Variation In Ecosystem Respiration Research Articles

Drought in the middle growing season inhibited carbon uptake more critical in an anthropogenic shrub ecosystem of Northwest China

Large-scale ecological restoration efforts of the degraded desert steppe ecosystem in northwestern China have effectively combated desertification but have also resulted in the encroachment of shrubs. However, the function of the anthropogenic shrub ecosystem is still unknown under climate change, especially the response of carbon fluxes to drought, which hinders our further understanding of its sustainability in the future. Hence, based on the eddy covariance and environment sensors, continuous carbon and water fluxes and biophysical factors were monitored during 2019–2022 to investigate the characteristics of carbon fluxes and their response to seasonal drought in an anthropogenic shrub ecosystem dominated by Caragana liouana in the southern Mu Us sandy land, northwestern China. The results showed the shrub ecosystem was a stable carbon sink over the 4 years despite large interannual variability, with annual net ecosystem production (NEP) ranging from 54.37 to 150.35 g C·m−2·y−1. Interannual variability in NEP was mainly due to variation in gross primary productivity (GPP), which is driven by the fact that interannual variability in GPP was higher than interannual variability in ecosystem respiration (Reco). While drought inhibited GPP more than Reco, thus weakening the carbon sequestration ability of the ecosystem during the middle growing season, and thereby carbon fluxes in the shrub ecosystem were significantly suppressed by drought in the middle growing season but slightly in the early growing season. Furthermore, the magnitude and variation of carbon fluxes were determined by the maximum apparent photosynthetic capacity of the canopy (Pmax), leaf area index (LAI), and canopy conductance (gc), all of which are limited by water stress during drought. The work suggests that to keep the carbon sink of anthropogenic shrub ecosystems in arid and semi-arid areas, managers should focus on drought-relief measures in the middle growing season that are better adapted to future climate change.

High rates of daytime river metabolism are an underestimated component of carbon cycling

River metabolism and, thus, carbon cycling are governed by gross primary production and ecosystem respiration. Traditionally river metabolism is derived from diel dissolved oxygen concentrations, which cannot resolve diel changes in ecosystem respiration. Here, we compare river metabolism derived from oxygen concentrations with estimates from stable oxygen isotope signatures (δ18O2) from 14 sites in rivers across three biomes using Bayesian inverse modeling. We find isotopically derived ecosystem respiration was greater in the day than night for all rivers (maximum change of 113 g O2 m−2 d−1, minimum of 1 g O2 m−2 d−1). Temperature (20 °C) normalized rates of ecosystem respiration and gross primary production were 1.1 to 87 and 1.5 to 22-fold higher when derived from oxygen isotope data compared to concentration data. Through accounting for diel variation in ecosystem respiration, our isotopically-derived rates suggest that ecosystem respiration and microbial carbon cycling in rivers is more rapid than predicted by traditional methods.

CO2 and CH4 exchanges between moist moss tundra and atmosphere on Kapp Linné, Svalbard

Abstract. We measured CO2 and CH4 fluxes using chambers and eddy covariance (only CO2) from a moist moss tundra in Svalbard. The average net ecosystem exchange (NEE) during the summer (9 June–31 August) was negative (sink), with −0.139 ± 0.032 µmol m−2 s−1 corresponding to −11.8 g C m−2 for the whole summer. The cumulated NEE over the whole growing season (day no. 160 to 284) was −2.5 g C m−2. The CH4 flux during the summer period showed a large spatial and temporal variability. The mean value of all 214 samples was 0.000511 ± 0.000315 µmol m−2 s−1, which corresponds to a growing season estimate of 0.04 to 0.16 g CH4 m−2. Thus, we find that this moss tundra ecosystem is closely in balance with the atmosphere during the growing season when regarding exchanges of CO2 and CH4. The sink of CO2 and the source of CH4 are small in comparison with other tundra ecosystems in the high Arctic. Air temperature, soil moisture and the greenness index contributed significantly to explaining the variation in ecosystem respiration (Reco), while active layer depth, soil moisture and the greenness index were the variables that best explained CH4 emissions. An estimate of temperature sensitivity of Reco and gross primary productivity (GPP) showed that the sensitivity is slightly higher for GPP than for Reco in the interval 0–4.5 ∘C; thereafter, the difference is small up to about 6 ∘C and then begins to rise rapidly for Reco. The consequence of this, for a small increase in air temperature of 1∘ (all other variables assumed unchanged), was that the respiration increased more than photosynthesis turning the small sink into a small source (4.5 g C m−2) during the growing season. Thus, we cannot rule out that the reason why the moss tundra is close to balance today is an effect of the warming that has already taken place in Svalbard.

