OSID：
甲烷(CH4)、氧化亚氮(N2O)、二氧化碳(CO2)等温室气体增多引起的全球变暖对生态可持续性和粮食生产安全构成了重大挑战[1]。农业生产活动是温室气体排放的最主要途径之一[2],约占全球人为温室气体排放总量的23%。在100年时间尺度上,CH4和N2O全球增温潜势分别为CO2的25倍和298倍[3]。因此,减少农业生产中的温室气体排放对于减缓温室效应、确保生态可持续性和粮食生产安全至关重要[4]。
农业生产中影响温室气体排放的因素众多,主要包括气候条件、土壤性质、田间管理措施以及田间作物特性[5]。农作物秸秆是农业活动的重要副产品,含有对作物生长至关重要的氮、磷、钾等营养物质[6]。中国农作物秸秆年产量约占世界总产量的1/4[7],秸秆还田作为中国农业的一项基本实践,其采用率稳步上升,2010—2015年秸秆直接还田率达到61%[8]。秸秆还田主要是通过机械粉碎或覆盖将秸秆重新返还到农田中[9],长期试验研究结果表明,秸秆还田能够增加土壤有机碳和养分含量[6,10],降低土壤容重,改善土壤结构[11],还会显著影响农田温室气体的产生和排放。秸秆还田进入土壤后,作为有机质来源会导致土壤CO2排放量增加[12],还田秸秆分解刺激有机碳矿化也能够增加CO2排放[13]。部分研究则表明,秸秆覆盖能够通过减少土壤干扰和延缓土壤碳分解来减少CO2排放[14]。秸秆还田增加了碳输入,为产CH4活动提供了底物,同时创造了有利于增加CH4排放的厌氧环境[15-16]。部分研究则表明,在土壤湿度低的干旱地区,可能不会形成严格的厌氧条件,导致甲烷氧化菌活性增强,从而促进CH4负排放[17]。秸秆分解过程中产生的土壤无机氮和水溶性有机碳可为硝化反硝化作用提供充足底物,同时增强氧气消耗形成厌氧环境从而促进反硝化作用及N2O排放[18-19]。部分研究则表明,秸秆会固化土壤中的有效氮,减少硝化和反硝化细菌的氮源,从而减少N2O排放[20]。
学者就秸秆还田对农田温室气体排放的影响开展了大量试验研究,但由于气候条件、土壤性质和田间管理措施的不同,区域尺度结论尚未见报道。因此,本研究采用Meta分析方法,综合分析不同气候条件、土壤性质和田间管理措施下秸秆还田对中国小麦农田土壤温室气体排放的影响,以期为优化秸秆管理实践和减少农田生态系统温室气体排放提供参考。
通过Web of Science、ScienceDirect和中国知网等数据库,检索“秸秆还田(straw returning)、温室气体(greenhouse gas)、氧化亚氮(nitrous oxide或N2O)、二氧化碳(carbon dioxide或CO2)、甲烷(methane或CH4)”等关键词,收集筛选2024年10月以前发表的关于秸秆还田对中国小麦农田土壤温室气体排放影响的研究论文。筛选标准如下:①试验区域位于中国,田间管理措施明确。②小麦田间试验(不包括盆栽和防雨试验)。③试验需同时包含秸秆还田和不还田处理,每个处理3个重复。④文中至少提供1种温室气体数据,以及平均值和标准差(或标准误差)。基于以上筛选标准,收集了来自国内已发表的403篇中英文文献,在其中筛选出49篇文献,共计216组数据,其中N2O排放数据127组,CO2排放数据47组,CH4吸收数据42组。筛选出的文献研究地点分布在华北地区(18篇)、西北地区(16篇)、长江中下游地区(11篇)、西南地区(4篇),基本涵盖了中国小麦主产区。从筛选出的文献中试验研究时间分布情况看,2000年前1篇,2000—2010年12篇,2011—2020年28篇,2021—2024年8篇。
