Peatland carbon efflux partitioning reveals that Sphagnum photosynthate contributes to the DOC pool

Over half of the world's peat originated from Sphagnum, representing 10–15% of the terrestrial carbon stock. However, information regarding the release and exudation of organic carbon by living Sphagnum plants into the surface peat is scarce. In this study, we examined the contribution of recent Sphagnum subnitens (Russ. and Warnst.) photosynthate carbon to the peatland dissolved organic carbon (DOC) pool. This was done using a ¹³CO₂ pulse-chase experimental approach during the growing season. Despite the importance of Sphagnum in long-term carbon accumulation, results showed that the Sphagnum community rapidly contributes recently synthesized carbon to the peatland DOC pool. We estimate that by 4 h up to 4% of the total DOC in peat leachate was derived from ¹³CO₂ pulse labelling at ambient CO₂ concentrations. Nonetheless, a huge 64% of the ¹³C initially assimilated by photosynthesis was retained in Sphagnum subnitens for 23 days after labelling, consistent with the role of Sphagnum in peatland carbon accumulation. The majority of ¹³C loss as respired CO₂ came within the few days post ¹³CO₂ labelling, suggesting that it was derived from plant respiration of photosynthates.     

Influence of net ecosystem metabolism in transferring riverine organic carbon to atmospheric CO<Subscript>2</Subscript> in a tropical coastal lagoon (Chilka Lake,India)

Studies on biogeochemical cycling of carbon in the Chilka Lake, Asia’s largest brackish lagoon on the east coast of India, revealed, for the first time, strong seasonal and spatial variability associated with salinity distribution. The lake was studied twice during May 2005 (premonsoon) and August 2005 (monsoon). It exchanges waters with the sea (Bay of Bengal) and several rivers open into the lake. The lake showed contrasting levels of dissolved inorganic carbon (DIC) and organic carbon (DOC) in different seasons; DIC was higher by ∼22% and DOC was lower by ∼36% in premonsoon than in monsoon due to seasonal variations in their supply from rivers and in situ production/mineralisation. The DIC/DOC ratios in the lake during monsoon were influenced by physical mixing of end member water masses and by intense respiration of organic carbon. A strong relationship between excess DIC and apparent oxygen utilisation showed significant control of biological processes over CO₂ production in the lake. Surface partial pressure of CO₂ (pCO₂), calculated using pH–DIC couple according to Cai and Wang (Limnol and Oceanogr 43:657–668, 1998), exhibited discernable gradients during monsoon through northern (1,033–6,522 μatm), central (391–2,573 μatm) and southern (102–718 μatm) lake. The distribution pattern of pCO₂ in the lake seems to be governed by pCO₂ levels in rivers and their discharge rates, which were several folds higher during monsoon than premonsoon. The net CO₂ efflux, based on gas transfer velocity parameterisation of Borges et al. (Limnol and Oceanogr 49(5):1630–1641, 2004), from entire lake during monsoon (141 mmolC m⁻² d⁻¹ equivalent to 2.64 GgC d⁻¹ at basin scale) was higher by 44 times than during premonsoon (9.8 mmolC m⁻² d⁻¹ ≈ 0.06 GgC d⁻¹). 15% of CO₂ efflux from lake in monsoon was contributed by its supply from rivers and the rest was contributed by in situ heterotrophic activity. Based on oxygen and total carbon mass balance, net ecosystem production (NEP) of lake (−308 mmolC m⁻² d⁻¹ ≈ −3.77 GgC d⁻¹) was found to be almost in consistent with the total riverine organic carbon trapped in the lake (229 mmolC m⁻² d⁻¹ ≈ 2.80 GgC d⁻¹) suggesting that the strong heterotrophy in the lake is mainly responsible for elevated fluxes of CO₂ during monsoon. Further, the pelagic net community production represented 92% of NEP and benthic compartment plays only a minor role. This suggests that Chilka lake is an important region in biological transformation of organic carbon to inorganic carbon and its export to the atmosphere.     

