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1.
We tested the hypothesis that the stable carbon isotope signature of ecosystem respiration (δ13CR) was regulated by canopy conductance (Gc) using weekly Keeling plots (n=51) from a semiarid old‐growth ponderosa pine (Pinus ponderosa) forest in Oregon, USA. For a comparison of forests in two contrasting climates we also evaluated trends in δ13CR from a wet 20‐year‐old Douglas‐fir (Pseudotsuga menziesii) plantation located near the Pacific Ocean. Intraannual variability in δ13CR was greater than 8.0‰ at both sites, was highest during autumn, winter, and spring when rainfall was abundant, and lowest during summer drought. The δ13CR of the dry pine forest was consistently more positive than the wetter Douglas‐fir forest (mean annual δ13CR: ?25.41‰ vs. ?26.23‰, respectively, P=0.07). At the Douglas‐fir forest, δ13CR–climate relationships were consistent with predictions based on stomatal regulation of carbon isotope discrimination (Δ). Soil water content (SWC) and vapor pressure deficit (vpd) were the most important factors governing δ13CR in this forest throughout the year. In contrast, δ13CR at the pine forest was relatively insensitive to SWC or vpd, and exhibited a smaller drought‐related enrichment (~2‰) than the enrichment observed during drought at the Douglas‐fir forest (~5‰). Groundwater access at the pine forest may buffer canopy–gas exchange from drought. Despite this potential buffering, δ13CR at the pine forest was significantly but weakly related to canopy conductance (Gc), suggesting that δ13CR remains coupled to canopy–gas exchange despite groundwater access. During drought, δ13CR was strongly correlated with soil temperature at both forests. The hypothesis that canopy‐level physiology is a critical regulator of δ13CR was supported; however, belowground respiration may become more important during rain‐free periods.  相似文献   

2.
The δ13C values of atmospheric carbon dioxide (CO2) can be used to partition global patterns of CO2 source/sink relationships among terrestrial and oceanic ecosystems using the inversion technique. This approach is very sensitive to estimates of photosynthetic 13C discrimination by terrestrial vegetation (ΔA), and depends on δ13C values of respired CO2 fluxes (δ13CR). Here we show that by combining two independent data streams – the stable isotope ratios of atmospheric CO2 and eddy‐covariance CO2 flux measurements – canopy scale estimates of ΔA can be successfully derived in terrestrial ecosystems. We also present the first weekly dataset of seasonal variations in δ13CR from dominant forest ecosystems in the United States between 2001 and 2003. Our observations indicate considerable summer‐time variation in the weekly value of δ13CR within coniferous forests (4.0‰ and 5.4‰ at Wind River Canopy Crane Research Facility and Howland Forest, respectively, between May and September). The monthly mean values of δ13CR showed a smaller range (2–3‰), which appeared to significantly correlate with soil water availability. Values of δ13CR were less variable during the growing season at the deciduous forest (Harvard Forest). We suggest that the negative correlation between δ13CR and soil moisture content observed in the two coniferous forests should represent a general ecosystem response to the changes in the distribution of water resources because of climate change. Shifts in δ13CR and ΔA could be of sufficient magnitude globally to impact partitioning calculations of CO2 sinks between oceanic and terrestrial compartments.  相似文献   

3.
In this paper we describe measurements and modeling of 18O in CO2 and H2O pools and fluxes at a tallgrass prairie site in Oklahoma. We present measurements of the δ18O value of leaf water, depth‐resolved soil water, atmospheric water vapor, and Keeling plot δ18O intercepts for net soil‐surface CO2 and ecosystem CO2 and H2O fluxes during three periods of the 2000 growing season. Daytime discrimination against C18OO, as calculated from measured above‐canopy CO2 and δ18O gradients, is also presented. To interpret the isotope measurements, we applied an integrated land‐surface and isotope model (ISOLSM) that simulates ecosystem H218O and C18OO stocks and fluxes. ISOLSM accurately predicted the measured isotopic composition of ecosystem water pools and the δ18O value of net ecosystem CO2 and H2O fluxes. Simulations indicate that incomplete equilibration between CO2 and H2O within C4 plant leaves can have a substantial impact on ecosystem discrimination. Diurnal variations in the δ18O value of above‐canopy vapor had a small impact on the predicted δ18O value of ecosystem water pools, although sustained differences had a large impact. Diurnal variations in the δ18O value of above‐canopy CO2 substantially affected the predicted ecosystem discrimination. Leaves dominate the ecosystem 18O‐isoflux in CO2 during the growing season, while the soil contribution is relatively small and less variable. However, interpreting daytime measurements of ecosystem C18OO fluxes requires accurate predictions of both soil and leaf 18O‐isofluxes.  相似文献   

