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温室气体浓度增加对全球气候变暖和环境变化会产生巨大影响,故有关温室气体排放问题倍受关注[1]。大气圈中CO2、CH4、N2O对温室效应的总贡献率高达80%左右[2],CH4与N2O是仅次于CO2的重要温室气体,两者增温潜势为CO2的25和298倍[3],且CH4、N2O与CO2正以每年0.6%、0.3%与0.4%的速率在增长[4],如3者继续按此速度增加在本世纪末全球地表平均温度有可能升高2~7 ℃[5]。因此,如何控制温室气体排放应是减缓气候变化的关键。
土壤是温室气体的主要排放源或吸收汇,大气中每年约有5%~20%的CO2、15%~30%的CH4和80%~90%的N2O来源于土壤[2]。森林土壤是陆地生态系统最重要的碳、氮储备库,农田土壤是温室气体的重要排放源[6],由于土壤性质不同两者温室气体排放通量有很大差异[7-8],故土地利用方式的改变是造成土壤温室气体排放通量变化的主要原因之一[2]。
退耕还林与森林采伐等土地利用方式的变化会使温室气体排放发生显著的改变[9]。现有研究结果表明:退耕还林既能降低土壤CO2排放量[10],也可能增加土壤CO2的排放量[11-12];一般会增加土壤CH4的吸收能力[13];既降低土壤N2O的排放量[13-14],也可能对其无显著影响[15]。抚育间伐是森林的主要经营方式之一,采伐能够通过改变林内微环境(土壤温度与湿度以及土壤有机质含量等),进而影响土壤温室气体的排放[16-17]。抚育间伐对土壤温室气体排放影响研究结果表明:采伐既可能降低土壤CO2排放量[18-19],也可能提高土壤CO2排放量[20-21];采伐既能够减弱土壤CH4的吸收能力[16, 22],也能够增强土壤CH4的吸收能力[23]或对其无显著影响[16];采伐既可增加土壤N2O的排放量[24],也可降低土壤N2O的排放量[25]。此外,仅有少数研究同步测定比较3种温室气体排放状况,并评价采伐对土壤增温潜势影响,如采伐对北美温带土壤增温潜势无显著影响[23]与采伐降低我国温带森林湿地土壤温室气体增温潜势[17]。因此,目前国内外有关退耕还林或森林采伐对土壤温室气体排放影响研究结果仍存在较大争议,且缺乏将退耕还林与抚育间伐或择伐经营两者结合起来探索对土壤温室气体排放及增温效果影响方面的相关研究。
土地利用变化尤其是退耕还林对土壤温室气体的影响不容忽视[26-27],目前对退耕还林的研究,大多数是退耕还林对土壤特性的影响[28-29],而退耕还林对温室气体排放的影响研究还不够完善,针对非生长季温室气体排放的研究甚少[30],退耕还林后再间伐或择伐经营对温室气体影响研究尚未见报道。本研究选择东北温带帽儿山弃耕地50年生落叶松(Larix gmelinii)人工林作为研究对象,采用静态箱取样-气相色谱分析方法,并于2015年5月至2016年4月,原位同步连续观测了不同间伐强度处理样地(未间伐、轻度间伐强度为25%、重度间伐强度为50%,强度指蓄积比及间伐试验已20年)与相应立地上农田样地的土壤3种温室气体(CO2、CH4、N2O)全年排放通量与相应环境因子(土壤温度、土壤湿度及养分含量等),以期揭示温带弃耕地造林及间伐经营对土壤温室气体排放及增温效果的影响规律,为定量评价我国退耕还林工程实施效果及经营管理退耕还林恢复森林的碳汇提供科学依据。
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研究地设在黑龙江省尚志市东北林业大学帽儿山森林生态系统定位站(45°20′ N、127°34′ E),该区属于长白山系支脉张广才岭西北部小岭的余脉,境内以山区丘陵地貌为主,平均海拔400 m。属温带大陆性季风气候,夏季温热多雨年,平均气温2.8 ℃,年平均降水量约为720 mm,秋季降温迅速,冬季漫长寒冷,早霜9月中旬,晚霜5月下旬,无霜期120 d左右。土壤以暗棕壤为主,地带性植被为红松阔叶混交林。帽儿山老爷岭生态定位站于1965年在弃耕地上营造了落叶松林(林龄50年),造林密度为2 500株/hm2,株行距为2 m×2 m,1995年对该落叶松人工林进行了不同强度带状(带宽60~80 m、带长150 m)间伐试验(未间伐为对照、轻度间伐强度为25%、重度间伐强度为50%,强度指蓄积比例),目前间伐试验地仍保存完好。
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本项研究借助1995年所建立的落叶松人工林间伐试验地,于2015年4月下旬在弃耕地(位于海拔高度相同地势平坦谷地)落叶松林未间伐为对照(造林)、轻度间伐强度为25%、重度间伐强度为50%(蓄积比)3种间伐处理样带内及附近相应立地上的农田地(玉米地)各设置3个20 m×30 m标准地,共计设置12块标准地。间伐后落叶松人工林各处理样地均为自然生长状态(未进行水、肥管理措施);农田样地(开垦于上世纪60年代初期,一直未进行水、施管理)于2015年5月中旬播种玉米,10月上旬收获,期间无水、肥管理措施。每个标准地内各设置1个静态箱,共计设置12个静态箱(4种处理3次重复),并于2015年5月上旬至2016年4月下旬(全年尺度)对各样地土壤温室气体及环境因子进行观测,以便揭示造林与间伐对弃耕地土壤温室气体排放的影响规律。各处理样地林分状况见表 1与土壤状况见表 2。
表 1 温带帽儿山弃耕地不同间伐处理落叶松人工林样地状况
Table 1. Conditions of planted larch forest under different thinning intensity treatments in Maoershen Mountains in northeastern China
样地
Sample
site密度/(株·hm-2)
Stand density/
(tree·ha-1)胸高断面积/(m2·hm-2)
Basal area at breast
height/(m2·ha-1)平均胸径
Average D
BH/cm胸径范围
Range of
DBH/cm林下灌木组成
Shrub compositionLW 2 502 52.50 16.35 3.5~25.2 卫矛Eronymus sp. LQ 1 867 48.54 18.20 5.6~28.3 暴马丁香Syringa reticulate LZ 1 283 40.77 20.12 6.2~32.8 五味子Schisandra chinesis 注:LW.未间伐样地;LQ.轻度间伐样地;LZ.重度间伐样地。下同。Notes: LW, no thinning site; LQ, mild thinning site; LZ, severe thinning site. The same below. 表 2 温带帽儿山弃耕地不同间伐处理落叶松人工林样地土壤理化性质
Table 2. Soil physicochemical property of planted larch forest under different thinning intensity treatments in Maoershan Mountains in northeastern China
土壤养分
Soil nutrient土壤层
