氮素类型和剂量对寒温带针叶林土壤N2O排放的影响

耿 静,程淑兰,方华军,于贵瑞,徐敏杰,王 磊,李晓玉,司高月,何 舜

1 中国科学院地理科学与资源研究所,生态系统观测与模拟重点实验室, 北京 100101 2 中国科学院大学资源与环境学院, 北京 100049

氮素类型和剂量对寒温带针叶林土壤N2O排放的影响

耿 静1,2,程淑兰2,方华军1,*,于贵瑞1,徐敏杰2,王 磊1,李晓玉1,司高月2,何 舜2

1 中国科学院地理科学与资源研究所,生态系统观测与模拟重点实验室, 北京 100101 2 中国科学院大学资源与环境学院, 北京 100049

大气氮沉降；土壤N2O通量；氮素有效性；主控因子；北方森林

氧化亚氮(N2O)是地球大气中三大温室气体之一,百年尺度上单分子N2O的增温潜势(GWP)是CH4和CO2的21倍和298倍,对全球变暖的贡献约占6%[1]。同时,对流层中的N2O通过扩散进入平流层,与O3反应生成NO破坏臭氧层,增加地面紫外辐射量[2]。IPCC第五次报告表明,2011年大气中N2O浓度为324 ppb,较工业化前的数值高出20%,近几十年平均增幅为0.25%/a。大气N2O的源估计为17.7 Tg N/a,自然源(农业、水体、生物燃烧等)和人为源(自然植被土壤、海洋)分别占37.8%和62.2%；大气N2O的汇估计为12.6 Tg N/a,主要在平流层被光解为NOx,最终转化成硝酸和硝酸盐等反应性氮[3]。其中,60%—70%的大气反应性氮以干湿沉降形式到达地表,导致当前全球大气氮沉降量高达105 Tg N/a[4-5],显著改变陆地生态系统碳、氮循环,降低生物多样性,进而影响陆地生态系统的结构和功能[6]。

北方森林(Boreal forests)是仅次于热带森林的第二大森林群区,占全球陆地面积的14.5%,其土壤碳密度平均为296 t C/hm2[17]；此外,由于该区温度较低,土壤氮素矿化缓慢,土壤有效氮极其匮乏,对外源性氮素响应十分敏感[18]。研究表明,外源性氮素输入会显著改变北方森林植物和微生物群落组成[19]、土壤碳氮转化与温室气体排放[20]、生态系统生产力和固碳潜力[21]。过去普遍认为,水热条件较好、土壤风化强烈的热带/亚热带森林土壤N2O排放较高,而温度较低、有效氮贫乏的高纬度地区森林土壤N2O排放量可以忽略不计[22-23]。然而,近年来一些研究发现,由于气温升高和氮沉降增加提高了高纬度地区森林、苔原等自然生态系统氮素的可利用性,导致该区土壤也大量排放N2O[24-25]。长期以来对高纬度地区自然植被土壤N2O排放的忽视,可能是导致全球N2O收支研究中诸多不确定性的原因之一[26]。大兴安岭寒温带针叶林是北方森林的南缘,面积占全国森林面积的29%,有关土壤N2O对外源性氮素输入尚未有实验报道。

1 材料与方法

1.1 研究区概况

研究区位于内蒙古大兴安岭森林生态系统国家野外科学观测研究站以东的开拉气林场(50°20′—50°30′N, 121°45′—122°00′E),属大兴安岭西北坡,海拔826 m。该地区是寒温带半湿润气候,冬季寒冷漫长,夏季凉爽多雨。年均气温-5.4℃,最高温出现在7月,最低温在1月。年降水量450—550 mm,其中60%集中于5—9月。年均日照2594 h,全年地表蒸发量800—1200 mm,无霜期80 d。该区主要物种为兴安落叶松(Larixgmelini)、白桦(Betulaplatyphylla)、杜鹃(Rhododendronsimsii)、杜香(Ledumpalustre)、红豆越橘(VacciniumVitisidaea)等。研究区的植被类型为杜香-落叶松林,林龄约150a。土壤类型为发育于花岗岩残积物上的棕色针叶林土,土壤腐殖质含量10%—30%,pH值为4.5—6.5。