Spatial patterns of foliar stable carbon isotope compositions of C<Subscript>3</Subscript> plant species in the Loess Plateau of China

The spatial pattern of foliar stable carbon isotope compositions (δ¹³C) of dominant species and their relationships with environmental factors in seven sites, Yangling, Yongshou, Tongchuan, Fuxian, Ansai, Mizhi and Shenmu, standing from south to north in the Loess Plateau of China, was studied. The results showed that in the 121 C₃ plant samples collected from the Loess Plateau, the foliar δ¹³C value ranged from −22.66‰ to −30.70‰, averaging −27.04‰. The foliar δ¹³C value varied significantly (P<0.01) among the seven sites, and the average δ¹³C value increased by about 1.69‰ from Yangling in the south to Shenmu in the north as climatic drought increased. There was a significant difference in foliar δ¹³C value among three life-forms categorized from all the plant samples in the Loess Plateau (P<0.001). The trees (−26.74‰) and shrubs (−26.68‰) had similar mean δ¹³C values, both significantly (P<0.05) higher than the mean δ¹³C value of herbages (−27.69‰). It was shown that the trees and shrubs had higher WUEs and employed more conservative water-use patterns to survive drier habitats in the Loess Plateau. Of all the C₃ species in the Loess Plateau, the foliar δ¹³C values were significantly and negatively correlated with the mean annual rainfall (P<0.001) and mean annual temperature (P<0.05), while being significantly and positively correlated with the latitude (P<0.001) and the annual solar radiation (P<0.01). In general, the foliar δ¹³C values increased as the latitude and solar radiation increased and the rainfall and temperature decreased. The annual rainfall as the main influencing factor could explain 13.3% of the spatial variations in foliar δ¹³C value. A 100 mm increment in annual rainfall would result in a decrease by 0.88‰ in foliar δ¹³C values.     

Rise in CO<Subscript>2</Subscript> affects soil water transport through repellency

Several studies have shown improved soil stability under elevated atmospheric CO₂ caused by increased plant and microbial biomass. These studies have not quantified the mechanisms responsible for soil stabilisation or the effect on water relations. The objective of this study was to assess changes in water repellency under elevated CO₂. We hypothesised that increased plant biomass will drive an increase in water repellency, either directly or through secondary microbial processes. Barley plants were grown at ambient (360 ppm) and elevated (720 ppm) CO₂ concentrations in controlled chambers. Each plant was grown in a separate tube of 1.2 m length constructed from 22 mm depth × 47 mm width plastic conduit trunk and packed with sieved arable soil to 55% porosity. After 10 weeks growth the soil was dried at 40°C before measuring water sorptivity, ethanol sorptivity and repellency at many depths with a 0.14 mm radius microinfiltrometer. This provided a microscale measure of the capacity of soil to rewet after severe drying. At testing roots extended throughout the depth of the soil in the tube. The depth of the measurement had no effect on sorptivity or repellency. A rise in CO₂ resulted in a decrease in water sorptivity from 1.13 ± 0.06 (s.e) mm s^−1/2 to 1.00 ± 0.05 mm s^−1/2 (P < 0.05) and an increase in water repellency from 1.80 ± 0.09 to 2.07 ± 0.08 (P < 0.05). Ethanol sorptivity was not affected by CO₂ concentration, suggesting a similar pore structure. Repellency was therefore the primary cause of decreased water sorptivity. The implications will be both positive and negative, with repellency potentially increasing soil stability but also causing patchier wetting of the root-zone.     

Vascular Plant Responses to Elevated CO2 in a Temperate Lowland Sphagnum Peatland

Vascular plant responses to experimental enrichment with atmospheric carbon dioxide (CO₂), using MINIFACE technology, were studied in a Dutch lowland peatland dominated by Sphagnum and Phragmites for 3 years. We hypothesized that vascular plant carbon would accumulate in this peatland in response to CO₂ enrichment owing to increased productivity of the predominant species and poorer quality (higher C/N ratios) and consequently lower decomposability of the leaf litter of these species. Carbon isotope signatures demonstrated that the extra 180 ppmv CO₂ in enriched plots had been incorporated into vegetation biomass accordingly. However, on the CO₂ sequestration side of the ecosystem carbon budget, there were neither any significant responses of total aboveground abundance of vascular plants, nor of any of the individual species. On the CO₂ release side of the carbon budget (decomposition pathway), litter quantity did not differ between ambient and CO₂ treatments, while the changes in litter quality (N and P concentration, C/N and C/P ratio) were marginal and inconsistent. It appeared therefore that the afterlife effects of significant CO₂-induced changes in green-leaf chemistry (lower N and P concentrations, higher C/N and C/P) were partly offset by greater resorption of mobile carbohydrates from green leaves during senescence in CO₂-enriched plants. The decomposability of leaf litters of three predominant species from ambient and CO₂-enriched plots, as measured in a laboratory litter respiration assay, showed no differences. The relatively short time period, environmental spatial heterogeneity and small plot sizes might explain part of the lack of CO₂ response. When our results are combined with those from other Sphagnum peatland studies, the common pattern emerges that the vascular vegetation in these ecosystems is genuinely resistant to CO₂-induced change. On decadal time-scales, water management and its effects on peatland hydrology, N deposition from anthropogenic sources and land management regimes that arrest the early successional phase (mowing, tree and shrub removal), may have a greater impact on the vascular plant species composition, carbon balance and functioning of lowland Sphagnum–Phragmites reedlands than increasing CO₂ concentrations in the atmosphere.     