Soil layer/cm样地Sample site NT LW LQ LZ 硝态氮 Ammonium nitrogen/(mg·L-1) 0~10 0.63±0.06a 1.11±0.37a 1.02±0.58a 2.20±0.80a 10~20 0.69±0.12a 0.62±0.52a 0.62±0.35a 1.13±0.37a 20~40 0.70±0.28a 0.35±0.14a 0.40±0.21a 0.60±0.21a 铵态氮 Nitrate nitrogen/(mg·L-1) 0~10 2.76±1.27a 3.01±0.96ab 5.01±1.59b 2.15±0.22a 10~20 3.20±1.11a 2.46±0.50ab 4.52±1.29b 2.03±0.09a 20~40 2.36±0.55a 3.84±1.00a 3.10±0.70a 2.93±1.17a 全氮 Total nitrogen/(g·kg-1) 0~10 6.60±0.43b 5.27±1.27a 4.99±0.36ab 4.79±0.33a 10~20 5.45±1.31a 3.61±1.19a 3.92±1.14a 3.92±0.34a 20~40 3.28±0.70a 2.32±0.47a 2.79±0.48a 2.55±0.58a 有机碳 Organic carbon/(g·kg-1) 0~10 61.21±5.67b 53.49±12.10ab 44.76±4.40a 44.17±5.62a 10~20 47.22±15.81a 31.51±11.34a 31.79±8.65a 31.07±2.48a 20~40 26.57±6.60b 19.46±5.08a 22.02±4.79a 18.56±5.85a pH 0~10 5.70±0.14a 5.88±0.53a 5.59±0.44a 5.71±0.16a 10~20 5.82±0.17a 5.22±0.69a 5.66±0.47a 5.67±0.08a 20~40 5.87±0.10a 5.78±0.03a 5.82±0.46a 5.60±0.51a 含水率 Soil moisture/% 0~10 33.37±1.32b 34.69±1.07b 36.63±0.76c 25.71±0.39a 10~20 32.76±0.04b 36.08±0.35c 35.54±0.45c 28.08±0.20a 20~40 33.58±0.32b 36.23±0.59c 36.63±0.39c 29.11±0.43a 温度 Temperature/℃ 0~10 5.32±0.11b 3.40±0.14a 3.59±0.30a 3.32±0.10a 10~20 5.48±0.04b 4.24±0.03a 4.14±0.19a 4.13±0.20a 20~40 5.59±0.11b 4.39±0.38a 4.50±0.27a 4.42±0.14a 注:NT.农田样地。下同。Notes: NT, farmland site. The same below. -
采用静态暗箱-气象色谱法对土壤温室气体(CO2、CH4和N2O)的采集分析,静态暗箱由不锈钢顶箱(50 cm×50 cm×50 cm)和不锈钢底座(50 cm×50 cm×10 cm)组成。为防止安插底座对样地土壤的扰动,首次取样前数天将底座插入土中10 cm,并切断底座周围根系及去除底座内植物,以确保对土壤异养呼吸的测定。采集气体样品时,各部分之间均用水密封。为减少箱内温度波动在顶箱外部设有保温材料,箱内顶部设有两个小风扇,用于使箱内的空气流通混合均匀。取样口设置在暗箱顶部直径为1 cm且内置橡胶塞。用60 mL医用注射器通过三通阀链接针头进行取样。取样时在底座的凹槽内注水密封后将顶箱插入底座凹槽内,用注射器在30 min内分别抽取0、10、20、30 min时的气体各60 mL,样品采集后装入500 mL的铝塑复合气袋中储存,并及时带入实验室,采用气相色谱仪(6820G)进行分析。取样时间为2015年5月6日至2016年4月29日的08:00—12:00,每月分上、中、下旬采样3次,共计采样36次。
气体通量(F)的计算公式[17]如下:
$$ F = \frac{M}{{{V_0}}} \cdot \frac{P}{{{P_0}}} \cdot \frac{T}{{{T_0}}} \cdot H \cdot \frac{{{\rm{dc}}}}{{{\rm{d}}\mathit{t}}} $$ (1) 式中:F为气体通量(mg/(m2·h)),正值为排放,负值为吸收;M为被测气体的摩尔质量;H为采样箱高度(cm);dc/dt为采样时气体浓度随时间变化的直线斜率;P、T为采样点的实际大气压和温度;V0、P0、T0分别为标准状态下的气体摩尔体积、标准大气压和绝对温度。
增温潜势估算:结合100年尺度上的全球增温潜势(global warming potential, GWP), CO2、CH4和N2O的增温潜势值依次为其排放总量的1、25和298倍[3],估算各处理样地的增温效果,计算公式如下:
$$ {\rm{GWP = }}\mathit{F}{\mathit{'}_{{\rm{C}}{{\rm{O}}_2}}} + F{\mathit{'}_{{\rm{C}}{{\rm{M}}_4}}} \times 25 + F{\mathit{'}_{{{\rm{N}}_2}{\rm{O}}}} \times 298 $$ (2) 式中:F′代表各温室气体年排放总量。
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在采取气体样品的同时,用T型热电偶温度探针及用TDR土壤水分速测仪(Spectrum Technologies,USA)原位同步测定空气温度(Ta),5、10、20、30和40 cm土壤层温度及体积含水率。冬季由于土壤冻结,土壤体积含水率未进行测定。用元素分析仪(Elementar Vario EL Ⅲ, 德国)测定土壤有机碳含量及全氮,用连续流动分析仪(AutoAnalyzer Ⅲ, Bran + Luebbe GmbH, Norderstedt, Germany)测定土壤硝态氮及铵态氮含量。
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采用SPSS19.0(SPSS Inc.,Chicago,Illinois,USA)和Excel 2013对数据进行统计分析,检验各处理样地土壤温室气体(CO2、CH4、N2O)排放通量的差异显著性及其与环境因子的相关性,图表绘制采用Sigmaplot12.0(Systat Software Inc.,San Jose,CA,USA)完成。
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如图 1和表 3所示,造林与间伐对帽儿山弃耕地土壤CO2排放通量的影响不同。4种处理样地土壤CO2排放通量(全年观测均值)分布在149.44~204.82 mg/(m2·h)之间,未间伐(造林)样地略高于农田样地11.6%(P>0.05),轻、重度间伐样地较农田降低11.4%和18.6%(P>0.05),且两者较未间伐样地显著降低20.6%和27.0%(P<0.05)。因此,弃耕地营造落叶松林提高了土壤CO2排放通量,间伐经营后降低了土壤CO2排放通量。
图 1 帽儿山弃耕地落叶松人工林CO2、CH4、N2O排放通量的季节变化
Figure 1. Seasonal variations of CO2, CH4 and N2O fluxes from planted larch forest and farmland in Maoershan Mountains in northeastern China
表 3 温带帽儿山弃耕地不同间伐处理落叶松人工林土壤温室气体排放通量
Table 3. Greenhouse gas fluxes of planted larch forest under different thinning intensity treatments in Maoershan Mountains in northeastern China
mg·m-2·h-1 气体
Air样地
