自Audsley等[66]和Huijbregts等[67]研究中的值.

1.3 環(huán)境影響評(píng)估

環(huán)境影響評(píng)估計(jì)算首先應(yīng)根據(jù)玉米種植過(guò)程中所產(chǎn)生的能源、水資源消耗和碳排放等環(huán)境問(wèn)題,并結(jié)合各地節(jié)能減排等政策目標(biāo),選擇相應(yīng)的環(huán)境影響指標(biāo),然后選擇計(jì)算方法和軟件工具.

1.3.1 環(huán)境影響指標(biāo) 環(huán)境影響指標(biāo)的選擇受到研究目標(biāo)與范圍的影響[68],圖2展示了所分析文獻(xiàn)中對(duì)不同環(huán)境影響指標(biāo)的選擇比例.

圖2 環(huán)境影響指標(biāo)選擇比例

由于農(nóng)業(yè)占全球溫室氣體排放量的13.5%[69],是溫室氣體的重要排放源,所以90%的相關(guān)文獻(xiàn)都對(duì)碳足跡指標(biāo)進(jìn)行了計(jì)算分析.其次由于玉米種植過(guò)程中施用的化肥,會(huì)造成氮、磷通過(guò)揮發(fā)、淋溶等方式進(jìn)入大氣和水體,以及農(nóng)藥的揮發(fā),所以有50%及以上的文獻(xiàn)關(guān)注了水體富營(yíng)養(yǎng)化、酸化、水生生態(tài)毒性和人類生態(tài)毒性4個(gè)環(huán)境影響指標(biāo).在機(jī)械作業(yè)和灌溉過(guò)程中,會(huì)消耗柴油和電力,所以有33%的文獻(xiàn)關(guān)注了玉米種植過(guò)程中的能源消耗.僅有22%和17%的文獻(xiàn)對(duì)臭氧層消耗?顆粒物形成這兩個(gè)環(huán)境影響指標(biāo)進(jìn)行了計(jì)算.

1.3.2 計(jì)算方法及軟件選擇 CML、ReCiPe和IPCC方法是應(yīng)用最多的3種方法.CML方法由萊頓大學(xué)環(huán)境科學(xué)研究所于1992年首次開發(fā),是一種面向中點(diǎn)的方法.該方法影響類別中酸化和光化學(xué)氧化劑形成兩種環(huán)境影響是基于歐洲平均值,其他影響類別的區(qū)域有效性是全球[70-71].ReCiPe方法是由許多LCA領(lǐng)域開發(fā)人員共同開發(fā),被認(rèn)為是CML2002和EI99方法的綜合,該方法同時(shí)具有中點(diǎn)建模和端點(diǎn)建模兩種方法.該方法的區(qū)域有效性是歐洲,但氣候變化、臭氧層損耗和資源消耗適用于全球[70-71].IPCC方法最早在2001年制定,2007年進(jìn)行了版本更新,它被認(rèn)為是用于計(jì)算氣候變化特定的LCIA方法[70-71].有文獻(xiàn)使用CML和ReCiPe方法同時(shí)對(duì)玉米青貯飼料生產(chǎn)系統(tǒng)的環(huán)境影響指標(biāo)進(jìn)行計(jì)算,結(jié)果表明氣候變化結(jié)果相差不大,CML方法得到的結(jié)果為383.26(kg CO2-eq/t),ReCiPe方法結(jié)果為329(kg CO2-eq/t).海洋生態(tài)毒性計(jì)算結(jié)果差異較大,分別為9.15′105和4.66′103(kg 1,4-DCB-eq/t,表示向土壤和空氣中排放1kgAs和Pb的效應(yīng)值),但兩種方法計(jì)算結(jié)果具有相同的靈敏度排行[72].這說(shuō)明由于不同方法所涵蓋的清單物質(zhì)數(shù)量、所采用的標(biāo)準(zhǔn)、以及所使用數(shù)據(jù)有效性等存在差別,對(duì)于相同清單數(shù)據(jù)同一環(huán)境影響類別的計(jì)算結(jié)果往往也存在差異.由于環(huán)境問(wèn)題的復(fù)雜性,很難得到一個(gè)被廣泛接受的LCIA方法,LCIA方法和科學(xué)的基準(zhǔn)體系仍在不斷發(fā)展中. Lindeijer[73]認(rèn)為國(guó)際上比較有代表性的影響評(píng)價(jià)方法有瑞典的產(chǎn)品設(shè)計(jì)的環(huán)境優(yōu)先戰(zhàn)略方法(EPS)、瑞士和荷蘭的生態(tài)稀缺性方法以及丹麥的工業(yè)產(chǎn)品環(huán)境設(shè)計(jì)方法(EDIP)等25種影響評(píng)價(jià)方法.

