火干扰对小兴安岭白桦沼泽温室气体排放的短期影响

利用静态箱-气相色谱法,研究了火干扰对小兴安岭白桦沼泽生长季CH4、CO2和N2O排放的季节变化、源/汇功能的影响,以及其与环境因子的关系.结果表明:轻度火干扰使白桦沼泽的气温和地表温度升高1.8℃～3.9℃,水位下降6.3 cm;重度火干扰使气温和0～40 cm土壤温度升高1.4 ℃～3.8℃,水位下降33.9 cm.轻度火烧、未火烧样地CH4呈春季吸收、夏秋季排放,重度火烧样地则呈春夏季吸收、秋季排放;未火烧样地CO2呈夏季>秋季≈春季,火干扰样地呈夏季>秋季>春季;未火烧样地N2O呈春季>夏季>秋季,轻度火烧样地呈秋季>春季>夏季,重度火烧样地呈夏季>秋季≈春季.未火烧样地CO2通量与气温、地表温度呈显著正相关,轻度火烧样地CO2通量与气温、5～10 cm土壤温度和水位呈显著正相关,重度火烧样地CO2通量与5～40 cm土壤温度呈显著正相关.火干扰使其CHM4排放量提高169.5%(轻度)或转变为弱吸收汇(重度),CO2和N2O排放量分别下降21.2%～34.7%和65.6%～95.8%,全球温暖潜势下降22.9%～36.6%.火干扰能够减少白桦沼泽温室气体排放,湿地管理实践中可适当开展有计划的火烧.     

小兴安岭阔叶林沼泽土壤CO_2、CH_4和N_2O排放规律及其影响因子

采用野外静态箱-气相色谱法,研究了小兴安岭典型阔叶林沼泽生长季节土壤CO2、CH4和N2O排放季节变化规律、源/汇功能及主要影响因子。结果表明:①苔草沼泽、毛赤杨沼泽和白桦沼泽生长季节土壤CO2、CH4、N2O排放分别集中在夏季、夏秋季、春夏季,平均排放通量依次为487.89、382.27、514.63 mg.m-2.h-1,1.88、1.03、0.04 mg.m-2.h-1,3.70、58.61、11.73μg.m-2.h-1。②三者生长季节土壤CO2排放通量与气温和0-20 cm土壤温度均呈显著正相关;苔草沼泽CH4排放通量与30-40cm土壤温度呈显著正相关,毛赤杨沼泽CH4排放通量与地表温度呈显著负相关;白桦沼泽N2O排放通量与地表温度呈显著正相关。苔草沼泽N2O排放与水位呈显著负相关;毛赤杨沼泽CH4排放与水位呈显著正相关;白桦沼泽CO2排放与水位呈显著负相关。③三者生长季节土壤均为CO2、CH4、N2O排放源(17.56、13.76、18.53 t.hm-2;67.54、37.05、1.30 kg.hm-2;0.13、2.11、0.42 kg.hm-2),三者CO2排放量相近(5.5%-21.6%);苔草沼泽为CH4的强排放源,毛赤杨沼泽为中排放源,白桦沼泽为弱排放源;毛赤杨沼泽为N2O的强排放源,白桦沼泽为中排放源,苔草沼泽为弱排放源。     

火干扰对小兴安岭毛赤杨沼泽温室气体排放动态影响及其影响因素

利用静态箱-气相色谱法,对小兴安岭轻度火烧毛赤杨沼泽CH4、CO2、N2O生长季排放通量进行研究.结果表明:火烧使毛赤杨沼泽生长季CH4排放通量提高485.2％,CO2和N2O排放通量分别下降45.5％、24.8％.火烧未改变CH4季节性排放规律,但改变了CO2、N2O季节性变化规律.火烧样地CH4排放通量与土壤15cm温度间存在显著正相关性关系而与水位相关性不显著,火烧样地CO2排放与土壤0-30 cm温度呈显著或极显著正相关,与水位极显著负相关.对照样地CO2排放通量与土壤0-15 cm温度呈显著或极显著正相关,与水位极显著负相关,火烧使毛赤杨沼泽CH4排放源的强度增强,CO2、N2O的排放消弱,全球温室潜势下降约为43.34％.     