4.
Conversion of grasslands to woodlands may alter the sensitivity of CO2 exchange of individual plants and entire ecosystems to air temperature and precipitation. We combined leaf‐level gas exchange and ecosystem‐level eddy covariance measurements to quantify the effects of plant temperature sensitivity and ecosystem temperature responses within a grassland and mesquite woodland across seasonal precipitation periods. In so doing, we were able to estimate the role of moisture availability on ecosystem temperature sensitivity under large‐scale vegetative shifts. Optimum temperatures (Topt) for net photosynthetic assimilation (A) and net ecosystem productivity (NEP) were estimated from a function fitted to A and NEP plotted against air temperature. The convexities of these temperature responses were quantified by the range of temperatures over which a leaf or an ecosystem assimilated 50% of maximum NEP (Ω50). Under dry pre‐ and postmonsoon conditions, leaf‐level Ω50 in C3 shrubs were two‐to‐three times that of C4 grasses, but under moist monsoon conditions, leaf‐level Ω50 was similar between growth forms. At the ecosystems‐scale, grassland NEP was more sensitive to precipitation, as evidenced by a 104% increase in maximum NEP at monsoon onset, compared to a 57% increase in the woodland. Also, woodland NEP was greater across all temperatures experienced by both ecosystems in all seasons. By maintaining physiological function across a wider temperature range during water‐limited periods, woody plants assimilated larger amounts of carbon. This higher carbon‐assimilation capacity may have significant implications for ecosystem responses to projected climate change scenarios of higher temperatures and more variable precipitation, particularly as semiarid regions experience conversions from C4 grasses to C3 shrubs. As regional carbon models, CLM 4.0, are now able to incorporate functional type and photosynthetic pathway differences, this work highlights the need for a better integration of the interactive effects of growth form/functional type and photosynthetic pathway on water resource acquisition and temperature sensitivity.  相似文献   

5.
Estimates of terrestrial carbon isotope discrimination are useful to quantify the terrestrial carbon sink. Carbon isotope discrimination by terrestrial ecosystems may vary on seasonal and interannual time frames, because it is affected by processes (e.g. photosynthesis, stomatal conductance, and respiration) that respond to variable environmental conditions (e.g. air humidity, temperature, light). In this study, we report simulations of the temporal variability of canopy‐scale C3 photosynthetic carbon isotope discrimination obtained with an ecophysiologically based model (ISOLSM) designed for inclusion in global models. ISOLSM was driven by half‐hourly meteorology, and parameterized with eddy covariance measurements of carbon and energy fluxes and foliar carbon isotope ratios from a pine forest in Metolius (OR). Comparing simulated carbon and energy fluxes with observations provided a range of parameter values that optimized the simulated fluxes. We found that the sensitivity of photosynthetic carbon isotope discrimination to the slope of the stomatal conductance equation (m, Ball–Berry constant) provided an additional constraint to the model, reducing the wide parameter space obtained from the fluxes alone. We selected values of m that resulted in similar simulated long‐term discrimination as foliar isotope ratios measured at the site. The model was tested with 13C measurements of ecosystem (δR) and foliar (δf) respiration. The daily variability of simulated 13C values of assimilated carbon (δA) was similar to that of observed δf, and higher than that of observed and simulated δR. We also found similar relationships between environmental factors (i.e. vapor pressure deficit) and simulated δR as measured in ecosystem surveys of δR. Therefore, ISOLSM reasonably simulated the short‐term variability of δA controlled by atmospheric conditions at the canopy scale, which can be useful to estimate the variability of terrestrial isotope discrimination. Our study also shows that including the capacity to simulate carbon isotope discrimination, together with simple ecosystem isotope measurements, can provide a useful constraint to land surface and carbon balance models.  相似文献   