Seasonal and diurnal variation in ecosystem respiration and environmental controls from an alpine wetland in arid northwest China

AbstractWetlands store large amounts of carbon stocks and are essential in both global carbon cycling and regional ecosystem services. Understanding the dynamics of wetland carbon exchange is crucial for assessing carbon budgets and predicting their future evolution. Although many studies have been conducted on the effects of climate change on the ecosystem carbon cycle, little is known regarding carbon emissions from the alpine wetlands in arid northwest China. In this study, we used an automatic chamber system (LI-8100A) to measure ecosystem respiration (ER) in the Bayinbuluk alpine wetland in northwest China. The ER showed a significant bimodal diurnal variation, with peak values appearing at 16:30 and 23:30 (Beijing time, UTC + 8). A clear seasonal pattern in ER was observed, with the highest value (19.38 µmol m−2 s−1) occurring in August and the lowest value (0.11 µmol m−2 s−1) occurring in late December. The annual ER in 2018 was 678 g C m−2 and respiration during the non-growing season accounted for 13% of the annual sum. Nonlinear regression revealed that soil temperature at 5 cm depth and soil water content (SWC) were the main factors controlling the seasonal variation in ER. The diurnal variation in ER was mainly controlled by air temperature and solar radiation. Higher temperature sensitivity (Q10) occurred under conditions of lower soil temperatures and medium SWC (25% ≤ SWC ≤ 40%). The present study deepens our understanding of CO2 emissions in alpine wetland ecosystems and helps evaluate the carbon budget in alpine wetlands in arid regions.

Interannual variation in ecosystem respiration in an Inner Mongolian meadow steppe in response to livestock grazing

Understanding the effects of livestock grazing on ecosystem respiration (Re) of grassland ecosystems is critical for accurately assessing the feedback of grazing management to climate change. We examined ecosystem respiration in response to varying cattle grazing intensities during growing seasons from 2009 to 2018 in a meadow steppe ecosystem of eastern Inner Mongolia. We found that ungrazed swards had the highest mean annual Re rate, with seasonal CVs in Re ranging from 37.53% to 46.04% for all treatments. When all treatments were analysed as a whole, we identified a significant positive relationship between the annual Re rate and annual peak value of standing plant aboveground biomass. Our findings showed that controlling factors on the mean annual Re differed substantially with grazing intensity. In ungrazed and lightly-grazed plots (G0.00 and G0.23), the mean annual Re rate was controlled mainly by canopy height and/or rainfall, while it was controlled more predominantly by contents of NH4+-N and available phosphorus in moderately and heavily grazed plots. We detected significant positive relationships of the annual Re rate with rainfall, soil moisture, ammonium nitrogen, and soil available phosphorus during the entire study period, whereas significant negative relationships were detected between the annual ecosystem respiration rate and the mean growing season temperature, irrespective of grazing intensity. Our findings revealed that grazing could substantially simplify the relationship between the mean annual Re rate and biotic and abiotic parameters. It may be concluded that the relationship between the annual Re rate and the standing crop aboveground biomass was a principal mechanism underlying the effects of gradient grazing on the Re of Chinese meadow steppe ecosystems.