Sample site温室气体平均通量Average flux of greenhouse gas 春季 Spring 夏季 Summer 秋季 Autumn 冬季 Winter 年均通量 Annual flux CO2 NT 234.81±41.20Cb 361.11±63.89Da 122.06±19.09Ba 15.92±2.61Aa 183.48±29.98ab LW 211.14±33.24Bb 400.41±57.30Ca 169.96±19.02Bb 37.76±12.30Ab 204.82±19.61b LQ 176.62±23.66Cab 334.81±33.44Da 116.15±33.34Ba 22.79±4.53Aa 162.59±6.23a LZ 125.88±13.27Ba 356.71±65.61Ca 99.15±10.67Ba 16.02±4.15Aa 149.44±21.39a CH4 NT -0.044±0.028ABa -0.058±0.016Aa -0.026±0.005BCb 0.002±0.003Ca -0.031±0.009a LW -0.032±0.024Aa -0.069±0.018Aa -0.066±0.016Aa 0.042±0.053Ba -0.031±0.014a LQ -0.033±0.016Aa -0.066±0.015Aa -0.053±0.007Aa 0.045±0.065Ba -0.027±0.010a LZ -0.047±0.003ABa -0.053±0.007Aa -0.031±0.008Bb 0.001±0.019Ca -0.033±0.005a N2O NT 0.113±0.099Ba 0.019±0.008ABa 0.007±0.005Aab 0.010±0.011Ab 0.037±0.030a LW 0.091±0.031Ba 0.016±0.007Aa -0.003±0.004Aa -0.005±0.043Aa 0.025±0.008a LQ 0.060±0.020Ca 0.029±0.004Ba 0.008±0.005Aab 0.005±0.001Aab 0.026±0.005a LZ 0.080±0.026Ba 0.017±0.010Aa 0.010±0.008Ab 0.007±0.005Aab 0.028±0.006a 注:表中给出的数据为平均值以及标准差,小写字母表示同一季节不同处理差异显著(P<0.05),大写字母表示同一处理不同季节差异显著(P<0.05)。Notes: data in the table are average and standard errors; different lowercase letters indicate there is a significant difference among different treatments in the same season (P<0.05); different capital letters indicate there is a significant difference among different seasons for the same treatment(P<0.05). 造林与间伐对弃耕地土壤CO2排放季节动态与季节分布也具有影响。未间伐、轻、重度间伐及农田样地土壤CO2排放季节动态趋势基本一致,均呈偏态型变化。但各处理样地土壤CO2排放峰值出现时间有所不同,未间伐与轻度间伐样地较农田提前了30~40 d,重度间伐样地较农田样地推迟了30 d。农田与轻度间伐样地土壤CO2排放通量呈现出夏>春>秋>冬的明显季节分布格局,而未间伐与重度间伐样地土壤CO2排放通量却呈现出夏>春≈秋>冬的季节分布格局。因此,营造落叶松林及对其加以间伐经营改变了弃耕地土壤CO2排放峰值的出现时间(提前或延迟)及其季节分布格局(造林与重度间伐)。
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如图 1和表 3所示,弃耕地造林与间伐对土壤CH4排放通量影响并不大。各处理样地CH4排放通量年平均值分布在-0.027~-0.033 mg/(m2·h)之间,均表现为CH4的弱吸收汇,未间伐与农田相近,轻度间伐略低于农田(-12.9%,P>0.05),重度间伐略高于农田(6.5%,P>0.05),但各处理样地间差异性均不显著。因此,温带弃耕地营造落叶松林和对其加以间伐经营对土壤CH4排放通量并无显著影响。
造林与间伐对弃耕地土壤CH4排放季节动态及季节分布却有较大影响。农田与重度间伐样地自3月中旬至11上旬吸收CH4,11月中旬至3上旬CH4排放与吸收交替发生;未间伐样地5月下旬至12月下旬吸收CH4,1月上旬至5月中旬CH4排放与吸收交替发生;轻度间伐样地4月中旬至12月中旬吸收CH4,自12月下旬至4月上旬CH4排放与吸收交替发生。此外,各样地CH4通量均呈现出春、夏、秋季吸收及冬季排放的季节分布格局,但农田与重度间伐样地CH4吸收通量呈夏>春>秋分布格局,未间伐与轻度间伐却呈春≈夏≈秋的季分布格局,且两者冬季CH4排放通量高于前两者。因此,弃耕地造林与轻度间伐使CH4吸收过程及排放与吸收交替过程延迟1~2个月,并改变了其季节分布格局。
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如图 1和表 3所示,造林与间伐对弃耕地土壤N2O排放通量具有一定程度的影响。4种处理样地土壤N2O排放通量年平均值分布在0.025~0.037 mg/(m2·h)之间,均表现为N2O弱排放源,且未间伐、轻度间伐与重度间伐依次低于农田样地32.4%、29.7%、24.3%(P>0.05),轻、重度间伐较未间伐提高4.0%和12.0%(P>0.05)。因此,温带弃耕地营造落叶松林及对其间伐经营均较大幅度降低了土壤N2O排放源强,间伐后土壤N2O源强略有提高但不显著。
4种处理样地土壤N2O排放季节动态趋势与季分布格局也有所不同。农田样地在春季开始时即达N2O排放峰值(3月中旬峰值0.356 mg/(m2·h)),随后逐渐降低,夏、秋、冬季均维持较低排放水平;而未间伐、轻度间伐与重度间伐样地在春季开始时N2O排放通量较低,随后迅速升高并于4月上旬达到各自排放峰值(0.350、0.170、0.263 mg/(m2·h)),随后迅速降低,且两个间伐样地在夏、秋、冬3季均维持低排放水平,而未间伐样地在夏季低排放,在秋、冬季呈现吸收与排放交替发生。此外,农田样地土壤N2O排放通量呈春≧夏≧秋≈冬的季节分布格局,未间伐、轻度间伐与重度间伐样地依次呈春>夏≈秋≈冬、春>夏>秋≈冬和春>夏≈秋≈冬的季分布格局。因此,弃耕地营造落叶松林与间伐经营改变了其土壤N2O排放季节动态趋势与季节分布格局。
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由表 4可以得到,造林与间伐对弃耕地土壤CO2、CH4、N2O排放通量与温度和土壤含水率相关性的影响并不同。未间伐、轻度间伐、重度间伐及农田样地土壤CO2排放通量与气温、5~40 cm土壤层温度均呈现极显著正相关;CH4排放通量与气温、5~40 cm土壤层温度均存在极显著负相关;N2O排放通量与气温、5~40 cm土壤层温度相关性均不显著。故造林与间伐并未改变弃耕地土壤CO2、CH4、N2O排放通量与温度的相关性。但造林与间伐改变了弃耕地土壤3种温室气体排放与土壤含水率的相关性,即由农田CO2与5~10 cm、N2O与20~30 cm土壤含水率显著正相关转化为不相关,农田CH4与土壤含水率不相关转化为轻度间伐与10 cm、重度间伐与5~10 cm显著正/负相关。可见,造林与间伐对弃耕地土壤CO2、CH4、N2O排放与温度相关性无影响,但对3者与土壤湿度相关性的有所改变。
表 4 间伐前后弃耕地落叶松人工林土壤温室气体排放通量与温度、湿度的相关性(温度为全年、湿度为生长季)
