Lakes are a major component of boreal landscapes, and whereas lake CO2 emissions are recognized as a major component of regional C budgets, there is still much uncertainty associated to lake CH4 fluxes. Here, we present a large-scale study of the magnitude and regulation of boreal lake summer diffusive CH4 fluxes, and their contribution to total lake carbon (C) emissions, based on in situ measurements of concentration and fluxes of CH4 and CO2 in 224 lakes across a wide range of lake type and environmental gradients in Québec. The diffusive CH4 flux was highly variable (mean 11.6 ± 26.4 SD mg m(-2)  d(-1) ), and it was positively correlated with temperature and lake nutrient status, and negatively correlated with lake area and colored dissolved organic matter (CDOM). The relationship between CH4 and CO2 concentrations fluxes was weak, suggesting major differences in their respective sources and/or regulation. For example, increasing water temperature leads to higher CH4 flux but does not significantly affect CO2 flux, whereas increasing CDOM concentration leads to higher CO2 flux but lower CH4 flux. CH4 contributed to 8 ± 23% to the total lake C emissions (CH4  + CO2 ), but 18 ± 25% to the total flux in terms of atmospheric warming potential, expressed as CO2 -equivalents. The incorporation of ebullition and plant-mediated CH4 fluxes would further increase the importance of lake CH4 . The average Q10 of CH4 flux was 3.7, once other covarying factors were accounted for, but this apparent Q10 varied with lake morphometry and was higher for shallow lakes. We conclude that global climate change and the resulting shifts in temperature will strongly influence lake CH4 fluxes across the boreal biome, but these climate effects may be altered by regional patterns in lake morphometry, nutrient status, and browning.

The static chamber method (non-flow-through-non-steady-state chambers) is the most common method to measure fluxes of methane (CH4) from soils. Laboratory comparisons to quantify errors resulting from chamber design, operation and flux calculation methods are rare. We tested fifteen chambers against four flux levels (FL) ranging from 200 to 2300 mu g CH4 M-2 II-1. The measurements were conducted on a calibration tank using three quartz sand types with soil porosities of 53% (dry fine sand, S1), 47% (dry coarse sand, S2), and 33% (wetted fine sand, S3). The chambers tested ranged from 0.06 to 1.8 m in height, and 0.02 to 0.195 m(3) in volume, 7 of them were equipped with a fan, and 1 with a vent-tube. We applied linear and exponential flux calculation methods to the chamber data and compared these chamber fluxes to the reference fluxes from the calibration tank. The chambers underestimated the reference fluxes by on average 33% by the linear flux calculation method (R-Iin), whereas the chamber fluxes calculated by the exponential flux calculation method (R-exp) did not significantly differ from the reference fluxes (p <0.05). The flux under- or overestimations were chamber specific and independent of flux level. Increasing chamber height, area and volume significantly reduced the flux underestimation (p <0.05). Also, the use of non-linear flux calculation method significantly improved the flux estimation; however, simultaneously the uncertainty in the fluxes was increased. We provide correction factors, which can be used to correct the under- or overestimation of the fluxes by the chambers in the experiment. (C) 2012 Elsevier B.V. All rights reserved

[1] Significant amounts of terrestrial carbon are processed in lakes and emitted into the atmosphere as CO 2 . However, due to lack of appropriate measurements the absolute role of lakes in the landscape as sinks or sources of CO 2 is still uncertain. We conducted the first long-term, ecosystem-level CO 2 flux measurements with eddy covariance technique in a boreal lake within a natural-state catchment covering 5 years. The aim was to reveal the natural level of CO 2 flux between a lake and the atmosphere and its role in regional carbon cycling. On average, the lake emitted ca 10% of the terrestrial net ecosystem production of the surrounding oldgrowth forest and the main immediate drivers behind the fluxes were physical rather than biological. Our results suggest that lakes are an integral part of terrestrial carbon cycling. Citation: Huotari, J., A. Ojala, E. Peltomaa, A. Nordbo, S. Launiainen, J. Pumpanen, T. Rasilo, P. Hari, and T. Vesala (2011), Long-term direct CO 2 flux measurements over a boreal lake: Five years of eddy covariance data, Geophys.