Eight years of variations in ecosystem respiration over a residue-incorporated rotation cropland and its controlling factors

The carbon dioxide emissions from cropland play important roles in the regional carbon budget. In this study, continuous measurements of the ecosystem respiration (RE) were obtained using the eddy covariance technique in a winter wheat–summer maize double cropping agroecosystem mainly between 2004 and 2012 in order to identify the among-year variations in RE and the related factors responsible. The annual RE, estimated by Lloyd and Taylor model, which was the most accurate, was 1866.4 ± 105.75 g C m−2 year−1 and it ranged from 1650.68 g C m−2 year−1 to 1945.57 g C m−2 year−1 during the eight years. The seasonal RE values were 867.98 ± 125.24 g C m−2 year−1 and 890.55 ± 131.34 g C m−2 year−1 for wheat and maize, respectively. Additionally, crop residue carbon ranged from 322.73 g C m−2 year−1 in 2012 and 453.49 g C m−2 year−1 in 2007. Correlation analysis indicated that the interannual variations in wheat and maize RE were correlated with the seasonal mean soil water content (W-Ws) and maximum leaf area index (W-LAImax) of wheat, and seasonal mean air temperature of maize (S-Ta), respectively. A rest method was attempted to investigate whether these relationships were occasional or inevitable. The rests of RE, i.e. the difference between simulated and observed RE values, were significantly influenced by LAI of wheat and hourly Ta of maize season but not by hourly Ws of maize season, indicating that the influence of W-LAImax and S-Ta on RE were inevitable outcomes and that of W-Ws on wheat RE was occasional. So we suggested that one should not confirm the controlling factors of interannual variations in carbon fluxes just from simple relationships, which may be statistical coincidences and do not correlated with biotical processes.

Seasonal and interannual variations in ecosystem respiration in relation to temperature, moisture, and productivity in a temperate semi-arid shrubland

Understanding the temporal dynamics and influencing factors of ecosystem respiration (Reco) in semi-arid shrublands is critical for predicting how their carbon balance may respond to climate change. Using the eddy-covariance technique, we quantified the net ecosystem CO2 exchange (NEE) in a semi-arid shrubland of northern China from July 2011 to December 2016, and partitioned NEE into Reco and gross primary productivity (GPP). Annual Reco varied from 300 g C m−2 yr−1 in 2014 to 426 g C m−2 yr−1 in 2012, and GPP ranged from 277 g C m−2 yr−1 in 2014 to 503 g C m−2 yr−1 in 2012. The relationship between half-hourly nighttime Reco and air temperature (Ta) was well-described by the Lloyd & Taylor model. Indicators of the seasonal temperature sensitivity (E0 and Q10) of Reco increased with both the annual integral and seasonal amplitude of GPP. However, when averaged into 1 °C Ta bins, nighttime Reco increased with Ta up to an optimal temperature of ~20 °C, above which it decreased with increasing Ta. Periods of low soil moisture in spring and summer markedly depressed Reco, contributing to its seasonal and interannual variations. In addition, low soil moisture had little effect on nighttime Reco when Ta was below 15 °C, but substantially reduced nighttime Reco when Ta was above 15 °C. Ecosystem respiration increased linearly with GPP at both seasonal and interannual scales, with the slope being 0.50 and 0.55, respectively. Our results have important implications for predicting Reco under climate change, considering continuous warming and increases in the frequency and intensity of extreme events (e.g., heat waves, droughts). Moreover, our results suggest that process-based carbon models should adequately represent the effects of substrate supply (e.g., by GPP) on Reco and its temperature sensitivity.

Carbon dynamics and environmental controls of a hilly tea plantation in Southeast China.