N2O排放规律及其影响因子

采用野外静态箱-气相色谱法,研究了小兴安岭典型阔叶林沼泽生长季节土壤CO2、CH4和N2O排放季节变化规律、源/汇功能及主要影响因子。结果表明：①苔草沼泽、毛赤杨沼泽和白桦沼泽生长季节土壤CO2、CH4、N2O排放分别集中在夏季、夏秋季、春夏季,平均排放通量依次为514.63、487.89、382.27 mgm-2h-1,1.88、1.03、0.04 mgm-2h-1,58.61、11.73、3.70µgm-2h-1。②三者生长季节土壤CO2排放通量与气温和0~20 cm土壤温度均呈显著正相关;苔草沼泽CH4排放通量与30~40 cm土壤温度呈显著正相关,毛赤杨沼泽CH4排放通量与地表温度呈显著负相关;白桦沼泽N2O排放通量与地表温度呈显著正相关。苔草沼泽N2O排放与水位呈显著负相关;毛赤杨沼泽CH4排放与水位呈显著正相关;白桦沼泽CO2排放与水位呈显著负相关。③三者生长季节土壤均为CO2、CH4、N2O排放源（17.56、13.76、18.53 thm-2;67.54、37.05、1.30 kghm-2;0.13、2.11、0.42 kg.hm-2）,三者CO2排放量相近（5.5%~21.6%）;苔草沼泽为CH4的强排放源,毛赤杨沼泽为中排放源,白桦沼泽为弱排放源;毛赤杨沼泽为N2O的强排放源,白桦沼泽为中排放源,苔草沼泽为弱排放源。     

火干扰对小兴安岭草丛、灌丛沼泽温室气体短期排放的影响

利用静态箱-气相色谱法,研究了火烧干扰对小兴安岭草丛、灌丛沼泽生长季CH4、CO2、N2O排放的季节变化及影响因子结果表明:火干扰使草丛、灌丛沼泽生长季的平均气温和各层土壤温度提高0.1—2.0℃,水位平均下降2.7 cm。火干扰使草丛、灌丛沼泽样地CH4排放通量提高了56%、524.9%,CO2排放通量分别下降了57.3%、14.5%,N2O排放通量分别下降27.1%,64.9%。火烧前后草丛沼泽CH4、N2O与灌丛沼泽CO2排放通量季节性规律未发生变化。火干扰改变了草丛沼泽生长季CO2、灌丛沼泽N2O排放通量的季节性变化规律。草丛沼泽对照样地CH4排放通量与5 cm土壤温度存在显著相关性,草丛沼泽CH4排放通量与水位相关性不显著。灌丛沼泽CH4排放通量与各层土壤温度及水位均无显著相关性。草丛、灌丛沼泽对照样地土壤CO2排放通量与0—15 cm土壤温度呈显著或极显著正相关,火烧样地与0—30 cm土壤温度呈显著或极显著正相关。草丛、灌丛沼泽对照、火烧样地土壤CO2排放通量与水位极显著负相关。火干扰使草丛、灌丛沼泽CH4排放源的强度增强,CO2、N2O的排放消弱,全球温室潜势下降约为23.3%。火干扰能够减少草丛、灌丛沼泽温室气体排放。     