6.
Accurate estimates of the δ13C value of CO2 respired from roots (δ13CR_root) and leaves (δ13CR_leaf) are important for tracing and understanding changes in C fluxes at the ecosystem scale. Yet the mechanisms underlying temporal variation in these isotopic signals are not fully resolved. We measured δ13CR_leaf, δ13CR_root, and the δ13C values and concentrations of glucose and sucrose in leaves and roots in the C4 grass Sporobolus wrightii and the C3 tree Prosopis velutina in a savanna ecosystem in southeastern Arizona, USA. Night‐time variation in δ13CR_leaf of up to 4.6 ± 0.6‰ in S. wrightii and 3.0 ± 0.6‰ in P. velutina were correlated with shifts in leaf sucrose concentration, but not with changes in δ13C values of these respiratory substrates. Strong positive correlations between δ13CR_root and root glucose δ13C values in P. velutina suggest large diel changes in δ13CR_root (were up to 3.9‰) influenced by short‐term changes in δ13C of leaf‐derived phloem C. No diel variation in δ13CR_root was observed in S. wrightii. Our findings show that short‐term changes in δ13CR_leaf and δ13CR_root were both related to substrate isotope composition and concentration. Changes in substrate limitation or demand for biosynthesis may largely control short‐term variation in the δ13C of respired CO2 in these species.  相似文献   

7.
Precipitation regimes are predicted to become more variable with more extreme rainfall events punctuated by longer intervening dry periods. Water‐limited ecosystems are likely to be highly responsive to altered precipitation regimes. The bucket model predicts that increased precipitation variability will reduce soil moisture stress and increase primary productivity and soil respiration in aridland ecosystems. To test this hypothesis, we experimentally altered the size and frequency of precipitation events during the summer monsoon (July through September) in 2007 and 2008 in a northern Chihuahuan Desert grassland in central New Mexico, USA. Treatments included (1) ambient rain, (2) ambient rain plus one 20 mm rain event each month, and (3) ambient rain plus four 5 mm rain events each month. Throughout two monsoon seasons, we measured soil temperature, soil moisture content (θ), soil respiration (Rs), along with leaf‐level photosynthesis (Anet), predawn leaf water potential (Ψpd), and seasonal aboveground net primary productivity (ANPP) of the dominant C4 grass, Bouteloua eriopoda. Treatment plots receiving a single large rainfall event each month maintained significantly higher seasonal soil θ which corresponded with a significant increase in Rs and ANPP of B. eriopoda when compared with plots receiving multiple small events. Because the strength of these patterns differed between years, we propose a modification of the bucket model in which both the mean and variance of soil water change as a consequence of interannual variability from 1 year to the next. Our results demonstrate that aridland ecosystems are highly sensitive to increased precipitation variability, and that more extreme precipitation events will likely have a positive impact on some aridland ecosystem processes important for the carbon cycle.  相似文献   

8.
Measurements of the carbon (δ13Cm) and oxygen (δ18Om) isotope composition of C3 plant tissue provide important insights into controls on water‐use efficiency. We investigated the causes of seasonal and inter‐annual variability in water‐use efficiency in a grassland near Lethbridge, Canada using stable isotope (leaf‐scale) and eddy covariance measurements (ecosystem‐scale). The positive relationship between δ13Cm and δ18Om values for samples collected during 1998–2001 indicated that variation in stomatal conductance and water stress‐induced changes in the degree of stomatal limitation of net photosynthesis were the major controls on variation in δ13Cm and biomass production during this time. By comparison, the lack of a significant relationship between δ13Cm and δ18Om values during 2002, 2003 and 2006 demonstrated that water stress was not a significant limitation on photosynthesis and biomass production in these years. Water‐use efficiency was higher in 2000 than 1999, consistent with expectations because of greater stomatal limitation of photosynthesis and lower leaf ci/ca during the drier conditions of 2000. Calculated values of leaf‐scale water‐use efficiency were 2–3 times higher than ecosystem‐scale water‐use efficiency, a difference that was likely due to carbon lost in root respiration and water lost during soil evaporation that was not accounted for by the stable isotope measurements.  相似文献   

