Winter Fluxes Research Articles

Origin and fluxes of atmospheric REE entering an ombrotrophic peat bog in Black Forest (SW Germany): Evidence from snow, lichens and mosses

The fate of the Rare Earth Elements (REE) were investigated in different types of archives of atmospheric deposition in the Black Forest, Southern Germany: (1) a 70 cm snow pack collected on the domed part of a raised bog and representing 2 months of snow accumulation, (2) a snow sample collected close to the road about 500 m from the peat bog, (3) two species of lichens and (4) a peat profile representing 400 years of peat accumulation as well as a “preanthropogenic” sample and the living moss layer from the top of the core. REE concentrations in peat are significantly correlated to Ti which is a lithogenic conservative element suggesting that REE are immobile in peat bog environments. Snow, lichens and peat samples show similar PAAS (Post Archean Australian Shale) normalized REE distributions suggesting that the complete atmospheric REE signal is preserved in the peat profile. However, the annual flux of REE accumulated by the peat is ca. 10 times greater than that of the bulk winter flux of REE. This difference probably indicates that the REE concentrations in the snowpack are not representative of the average REE flux over the whole year. Despite the pronounced geological differences between this site (granite host-rock) and a previously studied peat bog in Switzerland (limestone host-rock) similar REE distribution patterns and accumulation rates were found at both sites. Given that both sites confirm an Upper Continental Crust signature, the data suggests both sites are influenced by regional and not local, soil-derived lithogenic aerosols.

A Description of Interdecadal Time-Scale Propagating North Atlantic Sea Surface Temperature Anomalies and Their Effect on Winter European Climate, 1948–2002

Abstract Eastward-propagating interdecadal time-scale sea surface temperature (SST) winter anomalies have been shown to exist at the North Atlantic subpolar/subtropical gyre boundary. Heat flux and surface air temperature signatures of these anomalies are investigated using satellite- and ship-based SST observations and atmospheric reanalysis. Using bandpass filter analysis, retaining periods between 9 and 25 yr, a succession of coherent propagating SST anomalies is identified. The size, speed, propagation path, and decay characteristics of propagating anomalies detected during the period 1948–2002 are documented. The behavior of the propagations changes between the periods 1948–70 and 1970–2002. In the former period, SST anomalies propagated from the east coast of North America to the British Isles in ∼10 yr. The anomalies displayed a well-defined life cycle, growing in the western basin (west of 40°W) and decaying in the eastern basin. During the period 1970–2002, SST anomalies did not propagate deep into the eastern basin, but grew in the western basin and then ceased propagating. Oceanic anomalies have a comparable marked signature in surface sensible and latent heat fluxes and in surface air temperature. Winter surface heat flux anomalies act to amplify SST anomalies during the middle of their lifetimes, normally in the west-central Atlantic. At other times, heat flux anomalies are associated with decay of anomalies. Surface heat fluxes do not always act to cause propagation, and it is likely that other processes such as advection play a role in the propagation mechanism. North European winter surface air temperatures are raised or lowered by up to ±0.5°C over decadal time scales (∼1/3 of the total variation over the United Kingdom) when an SST anomaly reaches the eastern boundary. A variety of processes can cause SST variation on decadal time scales at the eastern boundary, but in the 1950s and 1960s the variability at these periods was the signature of features that had propagated across the Atlantic from the North American coast.

Seasonal Hydroclimate Variability over North America in Global and Regional Reanalyses and AMIP Simulations: Varied Representation