采伐对小兴安岭森林沼泽非生长季土壤温室气体排放的影响

中高纬度地区非生长季温室气体排放对生态系统碳、氮循环具有重要影响,但采伐干扰如何影响森林沼泽非生长季土壤温室气体排放尚不明确.本研究采用静态箱-气相色谱法,观测小兴安岭4种森林沼泽(毛赤杨沼泽、白桦沼泽、落叶松苔草沼泽、落叶松藓类沼泽)不同采伐方式下(对照、择伐45%、皆伐,试验处理已10年)非生长季土壤CO₂、CH₄、和N₂O通量及其相关环境因子(温度、湿度及碳氮含量等),分析采伐干扰对温带森林沼泽非生长季土壤温室气体排放的影响规律及主控因子.结果表明: 采伐干扰10年后,4种森林沼泽土壤CO₂、CH₄和N₂O非生长季平均通量分别在53.08～81.31 mg·m^-2·h^-1、0.09～3.07 mg·m^-2·h^-1和4.07～8.83 μg·m^-2·h^-1,其中,皆伐显著提高毛赤杨沼泽和落叶松藓类沼泽非生长季土壤CO₂、CH₄和N₂O排放量,择伐显著提高白桦沼泽、落叶松藓类沼泽及降低毛赤杨沼泽的CO₂排放量,且显著降低4种森林沼泽CH₄排放量及落叶松苔草沼泽的N₂O排放量;天然森林沼泽非生长季土壤CO₂排放受土壤温度、有机碳含量及C/N调控,CH₄受土壤温度、有机碳含量调控,N₂O受气温、土壤pH调控,采伐增加了CO₂排放与气温、土壤含水量及积雪深度的相关性,增加了CH₄排放与气温、土壤含水量、C/N的相关性,增加了N₂O排放与土壤全氮和C/N的相关性;温带天然森林沼泽非生长季土壤CO₂、CH₄和N₂O的年贡献率分别为33.2%～46.5%、6.3%～9.1%和61.5%～68.3%,皆伐提高了白桦沼泽和落叶松藓类沼泽CO₂年贡献率和除落叶松藓类沼泽外其他样地的N₂O年贡献率,择伐提高了落叶松苔草沼泽、落叶松藓类沼泽CO₂、CH₄和N₂O年贡献率,但降低了白桦沼泽3种气体年贡献率.温带天然森林沼泽非生长季土壤N₂O和CO₂的年贡献率相对较大,皆伐使两者年贡献率进一步提高,择伐却较大幅度提高了其CH₄的年贡献率.     

小兴安岭典型草丛沼泽湿地CO2、CH4和N2O的排放动态及其影响因素

2007年6～10月,采用静态箱-气相色谱法,同步研究了小兴安岭典型修氏苔草(Carex schmidtii)沼泽湿地CO2、CH4和N2O排放通量的季节动态及其与环境因子的关系,估算CO2、CH4和N2O的生长季排放量,探讨了沼泽湿地碳与氮的源汇关系.结果表明:草丛沼泽生长季节温室气体排放量以CO2占绝对优势(99.61%),CH4的排放量次之(0.39%),N2O的排放量最低(0.000 7%),且为碳、氮的吸收汇(分别为固定量的53.93%和0.04%);CO2、CH4和N2O生长季平均排放通量依次为487.89、1.88和0.004 mg·m-2·h-1,且具有明显的季节变化特征,CO2和N2O的最高排放量均出现在夏季(6月24日至8月14日和7月14日至8月14日),CH4的最高排放量出现在夏秋季(8月24日至9月24日),其中,CO2季节变化与空气温度和0～20 cm土壤温度具有显著相关性(p＜0.05),CH4与空气温度具有显著相关性(p<0.01),N2O与水位具有显著的负相关性(p＜0.05).     

剔除杂草对山核桃林地土壤温室气体排放的影响

森林土壤是CO2、N2O和CH4等温室气体的重要排放源,山核桃是中国特有的高档干果和木本油料树种,林下杂草管理对山核桃林地温室气体排放具有重要影响.采用静态箱-气相色谱法在浙江临安山核桃主产区进行了为期1年的原位试验,研究剔除林下杂草对山核桃林地土壤温室气体排放的影响.结果表明:剔除杂草和留养杂草山核桃林地土壤CO2排放通量呈现一致的季节变化规律:夏秋季高、冬春季低;N2O排放在夏季较高,其他季节变化平稳;CH4的排放无明显季节变化规律.剔除杂草显著降低了土壤CO2排放,促进了N2O排放和CH4吸收.剔除杂草对土壤水溶性有机碳和微生物生物量碳没有显著影响.剔除杂草的山核桃林地土壤排放温室气体的综合增温潜势为15.12 t CO2-e·hm-2·a-1,显著低于留养杂草处理(17.04 t CO2-e·hm-2·a-1).     