Tea plantations are widely distributed and continuously expanding across subtropical China in recent years. However, carbon flux exchanges from tea plantation ecosystems are poorly understood at the ecosystem level. In this study, we use the eddy covariance technique to quantify the magnitude and temporal variations of the net ecosystem exchange (NEE) in tea plantation in Southeast China over four years (2014–2017). The result showed that the tea plantation was a net carbon sink, with an annual NEE that ranged from −182.40 to −301.51 g C/m2, which was a much lower carbon sequestration potential than other ecosystems in subtropical China. Photosynthetic photon flux density (PPFD) explained the highest proportion of the variation in NEE and gross primary productivity (GPP) (for NEE: F = 389.89, p < .01; for GPP: F = 1,018.04, p < .01), and air temperature (Ta) explained the highest proportion of the variation in ecosystem respiration (RE) (F = 13,141.81, p < .01). The strong pruning activity in April not only reduced the carbon absorption capacity but also provided many plant residues for respiration, which switched the tea plantation to a carbon source from April to June. Suppression of NEE at higher air temperatures was due to the decrease in GPP more than the decrease in RE, which indicated that future global warming may transform this subtropical tea plantation from a carbon sink to carbon source.

Predicting peatland carbon fluxes from non‐destructive plant traits

Summary Determining the plant traits that best predict carbon (C) storage is increasingly important as global change drivers will affect plant species composition and ecosystem C cycling. Despite the critical role of peatlands in the global C cycle, trait–flux relationships in peatlands are relatively unknown. We assessed the ability of four non‐destructive plant traits to predict carbon dioxide (CO2) and methane (CH4) fluxes over two growing seasons in a temperate peatland in Ontario, Canada. We examined relationships between C‐fluxes and leaf area, leaf persistence (deciduous, evergreen), growth form (woody, herbaceous) and aerenchyma tissue. To explore potential inconsistencies between different scales of data aggregation, traits were analysed at the level of plots, species and microsites. CO2 fluxes showed a positive relationship with leaf area and leaf persistence, and a negative relationship with proportion of woody species. CH4 fluxes showed a positive relationship with aerenchyma and leaf area. The significance of trait–flux relationships differed based on whether data were averaged at the level of plot, species or microsite. We recommend applying leaf area as a non‐destructive trait to other systems where it is not ideal to measure traits destructively. A better understanding of the relationships between above and below‐ground traits is likely needed to further explain variation in ecosystem respiration and CH4 fluxes from plant traits. A lay summary is available for this article.

Spatiotemporal variability in carbon exchange fluxes across the Sahel

Semi-arid regions play an increasingly important role as a sink within the global carbon (C) cycle and is the main biome driving inter-annual variability in carbon dioxide (CO2) uptake by terrestrial ecosystems. This indicates the need for detailed studies of spatiotemporal variability in C cycling for semi-arid ecosystems. We have synthesized data on the land-atmosphere exchange of CO2 measured with the eddy covariance technique from the six existing sites across the Sahel, one of the largest semi-arid regions in the world. The overall aim of the study is to analyse and quantify the spatiotemporal variability in these fluxes and to analyse to which degree spatiotemporal variation can be explained by hydrological, climatic, edaphic and vegetation variables. All ecosystems were C sinks (average±total error −162±48gCm−2y−1), but were smaller when strongly impacted by anthropogenic influences. Spatial and inter-annual variability in the C flux processes indicated a strong resilience to dry conditions, and were correlated with phenological metrics. Gross primary productivity (GPP) was the most important flux process affecting the sink strength, and diurnal variability in GPP was regulated by incoming radiation, whereas seasonal dynamics was closely coupled with phenology, and soil water content. Diurnal variability in ecosystem respiration was regulated by GPP, whereas seasonal variability was strongly coupled to phenology and GPP. A budget for the entire Sahel indicated a strong C sink mitigating the global anthropogenic C emissions. Global circulation models project an increase in temperature, whereas rainfall is projected to decrease for western Sahel and increase for the eastern part, indicating that the C sink will possibly decrease and increase for the western and eastern Sahel, respectively.

