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Lotic and lentic ecosystems are traditionally viewed as dominated by either benthic or water column processes. However, mid-sized rivers represent a transition zone where both benthic and water column processes may both contribute substantially to ecosystem dynamics. Ecosystem processes such as gross primary production (GPP), ecosystem respiration (ER), or nutrient uptake, and the relative contribution of the water column to these processes at the reach scale, are poorly understood in non-wadeable, mid-sized rivers. To clarify the role of the water column at the reach-scale, and to quantify controls on water column processes, we measured GPP, ER, and uptake of nitrate (NO3−), ammonium (NH4+), and soluble reactive phosphorus (SRP) in the water columns of 15 mid-sized rivers (discharge: 13.5–83.3 m3 s−1) spanning nutrient and total suspended solids gradients. We compared water column metabolic and nutrient uptake rates to reach-scale rates to estimate the contribution of the water column to the entire river. Water column metabolism was autotrophic on the day when measured, GPP increased with nutrient availability, and the water column contributed more to whole river GPP than to ER. Water column nutrient uptake increased with GPP across solutes, and there was a positive relationship between human land use and water column uptake of NO3−–N and SRP. The water column accounted for a substantial proportion of reach-scale metabolism and nutrient uptake, but this contribution depended on suspended material and nutrient availability. Integrating the water column into theory describing lotic ecosystem function should clarify mechanisms controlling metabolism and nutrient processing and enhance management of non-wadeable rivers.

India has committed to increase carbon sequestration by forests under the Nationally Determined Contributions (NDC). However, the CO2 carbon exchange characteristics (Gross Primary Productivity, Net Ecosystem Exchange, and Ecosystem respiration) of Indian forests are poorly understood. Immense carbon sequestration opportunities exist with the natural as well as planted forests. Therefore, it becomes vital to assess the potential of existing forests (natural and planted) by means of the most accurate and reliable methods of eddy covariance (EC). It is also important to know the eco-physiological response of the forests towards diurnal, seasonal, and yearly changes in the prevailing environmental conditions. We analyzed CO2 carbon exchange characteristics of a mature moist deciduous forest and a young mixed deciduous plantation located in the sub-tropical climate regime in the western Himalayan foothills of India with the help of co-located EC measurements (2016 to 2018). The study reveals that both the mature forest (− 719.43 gCm− 2year− 1) and young plantation (− 467.49 gC m− 2 year− 1) are absorbing a significant amount of atmospheric CO2 carbon. The maximum uptake of atmospheric CO2 occurred during the post-monsoon season in both the forests. The minimum NEE of the mixed deciduous plantation was observed during the leaf fall period while the NEE of the moist deciduous forest was the lowest in monsoon due to a higher respiration rate during monsoon. Among the environmental factors, soil moisture showed maximum control on the productivity of both the forest ecosystems. The value reported in this study can help Forest Survey of India in establishing/assessing emission factor for these particular forest types. This study can also help forest managers in carbon assessment of plantation under NDC.

Salt marsh biogeochemical processes are regulated by ecosystem structure (e.g. plant community composition). However, plant-specific responses to stressors such as elevated nutrient inputs can have differing impacts on nitrogen (N) removal and carbon (C) sequestration. We conducted a field manipulation to investigate the impact of elevated nutrient loading on ecosystem C dynamics and nitrate reduction pathways (denitrification and dissimilatory nitrate reduction to ammonium (DNRA)) in plots dominated by either Juncus roemerianus or Spartina alterniflora that were collocated in a northern Gulf of Mexico salt marsh. We increased N and phosphorus (P) inputs by two- and three-times current levels in the region. Nutrient enrichment had no effect on net ecosystem exchange. However, a three-fold increase in nutrient input resulted in nearly one-third increases in gross primary productivity ($$GPP$$) and ecosystem respiration in S. alterniflora plots, whereas there was no impact in J. roemerianus plots. Denitrification increased in S. alterniflora plots tenfold at both treatment levels relative to controls, but as with $$GPP$$, there was no response in J. roemerianus plots to higher nutrient inputs. In contrast, a three-fold increase in nutrients reduced DNRA by half in J. roemerianus plots. This work demonstrates that plant species-specific responses in marshes need to be considered for determining the impact of higher nutrient inputs on plant productivity and N-removal and retention.