No abstract

Carbon dioxide is released from the soil to the atmosphere in heterotrophic respiration when the dead organic matter is used for substrates for soil micro-organisms and soil animals. Respiration of roots and mycorrhiza is another major source of carbon dioxide in soil CO 2 efflux. The partitioning of these two fluxes is essential for understanding the carbon balance of forest ecosystems and for modelling the carbon cycle within these ecosystems. In this study, we determined the carbon balance of three common tree species in boreal forest zone, Scots pine, Norway spruce, and Silver birch with gas exchange measurements conducted in laboratory in controlled temperature and light conditions. We also studied the allocation pattern of assimilated carbon with 14 C pulse labelling experiment. The photosynthetic light responses of the tree species were substantially different. The maximum photosynthetic capacity (P max ) was 2.21 lg CO 2 s -1 g -1 in Scots pine, 1.22 lg CO 2 s -1 g -1 in Norway spruce and 3.01 lg CO 2 s -1 g -1 in Silver birch seedlings. According to the pulse labelling experiments, 43-75% of the assimilated carbon remained in the aboveground parts of the seedlings. The amount of carbon allocated to root and rhizosphere respiration was about 9-26%, and the amount of carbon allocated to root and ectomycorrhizal biomass about 13-21% of the total assimilated CO 2 . The 14 CO 2 pulse reached the root system within few hours after the labelling and most of the pulse had passed the root system after 48 h. The transport rate of carbon from shoot to roots was fastest in Silver birch seedlings.

The numerous water bodies and their riparian zones in the boreal zone are crucial to lateral carbon transport and can thus be very signifi cant for carbon cycle on the landscape level. Therefore, we installed automa c measurement systems with Vaisala CARBOCAP CO 2 probes (Vaisala Oyj, Vantaa, Finland) in the riparian zone soil matrix around a small headwater lake, in the lake itself, and in the ou lowing brook and followed the seasonal and diurnal varia on in CO 2 concentra on as well as rain event-driven changes in this naturalstate soil-lake-brook con nuum in Southern Finland. Seasonal varia on was greatest and concentra ons highest deep in the soil and in the lake, but were also no ceable in the brook. On the other hand, diurnal varia on was highest in shallow soil layers and the lake surface. In the brook, the infl uence of the riparian zone superseded that of the lake at less than 150 m distance, which resulted in wider varia on and higher concentra ons of CO 2 . A er the rain event, the normal diurnal pa ern was changed, and the CO 2 concentra on in the soil increased as water fi lled the soil pores and slowed down the diff usion. Even though the water with lower CO 2 concentra on from the lake diluted the CO 2 concentra on in the brook, the input from the soil dominated the fl ow a er the rain and concentra ons increased. On an annual basis, the fl ow of terrestrial dissolved inorganic carbon (DIC) into the aqua c ecosystem was 1-13% of terrestrial net ecosystem produc on (NEP).Abbrevia ons: CI, confi dence interval; DIC, dissolved inorganic carbon; DOC, dissolved organic carbon; NEP, net ecosystem produc on; POC, par culate organic carbon.

BioOne Complete (complete.BioOne.org) is a full-text database of 200 subscribed and open-access titles in the biological, ecological, and environmental sciences published by nonprofit societies, associations, museums, institutions, and presses.

Soil temperature is proposed to affect the photosynthetic rate and carbon allocation in boreal trees through sink limitation. The aim of this study was to investigate the effect of temperature on CO(2) exchange, biomass partitioning and ectomycorrhizal (ECM) fungi of boreal tree species. We measured carbon allocation, above- and below-ground CO(2) exchange and the species composition of associated ECM fungi in the rhizosphere of Scots pine (Pinus sylvestris L.), Norway spruce (Picea abies K.) and silver birch (Betula pendula Roth) seedlings grown in soil maintained at 7-12, 12-15 and 16-22 °C. We found increased root biomass and photosynthetic rate at higher soil temperatures, but simultaneously with photosynthesis rate, higher temperature generally increased soil respiration as well as shoot, and root and rhizosphere respiration. The net CO(2) exchange and seedling biomass did not increase significantly with increasing temperature due to a concomitant increase in carbon assimilation and respiration rates. The 2-month-long growth period in different soil temperatures did not alter the ECM fungi species composition and the below-ground carbon sink strength did not seem to be directly related to ECM biomass and species composition in any of the tree species. Ectomycorrhizal species composition and number of mycorrhiza did not explain the CO(2) exchange results at different temperatures.