Effects of elevated CO<Subscript>2</Subscript> and/or O<Subscript>3</Subscript> on hormone IAA in needles of Chinese pine

This study examined the effects of season-long exposure of Chinese pine (Pinus tabulaeformis) to elevated carbon dioxide (CO₂) and/or ozone (O₃) on indole-3-acetic acid (IAA) content, activities of IAA oxidase (IAAO) and peroxidase (POD) in needles. Trees grown in open-top chambers (OTC) were exposed to control (ambient O₃, 55 nmol mol⁻¹ + ambient CO₂, 350 μmol mol⁻¹, CK), elevated CO₂ (ambient O₃ + high CO₂, 700 μmol mol⁻¹, EC) and elevated O₃ (high O₃, 80 ± 8 nmol mol⁻¹ + ambient CO₂, EO) OTCs from 1 June to 30 September. Plants grown in elevated CO₂ OTC had a growth increase of axial shoot and needle length, compared to control, by 20% and 10% respectively, while the growth in elevated O₃ OTC was 43% and 7% less respectively, than control. An increase in IAA content and POD activity and decrease in IAAO activity were observed in trees exposed to elevated CO₂ concentration compared with control. Elevated O₃ decreased IAA content and had no significant effect on IAAO activity, but significantly increased POD activity. When trees pre-exposed to elevated CO₂ were transferred to elevated O₃ (EC–EO) or trees pre-exposed to elevated O₃ were transferred to elevated CO₂ (EO–EC), IAA content was lower while IAAO activity was higher than that transferred to CK (EC–CK or EO–CK), the change in IAA content was also related to IAAO activity. The results indicated that IAAO and POD activities in Chinese pine needles may be affected by the changes in the atmospheric environment, resulting in the change of IAA metabolism which in turn may cause changes in Chinese pine’s growth. An erratum to this article can be found at     

Effect of salinity and plant species on CO2 flux and leaching of dissolved organic carbon during decomposition of plant residue

Mitigation of increased concentrations of CO₂ in the atmosphere by plants may be more efficient in saline systems with soils lower in organic matter than in other freshwater systems. In saline systems, decomposition rates may be lower and potential soil carbon storage higher than in fresh water systems. The effects of salinity, plant species and time on CO₂ surface flux and dissolved organic carbon (DOC) leached during irrigation were determined in the laboratory in microcosms containing sand amended with residues of two halophytes, Atriplex nummularia and Salicornia bigelovii, and one glycophyte, Triticum aestivum. Surface flux of CO₂ and DOC leached during decomposition were monitored for 133 days at 24 °C in microcosms containing different plant residue (5% w/w). Microcosms were irrigated every 14 days with distilled water or seawater adjusted to 10, 20, or 40 g L^-1 salts. CO₂ flux and DOC leached were significantly higher from microcosms amended with A. nummularia residue compared to S. bigelovii or T. aestivum at all salinities and decreased significantly over time for all plant species. Irrigating with water of high salinity, 40 g L^-1, compared to distilled water resulted in a decrease in CO₂ surface flux and DOC in leachate, but differences were not significant at all sampling dates. Results indicate that plant residue composition, as well as increased salinity, affect CO₂ surface flux and DOC in leachate during plant residue decomposition and may be an important consideration for C storage in saline systems.     