9.
In mixed forests, interactions among species influence ecosystem functioning but environmental conditions also play an important role in shaping relationships between biodiversity and ecosystem functioning. In the context of climate change, the carbon and water balance in pure versus mixed forest stands may be differentially influenced by changing soil water availability. To test this hypothesis, we compared the influence of biodiversity on stand water use efficiency (WUES) in boreal forests between wet and dry years. We assessed the carbon isotope composition (δ 13C) of tree rings in Betula pendula, Pinus sylvestris, and Picea abies growing in pure versus mixed stands. In addition, we tested whether differences in WUES affected patterns of stand basal area increment (BAIS). No biodiversity effect was found for stand δ 13C (δ 13CS) during the wet year. However, there was a significant increase in δ 13CS between the wet and the dry year and a significant effect of biodiversity on δ 13CS in the dry year. The increase in δ 13CS in mixed stands was associated with both selection and complementarity effects. Although BAIS decreased significantly in the dry year, changes in δ 13CS did not translate into variations in BAIS along the biodiversity gradient. Our results confirmed that the physiological response of boreal forest ecosystems to changing soil water conditions is influenced by species interactions and that during dry growing seasons, species interactions in mixed stands can lead to lower soil moisture availability. This illustrates that biodiversity effects can also be negative in mixed stands in the sense that soil resources can be more intensively exhausted. Overall, our results confirm that in boreal forests, the biodiversity–ecosystem functioning relationship depends on local environmental conditions.  相似文献   

10.
An increase in mean soil surface temperature has been observed over the last century, and it is predicted to further increase in the future. The effect of increased temperature on ecosystem carbon fluxes in a permanent temperate grassland was studied in a long‐term (6 years) field experiment, using multiple temperature increments induced by IR lamps. Ecosystem respiration (R‐eco) and net ecosystem exchange (NEE) were measured and modeled by a modified Lloyd and Taylor model including a soil moisture component for R‐eco (average R2 of 0.78) and inclusion of a photosynthetic component based on temperature and radiation for NEE (R2 = 0.65). Modeled NEE values ranged between 2.3 and 5.3 kg CO2 m?2 year?1, depending on treatment. An increase of 2 or 3°C led to increased carbon losses, lowering the carbon storage potential by around 4 tonnes of C ha?1 year?1. The majority of significant NEE differences were found during night‐time compared to daytime. This suggests that during daytime the increased respiration could be offset by an increase in photosynthetic uptake. This was also supported by differences in δ13C and δ18O, indicating prolonged increased photosynthetic activity associated with the higher temperature treatments. However, this increase in photosynthesis was insufficient to counteract the 24 h increase in respiration, explaining the higher CO2 emissions due to elevated temperature.  相似文献   

11.
Comparisons were made among Douglas‐fir forest, aspen (broad leaf deciduous) forest and wheatgrass (C3) grassland for ecosystem‐level water‐use efficiency (WUE). WUE was defined as the ratio of photosynthetic CO2 assimilation rate and evapotranspiration (ET) rate. The ET data measured by eddy covariance were screened so that they overwhelmingly represented transpiration. The three sites used in this comparison spanned a range of vegetation (plant functional) types and environmental conditions within western Canada. When compared in the relative order Douglas‐fir (located on Vancouver Island, BC), aspen (northern Saskatchewan), grassland (southern Alberta), the sites demonstrated a progressive decline in precipitation and a general increase in maximum air temperature and atmospheric saturation deficit (Dmax) during the mid‐summer. The average (±SD) WUE at the grassland site was 2.6±0.7 mmol mol?1, which was much lower than the average values observed for the two other sites (aspen: 5.4±2.3, Douglas‐fir: 8.1±2.4). The differences in WUE among sites were primarily because of variation in ET. The highest maximum ET rates were approximately 5, 3.2 and 2.7 mm day?1 for the grassland, aspen and Douglas‐fir sites, respectively. There was a strong negative correlation between WUE and Dmax for all sites. We also made seasonal measurements of the carbon isotope ratio of ecosystem respired CO2 (δR) in order to test for the expected correlation between shifts in environmental conditions and changes to the ecosystem‐integrated ratio of leaf intercellular to ambient CO2 concentration (ci/ca). There was a consistent increase in δR values in the grassland, aspen forest and Douglas‐fir forest associated with a seasonal reduction in soil moisture. Comparisons were made between WUE measured using eddy covariance with that calculated based on D and δR measurements. There was excellent agreement between WUE values calculated using the two techniques. Our δR measurements indicated that ci/ca values were quite similar among the Douglas‐fir, aspen and grassland sites, despite large variation in environmental conditions among sites. This implied that the shorter‐lived grass species had relatively high ci/ca values for the D of their habitat. By contrast, the longer‐lived Douglas‐fir trees were more conservative in water‐use with lower ci/ca values relative to their habitat D. This illustrates the interaction between biological and environmental characteristics influencing ecosystem‐level WUE. The strong correlation we observed between the two independent measurements of WUE, indicates that the stable isotope composition of respired CO2 is a useful ecosystem‐scale tool to help study constraints to photosynthesis and acclimation of ecosystems to environmental stress.  相似文献   

