氮肥高效施用在低碳农业中的关键作用

熊正琴，张晓旭

（南京农业大学资源与环境科学学院，江苏南京 210095）

氮肥高效施用在低碳农业中的关键作用

熊正琴，张晓旭

（南京农业大学资源与环境科学学院，江苏南京 210095）

低碳农业是我国集约化农业发展的必然趋势。深入理解氮肥高效施用是实现低碳农业的关键，可以更加明确如何集成优化农业管理措施增加产量、减少农田生态系统碳排放、提高土壤固碳效应，综合实现固碳、减排、增产的低碳农业发展目标。本文概述了低碳农业评价指标的三个阶段性研究特点，从田间温室气体排放的综合温室效应拓展为涵盖固碳效应的净温室效应，再拓展为涵盖生命周期评价碳排放的综合净温室效应以及兼顾作物产量的温室气体强度。提出了如何利用当季作物试验来估算农田生态系统净碳收支、结合生命周期评价当季作物综合净温室效应和单位产品温室气体强度的方法。按照现阶段低碳农业的评价指标，以我国稻–麦轮作生态系统集约化生产的低碳农业模式为案例，解析氮肥施用在低碳农业各组成包括作物产量、固碳效应、CH4和N2O排放、农业措施碳排放中的重要作用，明确氮肥高效施用在农田生态系统综合净温室效应和温室气体强度中的关键作用，从而实现低碳农业可持续发展。

低碳农业；生态系统净碳收支；土壤固碳效应；生命周期评价；净温室效应

联合国政府间气候变化专门委员会 (IPCC) 第五次评估报告指出，大气二氧化碳 (CO2)、甲烷 (CH4)和氧化亚氮 (N2O) 等温室气体浓度增加导致全球气候变暖已经成为无可争议的事实[1]。如何缓减气候变化对人类社会发展的影响受到世界各国政府和人民的高度重视。

1 低碳农业评价指标内涵不断扩展的三个发展阶段

1.1 低碳农业第一阶段评价指标通常仅考虑田间温室气体直接排放的综合温室效应

农田生态系统以光合作用生产农作物为主要目的，是一种特殊的CO2交换系统，通常其净碳交换可以近似为零，将CH4和N2O两种温室气体的排放作为农田生态系统的综合温室效应，是低碳农业研究第一阶段的评价指标。综合温室效应GWP (global warming potential) 指在特定时间尺度 (通常以100年时间尺度计) 内，单位质量的某一种温室气体相对于单位质量CO2的辐射潜力；作为一种相对指标，可以全面评价农田生态系统排放的温室气体对全球变暖温室效应的贡献[2]。在100年时间尺度上，单位质量的CH4和N2O的全球增温潜势分别为单位质量CO2的28倍和265倍[1]。分别表示作物生长周期内计算的季节累积排放量，根据单位质量增温潜势换算为CO2当量排放量。

1.2 低碳农业第二阶段评价指标拓展为涵盖固碳效应的净温室效应

农田生态系统固碳是当前国际社会公认的减缓大气CO2浓度升高的重要途径之一，如何提高农田生态系统碳储量和固碳速率，是当前国际社会广泛关注的焦点[3]。因此，考虑农田生态系统固碳效应，低碳农业研究第二阶段的评价指标拓展为农田生态系统净温室效应其中，以多种不同途径估算农田固碳效应，均存在较大的系统误差。目前，很多研究以测定土壤呼吸的不同比例直接表征农田生态系统CO2净交换[4]，不仅误差较大，也没有考虑作物系统生产力对CO2的固定，不能真实表征土壤的固碳效应。利用气象资料、土壤基本理化性质、农业管理措施等作为基本参数的模型预测也是目前区域固碳效应的研究方法，应用比较广泛的有DNDC[5]、DAYCENT[6]以及CENTURY[7]。估算农田固碳效应的主要方法是测量土壤有机碳 (SOC) 的变化[8]。目前比较普遍的方法是基于长期定位试验，测定SOC含量的年际变化，再外推演绎估算土壤固碳效应[9]。通过土壤调查基础数据库[10]或前人研究汇总估算大尺度范围农田有机碳变化[11]。