Quantifying ecosystem respiration remains challenging in aquatic ecosystems. Most investigators assume that nighttime and daytime respiration are equal. Recent studies suggest measuring dissolved oxygen isotopes during periods with and without photosynthesis can account for variations in daytime and nighttime respiration. These models are extremely sensitive to the oxygen isotopic fractionation factor (α) value used for respiration, yet almost nothing is known about the variability of α and factors driving that variability. We quantified how α varies with temperature and flow velocity using field measurements, laboratory experiments, and a modeling approach. We measured α in the field using sealed recirculating chambers in 16 rivers from different biomes (temperate, tropical, and sub-arctic) to assess a range of possible α values. The α values were widely variable, and variation was higher among sites in the same biome or ecoregion (e.g. 0.9780 ± 0.005 to 0.9898 ± 0.002 among six desert sites) than across different biomes. Our data revealed that both temperature, flow, and biofilm characteristics produced variations in α, with temperature decreasing and flow increasing it, until leveling off at high flow velocities. Biological and physical processes occurring in the diffusion boundary layer produced variations in α. Our results highlight that environmental conditions produce variable α values, the need for site-specific α measurements, and practical implications for consideration when measuring α in the field. More generally we illustrate an array of factors that can influence isotopic fractionation associated with metabolic activity of biologically active layers that could be important in any diffusion-limited environment.

Wetlands play a disproportionately large role in global terrestrial carbon stocks, and from 1 year to the next individual wetlands can fluctuate between carbon sinks and sources depending on factors such as hydrology, temperature, and land use. Although much research has been done on short-term seasonal to annual wetland biogeochemical cycles, there is a lack of experimental evidence concerning how the reversibility of wetland hydrological changes will influence these cycles over longer time periods. Five years of drought-induced declining water table at Lost Creek, a shrub fen wetland in northern Wisconsin, coincided with increased ecosystem respiration (Reco) and gross primary production (GPP) as derived from long-term eddy covariance observations. Since then, however, the average water table level at this site has increased, providing a unique opportunity to explore how wetland carbon fluxes are affected by interannual air temperature differences as well as changing water table levels. Water table level, as measured by water discharge, was correlated with Reco and GPP at interannual time scales. However, air temperature had a strong correlation with Reco, GPP, and net ecosystem productivity (NEP) at monthly time scales and correlated with NEP at inter-annual time scales. Methane flux was strongly temperature-controlled at seasonal time scales, increasing an order of magnitude from April to July. Annual methane emissions were 51 g C m−2. Our results demonstrate that over multi-year timescales, water table fluctuations can have limited effects on wetland net carbon fluxes and instead at Lost Creek annual temperature is the best predictor of interannual variation.

Frost-patterned grounds, such as mostly barren frost boils surrounded by denser vegetation, are typical habitat mosaics in tundra. Plant and microbial processes in these habitats may be susceptible ...

Headwater streams influence the biogeochemical characteristics of large rivers and play important roles in regional and global carbon budgets. The combined effects of seasonality and land use change on the biogeochemistry of headwater streams, however, are not well understood. In this study we assessed the influence of catchment land use and seasonality on the composition of dissolved organic matter (DOM) and ecosystem metabolism in headwater streams of a Kenyan river. Fifty sites in 34 streams draining a gradient of catchment land use from 100% natural forest to 100% agriculture were sampled to determine temporal and spatial variation in DOM composition. Gross primary production (GPP) and ecosystem respiration (ER) were determined in 10 streams draining primarily forest or agricultural catchments. Absorbance and fluorescence spectrophotometry of DOM reflected notable shifts in composition along the land use gradient and with season. During the dry season, forest streams contained higher molecular weight and terrestrially derived DOM, whereas agricultural streams were dominated by autochthonous production and low molecular weight DOM. During the rainy season, aromaticity and high molecular weight DOM increased in agricultural streams, coinciding with seasonal erosion of soils and inputs of organic matter from farmlands. Most of the streams were heterotrophic. However, GPP and ER were generally greater in agricultural streams, driven by higher dissolved nutrient (mainly TDN) concentrations, light availability (open canopy) and temperature compared with forest streams. There were correlations between freshly and autochthonously produced DOM, GPP and ER during both the dry and wet seasons. This is one of the few studies to link land-use with organic carbon dynamics and DOM composition. Measures of ecosystem metabolism in these streams help to affirm the role of tropical streams and rivers as important components of the global carbon cycle and demonstrate that even semi-intensive, smallholder agriculture can have measurable effects on riverine ecosystem functioning.

The net ecosystem carbon dioxide (CO2) exchange (NEE) between boreal bogs and the atmosphere and its environmental drivers remains understudied despite the large carbon store of these northern ecosystems. We present NEE measurements using the eddy covariance technique in a boreal ombrotrophic bog over five growing seasons and four winters. Inter-annual variability in CO2 uptake was most pronounced in June–September (−4 to −122 g CO2–C m−2), less in March–May (−1 to −21 g CO2–C m−2) and very small in October–November (−2 to −4 g CO2–C m−2). Variability in NEE between years was linked primarily to changes in water table depth (WTD). Strong and significant relationships (r2 > 0.89, p ≤ 0.05) were found between summer (June–September) maximum photosynthetic rate (A max), net ecosystem productivity (NEP), gross ecosystem productivity and WTD. Adding air temperature through multiple regression analysis further increased correlation between summer A max, NEP, and WTD (r2 = 0.96, p = 0.05). In contrast to previous studies examining controls on peatland CO2 exchange, no relationships were found between productivity or cumulative exchange and early season temperature, timing of the snowmelt or growing season length.