鉴于中国不同农业区域气候条件、土壤性质和田间管理措施的差异,秸秆还田对小麦农田土壤温室气体排放的影响可能存在差异。因此,将获取的数据进行分类统计(表1),分类标准如下:气候条件(年降雨量、年均气温);土壤性质(土壤有机碳、土壤全氮含量);田间管理措施(氮肥施用量、耕作类型、还田年限、还田方式、还田比例)。
表1 数据分类
Tab.1 Data classification
一级因子二级因子分组1234气候条件年降雨量/mm0～500500～1000>1000年均气温/℃0～1010～15>15土壤性质土壤有机碳含量(质量比)/(g·kg-1)0～1010～1515～20>20土壤全氮含量(质量比)/(g·kg-1)0～0.90.9～1.5>1.5氮肥施用量/(kg·hm-2)0～120120～180180～240>240耕作类型免耕翻耕旋耕田间管理措施还田年限/a0～33～7≥7还田方式覆盖还田掺入还田还田比例全部还田部分还田
1.3.1 标准差计算
当筛选的文献中提供有温室气体排放数据标准差(SD)时,则直接使用该值;若文献中无标准差,但提供有标准误差(SE)和样本重复次数m时,则采用公式计算,计算式为
(1)
若文献中标准差和标准误差均未提供,则假设标准差为平均值的1/10[21]。
1.3.2 效应值计算与整合
利用各研究中秸秆还田处理(试验组)和秸秆不还田(对照组)处理下小麦生长季农田土壤温室气体平均排放量、其对应的标准差、重复次数,采用生态学中常用的对数响应比作为效应值lnR[22],每个效应值权重因子wi是其方差的倒数。
所有秸秆还田与不还田处理平均效应值估计为
(2)
式中 lnRi——秸秆还田相对不还田处理的效应值
95%置信区间CI计算式为
CI=lnR++±1.96ElnR++
(3)
其中
(4)
获得的95%置信区间若包括0,表示秸秆还田对小麦农田土壤温室气体排放无显著影响(P>0.05);若大于0,表示秸秆还田会显著增加小麦农田土壤温室气体排放(P<0.05);若小于0,表示秸秆还田会显著减少小麦农田土壤温室气体排放(P<0.05);以上原则对亚组分析同样适用。
为更加直观地反映秸秆还田对小麦农田土壤温室气体排放的影响,将效应值转换为秸秆还田相对于秸秆不还田条件下温室气体排放量变化率Z,计算式为[23]
Z=(exp(lnR++)-1)×100%
(5)
1.3.3 发表偏倚检验
运用图形分析法绘制频率分布直方图检验所收集筛选数据的正态符合性。采用罗森博格失安全数检验文献是否存在发表偏倚,当失安全数大于5n+10(n为样本量)时,认为无发表偏倚,反之存在发表偏倚。
采用GetData Graph Digitizer 2.26软件提取以图像形式呈现的温室气体排放量数据,Excel 2016收集与处理数据,MetaWin 2.1软件进行Meta分析和发表偏倚检验,Origin 2022软件作图,利用R中的“随机森林”包来评估各因子对温室气体排放的重要性。
秸秆还田对小麦农田土壤N2O排放、CO2排放和CH4吸收效应值频数分布见图1,研究数据通过正态性分布检验。发表偏倚检验结果表明,秸秆还田下土壤N2O排放、CO2排放和CH4吸收的罗森博格失安全系数分别为25 251.2、156 173.8和2 311.5,均大于5n+10,故不存在发表偏倚。分别对土壤N2O排放、CO2排放和CH4吸收进行效应值计算,结果表明,与秸秆不还田相比,秸秆还田可使N2O排放量显著增加15.50%(P<0.05),CO2排放量显著增加10.68%(P<0.05),CH4吸收量显著增加26.45%(P<0.05)(图2,误差线右侧数字为样本数,误差线表示95%置信区间,若误差线与虚线相交,表示秸秆还田和不还田处理之间差异不显著(P>0.05),下同)。
图1 N2O排放、CO2排放和CH4吸收对秸秆还田效应值的频数分布