1.3 土壤固碳效应由依赖长期试验尺度发展到当季作物尺度

对于非长期定位试验，很难检测农田有机碳的变化[4]，尤其是当时间尺度缩短为一年或当季作物时，估算土壤有机碳变化的方法较少[12]。为了及时评价新研发的农田管理措施或种植技术等对农田生态系统固碳效应的潜力，本文作者提出了当季作物时间尺度上估算农田生态系统净碳收支 (NECB) 的方法，且得到了长期定位测量土壤有机碳变化方法的有效验证，为基于作物生长季节时间尺度的短期试验提供了土壤固碳效应的研究方法。该方法通过测定农田异养呼吸 (Rh) 和作物生态系统净初级生产力(NPP) 或生态系统呼吸 (Re) 和总初级生产力 (GPP)两种途径来计算生态系统净生产力 (NEP)，即NEP =NPP – Rh = GPP – Re；然后，根据 NECB = NEP –H – CH4+ M计算农田生态系统净碳收支；再根据生态系统净碳收支与土壤有机碳之间的内在关系估算土壤有机碳 (SOC) 的变化速率[13]。上述公式中Rh与Re为静态暗箱法测得的CO2累积排放碳量；H(Harvest) 表示因农田收获物移出农田生态系统的总碳量，包括秸秆和籽粒碳量；CH4代表作物全生长周期内CH4累积排放碳量；M (Manure) 表示农田施入外源有机肥碳量；NPP代表作物全生长周期内作物地上、地下部分增加的总碳量[13]。

1.4 低碳农业第三阶段即现阶段的评价指标拓展为涵盖生命周期评价碳排放的综合净温室效应以及兼顾作物产量的温室气体强度

除了农田生态系统直接排放的温室气体CH4和N2O引发温室效应外，在农业生产过程中化学品投入 (Ei) 和农事操作 (Eo) 也会直接或间接引起CO2排放，从而增加农田生态系统的温室效应[14]。因此，应用生命周期评价法LCA (life cycle assessment) 评估综合净温室效应时，除了前述农田生态系统CH4和N2O排放以及农田固碳效应外，还应当考虑农业措施导致的碳排放[15–16]，成为低碳农业研究第三阶段的评价指标。综合净温室效应计算公式为：net GWP =CH4× 28 + N2O × 265 + Eo + Ei – δSOC × 44/12 (kg CO2eq./hm2)。农业措施碳排放一方面来自化学品投入 (Ei) 如肥料、农药等的生产、储存、运输、施用等过程；另一方面则来自农事操作 (Eo) 如灌溉、翻耕和收获等消耗燃料或其他形式能源的过程。沿用国际标准化组织ISO (international organization for standardization) 对产品碳足迹的定义，低碳农业则是基于生命周期评价方法，计算农产品生产系统内各种温室气体排放与消纳之和，并以CO2当量形式表示，评价对气候变化的单一影响[17]。单位产品的综合净温室效应即为温室气体强度GHGI [greenhouse gas intensity (CO2eq. kg/kg, yield)]，其计算公式为：GHGI =net GWP/作物产量 。由于温室气体强度兼顾作物产量和综合净温室效应，是现阶段低碳农业的评价指标。

