Estimating the contribution of leaf litter decomposition to soil CO2 efflux in a beech forest using 13C-depleted litter

The contribution of leaf litter decomposition to total soil CO₂ efflux (F_L/F) was evaluated in a beech (Fagus sylvatica L.) forest in eastern France. The Keeling‐plot approach was applied to estimate the isotopic composition of respired soil CO₂ from soil covered with either control (?30.32‰) or ¹³C‐depleted leaf litter (?49.96‰). The δ¹³C of respired soil CO₂ ranged from ?25.50‰ to ?22.60‰ and from ?24.95‰ to ?20.77‰, respectively, with depleted or control litter above the soil. The F_L/F ratio was calculated by a single isotope linear mixing model based on mass conservation equations. It showed seasonal variations, increasing from 2.8% in early spring to about 11.4% in mid summer, and decreasing to 4.2% just after leaf fall. Between December 2001 and December 2002, cumulated F and F_L reached 0.98 and 0.08 kg_C m^?2, respectively. On an annual basis, decomposition of fresh leaf litter accounted for 8% of soil respiration and 80% of total C loss from fresh leaf litter. The other fraction of carbon loss during leaf litter decomposition that is assumed to have entered the soil organic matter pool (i.e. 20%) represents only 0.02 kg_C m^?2.     

Influence of soil O2 and CO2 on root respiration for Agave deserti

Respiration measured as CO₂ efflux was determined at various soil O₂ and CO₂ concentrations for individual, attached roots of a succulent perennial from the Sonoran Desert, Agave deserti Engelm. The respiration rate increased with increasing O₂ concentration up to about 16% O₂ for established roots and 5% O₂ for rain roots (fine branch roots on established roots induced by wetting of the soil) and then remained fairly constant up to 21% O₂. When O₂ was decreased from 21 to 0%, the respiration rates were similar to those obtained with increasing O₂ concentration. The CO₂ concentration in the root zone, which for the shallow-rooted A. deserti in the field was about 1 000 μl l^-1, did not affect root respiration at concentrations up to 2 000 μl l^-1, but higher concentrations reduced it, respiration being abolished at 20 000 μl l^-1 (2%) CO₂ for both established and rain roots. Upon lowering CO₂ to 1 000 μl l^-1 after exposure to concentrations up to 10000 μl l^-1 CO₂, inhibition of respiration was reversible. Uptake of the vital stain neutral red by root cortical cells was reduced to zero, indicating cell death, in about 4 h at 2% CO₂, substantiating the detrimental effects of high soil CO₂ concentrations on roots of A. deserti . This CO₂ response may explain why roots of desert succulents tend to occur in porous, well-aerated soils.     

Precipitation pulses and soil CO2 flux in a Sonoran Desert ecosystem

Precipitation is a major driver of biological processes in arid and semiarid ecosystems. Soil biogeochemical processes in these water‐limited systems are closely linked to episodic rainfall events, and the relationship between microbial activity and the amount and timing of rainfall has implications for the whole‐system carbon (C) balance. Here, the influences of storm size and time between events on pulses of soil respiration were explored in an upper Sonoran Desert ecosystem. Immediately following experimental rewetting in the field, CO₂ efflux increased up to 30‐fold, but generally returned to background levels within 48 h. CO₂ production integrated over 48 h ranged from 2.5 to 19.3 g C m⁻² and was greater beneath shrubs than in interplant spaces. When water was applied on sequential days, postwetting losses of CO₂ were only half a large as initial fluxes, and the size of the second pulse increased with time between consecutive events. Soil respiration was more closely linked to the organic matter content of surface soils than to rainfall amount. Beneath shrubs, rates increased nonlinearly with storm size, reaching an asymptote at approximately 0.5 cm simulated storms. This nonlinear relationship stems from (1) resource limitation of microbial activity that is manifest at small time scales, and (2) greatly reduced process rates in deeper soil strata. Thus, beyond some threshold in storm size, increasing the duration or depth of soil moisture has little consequence for short‐term losses of CO₂. In addition, laboratory rewetting across a broad range in soil water content suggest that microbial activity and CO₂ efflux following rainfall may be further modified by the routing and redistribution of water along hillslopes. Finally, analysis of long‐term precipitation data suggests that half the monsoon storms in this system are sufficient to induce soil heterotrophic activity and C losses, but are not large enough to elicit autotrophic activity and C accrual by desert shrubs.     