Table 4. Correlation between greenhouse gas fluxes and soil temperature and moisture of planted larch forest under different thinning intensity treatments
气体
Air样地
Sample
site气温
Air
temperature土壤温度 Soil temperature 土壤含水率 Soil moisture 5 cm 10 cm 20 cm 30 cm 40 cm 5 cm 10 cm 20 cm 30 cm 40 cm CO2 NT 0.91** 0.92** 0.91** 0.88** 0.85** 0.83** 0.55** 0.59** 0.16 0.12 0.06 LW 0.81** 0.86** 0.86** 0.87** 0.85** 0.83** -0.16 0.13 0.34 0.30 0.21 LQ 0.75** 0.84** 0.84** 0.85** 0.83** 0.81** 0.36 0.43 0.32 0.14 0.16 LZ 0.69** 0.82** 0.83** 0.85** 0.86** 0.85** 0.17 0.08 -0.17 -0.25 -0.29 CH4 NT -0.71** -0.68** -0.66** -0.63** -0.62** -0.59** -0.36 -0.38 0.02 0.04 0.23 LW -0.54** -0.63** -0.63** -0.61** -0.62** -0.62** 0.14 0.29 0.26 0.32 0.37 LQ -0.53** -0.56** -0.55** -0.52** -0.50** -0.47** 0.32 0.45* 0.22 0.20 0.20 LZ -0.62** -0.58** -0.57** -0.55** -0.51** -0.49** -0.46* -0.50* -0.43 -0.31 -0.24 N2O NT 0.12 -0.08 -0.09 -0.13 -0.15 -0.17 0.40 0.40 0.56** 0.51* 0.21 LW 0.21 0.02 0.04 -0.07 -0.10 -0.13 -0.08 0.26 0.38 0.32 0.30 LQ 0.29 0.12 0.11 0.05 0.01 -0.03 0.19 0.14 0.14 0.06 0.02 LZ 0.19 -0.04 -0.03 -0.10 -0.13 -0.15 0.06 0.01 -0.13 -0.12 -0.16 注:**表示在0.01水平(双侧)上相关;*表示在0.05水平(双侧)上相关。Notes: ** means extremely significant correlation at P<0.01 level(double side); * means significant correlation at P<0.05 level(double side). -
由表 5可以得到,造林与间伐经营对弃耕地土壤3种温室气体源/汇功能影响程度不同。4种处理样地土壤CO2年排放量分布在13.21~18.07 t/(hm2·a),均表现为CO2排放源,未间伐略高于农田11.8%(P>0.05),轻、重度间伐低于农田10.9%~18.3%(P>0.05),两者也低于未间伐20.3%~26.7%(重度间伐降低显著);CH4年排放量分布在-2.34~-2.86 kg/(hm2·a),均表现为CH4吸收汇,未间伐与轻、重度间伐较农田样地略有增减但差异性均不显著(-14.9%~4.0%,P >0.05);N2O年排放量分布在2.16~3.29 kg/(hm2·a),均表现为N2O排放源,未间伐与轻、重度间伐样地低于农田样地23.7%~34.3%(P>0.05)。因此,造林与间伐并未引起弃耕地土壤3种温室气体源/汇功能发生转化,仅对3者源/汇强度产生了影响,即造林使其土壤CO2源强有所提高,间伐则使其CO2源强有所降低,造林与间伐均使N2O源强有较大幅度的降低,但两者对CH4汇强影响相对较弱。
表 5 温带帽儿山弃耕地不同间伐处理落叶松人工林土壤温室气体排放总量及GWP值
Table 5. Fluxes and GWP of greenhouse gas of planted larch forest under different thinning intensity treatments in Maoershan Mountains in northeastern China
样地
Sample
siteCO2 CH4 N2O GWP总和/
(t·hm-2·a-1)
Total GWP/
(t·ha-1·yr-1)排放总量/
(t·hm-2·a-1)
Total flux/
(t·ha-1·yr-1)GWPCO2/
(t·hm-2·a-1)
GWPCO2/
(t·ha-1·yr-1)排放总量/
(kg·hm-2·a-1)
Total flux/
(kg·ha-1·yr-1)GWPCH4/
(t·hm-2·a-1)
GWPCH4/
(t·ha-1·yr-1)排放总量/
(kg·hm-2·a-1)
Total flux/
(kg·ha-1·yr-1)GWPN2O/
(t·hm-2·a-1)
GWPN2O/
(t·ha-1·yr-1)NT 16.17±2.67ab 16.17±2.67ab -2.75±0.81a -0.07±0.02a 3.29±2.71a 0.98±0.81a 17.08±3.33ab LW 18.07±1.73b 18.07±1.73b -2.78±1.21a -0.07±0.02a 2.16±0.68a 0.64±0.20a 18.64±1.73b LQ 14.41±0.52ab 14.41±0.52ab -2.34±0.89a -0.06±0.02a 2.26±0.46a 0.67±0.14a 15.02±0.61ab LZ 13.21±1.90a 13.21±1.90a -2.86±0.38a -0.07±0.01a 2.51±0.55a 0.75±0.60a 13.89±2.06a 注:小写字母表示不同处理差异显著(P<0.05)。GWPCO2、GWPCH4、GWPN2O分别为CO2、CH4和N2O的增温潜势值(即CO2、CH4和N2O排放总量的1、25和298倍)。Notes: different lowercase letters indicate there is a significant difference among different treatments (P<0.05). GWPCO2, GWPCH4 and GWPN2O mean global warming potential (GWP) of CO2, CH4 and N2O, respectively (1, 25 and 298 times of CO2, CH4 and N2O total fluxes). 造林与间伐经营对弃耕地土壤温室气体增温潜势产生了较大影响。4种处理样地土壤温室气体增温潜势分布在13.89~18.64 t/(hm2·a),未间伐样地较农田样地提高9.1%(P>0.05),轻、重度间伐样地却较农田样地降低12.1%~18.7%(P>0.05),且轻度间伐较未间伐降低19.4%(P>0.05),重度间伐较未间伐显著降低25.5%(P < 0.05)。因此,东北温带帽儿山弃耕地营造落叶松林提高了土壤温室气体增温潜势,对其加以间伐经营后则降低了其土壤温室气体增温潜势。