2 高效施用氮肥是实现低碳农业的关键

粮食安全是目前世界各国面临的重大挑战之一[18]。据FAO预测，到2030年我国粮食总产必须在现有基础上提高40%以上、单产增加45%以上，以保障我国粮食安全[19]。目前化学氮肥利用率大多低于30%，我国氮肥用量在持续快速增长的同时，粮食产量增加缓慢[20]。如何同步提高作物产量与氮肥利用率是当前国际社会农业可持续发展的研究热点。Tilman[21]指出必须更有效地利用农田养分，以降低农业对环境的负效应；Swaminathan[22]提出“Evergreen Revolution”，适度增加外部投入，改善农田生产效率，增强农业可持续性，降低环境成本；Matson等[23]提出“集约化可持续农业”。本文设定的集约化栽培模式依托于稻–麦轮作体系土壤–作物综合管理系统ISSM (integrated soil-crop system management)[24]，根据专家推荐结合当地实际情况进行氮肥水平、施用比例、种类、有机肥配施、种植密度以及土壤水分管理等措施的不同整合，旨在实现水稻高产、氮肥高效利用、同时降低环境影响的可持续农业发展模式，已成功运行[16,25–26]。因此本文解析上述集约化栽培模式中氮肥施用对发展低碳农业温室气体强度各组成要素的重要贡献。

2.1 氮肥施用直接决定作物产量和土壤固碳效应的增加

由表1可见，氮肥施用对作物产量和生态系统净碳收支及固碳效应具有明显影响。因此，氮肥施用直接决定着低碳农业中单位农产品的综合净温室效应即温室气体强度。

2.2 氮肥施用影响稻田生态系统温室气体CH4和N2O的田间直接排放

氮肥对稻田生态系统CH4排放量的影响极其复杂，可能增加，可能减少，也可能没有影响，具体情况与土壤性质、水稻品种、肥料种类、施用时间、施用方式以及施用量有关[27]。施用氮肥促进植株生长，提高植株CH4传输速率，同时抑制土壤CH4氧化[28]，从而促进CH4排放。然而，铵态氮和CH4的共同存在也可能促进甲烷氧化菌的生长、促进CH4氧化，从而降低CH4排放量。施用有机肥则是促进稻田生态系统CH4排放的重要因素[26,29–30]，其促进程度取决于有机肥的成分、性质以及施用方法。有机肥和化肥结合施用或者避免在淹水条件下直接施用有机肥，均可有效减少稻田CH4排放。

表 1 2011～2014年稻–麦轮作周期中氮肥用量、作物产量、生态系统净碳收支、固碳效应及温室气体强度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

氮肥施用也直接影响稻田生态系统N2O的排放量。施用化学氮肥能够显著增加土壤中NH4+-N与NO3–-N的含量，继而增强硝化作用和反硝化作用的强度，从而促进土壤N2O的产生与排放。通常认为，随着化学氮肥用量增加，土壤N2O排放量呈线性增加[1]。减少氮肥施用量或应用硝化抑制剂均可减少土壤硝化和反硝化过程产生的N2O[31]。当作物地上部氮盈余量等于或小于作物最佳需氮量时，土壤N2O排放变化较小；当施氮量超出作物地上部最大需求量时，N2O排放量急剧增加。因此，越来越多的研究表明，N2O排放与施氮量之间呈指数关系[32–33]。依据作物需肥特征优化施肥时间与方式，调整氮、磷、钾施用比例，选用长效缓释氮肥[34]，提高氮肥利用率，可有效减少N2O排放[35]。有机肥施用对土壤N2O是正效应还是负效应影响比较复杂[36]，主要取决于不同种类有机肥的C/N[37]及施用方法。

2.3 氮肥施用对于农业措施引起的碳排放贡献突出

本文系统边界为水稻和小麦田间生产阶段，从播种到作物收获生命周期全过程，包括农用化学品投入 (Ei) 和农事操作 (Eo) 等农业措施引起的碳排放；各不同栽培模式化学品投入与农事操作通过生命周期评价方法估算结果见表2[16]。农用化学品投入(Ei) 包括水稻和小麦种植过程中施用的肥料包括氮肥、磷肥和钾肥和农药 (除草剂、杀虫剂与杀菌剂)在生产、储存和运输过程中产生的碳排放；农事操作 (Eo) 主要包括灌溉、翻耕与收获等过程中农业机械消耗燃料或其他形式能源所引起的碳排放。在稻-麦轮作生态系统中，来自化学品投入 (Ei) 引起的GWP变化范围为CO2eq.734～4362 kg/hm2；来自农事操作 (Eo) 引起的GWP变化范围为CO2eq.1296～1708 kg/hm2。