Partitioning sources of soil respiration in boreal black spruce forest using radiocarbon

Separating ecosystem and soil respiration into autotrophic and heterotrophic component sources is necessary for understanding how the net ecosystem exchange of carbon (C) will respond to current and future changes in climate and vegetation. Here, we use an isotope mass balance method based on radiocarbon to partition respiration sources in three mature black spruce forest stands in Alaska. Radiocarbon (Δ¹⁴C) signatures of respired C reflect the age of substrate C and can be used to differentiate source pools within ecosystems. Recently‐fixed C that fuels plant or microbial metabolism has Δ¹⁴C values close to that of current atmospheric CO₂, while C respired from litter and soil organic matter decomposition will reflect the longer residence time of C in plant and soil C pools. Contrary to our expectations, the Δ¹⁴C of C respired by recently excised black spruce roots averaged 14‰ greater than expected for recently fixed photosynthetic products, indicating that some portion of the C fueling root metabolism was derived from C storage pools with turnover times of at least several years. The Δ¹⁴C values of C respired by heterotrophs in laboratory incubations of soil organic matter averaged 60‰ higher than the contemporary atmosphere Δ¹⁴CO₂, indicating that the major contributors to decomposition are derived from a combination of sources consistent with a mean residence time of up to a decade. Comparing autotrophic and heterotrophic Δ¹⁴C end members with measurements of the Δ¹⁴C of total soil respiration, we calculated that 47–63% of soil CO₂ emissions were derived from heterotrophic respiration across all three sites. Our limited temporal sampling also observed no significant differences in the partitioning of soil respiration in the early season compared with the late season. Future work is needed to address the reasons for high Δ¹⁴C values in root respiration and issues of whether this method fully captures the contribution of rhizosphere respiration.     

14C – a tool for separation of autotrophic and heterotrophic soil respiration

We assessed the potential of using ¹⁴C contents of soil respired CO₂ to calculate the contributions of heterotrophic and autotrophic respiration to total soil respiration. The partitioning of these fluxes is of utmost importance to evaluate implications of environmental change on soil carbon cycling and sequestration. At three girdled forest stands in Sweden and Germany, where the tree root (autotrophic) respiration had been eliminated, we measured both flux rates and ¹⁴C contents of soil respired CO₂ in girdled and control plots in the summers of 2001 or 2002. At all stands, CO₂ flux rates were slightly higher in the control plots, whereas the ¹⁴C contents of respired CO₂ tended to be higher in the girdled plots. This was expected and confirmed that heterotrophically respired CO₂ cycles more slowly through the forest ecosystem than autotrophically respired CO₂. On the basis of these data, the contributions of hetero‐ and autotrophic respiration to total soil respiration were calculated using two separate approaches (i.e. based on flux rates or ¹⁴C). Fractions of heterotrophic respiration ranged from 53% to 87%. Values calculated by both approaches did not differ significantly from each other. Finally, we compared the ¹⁴C contents of soil respired CO₂ in the girdled plots with the ¹⁴C contents of heterotrophically respired CO₂ calculated by three different ¹⁴C models. None of the models matched the measured data sufficiently. In addition, we suspect that inherent effects of girdling may cause the ¹⁴C content of CO₂ respired in the girdled plots to be lower than ‘true’ heterotrophically respired CO₂ in an undisturbed plot. Nevertheless, we argue that measurements and modeling of ¹⁴C can be developed into a valuable tool for separating heterotrophic and autotrophic soil respiration (e.g. when girdling cannot be performed).     

