Photosynthetic acclimation to elevated CO2 is modified by source:sink balance in three component species of chalk grassland swards grown in a free air carbon dioxide enrichment (FACE) experiment

Artificial chalk grassland swards were exposed to either ambient air or air enriched to 600 μ mol mol^–1 CO₂, using free-air CO₂ enrichment technology, and subjected to an 8 week simulated grazing regime. After 14 months of treatment, ribulose-1,5-bisphosphate carboxylase (Rubisco) activity ( V _c,max) and electron transport mediated ribulose-1,5-bisphosphate (RuBP) regeneration capacity ( J _max), estimated from leaf gas exchange, were significantly lower in fully expanded leaves of Anthyllis vulneraria L. (a legume) and Sanguisorba minor Scop. grown in elevated CO₂. After a change in source:sink balance brought about by defoliation, photosynthetic capacity was fully restored in A. vulneraria and S. minor, but acclimation continued in the grass Bromopsis erecta (Hudson) Fourr. Changes in net photosynthesis ( P _n) with growth at elevated CO₂ ranged from a 1·6% reduction in precut leaves of A. vulneraria to a 47·1% stimulation in postcut leaves of S. minor . Stomatal acclimation was observed in leaves of A. vulneraria (reduced stomatal density) and B. erecta (reduced stomatal conductance). The results are discussed in terms of whole-plant resource-use optimization and chalk grassland community competitive interactions at elevated CO₂.     

Reduced water repellency of a grassland soil under elevated atmospheric CO2

Water repellency is a widespread characteristic of soils that can modify soil moisture content and distribution and is implicated in important processes such as aggregation and carbon sequestration. Repellency arises as a consequence of organic matter inputs; as elevated atmospheric CO₂ is known to modify such inputs, we tested the repellency of a grassland soil after 5 years of exposure to elevated CO₂ in a free air carbon dioxide enrichment experiment. Using a water droplet penetration time test, we found a significant reduction in repellency at elevated CO₂ in samples at field moisture content. As many of the processes potentially influenced by repellency have been shown to be modified at elevated CO₂ (e.g. soil aggregation, C sequestration, recruitment from seed), we suggest that further exploration of this phenomenon could enhance our understanding of CO₂ effects on ecosystem function. The mechanism responsible for the change in repellency has not been identified.     

The interaction of rising CO2 and temperatures with water use efficiency

Abstract. Recent data concerning the impact of elevated atmospheric CO₂ upon water use efficiency (WUE) and the related measure, instantaneous transpiration efficiency (ITE), are reviewed. It is concluded from both short and long-term studies that, at the scale of the individual leaf or plant, an increase in WUE or ITE is generally observed in response to increased atmospheric CO₂ levels. However, the magnitude of this increase may decline with time. The opinion that elevated CO₂ may substantially decrease transpiration at the regional scale is discussed. The mechanisms by which elevated CO₂ may cause a change in these measures are discussed in terms of stomatal conductance, assimilation and respiration responses to elevated CO₂. Finally, recent experimental data and model outputs concerning the impact of the interaction of increased temperature with elevated CO₂ on WUE, ITE and yield are reviewed. It is concluded that substantially more data is required before reliable predictions about the regional scale response of WUE and catchment hydrology can be made.     

Soil CO2 efflux of two silver birch clones exposed to elevated CO2 and O3 levels during three growing seasons