Simapro和eBalance(現(xiàn)更新為eFootprint)是在玉米種植LCA研究中使用最多的兩種軟件工具.Simpro由荷蘭Leiden大學(xué)環(huán)境科學(xué)中心開發(fā),集成了Eco-indicator99、CML1992v2.1等多種生命周期評(píng)價(jià)方法,以及Ecoinvent、DataArchive等多個(gè)數(shù)據(jù)庫(kù)[74].eBalance是由中國(guó)億科環(huán)境科技有限公司(IKE)設(shè)計(jì)開發(fā),內(nèi)置Ecoinvent、ELCD和中國(guó)生命周期數(shù)據(jù)庫(kù)CLCD.相比于其他數(shù)據(jù)庫(kù),CLCD數(shù)據(jù)庫(kù)更能代表中國(guó)實(shí)際情況[75-76].Herrmann等[77]對(duì)比了使用不同生命周期評(píng)價(jià)軟件計(jì)算結(jié)果的差異,發(fā)現(xiàn)通常結(jié)果是相同或相近的,但由于不同軟件內(nèi)置的數(shù)據(jù)庫(kù)不同,在選擇不同的背景數(shù)據(jù)來(lái)源時(shí),有的結(jié)果也會(huì)存在較大差異.

1.3.3 計(jì)算結(jié)果 本文對(duì)玉米種植LCA研究特征化計(jì)算結(jié)果中的碳足跡值進(jìn)行分析.

圖3展示了所分析文獻(xiàn)中國(guó)內(nèi)外以單位質(zhì)量玉米為功能單位的玉米種植碳足跡結(jié)果.由于各地區(qū)玉米種植存在差異,也有文獻(xiàn)[3]計(jì)算得到玉米種植單位產(chǎn)量碳足跡為20.2和-6.4kg CO2-eq/kg.其中碳足跡為負(fù)值的原因在于該地區(qū)土壤固碳量大于玉米種植生命周期內(nèi)碳釋放量[3].我國(guó)碳排放高于國(guó)外,主要在于氮肥生產(chǎn)、田間N2O排放、灌溉耗電和機(jī)械作業(yè)的化石能源消耗排放等環(huán)節(jié),我國(guó)在這些環(huán)節(jié)比美國(guó)碳排放量多1535kg CO2-eq/hm2[78].同時(shí)由于我國(guó)免耕比例低,造成我國(guó)土壤固碳效率低于國(guó)外.經(jīng)分析計(jì)算,國(guó)外玉米種植單位產(chǎn)量碳足跡均值為(0.50±0.30)kg CO2-eq/kg,我國(guó)玉米種植單位產(chǎn)量碳足跡均值為(0.58±0.34)kg CO2-eq/kg.