The aim of this study was to establish the main drivers of the spatial variability in growing season CH4 flux within an arctic wetland ecosystem. During 3 years (2011–2013) we measured CH4 flux and potential drivers, e.g., CO2 fluxes (net ecosystem exchange (NEE), gross primary productivity (GPP) and ecosystem respiration), temperature, water table depth, pore-water concentration of organic acids (e.g., acetate) and the vascular plant composition and density. The study included 16–20 main plots (Cmain) and in 2013 also experimental plots (10 excluded muskoxen grazing, 9 snow fence and 10 automated chamber plots) distributed over 0.3 km2. The results show a 1.8-times difference in CH4 flux magnitude inter-annually and 9- to 35-times spatially (depending on year and treatment). During all 3 years GPP was a strong driver of the variability in Cmain plots. Accordingly, the plant productivity related variables NEE, GPP and acetate were singled out as the strongest drivers of the variability in 2013, when all variables were measured on a majority of the plots. These variables were equally strong drivers of the spatial variability in CH4 flux regardless of whether experimental plots were included in the analysis or not. The density of Eriophorum scheuchzeri was the strongest driver of the spatial variability in NEE, GPP and acetate. In conclusion, changes in vegetation composition or productivity of wet arctic ecosystems will have large impacts on their carbon balance and CH4 flux, irrespective of whether these changes are driven directly by climate change or by biotic interactions, such as grazing.

Carbon gas fluxes in tidal marshes vary spatially and temporally because of vegetation cover, subsurface biogeochemical processes, and environmental forcing. The objective of this study was to examine how ecosystem carbon gas exchange changes along an estuarine salinity gradient. We measured carbon dioxide (CO2) and methane (CH4) gas fluxes from three marshes representing a salinity gradient (0–32 ppt) in the Mobile Bay estuary, Alabama, USA. CH4 flux was relatively small with no significant differences across sites despite salinity differences. Interestingly, sediment porewater CH4 concentrations were significantly higher at the high salinity salt marsh and decreased with decreasing salinity. Midday net ecosystem exchange (where a positive rate indicates net carbon assimilated through photosynthesis) was greatest at the most fresh site (4.8 ± 0.3 µmol CO2 m−2 s−1), followed by the saline (2.8 ± 1.0 µmol CO2 m−2 s−1) and brackish (1.4 ± 0.6 µmol CO2 m−2 s−1) sites. However, net ecosystem exchange integrated diurnally revealed each marsh to be a net CO2 source to the atmosphere as a result of high ecosystem respiration with the freshwater marsh emitting more CO2 (−893.4 ± 187.9 g C m−2 year−1) than the brackish (−517.8 ± 85.2 g C m−2 year−1) and salt marsh (−410.2 ± 98.2 g C m−2 year−1). This finding leads to the conclusion that either the marshes are losing carbon or that they receive a subsidy of respirable carbon, possibly via tidal deposition. The extent to which sedimentation from tidal deposition contributes carbon to these ecosystems, however, remains unknown. Without such a subsidy, marshes in the study area will not be able to keep up with sea level rise.

Tidal wetlands are productive ecosystems with the capacity to sequester large amounts of carbon (C), but we know relatively little about the impact of climate change on wetland C cycling in lower salinity (oligohaline and tidal freshwater) coastal marshes. In this study we assessed plant production, C cycling and sequestration, and microbial organic matter mineralization at tidal freshwater, oligohaline, and salt marsh sites along the salinity gradient in the Delaware River Estuary over four years. We measured aboveground plant biomass, carbon dioxide (CO2) and methane (CH4) exchange between the marsh and atmosphere, microbial sulfate reduction and methanogenesis in marsh soils, soil biogeochemistry, and C sequestration with radiodating of soils. A simple model was constructed to estimate monthly and annually integrated rates of gross ecosystem production (GEP), ecosystem respiration (ER) to carbon dioxide (\( {\text{ER}}_{{{\text{CO}}_{2} }} \)) or methane (\( {\text{ER}}_{{{\text{CH}}_{4} }} \)), net ecosystem production (NEP), the contribution of sulfate reduction and methanogenesis to ER, and the greenhouse gas (GHG) source or sink status of the wetland for 2 years (2007 and 2008). All three marsh types were highly productive but evidenced different patterns of C sequestration and GHG source/sink status. The contribution of sulfate reduction to total ER increased along the salinity gradient from tidal freshwater to salt marsh. The Spartina alterniflora dominated salt marsh was a C sink as indicated by both NEP (~140 g C m−2 year−1) and 210Pb radiodating (336 g C m−2 year−1), a minor sink for atmospheric CH4, and a GHG sink (~620 g CO2-eq m−2 year−1). The tidal freshwater marsh was a source of CH4 to the atmosphere (~22 g C–CH4 m−2 year−1). There were large interannual differences in plant production and therefore C and GHG source/sink status at the tidal freshwater marsh, though 210Pb radiodating indicated modest C accretion (110 g C m−2 year−1). The oligohaline marsh site experienced seasonal saltwater intrusion in the late summer and fall (up to 10 mS cm−1) and the Zizania aquatica monoculture at this site responded with sharp declines in biomass and GEP in late summer. Salinity intrusion was also linked to large effluxes of CH4 at the oligohaline site (>80 g C–CH4 m−2 year−1), making this site a significant GHG source (>2,000 g CO2-eq m−2 year−1). The oligohaline site did not accumulate C over the 2 year study period, though 210Pb dating indicated long term C accumulation (250 g C m−2 year−1), suggesting seasonal salt-water intrusion can significantly alter C cycling and GHG exchange dynamics in tidal marsh ecosystems.