1.2 试验设计

1.3 土壤N2O排放通量监测

土壤N2O排放量采用静态箱-气相色谱法测定,测定时段为2013年生长季(5—10月)。在每个样地中分别设置带槽的底座(50 cm×50 cm×10 cm)和盖箱(50 cm×50 cm×20 cm),在测定时,将槽内灌满水,打开风扇的电源,然后小心地把带有温度计和小风扇的盖箱沿槽放入。在40 min 时间段内,每隔10 min用100mL注射器抽取1次气样,同时记录大气温度、箱内温度和地下5 cm的温度值。测定N2O时气相色谱的柱箱温度为55 ℃,检测器ECD的温度为250℃；载气(干空气及高纯H2)流量分别为300 mL/min和50 mL/min,尾吹气(N2)流量为10 mL/min。利用土壤水分仪(TDR200,Spectrum Technologies, USA)测定10 cm土壤体积含水量。用气相色谱仪(7890A,Agilent,USA)分析N2O气体浓度,利用下述公式计算土壤N2O气体通量：

(1)

式中,Qt为t时刻N2O的排放通量(μg N m-2h-1)；V为箱体的体积(m3)；A为取样时箱体所覆盖的面积(m2)；Ta为取样时的大气温度(K)；P为取样时的大气压值(kPa)；ΔC为Δt时间内箱体内N2O浓度增量(ppb)；Δt为时间变化量(s)。

1.4 土壤采集与分析

1.5 统计分析

利用重复测定方差分析(RANOVA)比较不同施氮水平和施氮类型对土壤温度、含水量、无机氮含量和土壤N2O通量的影响,利用Tukey′s HSD进行均值间的多重比较。采用一元和多元逐步回归分析方法探讨土壤N2O通量与土壤环境因子之间的关系。所有数据利用SPSS 16.0软件进行分析,利用SigmaPlot 12.5软件进行绘图。

2 结果与分析

2.1 土壤温度和水分

整个生长季,土壤5cm温度季节变化显著,整体上呈现单峰季节变化(表1,P< 0.001)。对照处理土壤温度最高值和最低值分别出现在7月初和5月初,平均变化范围为0.70—15.03℃(图1)。不同施氮处理下,土壤温度变化格局相似,增氮对土壤温度无明显影响(表1)。

表1 月份、施氮水平、施氮类型对土壤N2O通量、土壤温度、水分和无机氮含量影响的重复测量方差分析

Table 1 Repeated measures ANOVA of effects of month, N level and N form on soil N2O fluxes, soil temperature, soil moisture and inorganic N contents

变异来源Sourceofvariance土壤温度Soiltemperature土壤水分Soilmoisture土壤N2O通量SoilN2Oflux土壤NO-3-N含量SoilNO-3-Ncontent土壤NH+4-N含量SoilNH+4-NcontentO层OlayerM层MlayerO层OlayerM层Mlayer组内差异Withinsubjects(Multivariate)月份Month<0.001<0.001<0.0010.120.002<0.001<0.001月份×施氮水平Month×Nlevel0.980.020.180.430.160.710.06月份×施氮类型Month×Nform0.840.110.330.320.090.480.33组间差异Betweensubjects施氮水平Nlevel0.790.030.0030.050.14<0.001<0.001施氮类型Nform<0.0010.040.010.160.12<0.0010.003

图1 土壤温度和水分的季节变化及其对增氮的响应Fig.1 The seasonal variations and responses of soil temperature and soil moisture to N additionCK：对照 control；LAC：低氮氯化铵 low-NH4Cl；LPN：低氮硝酸钾 low-KNO3；LAN：低氮硝酸铵 low-NH4NO3；MAC：中氮氯化铵 medium-NH4Cl；MPN：中氮硝酸钾 medium-KNO3；MAN：中氮硝酸铵 medium-NH4NO3；HAC：高氮氯化铵 high-NH4Cl；HPN：高氮硝酸钾 high-KNO3；HAN：高氮硝酸铵 high-NH4NO3