Species control variation in litter decomposition in a pine forest exposed to elevated CO2

Net primary production and the flux of dry matter and nutrients from vegetation to soils has increased following four years of exposure to elevated CO₂ in a southern pine forest in NC, USA. This has increased the demand for nutrients to support enhanced rates of NPP and altered the conditions for litter decomposition on the forest floor. We quantified the chemistry and decomposition dynamics of leaf litter produced by five of the most abundant tree species in this ecosystem during the third and fourth growing seasons under elevated CO₂. The objectives of this study were to determine (i) if there were systemic or species‐specific changes in leaf litter chemistry associated with a sustained enhancement of plant growth under elevated CO₂; and (ii) whether the process of litter decomposition was altered by increased inputs of energy and nutrients to the forest floor in the plots under elevated CO₂. Leaf litter chemistry, including various C fractions and N concentration, was virtually unchanged by elevated CO₂. With few exceptions, plant litter produced under elevated CO₂ lost mass or N at the same relative rate as that produced under ambient CO₂. The relationship between initial litter chemistry and decomposition was not altered by elevated CO₂. The greater forest floor mass and nutrient content in the plots under elevated CO₂ had no consistent or long‐term effect on litter decomposition. Thus, we found no evidence that plant and microbial processes under elevated CO₂ resulted in systemic changes in mass loss or N dynamics during decomposition. In contrast to the limited effects of elevated CO₂ on litter chemistry and decomposition, there were large differences among species in initial litter chemistry, mass loss and N dynamics during decomposition. If the species composition of this forest community is altered by elevated CO₂, the indirect effect of a change in species composition will exert greater control over the long‐term rate of nutrient cycling than the direct effect of elevated CO₂ on litter chemistry and decomposition dynamics alone.     

Elevated CO2 effects on decomposition processes in a grazed grassland

The effects of elevated atmospheric CO₂ (475 μL L^?1) on in situ decomposition of plant litter and animal faecal material were studied over 2 years in a free air CO₂ enrichment (FACE) facility. The pasture was grazed by sheep and contained a mixture of C₃ and C₄ grasses, legumes and forbs. There was no effect of elevated CO₂ on decomposition within plant species but marked differences between species with faster decomposition in dicots; a group that increased in abundance at elevated CO₂. Decomposition of mixed herbage root material occurred at a similar rate to that of leaf litter suggesting that any CO₂‐induced increase in carbon allocation to roots would not reduce rates of decomposition. Sheep faeces resulting from a ‘high‐CO₂ diet’ decomposed significantly slower during summer but not during winter. The overall outcome of these experiments were explored using scenarios that took account of changes in botanical composition, allocation to roots and the presence of herbivores. In the absence of herbivores, elevated CO₂ led to a 15% increase in the rate of mass loss and an 18% increase in the rate of nitrogen (N) release. In the presence of herbivores, these effects were partially removed (11% increase in rate of mass loss and 9% decrease in N release rate) because of the recycling occurring through the animals in the form of faeces.     

The turnover of carbon pools contributing to soil CO2 and soil respiration in a temperate forest exposed to elevated CO2 concentration

Soil carbon is returned to the atmosphere through the process of soil respiration, which represents one of the largest fluxes in the terrestrial C cycle. The effects of climate change on the components of soil respiration can affect the sink or source capacity of ecosystems for atmospheric carbon, but no current techniques can unambiguously separate soil respiration into its components. Long‐term free air CO₂ enrichment (FACE) experiments provide a unique opportunity to study soil C dynamics because the CO₂ used for fumigation has a distinct isotopic signature and serves as a continuous label at the ecosystem level. We used the ¹³C tracer at the Duke Forest FACE site to follow the disappearance of C fixed before fumigation began in 1996 (pretreatment C) from soil CO₂ and soil‐respired CO₂, as an index of belowground C dynamics during the first 8 years of the experiment. The decay of pretreatment C as detected in the isotopic composition of soil‐respired CO₂ and soil CO₂ at 15, 30, 70, and 200 cm soil depth was best described by a model having one to three exponential pools within the soil system. The majority of soil‐respired CO₂ (71%) originated in soil C pools with a turnover time of about 35 days. About 55%, 50%, and 68% of soil CO₂ at 15, 30, and 70 cm, respectively, originated in soil pools with turnover times of less than 1 year. The rest of soil CO₂ and soil‐respired CO₂ originated in soil pools that turn over at decadal time scales. Our results suggest that a large fraction of the C returned to the atmosphere through soil respiration results from dynamic soil C pools that cannot be easily detected in traditionally defined soil organic matter standing stocks. Fast oxidation of labile C substrates may prevent increases in soil C accumulation in forests exposed to elevated [CO₂] and may consequently result in shorter ecosystem C residence times.     