Urbanization has resulted in the extensive burial and channelization of headwater streams, yet little is known about the impacts of stream burial on ecosystem functions critical for reducing downstream nitrogen (N) and carbon (C) exports. In order to characterize the biogeochemical effects of stream burial on N and C, we measured NO3 − uptake (using 15N-NO3 − isotope tracer releases) and gross primary productivity (GPP) and ecosystem respiration (ER) (using whole stream metabolism measurements). Experiments were carried out during four seasons, in three paired buried and open stream reaches, within the Baltimore Ecosystem Study Long-term Ecological Research site. Stream burial increased NO3 − uptake lengths by a factor of 7.5 (p

Piped streams, or streams that run underground, are often associated with urbanization. Despite the fact that they are ubiquitous in many urban watersheds, there is little empirical evidence regarding the ecological structure and function of piped stream reaches. This study measured ecosystem metabolism, nutrient uptake, and related characteristics of Pettee Brook—an urban stream that flows through several piped sections in Durham, New Hampshire, USA. Pettee Brook had high chloride and nutrient concentrations, low benthic biomass, and low rates of gross primary productivity (GPP), ecosystem respiration (ER), and nutrient uptake along its entire length during summer. Spring was a period of elevated biological activity, as increased light availability in the un-piped sections of the stream led to substantially higher GPP, ER, NH4 uptake, and PO4 uptake in these open reaches. Piped reaches of Pettee Brook were similar to open reaches in terms of water quality, dissolved O2 concentration, temperature, and discharge. Piped reaches did, however, have significantly less light, shallower sediments, and no debris dams. The absence of light inhibited autotrophic activity in piped reaches, resulting in the complete loss of GPP as well as a significant reduction in benthic AFDM and chlorophyll a biomass. Heterotrophic activity in piped reaches was not impaired to the same extent as autotrophic activity. Reduced ER was observed in piped reaches during the summer, but we failed to find significantly lower DOC or nutrient uptake rates in piped reaches than in open reaches. Carbon consumption in piped reaches, which do not have significant autochthonous or allochthonous carbon replenishment, must rely primarily on upstream inputs of organic matter. These results suggest that although ecological conditions in piped streams may be degraded beyond the extent of other urban stream reaches, piped reaches may still sustain some measurable ecosystem function.

We evaluated the nighttime CO2 flux (ecosystem respiration) on Rishiri Island, located at the northern tip of Hokkaido, Japan, from 2009 to 2011, by using the relationship between atmospheric 222Rn and CO2 concentrations. The annual mean CO2 flux was 1.8 μmol m−2 s−1, with a maximum monthly mean in July (4.6 ± 2.6 μmol m−2 s−1) and a broad minimum from December to March (0.33 ± 0.29 μmol m−2 s−1). The annual mean was comparable to fluxes at the JapanFlux sites in northern Japan. During the season of snow cover (mid-December to early April), the CO2 flux was low (0.45 ± 0.43 μmol m−2 s−1). Total annual respiration was estimated at 679 ± 174 g cm−2, about 8 % of which occurred during the season of snow cover.

The rate at which CO2 is released from woody debris post-clearcut affects the long term carbon consequences of such disturbances. Changes in microclimate post-clearcut may alter the rate of woody debris decomposition from that in a mature forest. However, very few studies have explored post-disturbance rates of woody debris respiration and the possible influence of an altered microclimate, and even fewer have considered the role of log position in influencing rates of respiration. This study explored the effects of log position and microclimate variability on the rates of coarse woody debris (CWD) respiration. The rates of respiration of downed Norway spruce (Picea abies) logs were repeatedly measured in situ using an LI-6200 gas analyzer. Treatments included native logs in the clearcut site, native logs in a neighboring mature spruce stand, and logs transferred from the clearcut site to the mature spruce stand. The transfer logs showed the highest rates of respiration (0.44 ± 0.03 g CO2 m−2 log surface h−1), followed by the clearcut logs (0.36 ± 0.02 g CO2 m−2 log surface h−1), and spruce stand logs (0.30 ± 0.02 g CO2 m−2 log surface h−1) (P