Fig.1 Frequency distributions of effect value of N2O emission,CO2 emission and CH4 uptake in response to straw returning
图2 秸秆还田下N2O排放、CO2排放和CH4吸收综合效应
Fig.2 Overall response of change in N2O emissions,CO2 emissions and CH4 uptake for straw returning
不同气候条件下秸秆还田对农田土壤温室气体排放效应的影响见图3。在不同年降雨量下,秸秆还田均增加了农田土壤N2O和CO2排放,且排放效应值随年降雨量的增加而减小;CH4吸收效应值随年降雨量的增加而增大。当年降雨量为0～500 mm时,与秸秆不还田相比,秸秆还田下土壤N2O和CO2排放显著增加19.94%和23.47%(P<0.05),95%置信区间分别为13.08%～28.09%和10.58%～39.92%;秸秆还田对土壤CH4吸收影响不显著。当年降雨量为500～1 000 mm时,秸秆还田下土壤N2O、CO2排放量显著增加18.82%和12.66%(P<0.05),95%置信区间分别为14.57%～24.46%和10.01%～15.37%;秸秆还田对土壤CH4吸收影响不显著。当年降雨量大于1 000 mm时,秸秆还田下土壤 N2O 和CO2排放量增加最少,CH4吸收量增加最多,分别增加5.02%、9.88%和381.63%,95%置信区间分别为-12.11%～25.50%、7.62%～11.15%和201.41%～669.60%。秸秆还田下各年降雨量亚组之间N2O和CO2排放效应没有显著差异,而CH4吸收效应在年降雨量大于1 000 mm时显著大于500～1 000 mm和0～500 mm。
图3 不同年降雨量和年均气温下秸秆还田温室气体排放效应
Fig.3 Effects of different precipitation and temperature on greenhouse gas emissions from straw returning to field
随着年均气温的增加,秸秆还田下土壤N2O排放效应值减小,CO2排放效应值和CH4吸收效应值增大。当年均气温为0～10℃时,与秸秆不还田相比,秸秆还田下土壤N2O排放量显著增加34.14%(P<0.05),95%置信区间为18.82%～51.45%;秸秆还田抑制了土壤CO2排放和CH4吸收,与不还田处理差异不显著,在此气温条件下秸秆还田增加土壤CO2排放最少。当年均气温为10～15℃时,秸秆还田下土壤N2O和CO2排放量显著增加18.41%和13.76%(P<0.05),95%置信区间分别为12.60%～25.17%和11.03%～16.56%;秸秆还田抑制了土壤对CH4吸收,与不还田处理差异不显著。当年均气温大于15℃时,秸秆还田下土壤N2O排放增加最少且与不还田处理差异不显著;CO2排放量、CH4吸收量显著增加18.13%和119.20%(P<0.05),95%置信区间分别为7.62%～32.21%和38.43%～247.09%。秸秆还田下各年均气温亚组之间N2O和CO2排放效应没有显著差异,而CH4吸收效应在年均气温大于15℃时显著大于10～15℃。