12.
At eight different dates during the 2000 growing season, δ13C and δ18O were determined in the phloem of adult beech trees growing in natural beech stands in south‐west Germany differing in stand density and local climate. In addition, stand transpiration, precipitation, photosynthetic active radiation, relative air humidity, water pressure deficit of the air, air and soil temperature, soil water potential, and sugar concentration of the phloem sap were determined directly and evaporation and canopy stomatal conductance were modelled. All parameters were related to δ13C. The study aimed to identify the time integral within which the δ13C of organic compounds transported in the phloem is an indicative measure of these environmental influences. δ13C of soluble carbon transported in the phloem was well correlated with mean stomatal conductance in a two‐day integral prior to phloem sampling but did not depend on either light intensity or soil water availability. A strong positive relationship between δ13C and δ18O pointed to observed variation in δ13C of phloem sap being a result of variation in stomatal conductance. Bulk leaf δ13C was a poor indicator of changes in environmental conditions during the growing season. From these results we conclude that the analysis of δ13C in soluble carbon transported in the phloem is a reliable indicator of short‐term changes in Ci/Ca. In contrast, the δ13C of structural carbon in beech foliage represents an integration of a range of factors that mask short‐term influences responsible for Ci/Ca.  相似文献   

13.
Leaf respiration in the dark and its C isotopic composition (δ13CR) contain information about internal metabolic processes and respiratory substrates. δ13CR is known to be less negative compared to potential respiratory substrates, in particular shortly after darkening during light enhanced dark respiration (LEDR). This phenomenon might be driven by respiration of accumulated 13C‐enriched organic acids, however, studies simultaneously measuring δ13CR during LEDR and potential respiratory substrates are rare. We determined δ13CR and respiration rates (R) during LEDR, as well as δ13C and concentrations of potential respiratory substrates using compound‐specific isotope analyses. The measurements were conducted throughout the diel cycle in several plant species under different environmental conditions. δ13CR and R patterns during LEDR were strongly species‐specific and showed an initial peak, which was followed by a progressive decrease in both values. The species‐specific differences in δ13CR and R during LEDR may be partially explained by the isotopic composition of organic acids (e.g., oxalate, isocitrate, quinate, shikimate, malate), which were 13C‐enriched compared to other respiratory substrates (e.g., sugars and amino acids). However, the diel variations in both δ13C and concentrations of the organic acids were generally low. Thus, additional factors such as the heterogeneous isotope distribution in organic acids and the relative contribution of the organic acids to respiration are required to explain the strong 13C enrichment in leaf dark‐respired CO2.  相似文献   

14.
Conversion of tropical rainforests to pastures and plantations is associated with changes in soil properties and biogeochemical cycling, with implications for carbon cycling and trace gas fluxes. The stable isotopic composition of ecosystem respiration (δ13CR and δ18OR) is used in inversion models to quantify regional patterns of CO2 sources and sinks, but models are limited by sparse measurements in tropical regions. We measured soil respiration rates, concentrations of CO2, CH4, CO, N2O and H2 and the isotopic composition of CO2, CH4 and H2 at four heights in the nocturnal boundary layer (NBL) above three common land‐use types in central Panama, during dry and rainy seasons. Soil respiration rates were lowest in Plantation (average 3.4 μmol m?2 s?1), highest in Pasture (8.3 μmol m?2 s?1) and intermediate in Rainforest (5.2 μmol m?2 s?1). δ13CR closely reflected land use and increased during the dry season where C3 vegetation was present. δ18OR did not differ by land use but was lower during the rainy than the dry season. CO2 was correlated with other species in approximately half of the NBL profiles, allowing us to estimate trace gas fluxes that were generally within the range of literature values. The Rainforest soil was a sink for CH4 but emissions were observed in Pasture and Plantation, especially during the wet season. N2O emissions were higher in Pasture and Plantation than Rainforest, contrary to expectations. Soil H2 uptake was highest in Rainforest and was not observable in Pasture and Plantation during the wet season. We observed soil CO uptake during the dry season and emissions during the wet season across land‐use types. This study demonstrated that strong impacts of land‐use change on soil–atmosphere trace gas exchange can be detected in the NBL, and provides useful observational constraints for top‐down and bottom‐up biogeochemistry models.  相似文献   