Responses of ecosystem respiration to short-term experimental warming in the alpine meadow ecosystem of a permafrost site on the Qinghai–Tibetan Plateau

As the largest high-altitude permafrost region in the world, the Qinghai–Tibetan Plateau (QTP) has experienced pronounced warming, which may exert a profound effect on ecosystem's CO2 emissions due to the large amounts of organic carbon in the permafrost soil. In this study, response of ecosystem respiration to short-term experimental warming was investigated in the alpine meadow ecosystem of a permafrost site on the QTP during the 2012 growing season. The results showed that cumulative growing season ecosystem CO2 emissions were 277.09 and 325.33gCm−2 for the control and warming plots, respectively. Thus, a 2.18°C increase in ground surface temperature and a 0.62°C increase in soil temperature prompted a 17.41% increase in the ecosystem's CO2 emissions. The relationships between temporal variations in ecosystem respiration and soil temperature can be described through exponential equation. Soil temperature accounted for 50 and 64% of the variations in ecosystem respiration on a diurnal scale, and 71 and 84% of the variations in ecosystem respiration on a seasonal scale in the control and warming plots, respectively. Moreover, the temperature sensitivity (i.e. apparent Q10 values) of the ecosystem respiration rates was significantly higher in the warming plots than in the control plots. Our results indicate that the effect of warming promoted CO2 emissions of the alpine meadow ecosystem at a permafrost site on the QTP, and that soil temperature is the key factor controlling ecosystems' CO2 emissions. In addition, while ecosystem respiration provides positive feedback on the rise in temperature through an increase in temperature sensitivity under warming conditions, at least in the short term, this feedback may be partially offset by an increase in above- and belowground biomass caused by warming.

Extreme Drought Effects on Carbon Dynamics of a Semiarid Pasture

Environmental and management factors are critical in determining the C source or sink status of agroecosystems. Information on the C dynamics of an ecosystem from source to sink and vice versa are critical in determining the role of that ecosystem in regional and global C balances. We investigated the impact of the 2011 mega‐drought on seasonal changes in net CO2 exchange of a WW‐B. Dahl Old World bluestem [Bothriochloa bladhii (Retz) S.T. Blake] pasture in the Texas High Plains and compared the results with those from 2010, a hydrologically wet year. Carbon dioxide flux between the vegetation and atmosphere was measured using an eddy covariance flux tower. Our results indicate that net ecosystem exchange, ecosystem respiration, and gross primary production for this agroecosystem were strongly affected by environmental variables and grazing. During the period of measurement in 2010 (Days of the Year 152–365), the pasture accumulated 164 g C m–2 and was a net C sink. During the same period in 2011, the severe drought changed the dynamics of the pasture from a C sink to a source, with a net cumulative loss of 142 g C m–2. Ecosystem respiration was an exponential function of soil temperature in both years. When extreme water‐limiting days were excluded, the exponential model explained 90% of the variation in ecosystem respiration in 2011 and 92% of the variation in ecosystem respiration in 2010. Incorporating the results from our study with ecosystem models can improve our understanding of the contributions of managed pastures to regional C balances.

Ecosystem Respiration Does not Differ Before and After Tidal Inundation in Brackish Marshes of the Min River Estuary, Southeast China

Respiration in tidal marshes plays an important role in the global carbon cycle, but is little investigated in Asia. In this study, ecosystem respiration in three brackish marshes dominated by Spartina alterniflora (smooth cordgrass), Phragmites australis (common reed) and Cyperus malaccensis (shichito matgrass) were examined monthly during 2009 in the subtropical tidal wetland of the Min River estuary of southeast China. Measurements were taken before and after tidal inundation. Soil porewater CO2 concentrations at soil depths of 5 and 10 cm were also measured. Differences in ecosystem respiration before and after tidal inundation stages were statistically indistinguishable in each vegetation type on an annual scale but differed by month. Monthly variation in ecosystem respiration was apparent and overall respiration in P. australis stand had the highest respiration of the vegetation types. Monthly average CO2 concentration in soil porewater did not differ significantly between the two depths. Ecosystem respiration in P. australis and C. malaccensis stands had a significant positive correlation with soil and air temperature, and a significant negative correlation with soil pH in S. alterniflora and C. malaccensis stands. Our results suggested that the brackish marsh ecosystem of subtropical estuary emits CO2 to the atmosphere at relatively higher level, but the emission pattern did not differ before and after tides.