不同土壤性质下秸秆还田对农田土壤温室气体排放效应的影响见图4。秸秆还田下土壤N2O排放效应值随土壤有机碳含量的增加而减小,而CH4吸收效应值随土壤有机碳含量的增加而增大;与秸秆不还田相比,当土壤有机碳含量小于20 g/kg时,秸秆还田均增加了农田土壤N2O和CO2排放。当土壤有机碳含量为0～10 g/kg时,秸秆还田下土壤N2O和CO2排放量显著增加71.57%和26.29%(P<0.05),95%置信区间分别为43.88%～104.60%和10.92%～43.79%,秸秆还田对CH4吸收影响不显著。当土壤有机碳含量为10～15 g/kg时与0～10 g/kg结果相似。当土壤有机碳含量为15～20 g/kg时,秸秆还田下土壤N2O和CO2排放量增加19.29%和29.62%,95%置信区间分别为-6.75%～52.61%和9.57%～53.33%;CH4吸收量显著减少82.68%(P<0.05),95%置信区间为-55.71%～-93.22%。当土壤有机碳含量大于20 g/kg时,秸秆还田下土壤N2O排放量最少且显著减少14.63%(P<0.05),CH4吸收量最多且显著增加150.48%(P<0.05),95%置信区间分别为-26.77%～-1.26%和44.83%～333.19%。
图4 不同土壤有机碳和全氮含量下秸秆还田温室气体排放效应
Fig.4 Effects of different soil organic carbon and total nitrogen contents on greenhouse gas emissions from straw returning to field
与秸秆不还田相比,当土壤全氮含量为0～0.9 g/kg时,秸秆还田下土壤N2O和CO2排放量显著增加26.97%和9.47%(P<0.05),95%置信区间分别为14.58%～40.71%和6.40%～12.61%;秸秆还田对CH4吸收影响不显著。当土壤全氮含量为0.9～1.5 g/kg时,秸秆还田下土壤N2O排放量增加17.80%,CO2排放量显著增加29.30%(P<0.05),95%置信区间分别为-0.48%～39.43%和21.02%～38.17%;秸秆还田对CH4吸收影响不显著。当土壤全氮含量大于1.5 g/kg时,秸秆还田下土壤CH4吸收量显著增加132.89%(P<0.05),95%置信区间为67.45%～193.09%,对N2O和CO2排放影响不显著。
不同田间管理措施下秸秆还田对农田土壤温室气体排放效应的影响见图5。秸秆还田下土壤N2O排放效应值随氮肥施用量增加而减小,CO2排放效应值随氮肥施用量增加先增大后减小。与秸秆不还田相比,当氮肥施用量为0～120 kg/hm2时,秸秆还田下土壤N2O排放量增加最多,显著增加N2O排放量达62.53%(P<0.05),95%置信区间为32.13%～99.95%。当氮肥施用量为120～180 kg/hm2时,秸秆还田下土壤CO2排放增加最多,显著增加CO2排放量达17.52%(P<0.05),95%置信区间为9.94%～25.60%。当施氮量大于240 kg/hm2时,秸秆还田下土壤CH4吸收增加最多,显著增加CH4吸收量达69.33%(P<0.05),95%置信区间为10.76%～224.04%。
图5 不同氮肥施用量、耕作类型、还田年限、还田方式和还田比例下秸秆还田温室气体排放效应
Fig.5 Effects of different nitrogen application amount,tillage types,returning years,returning types and returning proportions on greenhouse gas emissions from straw returning to field