Allocation and residence time of photosynthetic products in a boreal forest using a low-level 14C pulse-chase labeling technique

Much of our understanding about how carbon (C) is allocated in plants comes from radiocarbon (¹⁴C) pulse‐chase labeling experiments. However, the large amounts of ¹⁴C required for decay‐counting mean that these studies have been restricted for the most part to mesocosm or controlled laboratory experiments. Using the enhanced sensitivity for ¹⁴C detection available with accelerator mass spectrometry (AMS), we tested the utility of a low‐level ¹⁴C pulse‐chase labeling technique for quantifying C allocation patterns and the contributions of different plant components to total ecosystem respiration in a black spruce forest stand in central Manitoba, Canada. All aspects of the field experiment used ¹⁴C at levels well below regulated health standards, without significantly altering atmospheric CO₂ concentrations. Over 30 days following the label application in late summer (August and September), we monitored the temporal and spatial allocation patterns of labeled photosynthetic products by measuring the amount and ¹⁴C content of CO₂ respired from different ecosystem components. The mean residence times (MRT) for labeled photosynthetic products to be respired in the understory (feather mosses), canopy (black spruce), and rhizosphere (black spruce roots and associated microbes) were <1, 6, and 15 days, respectively. Respiration from the canopy and understory showed significantly greater influence of labeled photosynthates than excised root and intact rhizosphere respiration. After 30 days,∼65% of the label assimilated had been respired by the canopy,∼20% by the rhizosphere, and∼9% by the understory, with∼6% unaccounted for and perhaps remaining in tissues. Maximum ¹⁴C values in root and rhizosphere respiration were reached 4 days after label application. The label was still detectable in root, rhizosphere and canopy respiration after 30 days; these levels of remaining label would not have been detectible had a ¹³C label been applied. Our results support previous studies indicating that a substantial portion of the C fueling rhizosphere respiration in the growing season may be derived from stored C pools rather than recent photosynthetic products.     

Forest soil CO2 flux: uncovering the contribution and environmental responses of ectomycorrhizas

Forests play a critical role in the global carbon cycle, being considered an important and continuing carbon sink. However, the response of carbon sequestration in forests to global climate change remains a major uncertainty, with a particularly poor understanding of the origins and environmental responses of soil CO₂ efflux. For example, despite their large biomass, the contribution of ectomycorrhizal (EM) fungi to forest soil CO₂ efflux and responses to changes in environmental drivers has, to date, not been quantified in the field. Their activity is often simplistically included in the ‘autotrophic’ root respiration term. We set up a multiplexed continuous soil respiration measurement system in a young Lodgepole pine forest, using a mycorrhizal mesh collar design, to monitor the three main soil CO₂ efflux components: root, extraradical mycorrhizal hyphal, and soil heterotrophic respiration. Mycorrhizal hyphal respiration increased during the first month after collar insertion and thereafter remained remarkably stable. During autumn the soil CO₂ flux components could be divided into ∼60% soil heterotrophic, ∼25% EM hyphal, and ∼15% root fluxes. Thus the extraradical EM mycelium can contribute substantially more to soil CO₂ flux than do roots. While EM hyphal respiration responded strongly to reductions in soil moisture and appeared to be highly dependent on assimilate supply, it did not responded directly to changes in soil temperature. It was mainly the soil heterotrophic flux component that caused the commonly observed exponential relationship with temperature. Our results strongly suggest that accurate modelling of soil respiration, particularly in forest ecosystems, needs to explicitly consider the mycorrhizal mycelium and its dynamic response to specific environmental factors. Moreover, we propose that in forest ecosystems the mycorrhizal CO₂ flux component represents an overflow ‘CO₂ tap’ through which surplus plant carbon may be returned directly to the atmosphere, thus limiting expected carbon sequestration from trees under elevated CO₂.     