圖3 國(guó)內(nèi)外玉米種植碳足跡分析

目前核算區(qū)域尺度玉米種植碳足跡數(shù)據(jù)來(lái)源有3種,一是通過(guò)國(guó)家統(tǒng)計(jì)數(shù)據(jù),二是通過(guò)實(shí)測(cè)或農(nóng)戶調(diào)研,三是通過(guò)模型模擬.通過(guò)國(guó)家統(tǒng)計(jì)數(shù)據(jù)僅能獲得農(nóng)資投入信息,無(wú)法反映出不同地區(qū)生產(chǎn)方式的差異,例如灌溉方式?翻耕免耕等,這些都是影響碳足跡大小的關(guān)鍵因素.來(lái)源于實(shí)測(cè)?農(nóng)戶調(diào)研或模型模擬的數(shù)據(jù)則考慮了不同地區(qū)生產(chǎn)環(huán)節(jié)的差異,能更加精確的計(jì)算各地實(shí)際生產(chǎn)水平下的碳足跡值.圖3所示分析數(shù)據(jù)來(lái)源于實(shí)測(cè)?農(nóng)戶調(diào)研和模型模擬,但其中部分?jǐn)?shù)據(jù)未考慮土壤碳變化.張丹等[78]通過(guò)農(nóng)戶調(diào)研并通過(guò)模型模擬土壤碳變化,計(jì)算得到我國(guó)玉米種植單位產(chǎn)量碳足跡均值為0.48kg CO2-eq/kg,低于圖3所得均值.也有文獻(xiàn)通過(guò)國(guó)家統(tǒng)計(jì)數(shù)據(jù)核算,并未對(duì)土壤碳變化進(jìn)行考慮,計(jì)算得到我國(guó)玉米種植單位產(chǎn)量碳足跡均值為0.79kg CO2-eq/kg,高于圖3所示均值[45].

對(duì)于歸一化計(jì)算,理論上對(duì)于全球性環(huán)境影響應(yīng)采用全球尺度的基準(zhǔn),區(qū)域性環(huán)境影響則采用相應(yīng)的區(qū)域基準(zhǔn).為了將全球性?區(qū)域性的影響在同一水平上進(jìn)行比較,Liang等[41-42]和Cui等[47]采用的歸一化因子均為2000年全球人均環(huán)境影響的歸一化因子,來(lái)源于Sleeswijk等[79]的研究.該研究選擇2000年為基準(zhǔn)年,收集全球和歐洲一級(jí)的相關(guān)資料,在LCA方法學(xué)背景下進(jìn)行氣候變化?酸化?富營(yíng)養(yǎng)化?生態(tài)毒性等15個(gè)環(huán)境影響指標(biāo)的歸一化研究,結(jié)果被廣泛應(yīng)用于各類LCA研究歸一化計(jì)算[14].加權(quán)評(píng)估中權(quán)重因子的設(shè)置往往依賴于社會(huì)?政治等因素,具有主觀性.王明新等[80]的研究表明通過(guò)專家組評(píng)議方法確定的權(quán)重系數(shù),加權(quán)評(píng)估結(jié)果可以反應(yīng)不同種植制度對(duì)環(huán)境造成的總潛在影響.

1.4 生命周期解釋

1.4.1 不確定性分析 清單數(shù)據(jù)來(lái)源和LCIA方法等會(huì)影響LCA結(jié)果的準(zhǔn)確性[32].不確定性分析就是對(duì)造成結(jié)果不確定性的來(lái)源進(jìn)行識(shí)別以提高結(jié)果的準(zhǔn)確性和可靠性[6].首先是清單數(shù)據(jù)收集應(yīng)盡可能完整.土壤中碳固定可以抵消部分溫室氣體排放,土壤有機(jī)碳變化量的缺失會(huì)影響碳足跡計(jì)算結(jié)果.但土壤有機(jī)碳除了受耕作(免耕?旋耕等)、秸稈還田等人為因素的影響,還會(huì)受氣候、土壤條件等自然因素的影響[78,81-82].是否將土壤有機(jī)碳變化納入糧食產(chǎn)品碳足跡計(jì)算仍存在爭(zhēng)議[46].同時(shí),這也與所研究的系統(tǒng)邊界相聯(lián)系.有研究認(rèn)為,若研究系統(tǒng)邊界僅為單季玉米從播種到成熟收獲過(guò)程,不包括收獲后土壤的耕作管理及秸稈還田等過(guò)程,則土壤有機(jī)碳變化不是必須要考慮的因素.若是包括收獲后的相關(guān)處理過(guò)程,或是對(duì)不同耕作制度進(jìn)行比較研究,則必須要考慮土壤有機(jī)碳的變化[81,83].