整个生长季0—10 cm层土壤含水量季节波动明显,呈逐渐递减的趋势(图1)。由于5、6月份土壤处于冻融期,土壤含水量较高,5月初对照处理土壤含水量值最高(27.48%)。秋季降水明显减少,8月中旬和9月末土壤含水量较低,最低值为5.86%。就某个月份而言,施氮水平对土壤含水量有显著影响(表1,P=0.02)。施氮水平和施氮类型均显著改变了土壤体积含水量(表1,P=0.03,P=0.04)。

2.2 土壤N2O通量

整个生长季土壤N2O排放通量季节变化显著(表1,P< 0.001)。除7月份外,其他月份土壤N2O排放均很低,对照处理土壤N2O通量变化范围为-1.19—5.13 μg N m-2h-1。施氮后土壤N2O排放急剧增加,7月份出现明显的排放峰(47.77μg N m-2h-1)(图2)。施氮水平和施氮类型均对土壤N2O有极显著的影响(表1,P=0.003,P=0.01)。随着增氮水平增加,土壤N2O排放量逐渐增加。就施氮类型而言,KNO3和NH4NO3的促进效应显著高于NH4Cl,说明硝态氮比铵态氮肥对土壤N2O排放量的影响更为显著。与对照相比,施加NH4NO3对土壤N2O排放的促进效应最强,不同施氮剂量处理土壤N2O通量的增幅度为442%—677%。

图2 土壤N2O通量的季节变化及其对增氮的响应Fig.2 The seasonal variations and responses of soil N2O fluxes to N addition

2.3 土壤无机氮含量

2.4 土壤N2O通量与土壤变量之间的关系

土壤变量Soilvariables回归方程Equation决定系数R2P土壤温度Soiltemperature(Ts)FN2O=0.68+0.37Ts0.040.0012有机层NH+4-N含量NH+4-NcontentinOlayer(NH+4-NO)FN2O=1.24+0.01NH+4-NO0.040.0053多元回归MultipleregressionFN2O=-7.51+0.47Ts+0.011NH+4-NO+0.85LN+2.37MN+2.86HN0.27<0.001

3 讨论

3.1 施氮类型和剂量对土壤无机氮累积的影响

3.2 施氮类型和剂量对土壤N2O通量的影响

4 结论

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The effects of types and doses of nitrogen addition on soil N2O flux in a cold-temperate coniferous forest, northern China

GENG Jing1, 2, CHENG Shulan2, FANG Huajun1,*, YU Guirui1, XU Minjie2, WANG Lei1, LI Xiaoyu1, SI Gaoyue2, HE Shun2

1KeyLaboratoryofEcosystemNetworkObservationandModeling,InstituteofGeographicalSciencesandNaturalResourcesResearch,ChineseAcademyofSciences,Beijing100101,China2CollegeofResourcesandEnvironment,UniversityofChineseAcademyofSciences,Beijing100049,China

atmospheric N deposition; soil N2O flux; N availability; controlling factors; boreal forest

国家自然科学基金项目(41471212, 31470558, 31290221, 31130009, 31290222)；国家重点基础研究发展计划项目(2012CB417103)；中国科学院地理科学与资源研究所“秉维”优秀青年人才基金项目(2011RC202)；中国科学院战略性先导科技专项(XDA05050600)

2015-08-04;

日期:2016-06-13

10.5846/stxb201508041639

* 通讯作者Corresponding author.E-mail: fanghj@igsnrr.ac.cn

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Geng J, Cheng S L, Fang H J, Yu G R, Xu M J, Wang L, Li X Y, Si G Y, He S.The effects of types and doses of nitrogen addition on soil N2O flux in a cold-temperate coniferous forest, northern China.Acta Ecologica Sinica,2017,37(2):395-404.