Abstract The monotony of seasonal variability is often compensated by the complexity of its spatial structure—the case in North American hydroclimate. The structure of hydroclimate variability is analyzed to provide insights into the functioning of the climate system and climate models. The consistency of hydroclimate representation in two global [40-yr ECMWF Re-Analysis (ERA-40) and NCEP] and one regional [North American Regional Reanalysis (NARR)] reanalysis is examined first, from analysis of precipitation, evaporation, surface air temperature (SAT), and moisture flux distributions. The intercomparisons benchmark the recently released NARR data and provide context for evaluation of the simulation potential of two state-of-the-art atmospheric models [NCAR's Community Atmospheric Model (CAM3.0) and NASA's Seasonal-to-Interannual Prediction Project (NSIPP) atmospheric model]. Intercomparisons paint a gloomy picture: great divergence in global reanalysis representations of precipitation, with the eastern United States being drier in ERA-40 and wetter in NCEP in the annual mean by up to a third in each case; model averages are like ERA-40. The annual means, in fact, mask even larger but offsetting seasonal departures. Analysis of moisture transport shows winter fluxes to be more consistently represented. Summer flux convergence over the Gulf Coast and Great Plains, however, differs considerably between global and regional reanalyses. Flux distributions help in understanding the choice of rainy season, especially the winter one in the Pacific Northwest; stationary fluxes are key. Land–ocean competition for convection is too intense in the models—so much so that the oceanic ITCZ in July is southward of its winter position in the both simulations! The overresponsiveness of land is also manifest in SAT; the winter-to-summer change over the Great Plains is 5–9 K larger than in observations, with implications for modeling of climate sensitivity. The nature of atmospheric water balance over the Great Plains is probed, despite unbalanced moisture budgets in reanalyses and model simulations. The imbalance is smaller in NARR but still unacceptably large, resulting from excessive evaporation in spring and summer. Adjusting evaporation during precipitation assimilation could lead to a more balanced budget. Global and regional reanalysis will remain of limited use for hydroclimate studies until they comply with the operative water and energy balance constraints.

Trace metal variations in the shells of Ensis siliqua record pollution and environmental conditions in the sea to the west of mainland Britain

Shells of the pod razor shell ( Ensis siliqua) from 13 locations around the west coast of mainland Britain have been analysed by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) for a range of trace metals including Zn, Cd, Pb, U, Ba, Sr and Mg. The trace metal record in these shells is a proxy record for changes in seawater chemistry during the 1990s. Regional variations exist in the median concentrations of the analysed metals. Barium concentrations are related to increased productivity from sewage sludge dumping at sea. Strontium shows a local relationship to salinity, but there is no clear relationship over the study area, instead high Sr is often associated with high Ba, and may reflect ontogenetic factors such as growth rate. Magnesium shows a seasonal variation within individual shells and can be used to calculate sea surface temperatures from groups of shells. Contaminant metals show a clear regional relationship with known sources, thus high Pb and Zn are typically associated with former metal mining areas (e.g. Cardigan Bay, Anglesey), and high Pb, Zn, Cd and U are associated with industrial activity in Liverpool Bay. Anomalies such as the high U in shells from northern Scotland cannot at present be explained. A seasonal variation of Pb is also seen in Cardigan Bay and Liverpool Bay, relating to increased winter fluxes of these metals to the marine environment. The regional distribution of these metals is consistent with known sources of contamination and patterns of seawater migration around the coast of Britain.

Temporal coupling between surface and deep ocean biogeochemical processes in contrasting subtropical and subantarctic water masses, southwest Pacific Ocean

Surface to deep‐ocean coupling was investigated in subtropical (STW) and subantarctic (SAW) waters off eastern New Zealand. Moorings, comprising a near‐surface fluorometer, temperature loggers, current meters, and sediment trap at 1500 m depth, were deployed at 41° and 46°40′S along 178°30′E between October 2000 and October 2001. Locally validated, remotely sensed data provided areal estimates of surface chlorophyll that were representative of 1997–2004 annual cycles. In STW, early spring chlorophyll peaks were coupled with deposition of labile (molar C:N∼7–8), bio‐siliceous organic matter. Low winter chlorophyll concentrations were associated with high particulate organic carbon (POC) fluxes of moderately refractory material (C:N∼9–10). This indicates that winter flux was affected by heterotrophic recycling processes (zooplankton exuviae, fecal pellets) and/or slowly sinking particles from the preceding autumn. Deep‐ocean POC fluxes off New Zealand were similar to global estimates and Tasman Sea (0.8 cf. 1.0 g C m−2 yr−1). Elevated biogenic silica and lithogenic fluxes probably reflect processes within a warm‐core eddy near an eroding landmass, rather than STW in general. In SAW, POC and biogenic silica flux peaks occurred in spring with moderate surface chlorophyll concentrations. Decoupling between high chlorophyll and low flux in summer may reflect near‐surface organic matter recycling by the microbial‐dominated ecosystem. In spring, moderate chlorophyll levels in SAW, high POC and silica flux, and high C:N ratios (9–13) indicate some coupling with upper water column processes. SAW, east of New Zealand, was characterized by low POC (0.2 g C m−2 yr−1), high biogenic silica, and low carbonate fluxes, unlike other subantarctic sites, which are dominated by carbonate deposition with fivefold higher POC flux.