In the present open‐top chamber experiment, two silver birch clones (Betula pendula Roth, clone 4 and clone 80) were exposed to elevated levels of carbon dioxide (CO₂) and ozone (O₃), singly and in combination, and soil CO₂ efflux was measured 14 times during three consecutive growing seasons (1999–2001). In the beginning of the experiment, all experimental trees were 7 years old and during the experiment the trees were growing in sandy field soil and fertilized regularly. In general, elevated O₃ caused soil CO₂ efflux stimulation during most measurement days and this stimulation enhanced towards the end of the experiment. The overall soil respiration response to CO₂ was dependent on the genotype, as the soil CO₂ efflux below clone 80 trees was enhanced and below clone 4 trees was decreased under elevated CO₂ treatments. Like the O₃ impact, this clonal difference in soil respiration response to CO₂ increased as the experiment progressed. Although the O₃ impact did not differ significantly between clones, a significant time × clone × CO₂× O₃ interaction revealed that the O₃‐induced stimulation of soil respiration was counteracted by elevated CO₂ in clone 4 on most measurement days, whereas in clone 80, the effect of elevated CO₂ and O₃ in combination was almost constantly additive during the 3‐year experiment. Altogether, the root or above‐ground biomass results were only partly parallel with the observed soil CO₂ efflux responses. In conclusion, our data show that O₃ impacts may appear first in the below‐ground processes and that relatively long‐term O₃ exposure had a cumulative effect on soil CO₂ efflux. Although the soil respiration response to elevated CO₂ depended on the tree genotype as a result of which the O₃ stress response might vary considerably within a single tree species under elevated CO₂, the present experiment nonetheless indicates that O₃ stress is a significant factor affecting the carbon cycling in northern forest ecosystems.     

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.     

Elevated atmospheric CO2 alters stomatal responses to variable sunlight in a C4 grass

Native tallgrass prairie in NE Kansas was exposed to elevated (twice ambient) or ambient atmospheric CO₂ levels in open-top chambers. Within chambers or in adjacent unchambered plots, the dominant C₄ grass, Andropogon gerardii, was subjected to fluctuations in sunlight similar to that produced by clouds or within canopy shading (full sun > 1500 μmol m⁻² s⁻¹ versus 350 μmol m⁻² s⁻¹ shade) and responses in gas exchange were measured. These field experiments demonstrated that stomatal conductance in A. gerardii achieved new steady state levels more rapidly after abrupt changes in sunlight at elevated CO₂ when compared to plants at ambient CO₂. This was due primarily to the 50% reduction in stomatal conductance at elevated CO₂, but was also a result of more rapid stomatal responses. Time constants describing stomatal responses were significantly reduced (29–33%) at elevated CO₂. As a result, water loss was decreased by as much as 57% (6.5% due to more rapid stomatal responses). Concurrent increases in leaf xylem pressure potential during periods of sunlight variability provided additional evidence that more rapid stomatal responses at elevated CO₂ enhanced plant water status. CO₂-induced alterations in the kinetics of stomatal responses to variable sunlight will likely enhance direct effects of elevated CO₂ on plant water relations in all ecosystems.     

Elevated CO2 increases the long-term decomposition rate of Quercus myrtifolia leaf litter

Decomposition of Quercus myrtifolia leaf litter in a Florida scrub oak community was followed for 3 years in two separate experiments. In the first experiment, we examined the effects CO₂ and herbivore damage on litter quality and subsequent decomposition. Undamaged, chewed and mined litter generated under ambient and elevated (ambient+350 ppm V) CO₂ was allowed to decompose under ambient conditions for 3 years. Initial litter chemistry indicated that CO₂ levels had minor effects on litter quality. Litter damaged by leaf miners had higher initial concentrations of condensed tannins and nitrogen (N) and lower concentrations of hemicellulose and C : N ratios compared with undamaged and chewed litter. Despite variation in litter quality associated with CO₂, herbivory, and their interaction, there was no subsequent effect on rates of decomposition under ambient atmospheric conditions. In the second experiment, we examined the effects of source (ambient and elevated) of litter and decomposition site (ambient and elevated) on litter decomposition and N dynamics. Litter was not separated by damage type. The litter from both elevated and ambient CO₂ was then decomposed in both elevated and ambient CO₂ chambers. Initial litter chemistry indicated that concentrations of carbon (C), hemicellulose, and lignin were higher in litter from elevated than ambient CO₂ chambers. Despite differences in C and fiber concentrations, litter from ambient and elevated CO₂ decomposed at comparable rates. However, the atmosphere in which the decomposition took place resulted in significant differences in rates of decomposition. Litter decomposing under elevated CO₂ decomposed more rapidly than litter under ambient CO₂, and exhibited higher rates of mineral N accumulation. The results suggest that the atmospheric conditions during the decomposition process have a greater impact on rates of decomposition and N cycling than do the atmospheric conditions under which the foliage was produced.     