各施氮模式中氮肥施用对Ei的贡献率高达66%～75%，是Ei中最主要的碳排放来源 (图1)。一方面是因为氮肥本身在生产和运输过程中需要消耗大量的化石燃料，导致氮肥施用引起的碳排放高；另一方面，集约化农业生产中粮食产量的增加主要依靠氮肥的投入。氮肥不仅是Ei的主要组成部分，也是农业措施碳排放Ei+Eo的主要组成部分[16]。如图1所示，与CH4与N2O排放引起的温室效应相比，农业措施引起的碳排放对温室效应的贡献不容忽视。农业措施碳排放在CH4、N2O排放与农业措施碳排放引起的总温室效应中占25%～38%。各模式中Ei引起的温室效应占总温室效应的10%～25%；Eo则为6%～24%[38]。申建波等[39]研究发现，常规管理措施中农业管理的潜在温室效应占总温室效应的29%。梁龙等[40]研究表明，河北平原推荐管理措施中农业管理的潜在温室效应在总温室效应中的比重为31%。因此，合理施用氮肥，提高氮肥利用率，不仅可以降低氮肥施用后流失到环境中造成的环境污染，减缓稻田生态系统CH4和N2O的直接排放，还可以降低因氮肥施用造成的间接碳排放，同时增加作物产量和土壤固碳效应，降低单位产品的综合净温室效应，实现集约化生产下的低碳农业目标。

3 低碳农业研究展望

3.1 低碳农业旨在实现集约化农业生产方式下的低碳目标

随着农业现代化与集约化的进展，碳耗总量增加是必然的。农业提倡“低碳”不等于减少碳耗总量的所谓“低碳农业”，而是要努力追求以较低的单位产品耗碳率换取较高的固碳率。为此，需要集成优化农业管理措施、提高氮肥利用率，兼顾实现固碳、减排、增产的低碳农业发展目标，提高单位产品的碳效率、促进农业可持续发展[41]。虽然作物生产与温室效应之间存在复杂的交互作用和区域特征[42–43]，但随着国内外对资源利用效率和环境保护意识的逐渐增强和管理水平的逐步提高，我国农业集约化生产中单位产品的温室效应体现出逐渐减缓的趋势[44–45]。还应该在追求实现单位产品低碳农业的同时，获取更高的单位产品经济效益[44,46–47]。综合考虑低碳农业发展的评价指标和驱动因素，增强科普宣传，影响政府决策，已成为国内外低碳农业的研究趋势[46,48–49]。

3.2 氮肥高效施用是实现低碳农业的关键

reshing(kWh/hm2)0.94 horu 5 5.1粒79775为sp脱6.3、1413 Th 17 131727363937474773数utandfarms,n ho eat 07放5、系ual rice-wh .0 0.15、0排estimated carboal h)forthreshing获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翻获tsin thea或planting liedptio(D播st[C eq.kg/(cm·hm2)] as referred to L, as种vent)别C cost (C eq.k m2·a)]2为erapp consum移Crop(E ivalen Farm 22222232323 1.3、耕232323分) p栽数g/kg reandco 6 C 0.0725 g/(h 系i and xideequ 度 放O2eq.k 强 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剂5.16[C考atio p p[C 农 翻Plough iesel,kg/hm2)菌为参杀数数.9 C cost (C eq.k放(D 和系系排放s to carbon d剂放放and 3 t)forcro Ei) 碳2013 8080 Irrigation 55555555 1514排1514 0 1041 0 1041 0 1041 0 1041虫排排.1、的施.3, 5杀碳碳hina’sfertilizer m g/even o、(cm)2012 656565 80 tion 65 80 1514 1514剂溉措n coefficientforirrig(E施ntribu 溉草灌业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 饼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)；农e collected ere 1 e ly.T品op钾和、25(C eq.kg/kW, 0.1剂icid ha rvesting, 3 e业farm 学s from 2011 to 2014肥0.07 04).W化tion 菌44.4 Fung 6 4.49 4.45359595979129磷杀、取0.07肥数为ing and农nd al(20系uta rota e Insecticid 氮系数统剂855568。放系放排放fertilizer, respectiv态虫18202020274133杀3737375076排碳排Zn生碳，碳作m2)作后的操据粒轮Siand 2事数脱iesel)fortillage, rak 2麦e草22246除Herb ely,as referred to L 4646–icalinputlevel(kg/h 46 icalinput 剂icid 2 4646农的t)；hecarbon emission coefficients w稻和费年入消ns. T 2014饼manure Chem g/even racticesforchemical inp erapp肥投和50品产50 e,respectivlied) p入00000000学生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 学4 C化期艺为肥Zn 平0水锌0001515000066周工数st(C eq.kco表an 入 作产系holerice-wheat rotatio co ere 0.9 l m 肥Si 投000000005858轮生放ra 硅225 225 C emission fro麦的排稻肥碳0.1 C gricultu 000000放555588个锌的钾肥K 303030303645排碳161616161924整和种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 和根移据栽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