Carbon balance in a monospecific stand of an annual herb <Emphasis Type="Italic">Chenopodium album</Emphasis> at an elevated CO<Subscript>2</Subscript> concentration

Elevated CO₂ enhances carbon uptake of a plant stand, but the magnitude of the increase varies among growth stages. We studied the relative contribution of structural and physiological factors to the CO₂ effect on the carbon balance during stand development. Stands of an annual herb Chenopodium album were established in open-top chambers at ambient and elevated CO₂ concentrations (370 and 700 μmol mol⁻¹). Plant biomass growth, canopy structural traits (leaf area, leaf nitrogen distribution, and light gradient in the canopy), and physiological characteristics (leaf photosynthesis and respiration of organs) were studied through the growing season. CO₂ exchange of the stand was estimated with a canopy photosynthesis model. Rates of light-saturated photosynthesis and dark respiration of leaves as related with nitrogen content per unit leaf area and time-dependent reduction in specific respiration rates of stems and roots were incorporated into the model. Daily canopy carbon balance, calculated as an integration of leaf photosynthesis minus stem and root respiration, well explained biomass growth determined by harvests (r ² = 0.98). The increase of canopy photosynthesis with elevated CO₂ was 80% at an early stage and decreased to 55% at flowering. Sensitivity analyses suggested that an alteration in leaf photosynthetic traits enhanced canopy photosynthesis by 40–60% throughout the experiment period, whereas altered canopy structure contributed to the increase at the early stage only. Thus, both physiological and structural factors are involved in the increase of carbon balance and growth rate of C. album stands at elevated CO₂. However, their contributions were not constant, but changed with stand development.     

Interacting effects of elevated atmospheric CO<Subscript>2</Subscript> and hydrology on the growth and carbon sequestration of <Emphasis Type="Italic">Sphagnum</Emphasis> moss

Peatlands are a critical carbon store comprising 30% of the Earth’s terrestrial soil carbon. Sphagnum mosses comprise up to 90% of peat in the northern hemisphere but impacts of climate change on Sphagnum mosses are poorly understood, limiting development of sustainable peatland management and restoration. This study investigates the effects of elevated atmospheric CO₂ (eCO₂) (800 ppm) and hydrology on the growth of Sphagnum fallax, Sphagnum capillifolium and Sphagnum papillosum and greenhouse gas fluxes from moss–peat mesocosms. Elevated CO₂ levels increased Sphagnum height and dry weight but the magnitude of the response differed among species. The most responsive species, S. fallax, yielded the most biomass compared to S. papillosum and S. capillifolium. Water levels and the CO₂ treatment were found to interact, with the highest water level (1 cm below the surface) seeing the largest increase in dry weight under eCO₂ compared to ambient (400 ppm) concentrations. Initially, CO₂ flux rates were similar between CO₂ treatments. After week 9 there was a consistent three-fold increase of the CO₂ sink strength under eCO₂. At the end of the experiment, S. papillosum and S. fallax were greater sinks of CO₂ than S. capillifolium and the ? 7 cm water level treatment showed the strongest CO₂ sink strength. The mesocosms were net sources of CH₄ but the source strength varied with species, specifically S. fallax produced more CH₄ than S. papillosum and S. capillifolium. Our findings demonstrate the importance of species selection on the outcomes of peatland restoration with regards to Sphagnum’s growth and GHG exchange.     

Elevated [CO2], temperature increase and N supply effects on the turnover of below-ground carbon in a temperate grassland ecosystem

The effects of elevated [CO₂] (700 μl l^-1 CO₂) and temperature increase (+3 °C) on carbon turnover in grassland soils were studied during 2.5 years at two N fertiliser supplies (160 and 530 kg N ha^-1 y^-1) in an experiment with well-established ryegrass swards (Lolium perenne) supplied with the same amounts of irrigation water. During the growing season, swards from the control climate (350 μl l^-1 [CO₂] at outdoor air temperature) were pulse labelled by the addition of ¹³CO₂. The elevated [CO₂] treatments were continuously labelled by the addition of fossil-fuel derived CO₂ (^{13 C} of -40 to -50 ‰). Prior to the start of the experimental treatments, the carbon accumulated in the plant parts and in the soil macro-organic matter (‘old’ C) was at −32‰. During the experiment, the carbon fixed in the plant material (‘new’ C) was at −14 and −54‰ in the ambient and elevated [CO₂] treatments, respectively. During the experiment, the ¹³C isotopic mass balance method was used to calculate, for the top soil (0–15 cm), the carbon turnover in the stubble and roots and in the soil macro-organic matter above 200 μ (MOM). Elevated [CO₂] stimulated the turnover of organic carbon in the roots and stubble and in the MOM at N+, but not at N−. At the high N supply, the mean replacement time of ‘old’ C by ‘new’ C declined in elevated, compared to ambient [CO₂], from 18 to 7 months for the roots and stubble and from 25 to 17 months for the MOM. This resulted from increased rates of ‘new’ C accumulation and of ‘old’ C decay. By contrast, at the low N supply, despite an increase in the rate of accumulation of ‘new’ C, the soil C pools did not turnover faster in elevated [CO₂], as the rate of ‘old’ C decomposition was reduced. A 3 °C temperature increase in elevated [CO₂] decreased the input of fresh C to the roots and stubble and enhanced significantly the exponential rate for the ‘old’ C decomposition in the roots and stubble. An increased fertiliser N supply reduced the carbon turnover in the roots and stubble and in the MOM, in ambient but not in elevated [CO₂]. The respective roles for carbon turnover in the coarse soil OM fractions, of the C:N ratio of the litter, of the inorganic N availability and of a possible priming effect between C-substrates are discussed. This revised version was published online in June 2006 with corrections to the Cover Date.     