Elevated CO2 stimulates associative N2 fixation in a C3 plant of the Chesapeake Bay wetland

In this study, the response of N₂ fixation to elevated CO₂ was measured in Scirpus olneyi, a C₃ sedge, and Spartina patens, a C₄ grass, using acetylene reduction assay and ¹⁵N₂ gas feeding. Field plants grown in PVC tubes (25 cm long, 10 cm internal diameter) were used. Exposure to elevated CO₂ significantly (P < 0·05) caused a 35% increase in nitrogenase activity and 73% increase in ¹⁵N incorporated by Scirpus olneyi. In Spartina patens, elevated CO₂ (660 ± 1 μ mol mol ^{− 1}) increased nitrogenase activity and ¹⁵N incorporation by 13 and 23%, respectively. Estimates showed that the rate of N₂ fixation in Scirpus olneyi under elevated CO₂ was 611 ± 75 ng ¹⁵N fixed plant ^{− 1} h ^{− 1} compared with 367 ± 46 ng ¹⁵N fixed plant ^{− 1} h ^{− 1} in ambient CO₂ plants. In Spartina patens, however, the rate of N₂ fixation was 12·5 ± 1·1 versus 9·8 ± 1·3 ng ¹⁵N fixed plant ^{− 1} h ^{− 1} for elevated and ambient CO₂, respectively. Heterotrophic non-symbiotic N₂ fixation in plant-free marsh sediment also increased significantly (P < 0·05) with elevated CO₂. The proportional increase in ¹⁵N₂ fixation correlated with the relative stimulation of photosynthesis, in that N₂ fixation was high in the C₃ plant in which photosynthesis was also high, and lower in the C₄ plant in which photosynthesis was relatively less stimulated by growth in elevated CO₂. These results are consistent with the hypothesis that carbon fixation in C₃ species, stimulated by rising CO₂, is likely to provide additional carbon to endophytic and below-ground microbial processes.     

Biomass and compositional changes occur in chalk grassland turves exposed to elevated CO2 for two seasons in FACE

Artificial turves composed of 7 chalk grassland species (Festuca ovina L.; Briza media L.; Bromopsis erecta (Hudson) Fourr.; Plantago media L.; Sanguisorba minor Scop.; Anthyllis vulneraria L. and Lotus corniculatus L.) were grown from seed and exposed to two seasons of elevated (600 μmol mol^–1) and ambient (340 μmol mol^–1) CO₂ concentrations in free air CO₂ enrichment (ETH-FACE, Zurich). The turves were clipped regularly to a height of 5 cm and assessed for above ground biomass production and relative abundance based on accumulated clipped dry biomass as well as by point quadrat recording. Below ground biomass production was assessed with root in-growth bags during the second season of growth. Increases in total biomass (> 30%) were noted in elevated CO₂, but the differences did not become significant until the second season of growth. Individual species’ biomass varied in response to elevated CO₂, with significant increases in biomass in elevated CO₂ turves for both legume species, and no significant CO₂ effect on S. minor or P. media. An initial positive CO₂ effect on biomass of combined grass species was reversed by the end of the experiment with less biomass and a significantly smaller proportion of total biomass present in elevated CO₂, which was attributed primarily to changes in proportion of F. ovina. Species relative abundance was significantly affected by elevated CO₂ in the final 4 of the 6 clip events, with the legume species increasing in proportion at the expense of the other species, particularly the grasses. Root length and dry weight were both significantly increased in elevated CO₂ (77% and 89%, respectively), and these increases were greater than increases in shoot biomass (36%) from the same period. Species responses to elevated CO₂, within the model community, were not consistent with predictions made from data on individual species, leading to the conclusion that responses to elevated CO₂, at the community level, and species within the community level, are the result of direct physiological effects and indirect competitive effects. These conclusions are discussed with respect to the ecological responses of natural communities, and the chalk grassland community in particular, to elevated CO₂.     