鼎湖山针阔叶混交林地表温室气体排放的日变化

利用静态箱 -气相色谱法对鼎湖山针阔叶混交林地表 3种温室气体 CO2 、CH4 、N2 O通量进行了原位观测 .结果表明 ,鼎湖山针阔叶混交林地表为 CO2 、N2 O的排放源 ,为 CH4 的弱汇 ,通量日变幅分别是 4 88.99～ 70 0 .5 7,0 .0 4 9～ 0 .10 8mg/ (m2· h)和- 0 .0 2 5～ - 0 .0 5 3mg/ (m2 · h) ;地表凋落物分解释放 CO2 约占总排放的 1/ 3;凋落物层和林下灌木对 CH4 和 N2 O的通量无明显影响 ;CO2 、N2 O的 9∶ 0 0观测值与其日平均值有明显差异     

东北温带次生林和落叶松人工林土壤CH4吸收和N2O排放通量

2007年6月-2008年6月,在帽儿山用静态箱/气相色谱法测定了相邻次生林和落叶松人工林土壤CH4和N2O通量,结果表明:次生林转变为落叶松人工林后土壤年CH4吸收和年N2O排放通量均显著增加,分别为次生林的1.2倍和3.6倍.两林分CH4和N2O通量表现相似的季节动态,生长季土壤CH4吸收通量和N2O排放通量均高于非生长季.次生林和落叶松人工林土壤CH4吸收通量与土壤温度均呈正相关关系,而与土壤含水量呈负相关关系.土壤N2O排放通量与土壤温度和土壤铵态氮含量均呈正相关关系,而与土壤含水量没有明显相关性.次生林转变为落叶松人工林后,落叶松林地较厚的凋落物层改变了林地土壤水分的格局,影响了土壤的CH4和N2O通量.     

Effects of Elevated Atmospheric CO2 Concentrations on CH4 and N2O Emission from Rice Soil: An Experiment in Controlled-environment Chambers

The effects of elevated concentrations of atmospheric CO₂ on CH₄ and N₂O emissions from rice soil were investigated in controlled-environment chambers using rice plants growing in pots. Elevated CO₂ significantly increased CH₄ emission by 58% compared with ambient CO₂. The CH₄ emitted by plant-mediated transport and ebullition–diffusion accounted for 86.7 and 13.3% of total emissions during the flooding period under ambient level, respectively; and for 88.1 and 11.9% of total emissions during the flooding period under elevated CO₂ level, respectively. No CH₄ was emitted from plant-free pots, suggesting that the main source of emitted CH₄ was root exudates or autolysis products. Most N₂O was emitted during the first 3 weeks after flooding and rice transplanting, probably through denitrification of NO₃⁻ contained in the experimental soil, and was not affected by the CO₂ concentration. Pre-harvest drainage suppressed CH₄ emission but did not cause much N₂O emission (< 10 μg N m⁻² h⁻¹) from the rice-plant pots at both CO₂ concentrations.     