Interactions among shrub cover and the soil microclimate may determine future Arctic carbon budgets

Arctic and Boreal terrestrial ecosystems are important components of the climate system because they contain vast amounts of soil carbon (C). Evidence suggests that deciduous shrubs are increasing in abundance, but the implications for ecosystem C budgets remain uncertain. Using midsummer CO(2) flux data from 21 sites spanning 16° of latitude in the Arctic and Boreal biomes, we show that air temperature explains c. one-half of the variation in ecosystem respiration (ER) and that ER drives the pattern in net ecosystem CO(2) exchange across ecosystems. Woody sites were slightly stronger C sinks compared with herbaceous communities. However, woody sites with warm soils (>10°C) were net sources of CO(2) , whereas woody sites with cold soils (<10°C) were strong sinks. Our results indicate that transition to a shrub-dominated Arctic will increase the rate of C cycling, and may lead to net C loss if soil temperatures rise.

Quantifying the influence of climate and biological drivers on the interannual variability of carbon exchanges in European forests through process-based modelling

It is necessary to increase our understanding of the influence of climate and biological drivers on the interannual variations in carbon (C) exchange between forests and the atmosphere. To this aim, a process-based model (CASTANEA) was used for simulating C exchanges over four European forests, encompassing a broad latitudinal gradient (from Mediterranean to boreal climate zones), for the years 2000–2007. CASTANEA reproduced 74–95% of the daily and 35–84% of the annual variance in daytime net ecosystem productivity (NEP). Constrained simulations allowed us to isolate the individual contributions of climatic drivers (radiation, temperature and relative humidity), hydrological drivers (soil water) and biological drivers (canopy dynamics, the thermal acclimation of photosynthetic capacities in evergreens, and vegetative and microbial C pools) to the anomalies in C exchange for timescales ranging from daily to annual. Orthonormal wavelet transformation of these single-contribution time series allowed us to quantify the individual drivers’ influences on flux variance across time scales. High frequency departures from mean annual C exchange patterns were mostly caused by hydroclimate modulations (87–99% of the flux variance from daily to weekly scales). The integration of these anomalies at the annual scale yielded a higher contribution of biological drivers: we identified a primary controller of gross primary production (GPP) variations through modulations of soil water in both the Mediterranean Quercus ilex and the temperate Fagus sylvatica forests; we also identified at the annual scale a complex determinism for both coniferous sites, with an unexpectedly higher contribution of the thermal acclimation driver at the temperate (warmer) site. Although all but one site experienced structural perturbations during the study period, interannual variations in ecosystem respiration (Reco) were readily related to changes in temperature and soil water, with a low contribution from variations in C pools at the annual scale. As a result of the preponderant dependence of net ecosystem production (NEP) on GPP fluxes, the inferred determinism of simulated net exchanges appeared similar to that of GPP. However, compensations occurred, leading, for example, to a much lower influence of soil water modulations on NEP than on GPP or Reco at the Mediterranean site.