Photosynthesis, carboxylation and leaf nitrogen responses of 16 species to elevated pCO2 across four free-air CO2 enrichment experiments in forest, grassland and desert

The magnitude of changes in carboxylation capacity in dominant plant species under long‐term elevated CO₂ exposure (elevated pC_a) directly impacts ecosystem CO₂ assimilation from the atmosphere. We analyzed field CO₂ response curves of 16 C₃ species of different plant growth forms in favorable growth conditions in four free‐air CO₂ enrichment (FACE) experiments in a pine and deciduous forest, a grassland and a desert. Among species and across herb, tree and shrub growth forms there were significant enhancements in CO₂ assimilation (A) by +40±5% in elevated pC_a (49.5–57.1 Pa), although there were also significant reductions in photosynthetic capacity in elevated pC_a in some species. Photosynthesis at a common pC_a (A_a) was significantly reduced in five species growing under elevated pC_a, while leaf carboxylation capacity (V_cmax) was significantly reduced by elevated pC_a in seven species (change of ?19±3% among these species) across different growth forms and FACE sites. Adjustments in V_cmax with elevated pC_a were associated with changes in leaf N among species, and occurred in species with the highest leaf N. Elevated pC_a treatment did not affect the mass‐based relationships between A or V_cmax and N, which differed among herbs, trees and shrubs. Thus, effects of elevated pC_a on leaf C assimilation and carboxylation capacity occurred largely through changes in leaf N, rather than through elevated pC_a effects on the relationships themselves. Maintenance of leaf carboxylation capacity among species in elevated pC_a at these sites depends on maintenance of canopy N stocks, with leaf N depletion associated with photosynthetic capacity adjustments. Since CO₂ responses can only be measured experimentally on a small number of species, understanding elevated CO₂ effects on canopy N_m and N_a will greatly contribute to an ability to model responses of leaf photosynthesis to atmospheric CO₂ in different species and plant growth forms.     

Elevated atmospheric CO2 in open top chambers increases net nitrification and potential denitrification

The control of soil nitrogen (N) availability under elevated atmospheric CO₂ is central to predicting changes in ecosystem carbon (C) storage and primary productivity. The effects of elevated CO₂ on belowground processes have so far attracted limited research and they are assumed to be controlled by indirect effects through changes in plant physiology and chemistry. In this study, we investigated the effects of a 4‐year exposure to elevated CO₂ (ambient + 400 µmol mol^?1) in open top chambers under Scots pine (Pinus sylvestris L) seedlings on soil microbial processes of nitrification and denitrification. Potential denitrification (DP) and potential N₂O emissions were significantly higher in soils from the elevated CO₂ treatment, probably regulated indirectly by the changes in soil conditions (increased pH, C availability and NO₃^– production). Net N mineralization was mainly accounted for by nitrate production. Nitrate production was significantly larger for soil from the elevated CO₂ treatment in the field when incubated in the laboratory under elevated CO₂ (increase of 100%), but there was no effect when incubated under ambient CO₂. Net nitrate production of the soil originating from the ambient CO₂ treatment in the field was not influenced by laboratory incubation conditions. These results indicate that a direct effect of elevated atmospheric CO₂ on soil microbial processes might take place. We hypothesize that physiological adaptation or selection of nitrifiers could occur under elevated CO₂ through higher soil CO₂ concentrations. Alternatively, lower microbial NH₄ assimilation under elevated CO₂ might explain the higher net nitrification. We conclude that elevated atmospheric CO₂ has a major direct effect on the soil microbial processes of nitrification and denitrification despite generally higher soil CO₂ concentrations compared to atmospheric concentrations.