Soil CO2 efflux in a boreal pine forest under atmospheric CO2 enrichment and air warming

The response of forest soil CO₂ efflux to the elevation of two climatic factors, the atmospheric concentration of CO₂ (↑CO₂ of 700 μmol mol⁻¹) and air temperature (↑ T with average annual increase of 5°C), and their combination (↑CO₂+↑ T ) was investigated in a 4-year, full-factorial field experiment consisting of closed chambers built around 20-year-old Scots pines ( Pinus sylvestris L.) in the boreal zone of Finland. Mean soil CO₂ efflux in May–October increased with elevated CO₂ by 23–37%, with elevated temperature by 27–43%, and with the combined treatment by 35–59%. Temperature elevation was a significant factor in the combined 4-year efflux data, whereas the effect of elevated CO₂ was not as evident. Elevated temperature had the most pronounced impact early and late in the season, while the influence of elevated CO₂ alone was especially notable late in the season. Needle area was found to be a significant predictor of soil CO₂ efflux, particularly in August, a month of high root growth, thus supporting the assumption of a close link between whole-tree physiology and soil CO₂ emissions. The decrease in the temperature sensitivity of soil CO₂ efflux observed in the elevated temperature treatments in the second year nevertheless suggests the existence of soil response mechanisms that may be independent of the assimilating component of the forest ecosystem. In conclusion, elevated atmospheric CO₂ and air temperature consistently increased forest soil CO₂ efflux over the 4-year period, their combined effect being additive, with no apparent interaction.     

Elevated atmospheric CO2 increases fine root production, respiration, rhizosphere respiration and soil CO2 efflux in Scots pine seedlings

In this study, we investigated the impact of elevated atmospheric CO₂ (ambient + 350 μmol mol^–1) on fine root production and respiration in Scots pine (Pinus sylvestris L.) seedlings. After six months exposure to elevated CO₂, root production measured by root in-growth bags, showed significant increases in mean total root length and biomass, which were more than 100% greater compared to the ambient treatment. This increased root length may have lead to a more intensive soil exploration. Chemical analysis of the roots showed that the roots in the elevated treatment accumulated more starch and had a lower C/N-ratio. Specific root respiration rates were significantly higher in the elevated treatment and this was probably attributed to increased nitrogen concentrations in the roots. Rhizospheric respiration and soil CO₂ efflux were also enhanced in the elevated treatment. These results clearly indicate that under elevated atmospheric CO₂ root production and development in Scots pine seedlings is altered and respiratory carbon losses through the root system are increased.     

Trends and methodological impacts in soil CO2 efflux partitioning: A metaanalytical review

Partitioning soil carbon dioxide (CO₂) efflux (R_S) into autotrophic (R_A; including plant roots and closely associated organisms) and heterotrophic (R_H) components has received considerable attention, as differential responses of these components to environmental change have profound implications for the soil and ecosystem C balance. The increasing number of partitioning studies allows a more detailed analysis of experimental constraints than was previously possible. We present results of an exhaustive literature search of partitioning studies and analyse global trends in flux partitioning between biomes and ecosystem types by means of a metaanalysis. Across all data, an overall decline in the R_H/R_S ratio for increasing annual R_S fluxes emerged. For forest ecosystems, boreal coniferous sites showed significantly higher (P<0.05) R_H/R_S ratios than temperate sites, while both temperate or tropical deciduous forests did not differ in ratios from any of the other forest types. While chronosequence studies report consistent declines in the R_H/R_S ratio with age, no difference could be detected for different age groups in the global data set. Different methodologies showed generally good agreement if the range of R_S under which they had been measured was considered, with the exception of studies estimating R_H by means of root mass regressions against R_S, which resulted in consistently lower R_H/R_S estimates out of all methods included. Additionally, the time step over which fluxes were partitioned did not affect R_H/R_S ratios consistently. To put results into context, we review the most common techniques and point out the likely sources of errors associated with them. In order to improve soil CO₂ efflux partitioning in future experiments, we include methodological recommendations, and also highlight the potential interactions between soil components that may be overlooked as a consequence of the partitioning process itself.     