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本研究得到的东北温带弃耕地营造落叶松林50年后提高土壤CO2排放量与现有结论退耕还林增加我国亚热带与暖温带及欧洲南部温带土壤CO2排放量[11, 13, 31]相一致;但间伐(轻、重度间伐)20年后降低土壤CO2排放量与择伐初期提高我国东北阔叶红松混交林土壤CO2排放量[19, 32]并不相一致(人工林与天然林对采伐干扰的响应会不同,间伐20年后的长期效果也会不同于初期效果)。前者提高的原因可能在于退耕还林后树木根系和凋落物增加导致其土壤呼吸高于农田[11, 13], 后者降低的原因可能是由于间伐引起树木根系、凋落物及土壤微生物的减少限制了土壤呼吸速率[33-34]。本研究中的间伐样地生长季0~40 cm土壤层平均温度低于农田2.5~3.5 ℃,也可能限制了土壤微生物分解活性,导致土壤呼吸速率有所下降。
弃耕地造林及间伐改变土壤CO2排放峰值出现时间及其季节分布格局的原因可能在于造林后植被类型发生变化,导致群落内微环境不同[12, 35]所致,本研究中弃耕地造林与间伐样地在春季0~40 cm各土壤层平均温度低于农田2.1~4.6 ℃,致使春季土壤呼吸速率低于农田,夏季土壤温度相对较高(13.2~16.7 ℃),对土壤呼吸限制作用相对较弱,加之林地储存大量凋落物的分解,春末夏初林地迅速达到CO2排放峰值,故导致土壤CO2排放季节动态与分布格局发生改变。
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东北温带弃耕地营造落叶松林与间伐经营对CH4吸收汇影响均不显著。前者与北美及欧洲温带农田转化为森林土壤CH4吸收汇增加[13, 36]不同;后者与德国南部温带云杉林择伐后土壤CH4通量无显著变化[16]一致,但与择伐显著降低温带次生林土壤CH4吸收汇[37]和增加温带森林土壤CH4吸收汇[23]并不一致。这些差异性可能与各研究区气候、土壤及林型不同有关,也可能与间伐时间有关,本研究为间伐20年后的中长期效果,势必会不同于间伐后的短期效果。
至于弃耕地造林与间伐改变土壤CH4排放的季节动态趋势及分布格局的原因,可能与土壤冻融交替引起有氧环境与厌氧环境交替密切相关,如土壤解冻期间会释放出大量的CH4[38],冻融期土壤表层微生物活性增强[39],并能够促进凋落物的分解[40]。弃耕地营造落叶松林(50年后)与轻度间伐(间伐20年后)样地林冠层茂密遮阴强烈,春季升温缓慢,土壤解冻晚于农田,秋季降温慢,土壤结冻也晚于农田,故两者的土壤冻融交替过程也就不同于农田,使两者CH4吸收过程及排放与吸收交替过程较农田延迟1~2个月,并改变了其季节分布格局。而重度间伐样地林冠层稀疏遮阴或保温作用相对较弱,对土壤冻融交替过程的影响并不大,故其CH4排放的季节动态趋势及分布格局与农田相似。
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东北温带弃耕地营造落叶松林与间伐经营均降低了土壤N2O排放通量,且间伐后落叶松林土壤N2O源强略有提高。前者与现有退耕还林降低土壤N2O排放通量[13, 41]的研究结论相一致,后者与热带雨林采伐后N2O排放量增加[24]相一致。弃耕地造林降低土壤N2O排放量的原因可能是由于造林后不再继续施用氮肥影响土壤氮的有效性[10, 42],土壤碳氮比升高,土壤厌氧微生物的减少,也可能使土壤N2O的排放量降低[13]。间伐后由于林内土壤温度升高,增强了土壤的硝化作用,促进了N2O的排放[43-44],而且间伐所造成的树木根系死亡也可能加大土壤N2O的排放[17, 45]。
弃耕地造林与间伐同样改变了土壤N2O排放季节动态趋势及分布格局,特别是各样地在春季融冻期土壤N2O排放量迅速升高随后呈现下降趋势,这与北美阔叶林融冻期土壤N2O排放的研究结果[46]相一致,可能是由于在融冻期土壤水分呈现“固态-液态”频繁交替的状态,这种状态有利于土壤硝化作用和反硝化作用同时进行,促进土壤N2O的排放[47-48]。本研究中由于农田地空旷春季土壤升温较林地快,初春即进入冻融期并达到N2O排放峰值,随即开始呈降低趋势,而林地因林冠层遮阴升温慢,冻融过程迟缓,故土壤N2O排放呈先升高至峰值后再降低趋势。
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东北温带弃耕地造林与间伐对土壤CO2、CH4、N2O排放通量与气温、土壤温度的相关性并未产生影响,即各样地土壤CO2排放与气温及土壤温度显著正相关,CH4通量与土壤温度呈现极显著负相关,土壤N2O通量与温度均无显著相关性,这与目前有关温室气体主控因子影响的研究结果[17, 31, 49]相一致。其原因在于土壤呼吸主要来源于植物根系自养呼吸与土壤微生物异养呼吸,而这两个生物学过程直接受土壤温度影响[50];土壤CH4通量取决于甲烷产生菌与甲烷氧化菌综合作用,两者也受土壤温度的影响,低温会限制甲烷氧化菌活性,温度升高甲烷吸收能力随之增强[51];土壤N2O通量来源于土壤硝化作用与反硝化作用,两者的最适宜温度范围在25~35 ℃或30~67 ℃[41]。本研究中各样地的土壤年平均温度相对较低(2.9~6.6 ℃),远未达到其最适宜温度范围,可能也是土壤N2O通量与温度相关性不显著的原因之一。
农田土壤CO2、N2O通量与土壤含水率显著正相关,这与现有农田土壤温室气体排放的研究结果[52]一致,弃耕地造林与间伐后土壤CO2、N2O排放通量与含水率无显著相关性,与大西洋温带森林及我国温带阔叶红松混交林采伐前后土壤CO2通量与含水率无相关性[19, 23]及东北次生林土壤N2O通量与含水率无相关性[53]的研究结果一致。其原因可能是本研究中弃耕地造林和间伐后土壤含水率(24.5%~37.0%)对土壤微生物活动比较适宜,并不是其主要限制因子。
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东北温带帽儿山弃耕地营造落叶松林50年后提高了土壤温室气体增温潜势,对其加以间伐经营20年后则降低土壤温室气体增温潜势。前者与我国亚热带农田转化为森林降低土壤温室气体增温潜势[54]不一致;后者与采伐对加拿大温带森林土壤温室气体增温潜势无影响[23]也不一致,但与采伐(择伐与皆伐)降低我国东北温带森林湿地土壤温室气体增温潜势[17]相一致。其原因在于温带帽儿山弃耕地营造落叶松林提高了占土壤温室气体增温潜势主体的CO2源强(占增温潜势的96.9%)(表 3),尽管也降低了土壤N2O源强及对土壤CH4汇强无显著影响,但因两者仅占次要地位,故对土壤温室气体增温潜势影响并不大。对其进行间伐经营后不仅降低了土壤CO2源强,而且也较大幅度地降低了土壤N2O源强,加之对土壤CH4汇强无显著影响,故间伐较大幅度地降低了土壤温室气体增温潜势。
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1) 东北温带弃耕地营造落叶松人工林50年后提高了土壤CO2排放量(11.6%),较大幅度地降低了土壤N2O排放量(32.4%),但对土壤CH4吸收量几乎无影响;间伐(轻、重度)经营20年后则降低了土壤CO2排放量(11.4%~18.6%)和N2O排放量(24.3%~29.7%),对土壤CH4吸收量影响也不大。
2) 东北温带弃耕地营造落叶松林与间伐经营对土壤CO2、CH4、N2O排放通量的季节动态趋势及季节分布格局均产生了一定程度的影响,但并未改变土壤CO2、CH4、N2O排放与气温、土壤温度的相关性,却改变了3者排放通量与土壤湿度的相关性。
3) 东北温带弃耕地营造落叶松林50年后使其土壤温室气体增温潜势(18.64 t/(hm2·a))较农田(17.08 t/(hm2·a))提高9.1%(P>0.05),间伐(轻度与重度)20年后则使土壤温室气体增温潜势(13.89~15.02 t/(hm2·a))较农田降低12.1%~18.7%(P>0.05),较未间伐林分降低19.4%~25.5%(重度间伐降低显著)。因此,从减缓温室效应角度考虑,建议东北温带弃耕地营造落叶松林后采取强度间伐方式(50%左右)比较适宜。