城镇快速发展对河流温室气体溶存及扩散通量的影响——以重庆市黑水滩河流域场镇为例

场镇发展是西南山区城镇发展的重要模式,且大部分场镇沿河分布,快速城镇发展给河流水环境及生物地化过程带来了一系列影响,然而其对河流温室气体排放时空格局的影响及机制尚不清楚。选择流域场镇发展特征明显的黑水滩河为研究对象,于2014年9月、12月、2015年3月、6月,对流域内干、支流水体温室气体浓度及扩散通量进行分析,旨在阐明流域场镇式发展下河流温室气体排放时空特征及关键驱动因素。研究结果表明,黑水滩河干、支流水体年均二氧化碳分压(pCO_2)及甲烷(CH_4)、一氧化二氮(N_2O)浓度均处于过饱和状态,是大气温室气体的净排放源;流域内干、支流水体流经不同场镇区前后水体碳、氮、磷及叶绿素a含量均不同程度增加,从上游向下游呈现明显的污染累积;水体溶存pCO_2\\CH_4\\N_2O浓度及扩散通量在不同场镇前后也呈现显著增加的趋势,三种温室气体扩散通量平均增幅分别为25.88%、55.22%、99.64%;河流水体pCO_2与N_2O浓度及通量秋季高于其他季节,CH_4浓度及扩散通量春季最高,秋季次之,夏、冬季最低,温室气体浓度及排放的季节变化主要受温度和降雨格局共同影响。相关分析表明,pCO_2与水温和pH关系密切,而水体CH_4和N_2O浓度与水体碳、氮、磷等生源要素均呈显著的正相关关系,水体CH_4与N_2O浓度对生源要素输入极为敏感,流域场镇发展带来的河流污染负荷的增加可能对水体CH_4与N_2O排放产生明显的激发效应。本研究认为,山区河流流域内沿河串珠状场镇分布对河流水体生源要素及其他理化性质产生累积影响,进而改变了水体温室气体的产生与排放时空格局。     

Contribution of litter layer to soil greenhouse gas emissions in a temperate beech forest

Background and aims

The litter layer is a major source of CO₂, and it also influences soil-atmosphere exchange of N₂O and CH₄. So far, it is not clear how much of soil greenhouse gas (GHG) emission derives from the litter layer itself or is litter-induced. The present study investigates how the litter layer controls soil GHG fluxes and microbial decomposer communities in a temperate beech forest.

Methods

We removed the litter layer in an Austrian beech forest and studied responses of soil CO₂, CH₄ and N₂O fluxes and the microbial community via phospholipid fatty acids (PLFA). Soil GHG fluxes were determined with static chambers on 22 occasions from July 2012 to February 2013, and soil samples collected at 8 sampling events.

Results

Litter removal reduced CO₂ emissions by 30 % and increased temperature sensitivity (Q₁₀) of CO₂ fluxes. Diffusion of CH₄ into soil was facilitated by litter removal and CH₄ uptake increased by 16 %. This effect was strongest in autumn and winter when soil moisture was high. Soils without litter turned from net N₂O sources to slight N₂O sinks because N₂O emissions peaked after rain events in summer and autumn, which was not the case in litter-removal plots. Microbial composition was only transiently affected by litter removal but strongly influenced by seasonality.

Conclusions

Litter layers must be considered in calculating forest GHG budgets, and their influence on temperature sensitivity of soil GHG fluxes taken into account for future climate scenarios.

     

Diffusive greenhouse gases fluxes from the surface of the Three Gorges Reservoir: Study at a site in Fuling