Biotic, Abiotic, and Management Controls on the Net Ecosystem CO2 Exchange of European Mountain Grassland Ecosystems

The net ecosystem carbon dioxide (CO2) exchange (NEE) of nine European mountain grassland ecosystems was measured during 2002–2004 using the eddy covariance method. Overall, the availability of photosynthetically active radiation (PPFD) was the single most important abiotic influence factor for NEE. Its role changed markedly during the course of the season, PPFD being a better predictor for NEE during periods favorable for CO2 uptake, which was spring and autumn for the sites characterized by summer droughts (southern sites) and (peak) summer for the Alpine and northern study sites. This general pattern was interrupted by grassland management practices, that is, mowing and grazing, when the variability in NEE explained by PPFD decreased in concert with the amount of aboveground biomass (BMag). Temperature was the abiotic influence factor that explained most of the variability in ecosystem respiration at the Alpine and northern study sites, but not at the southern sites characterized by a pronounced summer drought, where soil water availability and the amount of aboveground biomass were more or equally important. The amount of assimilating plant area was the single most important biotic variable determining the maximum ecosystem carbon uptake potential, that is, the NEE at saturating PPFD. Good correspondence, in terms of the magnitude of NEE, was observed with many (semi-) natural grasslands around the world, but not with grasslands sown on fertile soils in lowland locations, which exhibited higher maximum carbon gains at lower respiratory costs. It is concluded that, through triggering rapid changes in the amount and area of the aboveground plant matter, the timing and frequency of land management practices is crucial for the short-term sensitivity of the NEE of the investigated mountain grassland ecosystems to climatic drivers.

Mechanism and bio‐environmental controls of ecosystem respiration in a cropland in the North China plains

CO2 flux was measured continuously using the eddy covariance technique in a wheat‐maize rotation system in the North China Plains from October 2002 to October 2006. The annual and seasonal variation of ecosystem respiration and the bio‐environmental controls on them were investigated. The results show that ecosystem respiration (Rec) in the cropland increased exponentially with soil temperature at 5 cm depth. The temperature sensitivity coefficient (Q10) for ecosystem respiration varied from 3.5 to 5.4 for wheat and from 2.4 to 4.5 for maize. In the wheat growing season, monthly average R0 (ecosystem respiration at 0°C) increased linearly with soil temperature and logarithmically with leaf area index (LAI). Monthly average Q10 decreased logarithmically with R 0. Residual R ec was significantly correlated with LAI. After considering LAI, the modified Q10 model could estimate R ec better than before. The simulation results show that annual ecosystem respiration in the wheat‐maize rotation system in the North China Plains was 1327, 1348, 1040 and 1171 gC m‐2 yr‐1 for the 4 years of the study. As a 4‐year average, seasonal mean ecosystem respiration in wheat (2.60 gC m‐2 day‐1) was much lower than in maize (6.09 gC m‐2 day‐1). However, integrated ecosystem respiration for the wheat growing season (566 gC m‐2) was slightly higher than that for maize (520 gC m‐2). These account for 46.4 and 42.6% of the annual values, respectively.

Effect of the conversion of grassland to spring wheat field on the CO 2 emission characteristics in Inner Mongolia, China