15.
Seasonal variation in δ13C and δ18O of cellulose (δ13Cc and δ18Oc) was measured within two annual rings of Pinus radiata growing at three sites in New Zealand. In general, both δ13Cc and δ18Oc increased to a peak over summer. The three sites differed markedly in annual water balance, and these differences were reflected in δ13Cc and δ18Oc. Average δ13Cc and δ18Oc from each site were positively related, so that the driest site had the most enriched cellulose. δ13Cc and δ18Oc were also related within each site, although both the slope and the closeness of fit of the relationship varied between sites. Supporting the theory, the site with the lowest average relative humidity also had the greatest change in δ18Oc‰ change in δ13Cc. Specific climatic events, such as drought or high rainfall, were recorded as a peak or a trough in enrichment, respectively. These results suggest that seasonal and between‐site variation in δ13Cc and δ18Oc are driven by the interaction between variation in climatic conditions and soil water availability, and plant response to this variation.  相似文献   

16.
Compound‐specific stable isotope analysis (CSIA) of amino acids (AA) has rapidly become a powerful tool in studies of food web architecture, resource use, and biogeochemical cycling. However, applications to avian ecology have been limited because no controlled studies have examined the patterns in AA isotope fractionation in birds. We conducted a controlled CSIA feeding experiment on an avian species, the gentoo penguin (Pygoscelis papua), to examine patterns in individual AA carbon and nitrogen stable isotope fractionation between diet (D) and consumer (C) (Δ13CC‐D and Δ15NC‐D, respectively). We found that essential AA δ13C values and source AA δ15N values in feathers showed minimal trophic fractionation between diet and consumer, providing independent but complimentary archival proxies for primary producers and nitrogen sources respectively, at the base of food webs supporting penguins. Variations in nonessential AA Δ13CC‐D values reflected differences in macromolecule sources used for biosynthesis (e.g., protein vs. lipids) and provided a metric to assess resource utilization. The avian‐specific nitrogen trophic discrimination factor (TDFGlu‐Phe = 3.5 ± 0.4‰) that we calculated from the difference in trophic fractionation (Δ15NC‐D) of glutamic acid and phenylalanine was significantly lower than the conventional literature value of 7.6‰. Trophic positions of five species of wild penguins calculated using a multi‐TDFGlu‐Phe equation with the avian‐specific TDFGlu‐Phe value from our experiment provided estimates that were more ecologically realistic than estimates using a single TDFGlu‐Phe of 7.6‰ from the previous literature. Our results provide a quantitative, mechanistic framework for the use of CSIA in nonlethal, archival feathers to study the movement and foraging ecology of avian consumers.  相似文献   

17.
Disentangling the autotrophic and heterotrophic components of soil CO2 efflux is critical to understanding the role of soil system in terrestrial carbon (C) cycling. In this study, we combined a stable C-isotope natural abundance approach with the trenched plot method to determine if root exclusion significantly affected the isotopic composition (δ13C) of soil CO2 efflux (RS). This study was performed in different forest ecosystems: a tropical rainforest and two temperate broadleaved forests, where trenched plots had previously been installed. At each site, RS and its δ13C (δ13CRs) tended to be lower in trenched plots than in control plots. Contrary to RS, δ13CRs differences were not significant. This observation is consistent with the small differences in δ13C measured on organic matter from root, litter and soil. The lack of an effect on δ13CRs by root exclusion could be from the small difference in δ13C between autotrophic and heterotrophic soil respirations, but further investigations are needed because of potential artefacts associated with the root exclusion technique.  相似文献   