有研究表明[20]施肥所帶來(lái)的氨揮發(fā)?氮磷淋溶流失對(duì)氣候變化和顆粒物形成結(jié)果貢獻(xiàn)分別為42%和78%,農(nóng)藥揮發(fā)對(duì)淡水生態(tài)毒性的貢獻(xiàn)率約為98%,所以在選擇這3種指標(biāo)進(jìn)行計(jì)算時(shí),相關(guān)數(shù)據(jù)如若缺失,則對(duì)結(jié)果不確定性影響較大.其次清單數(shù)據(jù)應(yīng)盡可能準(zhǔn)確,田間溫室氣體排放量會(huì)受到肥料類型、土壤性質(zhì)差異等影響,應(yīng)選擇最接近所研究區(qū)域條件的排放因子或模型進(jìn)行計(jì)算.

1.4.2 靈敏度分析、方案對(duì)比及改進(jìn)潛力分析 靈敏度分析就是進(jìn)一步分析關(guān)鍵流程對(duì)LCA結(jié)果的貢獻(xiàn)程度[6].氮肥和磷肥的施用是玉米生產(chǎn)系統(tǒng)中的關(guān)鍵清單物質(zhì),特別是對(duì)氣候變化、酸化和富營(yíng)養(yǎng)化3種環(huán)境影響類別.有研究表明[47],通過(guò)增施氮肥,玉米產(chǎn)量增長(zhǎng)相對(duì)較小.所以可以通過(guò)測(cè)土配肥來(lái)科學(xué)施肥,使用小型施肥設(shè)備提高氮肥利用率、降低氮肥施用量、提高施用率以減少流失,或者用液體糞肥來(lái)替代礦肥,是降低玉米生產(chǎn)系統(tǒng)環(huán)境影響的潛在解決方案.文獻(xiàn)表明,使用液體糞肥替代50%的礦肥可降低37.9%的端點(diǎn)水平環(huán)境影響[31].另外耕作方式也對(duì)碳足跡影響很大,留茬免耕是在提高玉米產(chǎn)量的同時(shí)降低碳足跡的最好方式[44].相比于冬小麥-夏玉米生產(chǎn)系統(tǒng)適當(dāng)進(jìn)行灌溉的玉米單作系統(tǒng)是有望平衡糧食安全和環(huán)境可持續(xù)性的種植制度,總潛在環(huán)境影響可降低39.59%～ 40.30%[47].

1.5 LCA方法與其他評(píng)價(jià)方法的結(jié)合

LCA研究計(jì)算出玉米種植所帶來(lái)的環(huán)境影響,為降低環(huán)境影響所采取的種植模式調(diào)整?耕作管理方式變更等措施,除了帶來(lái)環(huán)境影響的變化還會(huì)帶來(lái)資源利用效率、糧食安全、經(jīng)濟(jì)收入等一系列變化.單獨(dú)進(jìn)行環(huán)境影響研究會(huì)帶來(lái)建議或決策的片面性,而綜合分析則比單項(xiàng)影響評(píng)價(jià)更具有說(shuō)服力.Cui等人[47]在對(duì)華北平原小麥-玉米輪作轉(zhuǎn)向玉米單作種植模式進(jìn)行評(píng)價(jià)時(shí),采用經(jīng)濟(jì)分析?LCA方法和能值分析方法對(duì)環(huán)境與經(jīng)濟(jì)影響進(jìn)行了綜合評(píng)價(jià),結(jié)果表明玉米單作系統(tǒng)產(chǎn)量較低,但卻具有更高的可持續(xù)性水平,環(huán)境負(fù)荷比小麥-玉米輪作系統(tǒng)低8.16%～26.7%,能值可持續(xù)性指數(shù)高10.20%～ 30.52%,說(shuō)明單作玉米是解決目前華北地區(qū)小麥-玉米輪作系統(tǒng)造成的環(huán)境壓力的有效措施.Liang等人[41]采用經(jīng)濟(jì)分析和LCA研究對(duì)華北平原玉米-小麥輪作系統(tǒng)用有機(jī)肥替代礦肥的經(jīng)濟(jì)和環(huán)境可持續(xù)性進(jìn)行了評(píng)價(jià),結(jié)果表明使用液態(tài)有機(jī)肥替代50%的礦肥后,相比于100%使用礦肥,經(jīng)濟(jì)利潤(rùn)增加19.1%,端點(diǎn)水平環(huán)境影響降低了37.9%,生態(tài)效率提高了91.7%.表明使用液態(tài)有機(jī)肥替代50%的礦肥在促進(jìn)農(nóng)業(yè)可持續(xù)生產(chǎn)中的應(yīng)用前景.