与秸秆不还田相比,在不同耕作类型下秸秆还田均增加了农田土壤N2O排放,其中免耕条件下秸秆还田对土壤N2O排放影响不显著;耕作类型为翻耕和旋耕时,秸秆还田下土壤N2O排放量分别显著增加47.27%和12.89%(P<0.05),95%置信区间分别为23.08%～76.23%和0.59%～26.21%。耕作类型为免耕时,秸秆还田下土壤CO2排放量显著减少10.81%(P<0.05),95%置信区间为-18.73%～-2.13%;耕作类型为翻耕和旋耕时,秸秆还田下土壤CO2排放量分别显著增加29.71%和15.23%(P<0.05),95%置信区间分别为22.59%～31.94%和12.42%～18.10%。耕作类型为免耕时,秸秆还田下土壤CH4吸收量显著增加91.00%(P<0.05),95%置信区间为7.85%～238.24%;耕作类型为翻耕和旋耕时,秸秆还田对土壤CH4吸收影响不显著。
与秸秆不还田相比,当还田年限为0～3 a时,土壤CO2排放量显著增加14.21%(P<0.05),95%置信区间为11.44%～17.07%,对土壤N2O排放和CH4吸收影响不显著。当还田年限为3～7 a时,土壤N2O和CO2排放量分别显著增加38.21%和12.01%(P<0.05),95%置信区间分别为19.28%～60.14%和9.67%～14.40%,对CH4吸收影响不显著。当还田年限大于7 a时,土壤N2O排放量显著增加20.19%(P<0.05),95%置信区间为2.63%～48.28%,对土壤CO2排放和CH4吸收影响不显著。
与秸秆不还田相比,秸秆覆盖还田下土壤CO2排放量和CH4吸收量分别显著增加7.89%和202.04%(P<0.05),95%置信区间分别为4.70%～11.16%和26.83%～619.37%;土壤N2O排放量增加10.20%,95%置信区间为-6.21%～29.49%,呈不显著的增加效应。秸秆掺入还田下土壤N2O、CO2排放量分别显著增加26.16%和21.82%(P<0.05),95%置信区间分别为13.13%～40.69%和17.26%～26.57%;秸秆掺入还田下土壤CH4吸收量增加3.05%,95%置信区间为-17.77%～29.14%,呈不显著的增加效应。秸秆部分还田下CO2排放量和CH4吸收量分别显著增加9.95%和121.64%(P<0.05),95%置信区间分别为4.24%～15.99%和4.46%～370.35%;土壤N2O排放量减少11.33%,95%置信区间为-30.86%～13.41%,呈不显著的减少效应。秸秆全部还田下土壤N2O、CO2排放量显著增加27.95%和14.00%(P<0.05),95%置信区间分别为15.77%～41.43%和10.75%～17.33%;土壤CH4吸收量增加15.95%,95%置信区间为-17.92%～43.02%,呈不显著的增加效应。
秸秆还田下,气候条件、土壤性质和田间管理措施对土壤N2O、CO2排放和CH4吸收均有一定影响,各因素对温室气体排放影响的相对重要性见图6。对土壤N2O排放影响较大的前3位因素分别为年降雨量(15.75%)、土壤全氮含量(11.43%)和土壤有机碳含量(9.80%),对CO2排放影响较大的前3位因素分别为土壤有机碳含量(17.11%)、土壤全氮含量(15.91%)和年降雨量(11.47%),对CH4吸收影响较大的前3位因素分别为年降雨量(13.54%)、土壤全氮含量(11.19%)和氮肥施用量(8.52%)。还田年限、还田方式、还田比例和耕作措施对秸秆还田下温室气体排放的影响相对较弱。
图6 秸秆还田下各因素对温室气体排放影响的相对重要性
Fig.6 Relative importance of each factor on greenhouse gas emissions from returning straw to field