To investigate the water-air diffusive greenhouse gases (GHGs) fluxes from the Three Gorges Reservoir (TGR), a field experiment on carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O) fluxes from water surface was carried out from March 2011 to August 2012 by floating static chamber method. The results showed that CO₂ was released to the atmosphere all the time and was less in autumn than in other seasons (P < 0.05). CH₄ was also released to the atmosphere throughout the year but more in summer than other three seasons (P < 0.05). N₂O flux was higher in autumn than other seasons (P < 0.05), and N₂O was absorbed from the atmosphere mainly in summer. Moreover, correlation analysis illustrated that CO₂ flux had significantly negative correlation with wind velocity (P < 0.05), whereas positive correlation with pH (P < 0.01) had been found. There was no significant correlation between CH₄ (or N₂O) flux and the measured environmental variables respectively (P > 0.05). Additionally, the annual fluxes of CO₂, CH₄ and N₂O were 140.45 ± 12.57 mg CO₂·m^?2 h^?1, 1.35 ± 0.14 mg CH₄·m^?2 h^?1 and 34.34 ± 11.64 μg N₂O·m^?2 h^?1, respectively. When compared to other reservoirs worldwide, the CO₂ and N₂O fluxes from TGR were higher than those from boreal and temperate reservoirs, but much lower than those from tropical reservoirs. CH₄ flux was lower than those from boreal, temperate and most tropical reservoirs. In our study, the surface area of the TGR emitted 1.42 × 10⁶ t CO₂, 1.19 × 10⁴ t CH₄ and 589.93 t N₂O in a year. The total GWP was 17.68 t CO₂-eq ha^?1 yr^?1, of which CO₂ flux was dominant (74.38%). Therefore, CO₂ was the main contributor of GHGs fluxes in our study and thus future researches should focus on how to reduce CO₂ fluxes from the surface of the TGR. TGR has a considerable contribution to regional GHG emissions.     

Soil-atmospheric exchange of CO2, CH4, and N2O in three subtropical forest ecosystems in southern China

The magnitude, temporal, and spatial patterns of soil‐atmospheric greenhouse gas (hereafter referred to as GHG) exchanges in forests near the Tropic of Cancer are still highly uncertain. To contribute towards an improvement of actual estimates, soil‐atmospheric CO₂, CH₄, and N₂O fluxes were measured in three successional subtropical forests at the Dinghushan Nature Reserve (hereafter referred to as DNR) in southern China. Soils in DNR forests behaved as N₂O sources and CH₄ sinks. Annual mean CO₂, N₂O, and CH₄ fluxes (mean±SD) were 7.7±4.6 Mg CO₂‐C ha^?1 yr^?1, 3.2±1.2 kg N₂O‐N ha^?1 yr^?1, and 3.4±0.9 kg CH₄‐C ha^?1 yr^?1, respectively. The climate was warm and wet from April through September 2003 (the hot‐humid season) and became cool and dry from October 2003 through March 2004 (the cool‐dry season). The seasonality of soil CO₂ emission coincided with the seasonal climate pattern, with high CO₂ emission rates in the hot‐humid season and low rates in the cool‐dry season. In contrast, seasonal patterns of CH₄ and N₂O fluxes were not clear, although higher CH₄ uptake rates were often observed in the cool‐dry season and higher N₂O emission rates were often observed in the hot‐humid season. GHG fluxes measured at these three sites showed a clear increasing trend with the progressive succession. If this trend is representative at the regional scale, CO₂ and N₂O emissions and CH₄ uptake in southern China may increase in the future in light of the projected change in forest age structure. Removal of surface litter reduced soil CO₂ effluxes by 17–44% in the three forests but had no significant effect on CH₄ absorption and N₂O emission rates. This suggests that microbial CH₄ uptake and N₂O production was mainly related to the mineral soil rather than in the surface litter layer.     

Does hatchery-reared Siniperca chuatsi (Actinopterygii,Perciformes) compete significantly with two wild Siniperca populations for diets in a shallow lake?

Greenhouse gas emissions of Lake Neusiedl, the westernmost European shallow steppe lake, were analysed to identify differences between the seasons of the years and between different locations in the pelagic zone and reed belt. Emissions of CO₂, CH₄ and N₂O were measured in gas samples that had been recovered from the gas space of floating chambers operated as closed systems. Sampling periods covered all seasons except winter. Scaled up to the whole lake area (320 km²), the diffusive emissions of spring, summer and autumn totalled to about 79,500 t CO₂e, disregarding bubble emissions, winter emissions and plant-mediated emissions. The emission sum consisted of about 57,000 t CO₂, 760 t CH₄, and 12 t N₂O. Approximately one-third of the methane and carbon dioxide emissions originated in the pelagic zone and two-thirds in the reed belt (without plant emissions) whereas nitrous oxide emissions were similar in these two zones. An estimate of ebullitive emissions resulted in additional 1,765 t CH₄ that predominantly originated in or near the reed belt from spring to autumn.     