Seasonal variations in foraminiferal flux in the Southern Ocean, Campbell Plateau, New Zealand

Time-incremental sediment trap moorings were deployed at two sites across Campbell Plateau, New Zealand; one in the relatively quiet waters of the plateau interior (Pukaki Rise) and the other at the Antarctic Circumpolar Current-swept margin (eastern Campbell flank). A continuous record of particle flux over 416 days identifies marked differences in faunal composition and seasonal occurrence in response to these two dynamically different Southern Ocean settings. In the shallow and thermally isolated interior of the Pukaki Rise site, the flux of foraminifera was closely linked to the austral spring pulse of primary production, at which time 97% of the total foraminiferal production for the year occurred. This seasonal flux of foraminifera at Pukaki Rise was dominated by the species Globorotalia inflata and Globigerina bulloides. The collapse of phytoplankton production at the end of spring was most likely a result of a co-limitation of iron and silicic acid. Over the deep and dynamic eastern flank of Campbell Plateau there were four seasonal pulses of foraminiferal flux, the largest in spring when almost half of the foraminiferal production occurred. Most of the spring flux at this site consisted of Globorotalia inflata and Globigerinita glutinata. Summer at eastern Campbell flank, exhibited the lowest flux of foraminifera, but the highest species diversity, while autumn and winter fluxes were dominated by the deep-dwelling Globorotalia truncatulinoides. Although there was only a single pulse of foraminiferal flux at Pukaki Rise, its mass (g m − 2 d − 1 ) was still greater than that which occurred over the flank, summed across all four seasons. However, the overall standing stock (tests m − 2 d − 1 ) on the flank was more than twice that which occurred at the Pukaki Rise site. An examination of core-top material taken from gravity cores collected in the vicinity of the sediment trap locations, revealed typical Southern Ocean foraminiferal assemblages that were markedly dissimilar in proportion to those observed in the sediment traps.

Importance of seasonal variability and coastal processes on estuarine manganese and barium cycling in a Pacific Northwest estuary

Dissolved and particulate manganese (Mn) and barium (Ba) within the Tillamook Bay estuary are examined over seasonal cycles under a range of river discharge conditions from January to December 1999. Based on estuarine distributions and box model calculations, Mn and Ba behavior within the estuary is best quantified by considering two dominant factors: interaction with suspended particulate material (adsorption/desorption reactions) and input from benthic sources. Seasonal differences in elemental concentration patterns are dominated by variations in the relative importance of these two variables. While Mn exhibits estuarine behavior typical for this element, Ba shows an uncharacteristic lower estuary source. We hypothesize that this source results from the benthic dissolution of ocean-derived particulate Ba. The fluxes of dissolved Mn and Ba from the river to the coastal ocean are related to river discharge rates with the high fluxes observed under winter conditions. These winter flux maxima occur despite estuarine removal processes that also occur under winter conditions. During all other seasons, box model calculations indicate that the total (dissolved plus particulate) river fluxes of Mn and Ba to the coastal ocean are enhanced by internal estuarine processes—i.e., by transport across the sediment–water interface. On an annual basis, dissolved Mn and Ba fluxes to the coastal ocean are enriched by 390% and 170%, respectively, over what is expected from riverine inputs alone. The results of this research underscore the importance of estuarine processes in modifying the riverine flux of these elements to the ocean and the necessity of temporal sampling in evaluating processes controlling elemental distributions and delivery to the coastal ocean. Measurements of both the dissolved and particulate phases are also essential for describing and modeling these elemental budgets.

Radon-prone areas in the Lombard plain

This paper reports the results of indoor radon measurements carried out in the Lombard plain. The aim of this study, which is based on the geological context, was to identify radon high-risk areas. The underlying geology has been established by means of the available stratigraphies giving a schematic representation of the sites in which either occurrence of gravel or silt and clay predominate with depths ranging from 0 to 50 m. Radon measurements were performed in a sample of 411 one-family houses in seven villages located in the southern area of Bergamo and Brescia. The findings indicate that when the substrate is dominated by clay, radon concentration for dwellings on the ground floor are low, whereas a strong predominance of underlying gravel mixed up in sand gives in winter months high radon flux from underground.