18.
We present a study of soil organic carbon (SOC) inventories and δ13C values for 625 soil cores collected from well‐drained, coarse‐textured soils in eight areas along a 1000 km moisture gradient from Southern Botswana, north into southern Zambia. The spatial distribution of trees and grass in the desert, savannah and woodland ecosystems along the transect control large systematic local variations in both SOC inventories and δ13C values. A stratified sampling approach was used to smooth this variability and obtain robust weighted‐mean estimates for both parameters. Weighted SOC inventories in the 0–5 cm interval of the soils range from 7 mg cm?2 in the driest area (mean annual precipitation, MAP=225 mm) to 41±12 mg cm?2 in the wettest area (MAP=910 mm). For the 0–30 cm interval, the inventories are 37.8 mg cm?2 for the driest region and 157±33 mg cm?2 for the wettest region. SOC inventories at intermediate sites increase as MAP increases to approximately 400–500 mm, but remain approximately constant thereafter. This plateau may be the result of feedbacks between MAP, fuel load and fire frequency. Weighted δ 13C values decrease linearly in both the 0–5 and 0–30 cm depth intervals as MAP increases. A value of –17.5±1.0‰ characterizes the driest areas, while a value of ?25±0.7‰ characterizes the wettest area. The decrease in δ 13C value with increasing MAP reflects an increasing dominance of C3 vegetation as MAP increases. SOC in the deeper soil (5–30 cm depth) is, on average, 0.4±0.3‰ enriched in 13C relative to SOC in the 0–5 cm interval.  相似文献   

19.
This study investigated the impact of predicted future climatic and atmospheric conditions on soil respiration (RS) in a Danish Calluna‐Deschampsia‐heathland. A fully factorial in situ experiment with treatments of elevated atmospheric CO2 (+130 ppm), raised soil temperature (+0.4 °C) and extended summer drought (5–8% precipitation exclusion) was established in 2005. The average RS, observed in the control over 3 years of measurements (1.7 μmol CO2 m?2 sec?1), increased 38% under elevated CO2, irrespective of combination with the drought or temperature treatments. In contrast, extended summer drought decreased RS by 14%, while elevated soil temperature did not affect RS overall. A significant interaction between elevated temperature and drought resulted in further reduction of RS when these treatments were combined. A detailed analysis of short‐term RS dynamics associated with drought periods showed that RS was reduced by ~50% and was strongly correlated with soil moisture during these events. Recovery of RS to pre‐drought levels occurred within 2 weeks of rewetting; however, unexpected drought effects were observed several months after summer drought treatment in 2 of the 3 years, possibly due to reduced plant growth or changes in soil water holding capacity. An empirical model that predicts RS from soil temperature, soil moisture and plant biomass was developed and accounted for 55% of the observed variability in RS. The model predicted annual sums of RS in 2006 and 2007, in the control, were 672 and 719 g C m?2 y?1, respectively. For the full treatment combination, i.e. the future climate scenario, the model predicted that soil respiratory C losses would increase by ~21% (140–150 g C m?2 y?1). Therefore, in the future climate, stimulation of C storage in plant biomass and litter must be in excess of 21% for this ecosystem to not suffer a reduction in net ecosystem exchange.  相似文献   

20.
In semi-arid regions, where plants using both C3 and C4 photosynthetic pathways are common, the stable C isotope ratio (δ13C) of ecosystem respiration (δ13CR) is strongly variable seasonally and inter-annually. Improved understanding of physiological and environmental controls over these variations will improve C cycle models that rely on the isotopic composition of atmospheric CO2. We hypothesized that timing of precipitation events and antecedent moisture interact with activity of C3 and C4 grasses to determine net ecosystem CO2 exchange (NEE) and δ13CR. Field measurements included CO2 and δ13C fluxes from the whole ecosystem and from patches of different plant communities, biomass and δ13C of plants and soils over the 2000 and 2001 growing seasons. NEE shifted from C source to sink in response to rainfall events, but this shift occurred after a time lag of up to 2 weeks if a dry period preceded the rainfall. The seasonal average of δ13CR was higher in 2000 (−16‰) than 2001 (20‰), probably due to drier conditions during the 2000 growing season (79.7 mm of precipitation from April up to and including July) than in 2001 (189 mm). During moist conditions, δ13C averaged −22‰ from C3 patches, −16‰ from C4 patches, and −19‰ from mixed C3 and C4 patches. However, during dry conditions the apparent spatial differences were not obvious, suggesting reduced autotrophic activity in C4 grasses with shallow rooting depth, soon after the onset of dry conditions. Air and soil temperatures were negatively correlated with δ13CR; vapor pressure deficit was a poor predictor of δ13CR, in contrast to more mesic ecosystems. Responses of respiration components to precipitation pulses were explained by differences in soil moisture thresholds between C3 and C4 species. Stable isotopic composition of respiration in semi-arid ecosystems is more temporally and spatially variable than in mesic ecosystems owing to dynamic aspects of pulse precipitation episodes and biological drivers.  相似文献   

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