A new method to measure carbon isotope composition of CO2 respired by trees: stem CO2 equilibration

1.  Applying Keeling plot techniques to derive δ¹³C of respiratory input in a closed non-equilibrated chamber can lead to large errors because steady-state diffusion rules are violated in a non-steady-state environment. To avoid these errors, respiratory δ¹³C can be derived using equilibrated closed chambers.
2.  We introduce a new method to obtain stem respired CO₂δ¹³C (δ_{st - r}) with closed equilibrated stem chambers (E-SC). We present a theoretical model describing the equilibration process, test the model against field data and find excellent agreement. The method is further tested by comparing it with closed non-equilibrated stem chambers (NE-SC); we found no difference between these methods.
3.  Our theoretical model to describe CO₂ diffusion from the respiratory pool into the chamber and the equation to derive the δ¹³C of the efflux are general. They could be applied to other ecosystem components (e.g. soils).
4.  Our method is easy to implement, cost effective, minimizes sources of error and allows for rigorous leak detection. One major limitation is its inability to detect rapid change; the equilibration process requires 15 ± 2 h. A second limitation is that it cannot be used for species that produce abundant pitch at sites of stem wounding (e.g. Pseudotsuga menziesii ).
5.  Investigating δ¹³C of CO₂ respired by different ecosystem components is necessary to interpret δ¹³C of ecosystem respiration. This parameter has major implications with respect to global carbon cycle science.     

Diurnal and seasonal variation in the carbon isotope composition of leaf dark-respired CO2 in velvet mesquite (Prosopis velutina)

We evaluated diurnal and seasonal patterns of carbon isotope composition of leaf dark-respired CO₂ ( δ ¹³C_l) in the C₃ perennial shrub velvet mesquite ( Prosopis velutina ) across flood plain and upland savanna ecosystems in the south-western USA. δ ¹³C_l of darkened leaves increased to maximum values late during daytime periods and declined gradually over night-time periods to minimum values at pre-dawn. The magnitude of the diurnal shift in δ ¹³C_l was strongly influenced by seasonal and habitat-related differences in soil water availability and leaf surface vapour pressure deficit. δ ¹³C_l and the cumulative flux-weighted δ ¹³C value of photosynthates were positively correlated, suggesting that progressive ¹³C enrichment of the CO₂ evolved by darkened leaves during the daytime mainly resulted from short-term changes in photosynthetic ¹³C discrimination and associated shifts in the δ ¹³C signature of primary respiratory substrates. The ¹³C enrichment of dark-respired CO₂ relative to photosynthates across habitats and seasons was 4 to 6‰ at the end of the daytime period (1800 h), but progressively declined to 0‰ by pre-dawn (0300 h). The origin of night-time and daytime variations in δ ¹³C_l is discussed in terms of the carbon source(s) feeding respiration and the drought-induced changes in carbon metabolism.     

Seasonal patterns of soil CO2 efflux and soil air CO2 concentration in a Scots pine forest: comparison of two chamber techniques