Long-term effects of afforestation and thinning on greenhouse gas emissions from temperate abandoned-land soil in the Northeast of China
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摘要: 采用静态箱-气象色谱法,测定不同间伐强度温带弃耕地落叶松人工林(未间伐为对照、轻度间伐强度为25%、重度间伐强度为50%,林龄50年及间伐已20年)及相应立地上农田的土壤温室气体(CO2、CH4和N2O)排放年通量与相关环境因子(土壤温度、湿度及养分含量等),揭示造林与间伐对弃耕地土壤温室气体排放的影响规律,以便为定量评价退耕还林工程实施效果提供依据。结果表明:1)土壤CO2年均排放通量(149.44~204.82 mg/(m2·h))呈现未间伐>农田>轻度间伐>重度间伐的变化趋势,未间伐较农田提高11.6%,轻、重度间伐较农田降低11.4%~18.6%,较未间伐显著降低20.6%~27.0%;2)土壤CH4吸收通量(-0.027~-0.033 mg/(m2·h))呈现重度间伐>未间伐=农田>轻度间伐变化趋势,未间伐与农田相同,轻度间伐较农田降低12.9%,重度间伐较农田提高6.5%;3)土壤N2O排放通量(0.025~0.037 mg/(m2·h))呈现农田>重度间伐>轻度间伐>未间伐的变化趋势,未间伐较农田降低32.4%,轻、重度间伐较农田降低24.3%~29.7%;4)温带弃耕地造林与间伐经营并未改变土壤CO2、CH4、N2O排放通量与气温和土壤温度的相关性,但改变了3种温室气体与土壤湿度的相关性;5)土壤增温潜势(13.89~18.64 t/(hm2·a))呈现未间伐>农田>轻度间伐>重度间伐的变化趋势,未间伐较农田提高9.1%,轻、重度间伐较农田降低12.1%~18.7%,两者也较未间伐降低19.4%~25.5%。因此,东北温带弃耕地营造落叶松林提高了土壤增温潜势,间伐经营较大幅度降低了土壤增温潜势,故从控制气候变暖考虑对其采取强度间伐(50%)方式比较适宜。Abstract: CO2, CH4 and N2O annual emission fluxes from larch plantations (50 years old) on abandoned farmland under different thinning intensities (no thinning, contrast; mild thinning, 25% and severe thinning, 50%, tinning operation has been done for 20 years) and farmland on the corresponding site were measured by the static chamber method in temperate Maoershan Mountains in northeastern China to reveal the long-term effects of afforestation and thinning on greenhouse gas emissions from abandoned-land soil. The results showed that: 1) CO2 emission flux (149.44-204.82 mg/(m2·h)) took on a trend of no thinning > farmland > mild thinning > severe thinning, which increased by 11.6% at no thinning site than farmland site, yet they decreased by 11.4%-18.6% at mild and severe thinning sites compared with farmland site, and both also decreased by 20.6%-27.0% significantly compared with no thinning. 2) CH4 fluxes (-0.027-0.033 mg/(m2·h)) showed a trend of severe thinning > no thinning = farmland > mild thinning, there was no significant difference between no thinning site and farmland site, but it decreased by 12.9% at mild thinning site, and increased by 6.5% at severe thinning site than farmland site. 3) N2O emission fluxes (0.025-0.037 mg/(m2·h)) presented a trend of farmland > severe thinning > mild thinning > no thinning, which decreased by 32.4% at no thinning site, and decreased by 24.3%-29.7% at mild and severe thinning sites than farmland site. 4) the correlation between CO2, CH4 and N2O emission fluxes and the air temperature and soil temperature were not changed by afforestation and thinning, but the correlation among three kinds of greenhouse gases and soil moisture were changed. 5) The global warming potential (GWP) (13.89-18.64 t/(ha·yr)) showed the trend of no thinning > farmland > severe thinning > mild thinning, which increased by 9.1% at no thinning site, and decreased by 12.1%-18.7% at mild and severe thinning sites than farmland site, both also decreased by 19.4%-25.5% compared with no thinning site. Therefore, afforestation increased the global warming potential on the temperate abandoned land in northeastern China, thinning greatly reduced the global warming potential, so severe intensity thinning (50%) should be more suitable for larch plantation on temperate abandoned land to control the climate warming.