Carbon limitation of soil respiration under winter snowpacks: potential feedbacks between growing season and winter carbon fluxes

AbstractA reduction in the length of the snow‐covered season in response to a warming of high‐latitude and high‐elevation ecosystems may increase soil carbon availability both through increased litter fall following longer growing seasons and by allowing early winter soil frosts that lyse plant and microbial cells. To evaluate how an increase in labile carbon during winter may affect ecosystem carbon balance we investigated the relationship between carbon availability and winter CO2 fluxes at several locations in the Colorado Rockies. Landscape‐scale surveys of winter CO2 fluxes from sites with different soil carbon content indicated that winter CO2 fluxes were positively related to carbon availability and experimental additions of glucose to soil confirmed that CO2 fluxes from snow‐covered soil at temperatures between 0 and −3°C were carbon limited. Glucose added to snow‐covered soil increased CO2 fluxes by 52–160% relative to control sites within 24 h and remained 62–70% higher after 30 days. Concurrently a shift in the δ13C values of emitted CO2 toward the glucose value indicated preferential utilization of the added carbon confirming the presence of active heterotrophic respiration in soils at temperatures below 0°C. The sensitivity of these winter fluxes to substrate availability, coupled with predicted changes in winter snow cover, suggests that feedbacks between growing season carbon uptake and winter heterotrophic activity may have unforeseen consequences for carbon and nutrient cycling in northern forests. For example, published winter CO2 fluxes indicate that on average 50% of growing season carbon uptake currently is respired during the winter; changes in winter CO2 flux in response to climate change have the potential to reduce substantially the net carbon sink in these ecosystems.

Influence of Early Winter Upward Wave Activity Flux on Midwinter Circulation in the Stratosphere and Troposphere

Abstract Using NCEP–NCAR reanalysis data the authors show that the November–December averaged stratospheric eddy heat flux is strongly anticorrelated with the January–February averaged eddy heat flux in the midlatitude stratosphere and troposphere. This finding further emphasizes differences between early and midwinter stratospheric wave flux behavior, which has recently been found in long-term variations. Analysis suggests that the intraseasonal anticorrelation of stratospheric heat fluxes results from changes in the upward wave propagation in the troposphere. Stronger (weaker) upward wave fluxes in early winter lead to weaker (stronger) upward wave fluxes from the troposphere during midwinter. Also, enhanced equatorward wave refraction during midwinter (due to the stronger polar night jet) is associated with weak heat flux in the early winter. It is suggested that the effect of enhanced midwinter upward wave flux from the troposphere in the years with weak early winter heat flux overcompensates the effect of increased equatorward wave refraction in midwinter, leading to a net increase of midwinter upward wave fluxes into the stratosphere.

Contribution of vegetated littoral zone to winter fluxes of carbon dioxide and methane from boreal lakes

Littoral zones at the interface of the lake and the catchment are intensive sites for mineralization of organic matter, but the contribution of vegetated littoral zone to winter fluxes of carbon dioxide (CO2) and methane (CH4) from lake ecosystems into the atmosphere is poorly known. We studied littoral carbon gas fluxes and their spatial controls at five boreal lakes of varying trophic state during three consequent winters with contrasting snow conditions and flooding regimes. Lake‐wide estimates including littoral winter release and potential pelagic spring pulse of gases were calculated for three lakes. Large interannual and spatial differences in carbon gas fluxes were controlled by the interaction of climatic factors, ice and snow cover, on‐site hydrology, and apparently substrate supply from biomass production of the previous growing season. Littoral CO2 fluxes ranged from 0.9 to 7.5 mol m−2 winter−1, and the CH4 fluxes ranged from 0.04 to 0.38 mol m−2 winter−1, the latter being highest in eutrophic lakes. The vegetated littoral contributed the most (66–78%) to winter CH4 emissions from two lakes; in the smallest and most productive lake, pelagic accumulation exceeded littoral release. The large variation in littoral CO2 release could contribute to between‐winter differences of 82% in lake‐wide carbon gas emissions. The water level of the preceding summer and precipitation during early winter were found to be useful predictors for littoral carbon gas fluxes in winter. This suggests that the carbon gas exchange of a shallow boreal lake can be highly sensitive to changes in snowfall and subsequent flooding.