Characteristics of DOC Exported from Northern Hardwood Forests Receiving Chronic Experimental NO<Stack><Subscript>3</Subscript><Superscript>−</Superscript></Stack> Deposition

Abstract Sugar maple (Acer saccharum Marsh.)-dominated northern hardwood forests of the Great Lakes Region commonly receive elevated levels of atmospheric nitrate (NO₃⁻) deposition, which can alter belowground carbon (C) cycling. Past research has demonstrated that chronic experimental NO₃⁻ deposition (3 g N m⁻² y⁻¹ above ambient) elicits a threefold increase in the leaching loss of dissolved organic carbon (DOC). Here, we used DOC collected from tension-cup lysimeters to test whether increased DOC export under experimental NO₃⁻ deposition originated from forest floor or mineral soil organic matter (SOM). We used DOC radiocarbon dating to quantify C sources and colorimetric assays to measure DOC aromaticity and soluble polyphenolic content. Our results demonstrated that DOC exports are primarily derived from new C (<50-years-old) in the forest floor under both ambient and experimental NO₃⁻ deposition. Experimental NO₃⁻ deposition increased soluble polyphenolic content from 25.03 ± 4.26 to 49.19 ± 4.23 μg phenolic C mg DOC⁻¹, and increased total aromatic content as measured by specific UV absorbance. However, increased aromatic compounds represented a small fraction (<10%) of the total observed increased DOC leaching. In combination, these findings suggest that experimental NO₃⁻ deposition has altered the production or retention as well as phenolic content of DOC formed in forest floor, however exact mechanisms are uncertain. Further elucidation of the mechanism(s) controlling enhanced DOC leaching is important for understanding long-term responses of Great Lakes forests to anthropogenic N deposition and the consequences of those responses for aquatic ecosystems.     

青藏高原泥炭地水位下降促进土壤碳积累的影响机制

酚类物质作为泥炭地重要的碳分解抑制剂,植被作为泥炭地关键的碳输入来源,它们在土壤碳(可溶性有机碳(DOC)等)周转过程中都发挥着重要作用。然而,目前关于植被群落结构、酚类物质以及DOC含量对水位波动的响应存在较大争议。因此,为明确泥炭地水位下降对植被群落结构、酚类物质以及DOC含量的影响并探明三者间的潜在联系,以若尔盖高原泥炭地作为研究对象,选取红原县日干乔地区3处不同地下水位泥炭地(水位由高到低依次为S1(-1.9 cm)、S2(-10 cm)、S3(-19 cm)样地),调查不同水位条件下植被群落结构特征,并探究酚类物质及土壤碳含量对水位波动的响应。结果表明:(1)从S1到S3样地水位下降促进土壤DOC显著增加(P<0.05),土壤总碳从S1到S2显著增加(P<0.05),而从S2到S3无显著差异;(2)泥炭地水位下降促使禾本科(发草Deschampsia cespitosa)、莎草科(木里薹草Carex muliensis、乌拉草Carex meyeriana)植物大量出现,植被群落高度显著增加(P<0.05)。植被群落地上生物量由153.67 g/m~2增加至...     