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Key words:
- temperate abandoned land /
- larch plantation /
- greenhouse gas emission /
- thinning effects
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表 1 温带帽儿山弃耕地不同间伐处理落叶松人工林样地状况
Table 1. Conditions of planted larch forest under different thinning intensity treatments in Maoershen Mountains in northeastern China
样地
Sample
site密度/(株·hm-2)
Stand density/
(tree·ha-1)胸高断面积/(m2·hm-2)
Basal area at breast
height/(m2·ha-1)平均胸径
Average D
BH/cm胸径范围
Range of
DBH/cm林下灌木组成
Shrub compositionLW 2 502 52.50 16.35 3.5~25.2 卫矛Eronymus sp. LQ 1 867 48.54 18.20 5.6~28.3 暴马丁香Syringa reticulate LZ 1 283 40.77 20.12 6.2~32.8 五味子Schisandra chinesis 注:LW.未间伐样地;LQ.轻度间伐样地;LZ.重度间伐样地。下同。Notes: LW, no thinning site; LQ, mild thinning site; LZ, severe thinning site. The same below. 表 2 温带帽儿山弃耕地不同间伐处理落叶松人工林样地土壤理化性质
Table 2. Soil physicochemical property of planted larch forest under different thinning intensity treatments in Maoershan Mountains in northeastern China
土壤养分
Soil nutrient土壤层
Soil layer/cm样地Sample site NT LW LQ LZ 硝态氮 Ammonium nitrogen/(mg·L-1) 0~10 0.63±0.06a 1.11±0.37a 1.02±0.58a 2.20±0.80a 10~20 0.69±0.12a 0.62±0.52a 0.62±0.35a 1.13±0.37a 20~40 0.70±0.28a 0.35±0.14a 0.40±0.21a 0.60±0.21a 铵态氮 Nitrate nitrogen/(mg·L-1) 0~10 2.76±1.27a 3.01±0.96ab 5.01±1.59b 2.15±0.22a 10~20 3.20±1.11a 2.46±0.50ab 4.52±1.29b 2.03±0.09a 20~40 2.36±0.55a 3.84±1.00a 3.10±0.70a 2.93±1.17a 全氮 Total nitrogen/(g·kg-1) 0~10 6.60±0.43b 5.27±1.27a 4.99±0.36ab 4.79±0.33a 10~20 5.45±1.31a 3.61±1.19a 3.92±1.14a 3.92±0.34a 20~40 3.28±0.70a 2.32±0.47a 2.79±0.48a 2.55±0.58a 有机碳 Organic carbon/(g·kg-1) 0~10 61.21±5.67b 53.49±12.10ab 44.76±4.40a 44.17±5.62a 10~20 47.22±15.81a 31.51±11.34a 31.79±8.65a 31.07±2.48a 20~40 26.57±6.60b 19.46±5.08a 22.02±4.79a 18.56±5.85a pH 0~10 5.70±0.14a 5.88±0.53a 5.59±0.44a 5.71±0.16a 10~20 5.82±0.17a 5.22±0.69a 5.66±0.47a 5.67±0.08a 20~40 5.87±0.10a 5.78±0.03a 5.82±0.46a 5.60±0.51a 含水率 Soil moisture/% 0~10 33.37±1.32b 34.69±1.07b 36.63±0.76c 25.71±0.39a 10~20 32.76±0.04b 36.08±0.35c 35.54±0.45c 28.08±0.20a 20~40 33.58±0.32b 36.23±0.59c 36.63±0.39c 29.11±0.43a 温度 Temperature/℃ 0~10 5.32±0.11b 3.40±0.14a 3.59±0.30a 3.32±0.10a 10~20 5.48±0.04b 4.24±0.03a 4.14±0.19a 4.13±0.20a 20~40 5.59±0.11b 4.39±0.38a 4.50±0.27a 4.42±0.14a 注:NT.农田样地。下同。Notes: NT, farmland site. The same below. 表 3 温带帽儿山弃耕地不同间伐处理落叶松人工林土壤温室气体排放通量
Table 3. Greenhouse gas fluxes of planted larch forest under different thinning intensity treatments in Maoershan Mountains in northeastern China
mg·m-2·h-1 气体
Air样地
Sample site温室气体平均通量Average flux of greenhouse gas 春季 Spring 夏季 Summer 秋季 Autumn 冬季 Winter 年均通量 Annual flux CO2 NT 234.81±41.20Cb 361.11±63.89Da 122.06±19.09Ba 15.92±2.61Aa 183.48±29.98ab LW 211.14±33.24Bb 400.41±57.30Ca 169.96±19.02Bb 37.76±12.30Ab 204.82±19.61b LQ 176.62±23.66Cab 334.81±33.44Da 116.15±33.34Ba 22.79±4.53Aa 162.59±6.23a LZ 125.88±13.27Ba 356.71±65.61Ca 99.15±10.67Ba 16.02±4.15Aa 149.44±21.39a CH4 NT -0.044±0.028ABa -0.058±0.016Aa -0.026±0.005BCb 0.002±0.003Ca -0.031±0.009a LW -0.032±0.024Aa -0.069±0.018Aa -0.066±0.016Aa 0.042±0.053Ba -0.031±0.014a LQ -0.033±0.016Aa -0.066±0.015Aa -0.053±0.007Aa 0.045±0.065Ba -0.027±0.010a LZ -0.047±0.003ABa -0.053±0.007Aa -0.031±0.008Bb 0.001±0.019Ca -0.033±0.005a N2O NT 0.113±0.099Ba 0.019±0.008ABa 0.007±0.005Aab 0.010±0.011Ab 0.037±0.030a LW 0.091±0.031Ba 0.016±0.007Aa -0.003±0.004Aa -0.005±0.043Aa 0.025±0.008a LQ 0.060±0.020Ca 0.029±0.004Ba 0.008±0.005Aab 0.005±0.001Aab 0.026±0.005a LZ 0.080±0.026Ba 0.017±0.010Aa 0.010±0.008Ab 0.007±0.005Aab 0.028±0.006a 注:表中给出的数据为平均值以及标准差,小写字母表示同一季节不同处理差异显著(P<0.05),大写字母表示同一处理不同季节差异显著(P<0.05)。