A non‐vented non‐steady state flow‐through chamber and a non‐vented non‐steady state non‐flow‐through chamber technique were used to measure CO₂ efflux of a young Scots pine forest on a fertile till soil in southern Finland. Soil temperature, soil moisture and soil CO₂ concentration were measured concurrently with CO₂ efflux for two and a half successive years. The CO₂ efflux showed a seasonal pattern, effluxes ranging from low 0.0–0.1 g CO₂ m ^{? 2} h ^{? 1} in winter to peak values of 2.3 g CO₂ m ^{? 2} h ^{? 1} occurring in late June and in July. The daily average effluxes in July measured by flow through chambers were 1.23 and 0.98 g CO₂ m ^{? 2} h ^{? 1} in 1998 and 1999, respectively. The annual accumulated CO₂ efflux was 3117 and 3326 g CO₂ m ^{? 2} in 1998 and 1999, respectively. The spatial variation in CO₂ efflux was high (CV 0.18–0.45) and increased with increasing efflux. Soil air CO₂ concentration showed similar seasonal pattern the peak concentrations occurring in July–August. The CO₂ concentrations ranged from 580 to 780 µ mol mol ^{? 1} in the humus layer to 13 620–14 470 µ mol mol ^{? 1} in the C‐horizon. In winter the soil air CO₂ concentrations were lower, especially in deeper soil layers. Drought decreased CO₂ efflux and soil air CO₂ concentration. The in situ comparison on forest soil between the chamber methods showed the non‐flow‐through chamber to give ～～50% lower efflux values than that of the flow‐through chamber. When calibrated against known CO₂ efflux ranging from 0.4 to 0.8 g CO₂ m ^{? 2} h ^{? 1} generated with a diffusion box method developed by Widén and Lindroth [Acta Universitatis Agriculturae Suecia Silvestria, 2001], the flow‐through chamber gave equal effluxes at the lower end of the calibration range, but overestimated high effluxes by 20%. Non‐flow‐through chamber underestimated the CO₂ efflux by 30%.     

Diurnal, seasonal and annual variation in net ecosystem CO2 exchange of an alpine shrubland on Qinghai-Tibetan plateau

Thus far, grassland ecosystem research has mainly been focused on low‐lying grassland areas, whereas research on high‐altitude grassland areas, especially on the carbon budget of remote areas like the Qinghai‐Tibetan plateau is insufficient. To address this issue, flux of CO₂ were measured over an alpine shrubland ecosystem (37°36′N, 101°18′E; 325 above sea level [a. s. l.]) on the Qinghai‐Tibetan Plateau, China, for 2 years (2003 and 2004) with the eddy covariance method. The vegetation is dominated by formation Potentilla fruticosa L. The soil is Mol–Cryic Cambisols. To interpret the biotic and abiotic factors that modulate CO₂ flux over the course of a year we decomposed net ecosystem CO₂ exchange (NEE) into its constituent components, and ecosystem respiration (R_eco). Results showed that seasonal trends of annual total biomass and NEE followed closely the change in leaf area index. Integrated NEE were ?58.5 and ?75.5 g C m^?2, respectively, for the 2003 and 2004 years. Carbon uptake was mainly attributed from June, July, August, and September of the growing season. In July, NEE reached seasonal peaks of similar magnitude (4–5 g C m^?2 day^?1) each of the 2 years. Also, the integrated night‐time NEE reached comparable peak values (1.5–2 g C m^?2 day^?1) in the 2 years of study. Despite the large difference in time between carbon uptake and release (carbon uptake time < release time), the alpine shrubland was carbon sink. This is probably because the ecosystem respiration at our site was confined significantly by low temperature and small biomass and large day/night temperature difference and usually soil moisture was not limiting factor for carbon uptake. In general, R_eco was an exponential function of soil temperature, but with season‐dependent values of Q₁₀. The temperature‐dependent respiration model failed immediately after rain events, when large pulses of R_eco were observed. Thus, for this alpine shrubland in Qinghai‐Tibetan plateau, the timing of rain events had more impact than the total amount of precipitation on ecosystem R_eco and NEE.     

Root responses along a subambient to elevated CO2 gradient in a C3–C4 grassland

Atmospheric CO₂ (C_a) concentration has increased significantly during the last 20 000 years, and is projected to double this century. Despite the importance of belowground processes in the global carbon cycle, community‐level and single species root responses to rising C_a are not well understood. We measured net community root biomass over 3 years using ingrowth cores in a natural C₃–C₄ grassland exposed to a gradient of C_a from preglacial to future levels (230–550 μmol mol^?1). Root windows and minirhizotron tubes were installed below naturally occurring stands of the C₄ perennial grass Bothriochloa ischaemum and its roots were measured for respiration, carbohydrate concentration, specific root length (SRL), production, and lifespan over 2 years. Community root biomass increased significantly (P<0.05) with C_a over initial conditions, with linear or curvilinear responses depending on sample date. In contrast, B. ischaemum produced significantly more roots at subambient than elevated C_a in minirhizotrons. The lifespan of roots with five or more neighboring roots in minirhizotron windows decreased significantly at high C_a, suggesting that after dense root growth depletes soil resource patches, plants with carbon surpluses readily shed these roots. Root respiration in B. ischaemum showed a curvilinear response to C_a under moist conditions in June 2000, with the lowest rates at C_a<300 μmol mol^?1 and peak activity at 450 μmol mol^?1 in a quadratic model. B. ischaemum roots at subambient C_a had higher SRLs and slightly higher carbohydrate concentrations than those at higher C_a, which may be related to drier soils at low C_a. Our data emphasize that belowground responses of plant communities to C_a can be quite different from those of the individual species, and suggest that complex interactions between and among roots and their immediate soil environment influence the responses of root physiology and lifespan to changing C_a.     