2 LCA在玉米深加工產(chǎn)品中的應(yīng)用

2019年世界工業(yè)消費(fèi)玉米占玉米總產(chǎn)量的30.27%,玉米深加工產(chǎn)品超過(guò)1000多種.在食品?化工?醫(yī)藥等多領(lǐng)域廣泛應(yīng)用.目前LCA在玉米深加工產(chǎn)品中應(yīng)用有限,主要集中在葡萄糖[84]、乙醇[85-90]、谷氨酸鈉[37]等產(chǎn)品.

2.1 目的和范圍的確定

玉米深加工產(chǎn)品生產(chǎn)系統(tǒng)邊界如圖4所示.對(duì)于玉米燃料乙醇這類終端產(chǎn)品的LCA研究都是從“搖籃到墳?zāi)埂钡难芯?即包括從玉米種植到燃料乙醇的使用所有過(guò)程,對(duì)于葡萄糖?谷氨酸鈉這類中間產(chǎn)品的LCA研究系統(tǒng)邊界則都是從“搖籃到大門”,即從玉米種植開始到產(chǎn)品出廠為止.通常選擇單位質(zhì)量深加工產(chǎn)品為功能單位,Yang在對(duì)比玉米燃料乙醇與汽油生命周期環(huán)境影響時(shí),選擇汽車行駛一公里為功能單位[89].

2.2 環(huán)境影響指標(biāo)選擇及計(jì)算結(jié)果

對(duì)于玉米深加工產(chǎn)品來(lái)說(shuō),碳足跡仍然是重點(diǎn)關(guān)注的環(huán)境影響指標(biāo),有88%的文獻(xiàn)都對(duì)其進(jìn)行了計(jì)算.其次是能源消耗?酸化和富營(yíng)養(yǎng)化.玉米是深加工產(chǎn)品中最重要的原料,其生產(chǎn)過(guò)程包含在深加工產(chǎn)品系統(tǒng)邊界內(nèi),淡水作為玉米生產(chǎn)必需的資源,也有文獻(xiàn)對(duì)水足跡進(jìn)行了計(jì)算[88].Yang[89]在對(duì)比同作為燃料的玉米乙醇與汽油環(huán)境影響時(shí),選擇的環(huán)境影響指標(biāo)為水生生態(tài)毒性和人類健康毒性,發(fā)現(xiàn)用乙醇代替汽油,會(huì)進(jìn)一步降低水質(zhì),加劇非癌癥相關(guān)的人類健康風(fēng)險(xiǎn).該研究表明,在進(jìn)行環(huán)境決策時(shí),除了碳足跡和能源消耗等,還應(yīng)考慮更廣泛的環(huán)境影響,以避免或減少環(huán)境負(fù)擔(dān)的轉(zhuǎn)移.