Notes: data in the table are average and standard errors; different lowercase letters indicate there is a significant difference among different treatments in the same season (P<0.05); different capital letters indicate there is a significant difference among different seasons for the same treatment(P<0.05). 表 4 间伐前后弃耕地落叶松人工林土壤温室气体排放通量与温度、湿度的相关性(温度为全年、湿度为生长季)
Table 4. Correlation between greenhouse gas fluxes and soil temperature and moisture of planted larch forest under different thinning intensity treatments
气体
Air样地
Sample
site气温
Air
temperature土壤温度 Soil temperature 土壤含水率 Soil moisture 5 cm 10 cm 20 cm 30 cm 40 cm 5 cm 10 cm 20 cm 30 cm 40 cm CO2 NT 0.91** 0.92** 0.91** 0.88** 0.85** 0.83** 0.55** 0.59** 0.16 0.12 0.06 LW 0.81** 0.86** 0.86** 0.87** 0.85** 0.83** -0.16 0.13 0.34 0.30 0.21 LQ 0.75** 0.84** 0.84** 0.85** 0.83** 0.81** 0.36 0.43 0.32 0.14 0.16 LZ 0.69** 0.82** 0.83** 0.85** 0.86** 0.85** 0.17 0.08 -0.17 -0.25 -0.29 CH4 NT -0.71** -0.68** -0.66** -0.63** -0.62** -0.59** -0.36 -0.38 0.02 0.04 0.23 LW -0.54** -0.63** -0.63** -0.61** -0.62** -0.62** 0.14 0.29 0.26 0.32 0.37 LQ -0.53** -0.56** -0.55** -0.52** -0.50** -0.47** 0.32 0.45* 0.22 0.20 0.20 LZ -0.62** -0.58** -0.57** -0.55** -0.51** -0.49** -0.46* -0.50* -0.43 -0.31 -0.24 N2O NT 0.12 -0.08 -0.09 -0.13 -0.15 -0.17 0.40 0.40 0.56** 0.51* 0.21 LW 0.21 0.02 0.04 -0.07 -0.10 -0.13 -0.08 0.26 0.38 0.32 0.30 LQ 0.29 0.12 0.11 0.05 0.01 -0.03 0.19 0.14 0.14 0.06 0.02 LZ 0.19 -0.04 -0.03 -0.10 -0.13 -0.15 0.06 0.01 -0.13 -0.12 -0.16 注:**表示在0.01水平(双侧)上相关;*表示在0.05水平(双侧)上相关。Notes: ** means extremely significant correlation at P<0.01 level(double side); * means significant correlation at P<0.05 level(double side). 表 5 温带帽儿山弃耕地不同间伐处理落叶松人工林土壤温室气体排放总量及GWP值
Table 5. Fluxes and GWP of greenhouse gas of planted larch forest under different thinning intensity treatments in Maoershan Mountains in northeastern China
样地
Sample
siteCO2 CH4 N2O GWP总和/
(t·hm-2·a-1)
Total GWP/
(t·ha-1·yr-1)排放总量/
(t·hm-2·a-1)
Total flux/
(t·ha-1·yr-1)GWPCO2/
(t·hm-2·a-1)
GWPCO2/
(t·ha-1·yr-1)排放总量/
(kg·hm-2·a-1)
Total flux/
(kg·ha-1·yr-1)GWPCH4/
(t·hm-2·a-1)
GWPCH4/
(t·ha-1·yr-1)排放总量/
(kg·hm-2·a-1)
Total flux/
(kg·ha-1·yr-1)GWPN2O/
(t·hm-2·a-1)
GWPN2O/
(t·ha-1·yr-1)NT 16.17±2.67ab 16.17±2.67ab -2.75±0.81a -0.07±0.02a 3.29±2.71a 0.98±0.81a 17.08±3.33ab LW 18.07±1.73b 18.07±1.73b -2.78±1.21a -0.07±0.02a 2.16±0.68a 0.64±0.20a 18.64±1.73b LQ 14.41±0.52ab 14.41±0.52ab -2.34±0.89a -0.06±0.02a 2.26±0.46a 0.67±0.14a 15.02±0.61ab LZ 13.21±1.90a 13.21±1.90a -2.86±0.38a -0.07±0.01a 2.51±0.55a 0.75±0.60a 13.89±2.06a 注:小写字母表示不同处理差异显著(P<0.05)。GWPCO2、GWPCH4、GWPN2O分别为CO2、CH4和N2O的增温潜势值(即CO2、CH4和N2O排放总量的1、25和298倍)。Notes: different lowercase letters indicate there is a significant difference among different treatments (P<0.05). GWPCO2, GWPCH4 and GWPN2O mean global warming potential (GWP) of CO2, CH4 and N2O, respectively (1, 25 and 298 times of CO2, CH4 and N2O total fluxes). -
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