1200 years of fire impact on biogeochemistry as inferred from high resolution diatom analysis in a kettle lake from the Picea mariana-moss domain (Quebec, Canada)

The consequences of fire on water chemistry are important considering that major changes in the frequency and intensity of forest fires are anticipated as a result of global warming. Due to the important differences in succesionnal vegetative trends after fire between mixed-wood and coniferous-dominated forests in Quebec (Canada), we undertook a long-term paleoecological study of the impact of fires on the biogeochemistry of Lac à la Pessière, a small lake located in a conifer-dominated boreal forest ecoregion (Picea mariana-moss domain). The paleolimnological study was carried out using diatom assemblages (class:Bacillariophyceae) to reconstruct changes in environmental variables of limnological interest [pH, total phosphorus (TP), total nitrogen (TN), dissolved organic carbon (DOC) and, epilimnetic carbon dioxide (CO₂)] potentially associated with fire over the last 1200 calendar years. Diatom composition and related reconstructed limnological variables were compared before and after fire events. No significant changes were systematically observed in lake chemistry associated with fire events. However, diatom-inferred epilimnetic CO₂showed a clear decreasing trend over the last 400 cal. yrs BP. The results suggest that fire-induced changes in lake chemistry are limited in catchments dominated by black spruce (Picea mariana). We hypothesize that this fact result of excess moisture associated to the thick humus layer, which likely limits the mobilization of nutrients and major ions even during a fire event.     

巴丹吉林最高沙山区CO2浓度与昼夜变化规律

为查明沙漠区CO₂浓度和对大气CO₂的影响以及在全球碳循环中的作用,利用红外CO₂监测仪,在2009年5月对全球最高大的巴丹吉林诺尔图东大沙山等进行了17个钻孔CO₂浓度的昼夜观测。结果表明,巴丹吉林大沙山不同观测点CO₂浓度差异较大,各观测点CO₂浓度变化在0.01%~0.075％之间;夜间沙层CO₂浓度低,白天CO₂浓度高;CO₂浓度昼夜变化明显,从早7时到次日7时1~5m各深度CO₂浓度均呈现由低到高再到低的变化规律;在24h内,沙层CO₂浓度变化存在4个转折点,分别出现在凌晨5时、11时、18时和21时左右。在沙层水分一定的条件下,昼夜温度变化是造成沙层CO₂浓度昼夜规律变化的主要原因,两者呈正相关关系;含水量较高的沙层CO₂浓度明显高于含水量较低的沙层;5m深度以上沙层CO₂浓度均高于地表空气CO₂浓度,表明极端干旱的高大沙山区是CO₂的来源区,也指示环境恶劣的裸露高大沙山区微生物活动产生的沙层CO₂浓度仍然超过了大气CO₂浓度。