Subsurface CO<Subscript>2</Subscript> Dynamics in Temperate Beech and Spruce Forest Stands

Rates of soil respiration (CO₂ effluxes), subsurface pore gas CO₂/O₂ concentrations, soil temperature and soil water content were measured for 15 months in two temperate and contrasting Danish forest ecosystems: beech (Fagus sylvatica L.) and Norway spruce (Picea abies [L.] Karst.). Soil CO₂ effluxes showed a distinct seasonal trend in the range of 0.48–3.3 μmol CO₂ m⁻² s⁻¹ for beech and 0.50–2.92 μmol CO₂ m⁻² s⁻¹ for spruce and were well-correlated with near-surface soil temperatures. The soil organic C-stock (upper 1 m including the O-horizon) was higher in the spruce stand (184±23 Mg C ha⁻¹) compared to the beech stand (93±19 Mg C ha⁻¹) and resulted in a faster turnover time as calculated by mass/flux in soil beneath the beech stand (28 years) compared to spruce stand (60 years). Observed soil CO₂ concentrations and effluxes were simulated using a Fickian diffusion-reaction model based on vertical CO₂ production rates and soil diffusivity. Temporal trends were simulated on the basis of observed trends in the distribution of soil water, temperature, and live roots as well as temperature and water content sensitivity functions. These functions were established based on controlled laboratory incubation experiments. The model was successfully validated against observed soil CO₂ effluxes and concentrations and revealed that temporal trends generally could be linked to variations in subsurface CO₂ production rates and diffusion over time and with depths. However, periods with exceptionally high CO₂ effluxes (> 20 μmol CO₂ m⁻² s⁻¹) were noted in March 2000 in relation to drying after heavy rain and after the removal of snow from collars. Both cases were considered non-steady state and could not be simulated.     

Carbon allocation in shoots of alpine treeline conifers in a CO<Subscript>2</Subscript> enriched environment

With a new approach we assessed the relative contribution of stored and current carbon compounds to new shoot growth in alpine treeline conifers. Within a free air CO₂ enrichment experiment at the alpine treeline in Switzerland, ¹³C-depleted fossil CO₂ was used to trace new carbon in the two tree species Larix decidua L. and Pinus uncinata Ramond over two subsequent years. The deciduous L. decidua was found to supply new shoot growth (structural woody part) by 46% from storage. Surprisingly, the evergreen P. uncinata, assumed to use current-year photosynthates, also utilized a considerable fraction of storage (42%) for new wood growth. In contrast, the needles of P. uncinata were built up almost completely from current-year photosynthates. The isotopic composition of different wood carbon fractions revealed a similar relative allocation of current and stored assimilates to various carbon fractions. Elevated CO₂ influenced the composition of woody tissue in a species-specific way, e.g. the water soluble fraction decreased in pine in 2001 but increased in larch in 2002 compared to ambient CO₂. Heavy defoliation applied as an additional treatment factor in the second year of the experiment decreased the lipophilic fraction in current-year wood in both species compared to undefoliated trees. We conclude that storage may play an important role for new shoot growth in these treeline conifers and that altered carbon availability (elevated CO₂, defoliation) results in significant changes in the relative amount of mobile carbon fractions in woody tissue. In particular, stored carbon seems to be of greater importance in the evergreen P. uncinata than has been previously thought.     

Species-dependent responses of juniper and spruce to increasing CO<Subscript>2</Subscript> concentration and to climate in semi-arid and arid areas of northwestern China

Qilian juniper (Sabina przewalskii Kom.) and Qinghai spruce (Picea crassifolia Kom.) represent different tree functional types, which can be found extensively in northwestern China. The former is drought-tolerant, whereas the latter is hygrophilous and shade-tolerant. We compared their intrinsic water-use efficiency (iWUE, inferred from carbon isotopic discrimination, δ¹³C, in their wood) as a function of atmospheric CO₂ concentration, [CO₂], and climate. δ¹³C of spruce was consistently about higher than that of juniper in semi-arid areas but was lower in arid areas. This difference was stable over time and demonstrated strong cross-correlations between species, although some subtle high-frequency (2 or 3 years) variations existed in both species, suggesting that regional climate may control carbon isotope discrimination. The ratio (the [CO₂] values in leaf intercellular and the atmosphere, respectively) of the juniper increased steadily over time, whereas that of the spruce showed a long-term downward trend. IWUE increased at all sites over the 150-year study period, mainly caused by increasing [CO₂]. The relationship between iWUE and [CO₂] reveals that the spruce was more sensitive than the juniper to increasing [CO₂], suggesting a species-specific adaptation to long-term environmental changes. Correlations between the high-frequency variations in stable carbon discrimination (Δ) and climate indicate similar intra-site responses to climate in both species, but different response strengths. Overall, complex interactions of temperature and moisture on stable carbon discrimination during current growth seasons of both species were environmental-determined. Regulation of gas exchange and reduced transpiration may influence water and energy budgets directly; therefore species-dependent responses of trees to elevated CO₂ should be considered in future research on global plant physiological ecology.     