The carbon pool of peatlands has been considered as potentially unstable in a changing climate. This study is the first presenting carbon dioxide (CO2) net ecosystem exchange, CO2 efflux due to ecosystem respiration and CO2 uptake by gross primary production over a complete growing season for different microforms of a boreal peatland in Russia (61°56′N, 50°13′E). CO2 fluxes were measured using the closed chamber technique from the 25th April in the period of snow melt until the end of the vegetation period and the first frost on the 20th October 2008 at seven different microform types: minerogenous and ombrogenous hollows, lawns and hummocks, respectively, and Carex lawns situated in a transition zone between minerogenous and ombrogenous mire parts. The total number of chamber flux measurements was 5,517. Ombrogenous hummocks and lawns were sources of CO2 over the investigation period whereas hollows and minerogenous lawns were CO2 sinks. Some plots of Carex lawns and minerogenous hummocks were sinks while other plots of these microform types were sources. The CO2 fluxes were characterised by large variability not only between the microform types but also within the respective microform types. Of all microform types, the Carex, ombrogenous, and minerogenous lawns showed the highest variability in CO2 fluxes, which is probably related to a stronger within-microform heterogeneity in vegetation composition and coverage as well as in the water table level. Air temperature was one of the dominant controls on the CO2 flux dynamics. Water table and green area index were found to have strong influence on CO2 fluxes both within different patches of the same microform type as well as between different microforms.

We compared the CO2 exchange and its controls in the plant communities of a strongly patterned aapa mire, the Kaamanen fen in northern Finland. Based on a systematic vegetation inventory and an ordination analysis, four plant community types were chosen for the study: Ericales–Pleurozium string tops, Betula–Sphagnum string margins, Trichophorum tussock flarks and Carex–Scorpidium wet flarks. We measured plant community CO2 exchange with a closed chamber technique during the growing season of 2007 and early summer of 2008. Nonlinear regression models were used for simulating the CO2 exchange over the measurement period for different mire components and for the whole mire. The CO2 exchange dynamics distinguished two functional components in the mire: an ombrotrophic component (Ericales–Pleurozium string tops) and a minerotrophic component (other plant community types). Minerotrophic plant communities responded similarly to environmental controls, the most important of these being variation in leaf area and aerobic peat volume (water level). The ombrotrophic component dynamics were more obscure; frost and possibly peat moisture played a role. The minerotrophic communities functioned as effective CO2 sinks in the simulation, while the net CO2 exchange of the ombrotrophic community was close to zero. The smaller NEE of the ombrotrophic community was due to less efficient photosynthesis per unit leaf area in combination with high ecosystem respiration resulting from a large aerobic peat volume. Our study shows that a fen/bog functional dichotomy can also exist within one mire. Wet minerotrophic communities within northern mires can act as effective CO2 sinks.

The South Atlantic Bight (SAB) has been a focus for the study of continental shelf ecosystem respiration during the past two decades. However, two questions concerning respiration in this area have yet to be answered. First, why do previous estimates of respiration in the SAB exceed measured carbon fixation rates by almost an order of magnitude? Second, considering that bacteria are responsible for most of the pelagic community respiration in the SAB, why is respiration almost uniform from the coastline to the shelf break, while bacterial production estimates decrease offshore? This study addresses these critical questions by presenting new pelagic community respiration data that were collected across the entire width of the continental shelf off Georgia, USA from June 2003 to May 2006. The respiration was calculated as in vitro changes of dissolved oxygen and dissolved inorganic carbon concentrations during deck incubations. The measured respiration rates ranged from 0.3(±0.1) to 21.2(±1.4) mmol m−3 day−1. They followed a clear seasonal pattern, being lowest over the entire shelf in winter and reaching maxima in summer. Summertime respiration rates were highest on the inner shelf and decreased with distance offshore. Consistent with this trend, bacterial abundance measurements taken during the sampling month of July 2005 followed a pattern of seaward decline. The SAB organic carbon fluxes calculated from the respiration data are close to the estimates for primary production, which resolves a long-standing mystery regarding perceived carbon imbalance in the SAB.