年降雨量和年均气温可以影响土壤温度、含水率、孔隙度和微生物活动,进而影响秸秆分解和温室气体排放[24]。本研究中,当年均气温大于15℃时,秸秆还田对N2O排放的影响不显著。这是由于在高温地区,秸秆还田的降温作用会减缓微生物活动速率,进而减缓秸秆氮的释放,导致秸秆还田对N2O排放的影响降低。然而,当气温小于等于15℃时,秸秆还田通常表现为对土壤的保温作用,从而促进了N2O排放。当土壤温度大于等于10℃时,与秸秆不还田相比,秸秆还田对CO2排放的影响差异显著,虽然秸秆具有一定降温作用,但秸秆的输入增加了土壤碳含量,进而促进了CO2排放[25]。土壤温度增加促进了作物残体和有机物分解,最终增加了CO2排放[26]。土壤表面温度升高也可能增强硝化细菌活性,导致CO2排放。当年均气温大于15℃时,秸秆还田显著增加了土壤对CH4的吸收效应[27-28]。本研究表明,秸秆还田下降雨量对土壤N2O排放具有重要的影响(图6),当年降雨量为0～500 mm时,秸秆还田会产生更多的N2O排放(图3),这可能是由于与不还田相比,在降雨较少地区,秸秆还田保水效果更好,促进了反硝化作用,从而增加了N2O排放[29-31]。
不同土壤质地表现出不同的透气性和可耕性,而不同土壤性质,如土壤有机碳或全氮,对异养反硝化细菌非常重要,从而影响土壤温室气体排放[28]。本研究中,当土壤有机碳含量较低时,秸秆还田下土壤N2O排放显著增加(图4)。土壤有机碳效应值与土壤初始有机碳含量有关,此外,秸秆还田可直接提供有机碳,土壤微生物可将这些有机质转化为可供作物利用的养分,提高土壤有机碳总含量[32]。土壤有机碳在NH3和NO3同化过程中是硝化和反硝化微生物的关键底物,高土壤有机质含量促进异养硝化和反硝化,从而产生N2O[33]。秸秆还田下土壤CO2排放显著增加,这可能是由于秸秆还田直接增加了土壤有机碳,而土壤有机碳对土壤温室气体排放有重要影响(图6),有机碳直接增加了土壤微生物呼吸的底物有效性,促进了土壤微生物新陈代谢,并且秸秆还田增大了土壤孔隙,提高了土壤通透性,使得土壤有机质与土壤空气中的氧气充分接触,有机质分解速度增加,导致土壤呼吸速率增加[34]。此外,土壤有机碳含量显著影响土壤微生物丰度和活性,土壤微生物可以调节CH4吸收。
当氮肥施用量为0～180 kg/hm2时,秸秆还田显著增加了土壤N2O排放(图5),这可能是因为在适当施氮量下,秸秆掺入破坏了原有的物质和能量平衡,在土壤中造成更多分离和通风不良的缺氧微环境,这些条件增强了土壤硝化和反硝化作用,导致土壤N2O排放增加[35]。氮肥施用下,秸秆还田均显著增加了土壤CO2排放。氮肥施用量为0～120 kg/hm2时,CO2排放增加量低于其他氮肥施用量亚组,这是因为CO2排放与作物残茬分解、根系呼吸和有机碳矿化有关,氮肥施用量过低可能抑制土壤胞外酶活性、真菌生物量、细菌生物量和根系生物量,从而抑制土壤CO2排放[36-37]。本研究表明,氮肥施用量为0～240 kg/hm2时,秸秆还田对CH4吸收影响不显著,氮肥施用量大于240 kg/hm2时秸秆还田才显著增加土壤对CH4的吸收(图5),这可能是由于N的投入促进了作物生长、氧化和甲烷营养细菌活性,从而增加了CH4消耗[38]。然而,也有研究表明秸秆也可能将更多的作物碎屑带入土壤,提高碳水平和甲烷菌活性,并增加CH4排放[39]。
不同耕作类型下秸秆还田对温室气体排放的影响也存在差异,有研究表明免耕能显著增加温室气体排放[26,40],主要原因是免耕增加了土壤微生物活性,增强土壤异养呼吸[41-42],但也有研究表明免耕可以减少温室气体排放[43-44]。本研究中耕作类型为免耕时秸秆还田下N2O排放增加量低于翻耕和旋耕,但三者之间没有显著差异(图5),这与PETERSEN等[45]的研究结果一致。然而也有研究表明,与免耕相比翻耕和旋耕显著增加了秸秆还田下土壤N2O排放[46],原因是翻耕和旋耕在一定程度上改善了土壤透气透水性,并可能将土壤从厌氧状态转变成富氧状态,促使土壤的干湿循环变换过程加快,使土壤内营养物质和微生物混合更加均匀,加深了硝化反应程度,并且使产生的N2O更容易向外释放[47]。