Moss Responses to Elevated CO2 and Variation in Hydrology in a Temperate Lowland Peatland

We studied the effects of elevated CO₂ (180–200 ppmv above ambient) on growth and chemistry of three moss species (Sphagnum palustre, S. recurvum and Polytrichum commune) in a lowland peatland in the Netherlands. Thereto, we conducted both a greenhouse experiment with both Sphagnum species and a field experiment with all three species using MiniFACE (Free Air CO₂ Enrichment) technology during 3 years. The greenhouse experiment showed that Sphagnum growth was stimulated by elevated CO₂ in the short term, but that in the longer term (≥1 year) growth was probably inhibited by low water tables and/or down-regulation of photosynthesis. In the field experiment, we did not find significant changes in moss abundance in response to elevated CO₂, although CO₂ enrichment appeared to reduce S. recurvum abundance. Both Sphagnum species showed stronger responses to spatial variation in hydrology than to increased atmospheric CO₂ concentrations. Polytrichum was insensitive to changes in hydrology. Apart from the confounding effects of hydrology, the relative lack of growth response of the moss species may also have been due to the relatively small increase in assimilated CO₂ as achieved by the experimentally added CO₂. We calculated that the added CO₂ contributed at most 32% to the carbon assimilation of the mosses, while our estimates based on stable C isotope data even suggest lower contributions for Sphagnum (24–27%). Chemical analyses of the mosses showed only small elevated CO₂ effects on living tissue N concentration and C/N ratio of the mosses, but the C/N ratio of Polytrichum was substantially lower than those of the Sphagnum species. Continuing expansion of Polytrichum at the expense of Sphagnum could reduce the C sink function of this lowland Sphagnum peatland, and similar ones elsewhere, as litter decomposition rates would probably be enhanced. Such a reduction in sink function would be driven mostly by increased atmospheric N deposition, water table regulation for agricultural purposes and land management to preserve the early successional stage (mowing, tree and shrub removal), since these anthropogenic factors will probably exert a greater control on competition between Polytrichum and Sphagnum than increased atmospheric CO₂ concentrations.     

Light and growth temperature alter carbon isotope discrimination and estimated bundle sheath leakiness in C<Subscript>4</Subscript> grasses and dicots

We combined measurements of short-term (during gas exchange) and long-term (from plant dry matter) carbon isotope discrimination to estimate CO₂ leakiness from bundle sheath cells in six C₄ species (three grasses and three dicots) as a function of leaf insertion level, growth temperature and short-term irradiance. The two methods for determining leakiness yielded similar results (P > 0.05) for all species except Setaria macrostachya, which may be explained by the leaf of this species not being accommodating to gas exchange. Leaf insertion level had no effect on leakiness. At the highest growth temperature (36°C) leakiness was lower than at the two lower growth temperatures (16°C and 26°C), between which no differences in leakiness were apparent. Higher irradiance decreased leakiness in three species, while it had no significant effect on the others (there was an opposite trend in two species). The inverse response to increasing irradiance was most marked in the two NAD-ME dicots (both Amaranthus species), which both showed almost 50% leakiness at low light (300 μmol quanta m⁻² s⁻¹) compared to about 30% at high light (1,600 μmol quanta m⁻² s⁻¹). NADP-ME subtype grasses had lower leakiness than NAD-ME dicots. Although there were exceptions, particularly in the effect of irradiance on leakiness in Sorghum and Boerhavia, we conclude that conditions favourable to C₄ photosynthesis (high temperature and high light) lead to a reduction in leakiness.     

Long-term effects of elevated atmospheric CO<Subscript>2</Subscript> on species composition and productivity of a southern African C<Subscript>4</Subscript> dominated grassland in the vicinity of a CO<Subscript>2</Subscript> exhalation