Short-term effects of fire disturbance on greenhouse gases emission from Betula platyphylla-forested wetland in Xiaoxing'an Mountains, Northeast China

By the methods of static chamber and gas chromatography, this paper studied the effects of fire disturbance on the seasonal dynamics and source/sink functions of CH4, CO2 and N2O emissions from Betula platyphylla-forested wetland as well as their relations with environmental factors in Xiaoxing' an Mountains of China. In growth season, slight fire disturbance on the wetland induced an increase of air temperature and ground surface temperature by 1.8-3.9 degrees C and a decrease of water table by 6.3 cm; while heavy fire disturbance led to an increase of air temperature and 0-40 cm soil temperature by 1.4-3.8 degrees C and a decrease of water table by 33.9 cm. Under slight or no fire disturbance, the CH4 was absorbed by the wetland soil in spring but emitted in summer and autumn; under heavy fire disturbance, the CH4 was absorbed in spring and summer but emitted in autumn. The CO2 flux had a seasonal variation of summer > spring = autumn under no fire disturbance, but of summer > autumn > spring under fire disturbance; and the N2O flux varied in the order of spring > summer > autumn under no fire disturbance, but of autumn > spring > summer under slight fire disturbance, and of summer > spring = autumn under heavy fire disturbance. At unburned site, the CO2 flux was significantly positively correlated with air temperature and ground surface temperature; at slightly burned site, the CO2 flux had significant positive correlations with air temperature, 5-10 cm soil temperature, and water table; at heavily burned sites, there was a significant positive correlation between CO2 flux and 5-40 cm soil temperature. Fire disturbance made the CH4 emission increased by 169.5% at lightly burned site or turned into weak CH4 sink at heavily burned site, and made the CO2 and N2O emissions and the global warming potential (GWP) at burned sites decreased by 21.2% -34.7%, 65.6% -95.8%, and 22.9% -36.6% respectively, compared with those at unburned site. Therefore, fire disturbance could decrease the greenhouse gases emission from Betula platyphylla-forested wetland, and planned firing could be properly implemented in wetland management.     

Climate change reduces the net sink of CH4 and N2O in a semiarid grassland

Atmospheric concentrations of methane (CH₄) and nitrous oxide (N₂O) have increased over the last 150 years because of human activity. Soils are important sources and sinks of both potent greenhouse gases where their production and consumption are largely regulated by biological processes. Climate change could alter these processes thereby affecting both rate and direction of their exchange with the atmosphere. We examined how a rise in atmospheric CO₂ and temperature affected CH₄ and N₂O fluxes in a well‐drained upland soil (volumetric water content ranging between 6% and 23%) in a semiarid grassland during five growing seasons. We hypothesized that responses of CH₄ and N₂O fluxes to elevated CO₂ and warming would be driven primarily by treatment effects on soil moisture. Previously we showed that elevated CO₂ increased and warming decreased soil moisture in this grassland. We therefore expected that elevated CO₂ and warming would have opposing effects on CH₄ and N₂O fluxes. Methane was taken up throughout the growing season in all 5 years. A bell‐shaped relationship was observed with soil moisture with highest CH₄ uptake at intermediate soil moisture. Both N₂O emission and uptake occurred at our site with some years showing cumulative N₂O emission and other years showing cumulative N₂O uptake. Nitrous oxide exchange switched from net uptake to net emission with increasing soil moisture. In contrast to our hypothesis, both elevated CO₂ and warming reduced the sink of CH₄ and N₂O expressed in CO₂ equivalents (across 5 years by 7% and 11% for elevated CO₂ and warming respectively) suggesting that soil moisture changes were not solely responsible for this reduction. We conclude that in a future climate this semiarid grassland may become a smaller sink for atmospheric CH₄ and N₂O expressed in CO₂‐equivalents.     