There is considerable interest in how ecosystems will respond to changes in precipitation. Alterations in rain and snowfall are expected to influence the spatio-temporal patterns of plant and soil processes that are controlled by soil moisture, and potentially, the amount of carbon (C) exchanged between the atmosphere and ecosystems. Because grasslands cover over one third of the terrestrial landscape, understanding controls on grassland C processes will be important to forecast how changes in precipitation regimes will influence the global C cycle. In this study we examined how irrigation affects carbon dioxide (CO2) fluxes in five widely variable grasslands of Yellowstone National Park during a year of approximately average growing season precipitation. We irrigated plots every 2 weeks with 25% of the monthly 30-year average of precipitation resulting in plots receiving approximately 150% of the usual growing season water in the form of rain and supplemented irrigation. Ecosystem CO2 fluxes were measured with a closed chamber-system once a month from May-September on irrigated and unirrigated plots in each grassland. Soil moisture was closely associated with CO2 fluxes and shoot biomass, and was between 1.6% and 11.5% higher at the irrigated plots (values from wettest to driest grassland) during times of measurements. When examining the effect of irrigation throughout the growing season (May–September) across sites, we found that water additions increased ecosystem CO2 fluxes at the two driest and the wettest sites, suggesting that these sites were water-limited during the climatically average precipitation conditions of the 2005 growing season. In contrast, no consistent responses to irrigation were detected at the two sites with intermediate soil moisture. Thus, the ecosystem CO2 fluxes at those sites were not water-limited, when considering their responses to supplemental water throughout the whole season. In contrast, when we explored how the effect of irrigation varied temporally, we found that irrigation increased ecosystem CO2 fluxes at all the sites late in the growing season (September). The spatial differences in the response of ecosystem CO2 fluxes to irrigation likely can be explained by site specific differences in soil and vegetation properties. The temporal effects likely were due to delayed plant senescence that promoted plant and soil activity later into the year. Our results suggest that in Yellowstone National Park, above-normal amounts of soil moisture will only stimulate CO2 fluxes across a portion of the ecosystem. Thus, depending on the topographic location, grassland CO2 fluxes can be water-limited or not. Such information is important to accurately predict how changes in precipitation/soil moisture will affect CO2 dynamics and how they may feed back to the global C cycle.

Measurement of net ecosystem exchange was made using the eddy covariance method above three forests along a north-south climatic gradient in Sweden: Flakaliden in the north, Knottasen in central and Asa in south Sweden. Data were obtained for 2 years at Flakaliden and Knottasen and for one year at Asa. The net fluxes (Nep) were separated into their main components, total ecosystem respiration (Rt) and gross primary productivity (Pg). The maximum half-hourly net uptake during the heart of the growing season was highest in the southernmost site with −0.787 mg CO2 m−2 s−1 followed by Knottasen with −0.631 mg CO2 m−2 s−1 and Flakaliden with −0.429 mg CO2 m−2 s−1. The maximum respiration rates during the summer were highest in Knottasen with 0.245 mg CO2 m−2 s−1 while it was similar at the two other sites with 0.183 mg CO2 m−2 s−1. The annual Nep ranged between uptake of −304 g C m−2 year−1 (Asa) and emission of 84 g C m−2 year−1 (Knottasen). The annual Rt and Pg ranged between 793 to 1253 g C m−2 year−1 and −875 to −1317 g C m−2 year−1, respectively. Biomass increment measurements in the footprint area of the towers in combination with the measured net ecosystem productivity were used to estimate the changes in soil carbon and it was found that the soils were losing on average 96–125 g C m−2 year−1. The most plausible explanation for these losses was that the studied years were much warmer than normal causing larger respiratory losses. The comparison of net primary productivity and Pg showed that ca 60% of Pg was utilized for autotrophic respiration.

We measured net ecosystem CO2 flux (F n) and ecosystem respiration (R E), and estimated gross ecosystem photosynthesis (P g) by difference, for two years in a temperate heath ecosystem using a chamber method. The exchange rates of carbon were high and of similar magnitude as for productive forest ecosystems with a net ecosystem carbon gain during the second year of 293 ± 11 g C m−2 year−1 showing that the carbon sink strength of heather-dominated ecosystems may be considerable when C. vulgaris is in the building phase of its life cycle. The estimated gross ecosystem photosynthesis and ecosystem respiration from October to March was 22% and 30% of annual flux, respectively, suggesting that both cold-season carbon gain and loss were important in the annual carbon cycle of the ecosystem. Model fit of R E of a classic, first-order exponential equation related to temperature (second year; R 2 = 0.65) was improved when the P g rate was incorporated into the model (second year; R 2 = 0.79), suggesting that daytime R E increased with increasing photosynthesis. Furthermore, the temperature sensitivity of R E decreased from apparent Q 10 values of 3.3 to 3.9 by the classic equation to a more realistic Q 10 of 2.5 by the modified model. The model introduces R photo, which describes the part of respiration being tightly coupled to the photosynthetic rate. It makes up 5% of the assimilated carbon dioxide flux at 0°C and 35% at 20°C implying a high sensitivity of respiration to photosynthesis during summer. The simple model provides an easily applied, non-intrusive tool for investigating seasonal trends in the relationship between ecosystem carbon sequestration and respiration.