We describe the long-term effects of a CO₂ exhalation, created more than 70 years ago, on a natural C₄ dominated sub-tropical grassland in terms of ecosystem structure and functioning. We tested whether long-term CO₂ enrichment changes the competitive balance between plants with C₃ and C₄ photosynthetic pathways and how CO₂ enrichment has affected species composition, plant growth responses, leaf properties and soil nutrient, carbon and water dynamics. Long-term effects of elevated CO₂ on plant community composition and system processes in this sub-tropical grassland indicate very subtle changes in ecosystem functioning and no changes in species composition and dominance which could be ascribed to elevated CO₂ alone. Species compositional data and soil δ¹³C isotopic evidence suggest no detectable effect of CO₂ enrichment on C₃:C₄ plant mixtures and individual species dominance. Contrary to many general predictions C₃ grasses did not become more abundant and C₃ shrubs and trees did not invade the site. No season length stimulation of plant growth was found even after 5 years of exposure to CO₂ concentrations averaging 610 μmol mol⁻¹. Leaf properties such as total N decreased in the C₃ but not C₄ grass under elevated CO₂ while total non-structural carbohydrate accumulation was not affected. Elevated CO₂ possibly lead to increased end-of-season soil water contents and this result agrees with earlier studies despite the topographic water gradient being a confounding problem at our research site. Long-term CO₂ enrichment also had little effect on soil carbon storage with no detectable changes in soil organic matter found. There were indications that potential soil respiration and N mineralization rates could be higher in soils close to the CO₂ source. The conservative response of this grassland suggests that many of the reported effects of elevated CO₂ on similar ecosystems could be short duration experimental artefacts that disappear under long-term elevated CO₂ conditions.     

Costs of reproduction in circumpolar <Emphasis Type="Italic">Parnassia palustris</Emphasis> L. in light of global warming

In frequently burnt savannas, saplings face the formidable challenge of both recovering from, and eventually growing tall enough to escape from, frequent fire damage. The aim of this study was to explore how saplings allocate carbon to achieve these ends through carbon partitioning, storage and remobilization. Lignotuber total non-structural carbohydrate (TNC) concentrations and δ¹³C values of Acacia karroo (Fabaceae; Mimosoideae) were determined in plants from two different juvenile stages. These were one year after a fire when the plant consisted of numerous leafy shoots or coppices (“coppicing” stage), and three years after a fire when the plant consisted of one pole-like stem (“Gulliver” stage). Gulliver lignotubers were found to have significantly larger TNC pools (150 g vs. 97 g) and larger TNC concentrations (33% vs. 24%, w/w) than coppice lignotubers showing that post-coppice Gullivers recharged TNC in the lignotuber. δ¹³C values from the stems of plants in the Gulliver stage were significantly enriched (>1‰) in ¹³C compared to both coppicing (P < 0.01) and adult (P < 0.05) plants. Changes in both the amount of stored carbon and in the δ¹³C values indicated dependence on stored carbon reserves, and partially heterotrophic growth for initial resprouting. The plants appeared to use both current photosynthate and stored carbon reserves for growth of the Gulliver stem. The use of stored carbon is hypothesized to promote fast stem growth rates to a height where saplings escape fire injury.     

The effects of elevated atmospheric CO<Subscript>2</Subscript> and nitrogen amendments on subsurface CO<Subscript>2</Subscript> production and concentration dynamics in a maturing pine forest

Profiles of subsurface soil CO₂ concentration, soil temperature, and soil moisture, and throughfall were measured continuously during the years 2005 and 2006 in 16 locations at the free air CO₂ enrichment facility situated within a temperate loblolly pine (Pinus taeda L.) stand. Sampling at these locations followed a 4 by 4 replicated experimental design comprised of two atmospheric CO₂ concentration levels (ambient [CO₂]^a, ambient + 200 ppmv, [CO₂]^e) and two soil nitrogen (N) deposition levels (ambient, ambient + fertilization at 11.2 g_N m⁻² year⁻¹). The combination of these measurements permitted indirect estimation of belowground CO₂ production and flux profiles in the mineral soil. Adjacent to the soil CO₂ profiles, direct (chamber-based) measurements of CO₂ fluxes from the soil–litter complex were simultaneously conducted using the automated carbon efflux system. Based on the measured soil CO₂ profiles, neither [CO₂]^e nor N fertilization had a statistically significant effect on seasonal soil CO₂, CO₂ production, and effluxes from the mineral soil over the study period. Soil moisture and temperature had different effects on CO₂ concentration depending on the depth. Variations in CO₂ were mostly explained by soil temperature at deeper soil layers, while water content was an important driver at the surface (within the first 10 cm), where CO₂ pulses were induced by rainfall events. The soil effluxes were equal to the CO₂ production for most of the time, suggesting that the site reached near steady-state conditions. The fluxes estimated from the CO₂ profiles were highly correlated to the direct measurements when the soil was neither very dry nor very wet. This suggests that a better parameterization of the soil CO₂ diffusivity is required for these soil moisture extremes.