Intra-Seasonal Mapping of CO2 Flux in Rangelands of Northern Kazakhstan at One-Kilometer Resolution

Algorithms that establish relationships between variables obtained through remote sensing and geographic information system (GIS) technologies are needed to allow the scaling up of site-specific CO2 flux measurements to regional levels. We obtained Bowen ratio–energy balance (BREB) flux tower measurements during the growing seasons of 1998–2000 above a grassland steppe in Kazakhstan. These BREB data were analyzed using ecosystem light–curve equations to quantify 10-day CO2 fluxes associated with gross primary production (GPP) and total respiration (R). Remotely sensed, temporally smoothed normalized difference vegetation index (NDVIsm) and environmental variables were used to develop multiple regression models for the mapping of 10-day CO2 fluxes for the Kazakh steppe. Ten-day GPP was estimated (R2 = 0.72) by day of year (DOY) and NDVIsm, and 10-day R was estimated (R2 = 0.48) with the estimated GPP and estimated 10-day photosynthetically active radiation (PAR). Regression tree analysis estimated 10-day PAR from latitude, NDVIsm, DOY, and precipitation (R2 = 0.81). Fivefold cross-validation indicated that these algorithms were reasonably robust. GPP, R, and resulting net ecosystem exchange (NEE) were mapped for the Kazakh steppe grassland every 10 days and summed to produce regional growing season estimates of GPP, R, and NEE. Estimates of 10-day NEE agreed well with BREB observations in 2000, showing a slight underestimation in the late summer. Growing season (May to October) mean NEE for Kazakh steppe grasslands was 1.27 Mg C/ha in 2000. Winter flux data were collected during the winter of 2001–2002 and are being analyzed to close the annual carbon budget for the Kazakh steppe.

Alpine Grassland CO2 Exchange and Nitrogen Cycling: Grazing History Effects, Medicine Bow Range, Wyoming, U.S.A

Our study examined carbon dioxide exchange and nitrogen cycling over two consecutive years (winter and summer) in a grazed alpine grassland and in an embedded long-term grazing exclosure to ascertain whether grazing history had resulted in divergent soil carbon attributes, CO2 exchange rates, and different vegetation C and N and soil N processes. Soil C and N concentrations and masses were significantly higher in the grazed than in the ungrazed area, though grass leaf N was higher in the ungrazed area, as was vegetation biomass. Detectable amounts of CO2 were lost from the grazed and ungrazed areas of this grassland during the winters of 1998, 1999, and 2000, and at 6 of 15 winter flux sample dates, CO2 efflux was greater in the grazed area than in the ungrazed area. The ungrazed area consistently gained more C during the summer months than the grazed area, with net CO2 exchange peaking in mid-July 1998 at nearly 5 μmol m−2 s−1 in the ungrazed area compared to <2 μmol m−2 s−1 in the grazed area. During the 2-yr study period, the grazed area was a carbon source of 170 g C m−2, while the ungrazed area was a carbon sink of 83 g C m−2. Lower N mineralization rates early and late in the summer (1999) in the grazed site at Libby Flats corresponded to reductions in net CO2 exchange and lower plant N content compared to the ungrazed exclosure. Based on these results, we suggest that: (1) long-term grazing in high-altitude rangelands can alter annual CO2 exchange and N dynamics; (2) temporal synchrony in C and N processes occur during the summer; that is, increased C exchange rates accompany increased N mineralization rates; and (3) integrative (total soil C and N) and instantaneous (CO2 exchange and vegetation N) measures of C and N dynamics may not necessarily lead to the same interpretation regarding C sequestration and N cycling in alpine grasslands.

Temporal variability of fluxes of eolian‐transported freshwater diatoms, phytoliths, and pollen grains off Cape Blanc as reflection of land‐atmosphere‐ocean interactions in northwest Africa

Fluxes of airborne freshwater diatoms (FD), phytoliths (PH), and pollen grains (PO) collected with sediment traps off Cape Blanc, northwest Africa, from 1988 till 1991 are presented. Both continental rainfall variations and wind mean strength and direction play a key role in the temporal fluctuations of the fluxes of eolian traces in the pelagic realm. Drier conditions in Northern Africa in 1987 could have preceded the high lithogenic input and moderate FD flux in 1988. The PH peak in summer 1988 was probably caused by increased wind velocity. Wetter rainy seasons of 1988/89 might have promoted a significant pollen production in summer 1989, and FD in late 1989 and early 1990, as well as contributed to the reduction of the lithogenic flux in 1989/90. Decreased fluxes of FD, PH and PO, and higher contribution of the 6–11 μm lithogenic fraction in 1991 would mainly reflect minor intensity and decreased amount of continental trade winds. Air‐mass backward trajectories confirm that the Saharan Air Layer is predominantly involved in the spring/summer transport. Trade winds play a decisive role in the fall/winter months, but also contribute to the transport during late spring/summer. Origin of wind trajectories does not support a direct relationship between transporting wind‐layers and material source areas in Northern Africa. High winter fluxes of eolian tracers and high amount of trade winds with continental origin in summer warn against a simplistic interpretation of the seasonal eolian signal preserved in the sediments off Cape Blanc, and the wind layer involved in its transport.