We explored the influence of small-scale spatial variation in soil moisture on CO2 fluxes in the high Arctic. Of five sites forming a hydrological gradient, CO2 was emitted from the three driest sites and only the wettest site was a net sink of CO2. Soil moisture was a good predictor of net ecosystem exchange (NEE). Higher gross ecosystem photosynthesis (GEP) was linked to higher bryophyte biomass and activity in response to the moisture conditions. Ecosystem respiration (Re) rates increased with soil moisture until the soil became anaerobic and then Re decreased. At well-drained sites Re was driven by GEP, suggesting substrate and moisture limitation of soil respiration. We propose that spatial variability in soil moisture is a primary driver of NEE.

Changes in timing or amount of precipitation may be of great consequence for carbon cycling in the Mixedgrass Prairie of N. America, because CO2 fixation and efflux are tightly coupled to soil water properties. The objective of our project was to quantify how ecosystem respiration (Re) responds to experimental changes in winter and summer precipitation in a Mixedgrass Prairie using in situ field manipulations of snow depth and summer rain. Our study was conducted at the USDA-ARS High Plains Grasslands Research Station, west of Cheyenne, Wyoming. We installed three replicated 50 m snow fences to increase winter snow on the leeward side of the snow fence and experimentally manipulated summer precipitation by either increasing (+50%) or decreasing (−50%) precipitation amounts. We also measured ambient conditions. Re rates in May were around 2 g C m−2 d−1 for all treatments and increased to their greatest values in June, up to 10 g C m−2 d−1, with the ambient treatment having the largest flux rates. There were no treatment effects during the early summer, but by midsummer, Re rates were least in the reduced rainfall plots and greatest in the snow plots. Soil moisture and gross photosynthesis had strong influence on the daily Re rates, but soil temperature had little correlation with daily Re rates. In summary, the Re rates in this Mixedgrass Prairie are strongly influenced by changes in precipitation, especially winter snow accumulation. Thus, carbon cycle estimates under future climate change scenarios need to include not only the affects of changes in summer rain, but also, the consequences of deep snow in winter and itsȁ9 affect on carbon cycling processes in winter and subsequent summers.

This paper reviews the role of soil respiration in determining ecosystem carbon balance, and the conceptual basis for measuring and modeling soil respiration. We developed it to provide background and context for this special issue on soil respiration and to synthesize the presentations and discussions at the workshop. Soil respiration is the largest component of ecosystem respiration. Because autotrophic and heterotrophic activity belowground is controlled by substrate availability, soil respiration is strongly linked to plant metabolism, photosynthesis and litterfall. This link dominates both base rates and short-term fluctuations in soil respiration and suggests many roles for soil respiration as an indicator of ecosystem metabolism. However, the strong links between above and belowground processes complicate using soil respiration to understand changes in ecosystem carbon storage. Root and associated mycorrhizal respiration produce roughly half of soil respiration, with much of the remainder derived from decomposition of recently produced root and leaf litter. Changes in the carbon stored in the soil generally contribute little to soil respiration, but these changes, together with shifts in plant carbon allocation, determine ecosystem carbon storage belowground and its exchange with the atmosphere. Identifying the small signal from changes in large, slow carbon pools in flux dominated by decomposition of recent material and autotrophic and mycorrhizal respiration is a significant challenge. A mechanistic understanding of the belowground carbon cycle and of the response of different components to the environment will aid in identifying this signal. Our workshop identified information needs to help build that understanding: (1) the mechanisms that control the coupling of canopy and belowground processes; (2) the responses of root and heterotrophic respiration to environment; (3) plant carbon allocation patterns, particularly in different forest developmental stages, and in response to treatments (warming, CO2, nitrogen additions); and (4) coupling measurements of soil respiration with aboveground processes and changes in soil carbon. Multi-factor experiments need to be sufficiently long to allow the systems to adjust to the treatments. New technologies will be necessary to reduce uncertainty in estimates of carbon allocation, soil carbon pool sizes, and different responses of roots and microbes to environmental conditions.

Soil CO2 flux can contribute as much as 60-80% of total ecosystem respiration in forests. Although considerable research has focused on quantifying this flux during the growing season, comparatively little effort has focused on non-growing season fluxes. We measured soil CO2 efflux through snow in 50 and � 300 year old subalpine forest stands near Fraser CO. Our objectives were to quantify seasonal patterns in wintertime soil CO2 flux; determine if differences in soil CO2 flux between the two forest ages during the growing season persist during winter; and to quantify the sample size necessary to discern treatment differences. Soil CO2 flux during the 2002-2003 and 2003-2004 snow season averaged 0.31 and 0.35 lmols m � 2 s � 1 for the young and old forests respectively; similar to the relative difference observed during summer. There was a significant seasonal pattern of soil CO2 flux during the winter with fluxes averaging 0.22 lmols m � 2 s � 1 in December and January and increasing to an average of 0.61 lmols m � 2 s � 1 in May. Within-plot variability for measurements used in calculating flux was low. The coefficients of variation (CV) for CO2 concentration, snowpack density, and snow depth were 17, 8 and 14%, respectively, yielding a CV for flux measurements within-plot of 29%. A within plot CV of 29% requires 8 sub-samples per plot to estimate the mean flux with a standard error of ±10% of the mean. Variability in CO2 flux estimates among plots (size = 400 m 2 ) was similar to that within plot and was also low (CV = � 28%). With a CV of 28% among plots, ten plots per treatment would have a 50% probability of detecting a 25% difference in treatment means for a = 0.05.