玉米深加工產(chǎn)品生產(chǎn)過(guò)程中會(huì)產(chǎn)生玉米蛋白、飼料、酒糟等副產(chǎn)品,多產(chǎn)出系統(tǒng)不同產(chǎn)品之間共同承擔(dān)總環(huán)境影響,所以關(guān)于副產(chǎn)物的分配程序也是LCA研究中的重要環(huán)節(jié)[87].分配方法可以基于主副產(chǎn)品的物理參數(shù)(如質(zhì)量?營(yíng)養(yǎng)價(jià)值等)、經(jīng)濟(jì)關(guān)系(市場(chǎng)價(jià)格)進(jìn)行分配,也可以通過(guò)系統(tǒng)擴(kuò)展法進(jìn)行分配[90].在系統(tǒng)擴(kuò)展法中,假設(shè)副產(chǎn)品的環(huán)境負(fù)擔(dān)與功能等價(jià)的其他產(chǎn)品環(huán)境負(fù)擔(dān)相同,在總環(huán)境影響中扣除等價(jià)產(chǎn)品的環(huán)境影響,即為主產(chǎn)品環(huán)境影響.這幾種分配方法在玉米深加工產(chǎn)品的LCA研究中均得到了廣泛應(yīng)用[37].采用不同分配方法可能會(huì)導(dǎo)致不同的計(jì)算結(jié)果.

Tsiropoulos等[84]研究玉米濕磨法生產(chǎn)葡萄糖的環(huán)境影響及不同分配方法對(duì)環(huán)境的影響時(shí),采用經(jīng)濟(jì)價(jià)值和系統(tǒng)擴(kuò)展分配方法計(jì)算每生產(chǎn)1kg葡萄糖對(duì)不可再生資源消耗量為6.8～9MJ,溫室氣體排放量為0.7～1.1kg CO2-eq.對(duì)于不同分配方法,玉米都是最重要的清單物質(zhì),在經(jīng)濟(jì)價(jià)值和系統(tǒng)擴(kuò)展分配方法中,玉米種植過(guò)程對(duì)葡萄糖生產(chǎn)過(guò)程溫室氣體排放的貢獻(xiàn)都約為75%,對(duì)能源消耗貢獻(xiàn)都約為50%.Cheroennet等[88]對(duì)比了玉米乙醇與木薯乙醇的能源效率和水足跡,結(jié)果表明玉米種植階段的能源消耗量在玉米基乙醇生命周期僅次于工業(yè)生產(chǎn)過(guò)程,為總能耗的20%.Yang等[37]對(duì)中國(guó)味精生產(chǎn)過(guò)程中的清潔生產(chǎn)措施進(jìn)行評(píng)估,結(jié)果表明當(dāng)采用經(jīng)濟(jì)價(jià)值分配方法時(shí),味精生產(chǎn)過(guò)程對(duì)氣候變化的影響為6.26t CO2-eq,對(duì)水生生態(tài)毒性影響為848t TEG water-eq,對(duì)酸化影響為83.5kg SO2-eq,對(duì)富營(yíng)養(yǎng)化潛值貢獻(xiàn)為5.04kg PO4P-lim eq,采用質(zhì)量分配方法時(shí)結(jié)果分別為4.65t CO2-eq,630t TEG water eq?62.1kg SO2-eq和3.75kg PO4P-lim eq.玉米種植過(guò)程對(duì)水生生態(tài)毒性貢獻(xiàn)約90%,對(duì)酸化和富營(yíng)養(yǎng)化的貢獻(xiàn)分別約為75%和70%.

目前應(yīng)選擇哪種分配方法沒(méi)有直接的答案,分配方法的選擇取決于數(shù)據(jù)詳細(xì)程度、可用性等[90].不同分配方法影響環(huán)境影響的絕對(duì)值,但不會(huì)改變各種清單物質(zhì)對(duì)環(huán)境影響的貢獻(xiàn)排名[37].玉米種植環(huán)節(jié)始終是深加工產(chǎn)品環(huán)境影響的主要貢獻(xiàn)者.選用不同地區(qū)玉米為原料對(duì)結(jié)果貢獻(xiàn)大小也不同,深入研究玉米生產(chǎn),對(duì)于提高深加工產(chǎn)品環(huán)境性能具有重要意義.