Vertical partitioning of CO2 production within a temperate forest soil

The major driving factors of soil CO₂ production – substrate supply, temperature, and water content – vary vertically within the soil profile, with the greatest temporal variations of these factors usually near the soil surface. Several studies have demonstrated that wetting and drying of the organic horizon contributes to temporal variation in summertime soil CO₂ efflux in forests, but this contribution is difficult to quantify. The objectives of this study were to partition CO₂ production vertically in a mixed hardwood stand of the Harvard Forest, Massachusetts, USA, and then to use that partitioning to evaluate how the relative contributions of CO₂ production by genetic soil horizon vary seasonally and interannually. We measured surface CO₂ efflux and vertical soil profiles of CO₂ concentration, temperature, water content, and soil physical characteristics. These data were applied to a model of effective diffusivity to estimate CO₂ flux at the top of each genetic soil horizon and the production within each horizon. A sensitivity analysis revealed sources of uncertainty when applying a diffusivity model to a rocky soil with large spatial heterogeneity, especially estimates of bulk density and volumetric water content and matching measurements of profiles and surface fluxes. We conservatively estimate that the O horizon contributed 40–48% of the total annual soil CO₂ efflux. Although the temperature sensitivity of CO₂ production varied across soil horizons, the partitioning of CO₂ production by horizon did not improve the overall prediction of surface CO₂ effluxes based on temperature functions. However, vertical partitioning revealed that water content covaried with CO₂ production only in the O horizon. Large interannual variations in estimates of O horizon CO₂ production indicate that this layer could be an important transient interannual source or sink of ecosystem C.     

Source components and interannual variability of soil CO2 efflux under experimental warming and clipping in a grassland ecosystem

Partitioning soil CO₂ efflux into autotrophic (R_A) and heterotrophic (R_H) components is crucial for understanding their differential responses to climate change. We conducted a long‐term experiment (2000–2005) to investigate effects of warming 2°C and yearly clipping on soil CO₂ efflux and its components (i.e. R_A and R_H) in a tallgrass prairie ecosystem. Interannual variability of these fluxes was also examined. Deep collars (70 cm) were inserted into soil to measure R_H. R_A was quantified as the difference between soil CO₂ efflux and R_H. Warming treatment significantly stimulated soil CO₂ efflux and its components (i.e. R_A and R_H) in most years. In contrast, yearly clipping significantly reduced soil CO₂ efflux only in the last 2 years, although it decreased R_H in every year of the study. Temperature sensitivity (i.e. apparent Q₁₀ values) of soil CO₂ efflux was slightly lower under warming (P>0.05) and reduced considerably by clipping (P<0.05) compared with that in the control. On average over the 4 years, R_H accounted for approximately 65% of soil CO₂ efflux with a range from 58% to 73% in the four treatments. Over seasons, the contribution of R_H to soil CO₂ efflux reached a maximum in winter (∼90%) and a minimum in summer (∼35%). Annual soil CO₂ efflux did not vary substantially among years as precipitation did. The interannual variability of soil CO₂ efflux may be mainly caused by precipitation distribution and summer severe drought. Our results suggest that the effects of warming and yearly clipping on soil CO₂ efflux and its components did not result in significant changes in R_H or R_A contribution, and rainfall timing may be more important in determining interannual variability of soil CO₂ efflux than the amount of annual precipitation.