Carbon dioxide,methane, and nitrous oxide exchanges in an age‐sequence of temperate pine forests

We investigated soil carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O) exchanges in an age‐sequence (4, 17, 32, 67 years old) of eastern white pine (Pinus strobus L.) forests in southern Ontario, Canada, for the period of mid‐April to mid‐December in 2006 and 2007. For both CH₄ and N₂O, we observed uptake and emission ranging from ?160 to 245 μg CH₄ m^?2 h^?1 and ?52 to 21 μg N₂O m^?2 h^?1, respectively (negative values indicate uptake). Mean fluxes from mid‐April to mid‐December across the 4, 17, 32, 67 years old stands were similar for CO₂ fluxes (259, 246, 220, and 250 mg CO₂ m^?2 h^?1, respectively), without pattern for N₂O fluxes (?3.7, 1.5, ?2.2, and ?7.6 μg N₂O m^?2 h^?1, respectively), whereas the uptake rates of CH₄ increased with stand age (6.4, ?7.9, ?10.8, and ?23.3 μg CH₄ m^?2 h^?1, respectively). For the same period, the combined contribution of CH₄ and N₂O exchanges to the global warming potential (GWP) calculated from net ecosystem exchange of CO₂ and aggregated soil exchanges of CH₄ and N₂O was on average 4%, <1%, <1%, and 2% for the 4, 17, 32, 67 years old stand, respectively. Soil CO₂ fluxes correlated positively with soil temperature but had no relationship with soil moisture. We found no control of soil temperature or soil moisture on CH₄ and N₂O fluxes, but CH₄ emission was observed following summer rainfall events. LFH layer removal reduced CO₂ emissions by 43%, increased CH₄ uptake during dry and warm soil conditions by more than twofold, but did not affect N₂O flux. We suggest that significant alternating sink and source potentials for both CH₄ and N₂O may occur in N‐ and soil water‐limited forest ecosystems, which constitute a large portion of forest cover in temperate areas.     

Greenhouse gas fluxes over managed grasslands in Central Europe

Central European grasslands are characterized by a wide range of different management practices in close geographical proximity. Site‐specific management strategies strongly affect the biosphere–atmosphere exchange of the three greenhouse gases (GHG) carbon dioxide (CO₂), nitrous oxide (N₂O), and methane (CH₄). The evaluation of environmental impacts at site level is challenging, because most in situ measurements focus on the quantification of CO₂ exchange, while long‐term N₂O and CH₄ flux measurements at ecosystem scale remain scarce. Here, we synthesized ecosystem CO₂, N₂O, and CH₄ fluxes from 14 managed grassland sites, quantified by eddy covariance or chamber techniques. We found that grasslands were on average a CO₂ sink (−1,783 to −91 g CO₂ m⁻² year⁻¹), but a N₂O source (18–638 g CO₂‐eq. m⁻² year⁻¹), and either a CH₄ sink or source (−9 to 488 g CO₂‐eq. m⁻² year⁻¹). The net GHG balance (NGB) of nine sites where measurements of all three GHGs were available was found between −2,761 and −58 g CO₂‐eq. m⁻² year⁻¹, with N₂O and CH₄ emissions offsetting concurrent CO₂ uptake by on average 21 ± 6% across sites. The only positive NGB was found for one site during a restoration year with ploughing. The predictive power of soil parameters for N₂O and CH₄ fluxes was generally low and varied considerably within years. However, after site‐specific data normalization, we identified environmental conditions that indicated enhanced GHG source/sink activity (“sweet spots”) and gave a good prediction of normalized overall fluxes across sites. The application of animal slurry to grasslands increased N₂O and CH₄ emissions. The N₂O‐N emission factor across sites was 1.8 ± 0.5%, but varied considerably at site level among the years (0.1%–8.6%). Although grassland management led to increased N₂O and CH₄ emissions, the CO₂ sink strength was generally the most dominant component of the annual GHG budget.