Seasonal productivity dynamics in the pelagic central Benguela System inferred from the flux of carbonate and silicate organisms

Flux of bulk components, carbonate- and silicate-bearing skeleton organisms, and the δ 15N-isotopic signal were investigated on a 1-year time-series sediment trap deployed at the pelagic NU mooring site (Namibia Upwelling, ca. 29°S, 13°E) in the central Benguela System. The flux of bulk components mostly shows bimodal seasonality with major peaks in austral summer and winter, and moderate to low export in austral fall and spring. The calcium carbonate fraction dominates the export of particulates throughout the year, followed by lithogenic and biogenic opal. Planktonic foraminifera and coccolithophorids are major components of the carbonate fraction, while diatoms clearly dominate the biogenic opal fraction. Bulk δ 15N isotopic composition of particulate matter is positively correlated with the total mass flux during summer and fall, while negatively correlated during winter and spring. Seasonal changes in the intensity of the main oceanographic processes affecting the NU site are inferred from variations in bulk component flux, and in the flux and diversity patterns of individual species or group of species. Influence from the Namaqua (Hondeklip) upwelling cell through offshore migration of chlorophyll filaments is stronger in summer, while the winter flux maximum seems to reflect mainly in situ production, with less influence from the coastal and shelf upwelling areas. On a yearly basis, dominant microorganisms correspond well with the flora and fauna of tropical/subtropical waters, with minor contribution of near-shore organisms. The simultaneous occurrence of species with different ecological affinities mirrors the fact that the mooring site was located in a transitional region with large hydrographic variability over short-time intervals.

N2O emissions from snow-covered soils in the Swiss Alps

N2O emissions were investigated at two sites in the Swiss Alps, Alptal (SZ) and Piora Valley (TI), using the closed, vented chamber method during the snow-free period and the snowpack diffusion method during snow cover. Significant spatial variations in surface N2O effluxes between different sampling subsites were observed in both summer and winter. While in winter N2O fluxes from a raw humus site in Alptal and from a site of neutral pH in the Piora Valley were very low (0.01–0.02 mg N2O-N m−2 d−1) and similar to other published values, N2O emissions for acidic and alder-covered sites were 0.3–0.4 mg N2O-N m−2 d−1 and reached 1.7–1.9 mg N2O-N m−2 d−1 for nutrient-rich sites. This suggests that the soil temperatures close to the freezing point were not limiting microbial activity under snow. Snow-covered soils are insulated from extreme air temperatures and stay constant between −0.3 and −1.1°C. They represent an environment where micro-organisms may remain active while vegetation is dormant. N2O liberation is repressed at neutral and raw humus sites, as nitrification is the main source of N2O. During summer the soil microbial activity is strongly related to water availability and temperature: in contrast, in winter both factors are rather constant and the controlling factors of heterotrophic activity are unknown. Our data show that N2O is a significant component of alpine N cycling and that winter N2O fluxes should not be neglected in the calculation of the annual budget for alpine ecosystems, considering that snow covers (sub-) alpine regions for a large part of the year.

Winter CO2 flux from soil and snow surfaces in a cool‐temperate deciduous forest, Japan

We measured diurnal and wintertime changes in CO2 fluxes from soil and snow surfaces in a Japanese cool‐temperate Quercus/Betula forest between December 1994 and May 1995. To evaluate the relationship between these winter fluxes and temperature, flux measurements were made with the open‐flow infrared gas analyzer (IRGA) method rather than with the more commonly used closed chamber method or the snow CO2 profile method. The open‐flow IRGA method proved to be more successful in measurements of winter CO2 fluxes than the two standard methods. Despite colder air temperatures, soil temperature profiles were greater than 0°C because of the thermal insulation effect of deep snowpack. This reveals that soil temperature is satisfactory for microbial respiration throughout the winter. Unfrozen soils under the snowpack showed neither diurnal nor wintertime trends in CO2 fluxes or in soil surface temperature, although there was a daily snow surface CO2 flux of 0.18–0.32 g m−2. By combining this with other reference data, Japanese cool‐temperate forest soils in snowy regions can be estimated to emit &lt; 100 g m−2 carbon over an entire winter, and this value accounts for &lt; 15% of the annual emission. In the present study, when data for all winter fluxes were taken together, fluxes were most highly correlated with deep soil temperatures rather than the soil surface temperature. Such a high correlation can be attributed to the relatively increased respiration of the deep soil where the temperature was higher than the soil surface temperature. Thus, deeper soil temperature is a better predictor of winter CO2 fluxes in cold and snowy ecosystems.