Carbon pools and fluxes were quantified along an environmentalgradient in northern Arizona. Data are presented on vegetation, litter, andsoil C pools and soil CO2 fluxesfrom ecosystems ranging from shrub-steppe through woodlands to coniferousforest and the ecotones in between. Carbon pool sizes and fluxes in thesesemiarid ecosystems vary with temperature and precipitation and are stronglyinfluenced by canopy cover. Ecosystem respiration is approximately 50percent greater in the more mesic, forest environment than in the dryshrub-steppe environment. Soil respiration rates within a site varyseasonally with temperature but appear to be constrained by low soilmoisture during dry summer months, when approximately 75% of totalannual soil respiration occurs. Total annual amount of CO2 respired across all sites ispositively correlated with annual precipitation and negatively correlatedwith temperature. Results suggest that changes in the amount and periodicityof precipitation will have a greater effect on C pools and fluxes than willchanges in temperature in the semiarid Southwestern United States.

We estimated whole-ecosystem rates of respiration over a 40-km stretch of the tidally influenced freshwater Hudson River every 2 to 3 weeks from May through November. We measured in situ concentrations of oxygen over depth at dusk and dawn at 10 stations spaced over this interval. The use of multiple stations allowed for the consideration of the influence of tidal advection of water masses. Respiration was estimated from the decrease in oxygen overnight with a correction for diffusive exchange of oxygen with the atmosphere. We estimated this flux of oxygen to or from the atmosphere using the measured oxygen gradient and a transfer velocity model which is a function of wind velocity. Integration of the data for the period of May through November yields an estimate of whole-ecosystem respiration of 591 g C m−2 (S.E. = 66). That the standard error of this estimate is relatively low (11% of the estimate) indicates that the use of multiple stations adequately deals with error introduced through the advection of water between stations. The logarithm of average daily respiration rate was correlated with average daily temperature (p = 0.007;r 2 = 0.62). We used this temperature-respiration relationship to derive an estimate of the annual respiration rate of 755 g C m−2 yr−1 (S.E. = 72). This estimate is moderately sensitive to the estimated flux of oxygen between the atmosphere and water; using the lower and upper 95% confidence limits of our model relating the transfer velocity of oxygen to wind speed gives a range of annual respiration estimates from 665 g C m−2 yr−1 to 984 g C m−2 yr−1. The river is strongly heterotrophic, with most respiration driven by allochthonous inputs of organic matter from terrestrial ecosystems. The majority of the allochthonous inputs to the river (over 60%) are apparently metabolized within the river. Any change in allochthonous inputs due to changes in land use or climate patterns would be expected to alter the oxygen dynamics and energy flow within this tidally influenced river.

Herbivory is an important part of most ecosystems, and grazing alone can have a considerable impact on the ecosystems carbon balance with both direct and indirect effects. Removal of above-ground biomass by consumption of herbivores will change the below-ground carbon stock; the reduction of litter that goes into the ground will influence the total ecosystem carbon content. Little is however known about how plant-herbivory interactions effect the carbon balance, in particular methane emissions, of high arctic mires. We hypothesized that increased grazing pressure will change carbon allocation patterns resulting in decreased net ecosystem uptake of carbon and subsequently in lower methane emissions. An in-situ field experiment was conducted over 3 years in a high arctic mire at Zackenberg in NE Greenland. The experiment consisted of three treatments, with five replicates of each (1) control, (2) vascular plants were removed (NV), (3) clipped twice each growing season in order to simulate increased muskox grazing. Immediately after the initiation of the experiment net ecosystem uptake of CO2 decreased in clipped plots (mean total decrease for the three following years was 35 %). One year into the experiment a significantly lower CH4 emission was observed in these plots, the total mean reduction for the following 2 years was 26 %. Three years into the experiment significantly lower substrate (acetic acid) availability for CH4 production was observed (27 % reduction). NV plots had a mean decrease in CO2 uptake of 113 %, a 62 % decrease in ecosystem respiration and an 84 % decrease in CH4 emission (mean of all 3 years). Our study shows that increased grazing pressure in a high arctic mire can lead to significant changes in the carbon balance, with lower CO2 uptake leading to lower production of substrate for CH4 formation and in lower CH4 emission.