我国农田总施氮量世界第一，氮肥生产工艺比较落后，为了降低氮肥生产过程引起的间接碳排放，升级改造我国氮肥生产工艺势在必行[50]。据IPCC统计，农业措施所引起的CO2排放占全球CO2总排放量的20%[1]。随着农业现代化过程中化肥、农药的投入以及大型机械的运用，农业措施引起的碳排放对生态系统净温室效应的贡献将会越来越大。近来不同研究者针对我国主要农作物[42,45]、蔬菜[51–52]种植等开展的碳足迹研究，都体现出氮肥施用这一单因子在农业碳排放中的重要地位；即使不考虑田间N2O排放的温室效应，仅肥料施用占农业碳排放的比例就高达48%[53]。氮肥高效施用直接决定着作物产量、生态系统净碳收支、土壤固碳效应以及CH4和N2O排放，是农业措施碳排放的首要贡献者，也是实现集约化生产方式下低碳农业的关键驱动因子。

图1 不同集约化栽培模式下稻麦轮作生态系统CH4、N2O与农业措施碳排放 (Ei 和Eo) 温室效应百分比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 低碳农业需要从根本上注重提高农田土壤固碳效应、提高土壤生产力

近些年来，越来越多的研究开始考虑农田固碳效应[4,54]，许多研究表明稻田具有很强的固碳效应[55–56]。土壤有机碳含量的高低是农作物高产稳产的基础。国内外研究一致表明，农业管理措施如施肥、种植制度、灌溉、耕作等直接影响土壤有机碳的变化[57]；肥料、氮沉降和气候变化等也间接影响土壤有机碳库的变化[54,58]。据报道，近20年来我国大陆53%～59%的农田SOC含量呈增长趋势，30%～31%下降，4%～6%基本持平[56]。我国大陆农田表土有机碳贮量总体增加311.3～401.4 Tg，这主要归因于秸秆还田、有机肥施用和化肥投入的增加，合理的养分配比以及少 (免) 耕技术的推广。通过合理有效的农田管理措施，例如有机肥与化肥的合理配施[57]，可以调节农田土壤由碳源转变为碳汇，增强土壤固碳效应，同时提高土壤生产力[59]。然而，土壤固碳效应与氮肥施用之间的关系还存在很大的不确定性，需要针对特定的生态系统和生态环境开展长期研究[60]。生物质炭与氮肥的配合应用[61–62]则是提高农田土壤固碳效应、实现低碳农业的新趋势。

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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

公益性行业（农业）科研专项（201503106）；国家自然科学基金项目（41471192）资助。

熊正琴（1973—），女，重庆涪陵人，博士，教授，主要从事碳氮循环与生态环境研究。E-mail：zqxiong@njau.edu.cn