圖4 玉米深加工產(chǎn)品LCA研究系統(tǒng)邊界

3 結(jié)語(yǔ)

本文通過(guò)文獻(xiàn)梳理,綜述了LCA在國(guó)內(nèi)外玉米種植及深加工產(chǎn)品中的研究進(jìn)展.盡管LCA在玉米深加工產(chǎn)品中的研究數(shù)量有限,但從現(xiàn)有研究可以表明玉米種植過(guò)程是深加工產(chǎn)品環(huán)境影響的重要貢獻(xiàn)者,對(duì)環(huán)境影響的貢獻(xiàn)最高可達(dá)90%.我國(guó)玉米碳足跡均值比國(guó)外高約16%,所以在對(duì)玉米深加工產(chǎn)品進(jìn)行LCA研究時(shí),對(duì)于原料供應(yīng)地的選擇,除了考慮運(yùn)輸距離和成本外,應(yīng)盡可能收集供應(yīng)地的玉米種植數(shù)據(jù)進(jìn)行環(huán)境影響計(jì)算,避免使用數(shù)據(jù)庫(kù)中的玉米種植環(huán)境影響數(shù)據(jù)增大最終結(jié)果的不確定性.LCA在玉米種植環(huán)境影響研究中的應(yīng)用越來(lái)越多,但從目前的研究來(lái)看,尚存在一些需要改進(jìn)之處.

首先清單數(shù)據(jù)收集應(yīng)完整,環(huán)境影響指標(biāo)選擇要全面.除了對(duì)農(nóng)資投入與溫室氣體排放的考慮,氮磷淋溶流失、氨揮發(fā)、農(nóng)藥揮發(fā)、重金屬污染等排放數(shù)據(jù)也應(yīng)盡可能進(jìn)行收集,同時(shí)要注意提高所收集的清單數(shù)據(jù)質(zhì)量,以增強(qiáng)結(jié)果的準(zhǔn)確性.除了對(duì)碳足跡的關(guān)注,農(nóng)藥、化肥施用所帶來(lái)的生態(tài)毒性、臭氧層損耗等環(huán)境影響也不容忽視.另外在對(duì)生產(chǎn)系統(tǒng)進(jìn)行評(píng)價(jià)時(shí),不應(yīng)只局限于環(huán)境負(fù)荷方面,可以將LCA方法與能值分析?經(jīng)濟(jì)分析等方法結(jié)合,從多方面進(jìn)行評(píng)價(jià),增強(qiáng)結(jié)論的說(shuō)服力,以及決策的完整性.

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Application of life cycle assessment in maize planting and deep processing products.

DING Jia-ying, DONG Li-ming*, LIU Yan-feng, SUN Dong-xia

(State Environmental Protection Key Laboratory of Food Chain Pollution Control, Key Laboratory of Cleaner Production and Integrated Resource Utilization of China National Light Industry, School of Ecological Environment, Beijing Technology and Business University, Beijing 100048,China)., 2021,41(11)：5405～5415

This paper focused on the research of Life Cycle Assessment (LCA) in the environmental impact of maize planting, including the definition of goal and scope, inventory analysis, environmental impact indicators and calculation methods, results interpretation and so on. Carbon footprint was calculated in most literatures. CML(Institute of Environmental Sciences of the University of Leiden), ReCiPe (developed by many LCA researchers) and IPCC (Intergovernmental Panel on Climate Change) are the three most widely used methods. Through analysis and calculation, the average carbon footprint per unit yield of corn planting in foreign countries is 1.78kg CO2-eq/ kg, while that in China is 1.31kg CO2-eq / kg. Meanwhile, this research also reviewed the research of LCA in deep processing products from maize, and prospects the development direction of LCA in maize planting and deep processing products in the future.

life cycle assessment；maize planting；deep processing products from maize；environmental impact

X820.3

A

1000-6923(2021)11-5405-11

丁佳瑩(1995-),女,河北承德人,北京工商大學(xué)碩士研究生,主要從事食品行業(yè)LCA的研究.

2021-04-09

國(guó)家自然科學(xué)基金資助項(xiàng)目(41861124004)

* 責(zé)任作者, 教授, donglm@btbu.edu.cn