产生不一致结果的原因可能是在厌氧环境下反硝化反应增强以及硝化反应减弱的程度也会因为受到其他各因素(尤以施肥为最)的影响而发生改变,N2O排放变化的最终情况取决于其增强和减弱的程度[48]。本研究表明,秸秆还田下免耕比翻耕和旋耕减少了CO2排放、增加了土壤对CH4的吸收,这与之前的研究结果一致[49],主要由于免耕为作物残体腐殖质化提供了有利环境,并提高了植物生物量碳稳定速率[50]。此外,秸秆还田结合免耕措施还增加了土壤团聚体稳定性,降低酶和微生物对有机碳的分解,从而减少了有机碳矿化和CO2排放[51-52],而且部分秸秆在土壤表面发生好氧降解,其降解产物在土壤氧化层中还原为CH4的可能性较小[52-53]。
有研究表明,秸秆还田对土壤N2O排放有一定的抑制作用[54]。秸秆还田后的前1～2 a,向土壤中添加秸秆作为碳源,促进固氮,增加土壤总氮,同时减少N2O排放[55-57]。随着秸秆还田时间延长,土壤总氮含量持续增加,可能为氮转化提供更多的基质[58-59],并逐渐降低秸秆还田对N2O排放的抑制作用。本研究表明,短期秸秆还田对N2O排放有抑制作用但不显著,但长期秸秆还田促进了N2O排放(图5)。短期秸秆还田显著增加了CO2排放,长期秸秆还田下CO2排放和CH4吸收与秸秆不还田相比无显著差异,这与JIANG等[60]的研究结果一致。长期秸秆还田改变了土壤结构和质量,尤其是增加土壤团聚体,可以保护土壤有机碳(SOC)不被分解,从而降低CO2和CH4排放量[61]。其次,长期秸秆投入能提高土壤肥力,促进藻类生长,进而增加溶解氧浓度[62]。另外,长期秸秆投入促进植物良性快速生长,加快氧气输送到根际土壤,促进CH4氧化细菌生长[63]。
秸秆全部还田会形成局部厌氧环境,促进反硝化作用,增加土壤N2O排放[64]。与秸秆覆盖还田相比,秸秆掺入还田加速秸秆分解并消耗氧气,促进硝化反硝化作用,增加了土壤N2O排放[65-66]。本研究中,与秸秆不还田相比,秸秆掺入还田和全部还田下土壤对CH4的吸收增加不显著,这与HUANG等[64]的研究结论一致,原因可能是秸秆掺入还田和秸秆全部还田虽较秸秆覆盖和秸秆部分还田增加了甲烷菌底物,但硝化和反硝化的C、N反应底物也会增加,秸秆的好氧降解也呈增加趋势,从而增加土壤CO2的排放效应,但对CH4吸收效应没有显著影响[54,67]。
(1)与秸秆不还田相比,秸秆还田可使N2O排放量显著增加15.50%(P<0.05),CO2排放量显著增加10.68%(P<0.05),CH4吸收量显著增加26.45%(P<0.05)。
(2)当年降雨量大于1 000 mm时,秸秆还田下土壤N2O和CO2排放量增加最少,CH4吸收量增加最多。随着年均气温的增加,秸秆还田下土壤N2O排放效应值减小,CO2排放效应值和CH4吸收效应值增大。
(3)当土壤有机碳含量为0～10 g/kg时,秸秆还田下土壤N2O、CO2排放量显著增加71.57%和26.29%(P<0.05);当土壤有机碳含量大于20 g/kg时,秸秆还田对土壤N2O排放具有抑制作用,对CH4吸收具有促进作用。
(4)秸秆还田下土壤N2O排放效应值随氮肥施用量增加而减小,CO2排放效应值随氮肥施用量增加先增大后减小。免耕条件下秸秆还田显著减少了土壤CO2排放,增加了CH4吸收。秸秆部分还田减少了土壤N2O排放,全部还田增加了N2O排放。秸秆部分还田和覆盖还田显著增加了CO2排放和CH4吸收。因此,实现秸秆还田下温室气体排放的有效控制需综合考虑气候条件、土壤性质和田间管理措施等。
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