Winter fluxes of greenhouse gases from snow‐covered agricultural soil:intra‐annual and interannual variations

Despite the length of winter in cold temperate climates, few studies refer to greenhouse gas emissions from soils during the nongrowing season. In this study, N2O and CO2 fluxes from agricultural and forest soils in southeastern Quebec (Canada) were measured during winter and spring from 1994 to 1997, and the influences of climate, soil, and snow properties on the gaseous emissions were examined. N2O fluxes were far greater from the agricultural soil (2‐187 ng N2O m−2 s−1) than from the forest soil (&lt; 3 ng N2O m−2 s−1), but CO2 fluxes were equivalent for both soil systems (2‐102 μ CO2 m−2 s−1). The higher N2O concentrations in the lower soil horizons could be explained by positive temperature gradients with depth and concomitant negative gas solubility gradients. However, the higher N2O concentrations could also be explained by variations in the expression of N2O reductase with depth, which can modify the N2/N2O ratios in relation to the availability of O2. Calculated N2O‐N fluxes showed that N losses by gaseous emissions from soils during winter and spring were comparable to, or exceeded, similar reported N losses during the growing season. The highest winter fluxes observed in 1997 were interpreted to be due to favorable meteorological conditions that prevailed for denitrification through high soil water content in summer and fall of 1996. Although interannual and nterseasonal variations of fluxes are important, this study shows that wintertime losses of N2O from agricultural soil can be up to 2 to 4 times greater than emissions measured during the growing season in similar agroecosystems.

Fluxes of CO2and CH4in high latitude wetlands: measuring, modelling and predicting response to climate change

This review covers selected aspects of recent international efforts to measure and model greenhouse gas emission from northern wetlands, to identify the environmental factors that control gas emission, and to investigate wetlands’responses (particularly with respect to gas emission) to global change. Both bottom-up and top-to-bottom approaches, based respectively on local observations plus inventory of gas fluxes and inverse modelling of global circulation, agree on the size of the high latitude (>60°N) contribution to global methane, which should be about 13% or 70 Tg/year. It has been shown that winter and spring fluxes are an essential part in the annual budget of CH4 and especially CO2 exchange (varying from 5 to 50%). Soil micro-organisms were shown to be able to respire during winter even at-16°C. In comparison to aerobically respiring organisms, anaerobic methanogenic bacteria were less active in frozen soil, although they are subjected to significant stimulation by soil freeze-thaw cycles. The absence of immediate coupling of methanogenesis with plant photosynthesis implies that substrates for methane formation are derived from peat decomposition rather than from root exudation.

Temporal variations in nitrous oxide fluxes from urine-affected grassland

Fluxes of nitrous oxide (N 2O) from a permanent pasture were measured during a 1-y period using flux chambers. Intact soil cores from the same pasture were also treated with cow urine (19 g N m −2) in a laboratory incubation experiment. Diurnal variations in flux rates were determined using a mobile laboratory for continuous field measurements of N 2O fluxes at the field site. Nitrous oxide fluxes peaked after fertilizer (urea and NH 4NO 3) application, and winter fluxes were comparable to those measured during other parts of the year, with the annual N 2O flux estimated at 3.2 kg N ha −1 y −1 or 1.3% of the fertilizer-N applied. Nitrous oxide fluxes followed the same diurnal variation as soil temperature, with N 2O production peaking in late afternoon. Multiple linear regression analyses demonstrated weak relationships between N 2O flux, rainfall and soil temperature ( R 2 =0.28). There was an immediate increase in N 2O emission after urine application with rates reaching a peak of 89 mg N m −2 d −1 within 6 h, with an estimated 7% of the applied urine-N being lost as N 2O over 42 d. Results indicate that the large spatial and temporal variability in N 2O fluxes have to be assessed if accurate estimates of N 2O losses are to be derived for grazed grasslands.