Chinese grasslands have undergone great changes in land use in recent decades. Approximately 18.2% of the present arable land in China originated from the cultivation of grassland, but its impact on the carbon cycle has not been fully understood. This study was conducted in situ for 3 years to assess the comprehensive effects of cultivation of temperate steppe on soil organic carbon (SOC) and soil respiration rates as well as ecosystem respiration. As compared with those in the Stipa baicalensis steppe, the SOC concentrations at depths of 0–10 and 10–20 cm in the spring wheat field were found to have decreased by 38.3 and 17.4% respectively from 29.5 and 21.9 g kg −1 to 18.2 and 18.1 g kg −1 after a cultivation period of 30 years. Accordingly, the total amounts of soil respiration through the growing season (from April to September) in 2002, 2003 and 2004 were 265.2, 282.2 and 237.4 g C m −2 respectively in the spring wheat field, which were slightly lower than the values of 342.2, 412.0 and 312.1 g C m −2 in the S. baicalensis steppe, while ecosystem respiration of 690.9, 991.2 and 569.6 g C m −2 respectively in the spring wheat field were markedly higher than those of 447.0, 470.9 and 429.7 g C m −2 in the steppe plot. Similar seasonal variations of ecosystem respiration and soil respiration existed in both sample sites. Respiration rates were higher and greater differences existed in both ecosystem respiration and soil respiration during the exuberant growth stage of plants (from mid-June to mid-August). However, in the slower-growth period of the growing season (before late May and after late August), the CO 2 effluxes of the two sample sites were similar and remained at a relatively low level. The results also showed that ecosystem respiration and soil respiration were under similar environmental controls in both sample sites. Soil water content at a depth of 0–10 cm and soil temperatures at 5 and 10 cm were the main factors affecting the variations in ecosystem respiration and soil respiration rates in droughty years of 2002 and 2004 and in the rainy 2003, respectively. This study suggests that the conversion of the grassland to the spring wheat field has increased the carbon loss of the whole ecosystem due to the change of vegetation cover type and significantly reduced the carbon storage of surface soil. In addition, the tillage of grassland had different effects on ecosystem respiration and soil respiration. The effects were also dissimilar in different growth stages, which should be fully considered when assessing and predicting the effects of cultivation on the net CO 2 balance of grassland ecosystems.

Temperature and substrate controls on intra‐annual variation in ecosystem respiration in two subarctic vegetation types

AbstractArctic ecosystems are important in the context of climate change because they are expected to undergo the most rapid temperature increases, and could provide a globally significant release of CO2 to the atmosphere from their extensive bulk soil organic carbon reserves. Understanding the relative contributions of bulk soil organic matter and plant‐associated carbon pools to ecosystem respiration is critical to predicting the response of arctic ecosystem net carbon balance to climate change. In this study, we determined the variation in ecosystem respiration rates from birch forest understory and heath tundra vegetation types in northern Sweden through a full annual cycle. We used a plant biomass removal treatment to differentiate bulk soil organic matter respiration from total ecosystem respiration in each vegetation type.Plant‐associated and bulk soil organic matter carbon pools each contributed significantly to ecosystem respiration during most phases of winter and summer in the two vegetation types. Ecosystem respiration rates through the year did not differ significantly between vegetation types despite substantial differences in biomass pools, soil depth and temperature regime. Most (76–92%) of the intra‐annual variation in ecosystem respiration rates from these two common mesic subarctic ecosystems was explained using a first‐order exponential equation relating respiration to substrate chemical quality and soil temperature. Removal of plants and their current year's litter significantly reduced the sensitivity of ecosystem respiration to intra‐annual variations in soil temperature for both vegetation types, indicating that respiration derived from recent plant carbon fixation was more temperature sensitive than respiration from bulk soil organic matter carbon stores.Accurate assessment of the potential for positive feedbacks from high‐latitude ecosystems to CO2‐induced climate change will require the development of ecosystem‐level physiological models of net carbon exchange that differentiate the responses of major C pools, that account for effects of vegetation type, and that integrate over summer and winter seasons.

Scaling metabolism from organisms to ecosystems.

Understanding energy and material fluxes through ecosystems is central to many questions in global change biology and ecology. Ecosystem respiration is a critical component of the carbon cycle and might be important in regulating biosphere response to global climate change. Here we derive a general model of ecosystem respiration based on the kinetics of metabolic reactions and the scaling of resource use by individual organisms. The model predicts that fluxes of CO2 and energy are invariant of ecosystem biomass, but are strongly influenced by temperature, variation in cellular metabolism and rates of supply of limiting resources (water and/or nutrients). Variation in ecosystem respiration within sites, as calculated from a network of CO2 flux towers, provides robust support for the model's predictions. However, data indicate that variation in annual flux between sites is not strongly dependent on average site temperature or latitude. This presents an interesting paradox with regard to the expected temperature dependence. Nevertheless, our model provides a basis for quantitatively understanding energy and material flux between the atmosphere and biosphere.