Forest carbon balance under elevated CO2

Free-air CO₂ enrichment (FACE) technology was used to expose a loblolly pine (Pinus taeda L.) forest to elevated atmospheric CO₂ (ambient + 200 µl l^-1). After 4 years, basal area of pine trees was 9.2% larger in elevated than in ambient CO₂ plots. During the first 3 years the growth rate of pine was stimulated by ~26%. In the fourth year this stimulation declined to 23%. The average net ecosystem production (NEP) in the ambient plots was 428 gC m^-2 year^-1, indicating that the forest was a net sink for atmospheric CO₂. Elevated atmospheric CO₂ stimulated NEP by 41%. This increase was primarily an increase in plant biomass increment (57%), and secondarily increased accumulation of carbon in the forest floor (35%) and fine root increment (8%). Net primary production (NPP) was stimulated by 27%, driven primarily by increases in the growth rate of the pines. Total heterotrophic respiration (R_h) increased by 165%, but total autotrophic respiration (R_a) was unaffected. Gross primary production was increased by 18%. The largest uncertainties in the carbon budget remain in separating belowground heterotrophic (soil microbes) and autotrophic (root) respiration. If applied to temperate forests globally, the increase in NEP that we measured would fix less than 10% of the anthropogenic CO₂ projected to be released into the atmosphere in the year 2050. This may represent an upper limit because rising global temperatures, land disturbance, and heterotrophic decomposition of woody tissues will ultimately cause an increased flux of carbon back to the atmosphere.     

Ecosystem assembly and terrestrial carbon balance under elevated CO(2)

Research aimed at understanding how the global carbon balance will change with elevated CO(2) has largely ignored the responses of individual species and genotypes. Yet, plant traits strongly influence the biogeochemical cycling of carbon. Here, we illustrate how differences in inter- and intraspecific responses to elevated CO(2) affect not only physiology and growth, but also higher order biotic interactions and lifetime fitness, ultimately leading to new ecosystem assemblages. We assert that the unique combination of inter- and intraspecific traits in these ecosystem assemblages ultimately determine how ecosystems respond to elevated atmospheric CO(2). Thus, the identity of species and genotypes in an ecosystem is a crucial element to consider in forecasts of global carbon balance.     

不同尺度上植物叶气孔导度对升高CO2的响应

植物叶气孔导度对大气CO2浓度升高的响应可表现在以下几个层面：在叶水平上,叶气孔导度和气孔密度下降;在植物个体水平上,单位叶面积蒸腾下降,植株的水分利用率升高;在生态系统水平上,蒸散降低,土壤泾流和土壤水分含量增加;在全球尺度上,扩大了温室气体的增温效应,同时也降低了全球降雨量增加的趋势。正是因为植物叶气孔导度的变化会影响全球水循环,所以它在全球变化中起着非常重要的作用。但目前的研究结果还不能外推到更大的尺度上去。     

Increase in leaf mass per area benefits plant growth at elevated CO2 concentration

An increase in leaf mass per area (MLA) of plants grown at elevated [CO2] is often accompanied by accumulation of non-structural carbohydrates, and has been considered to be a response resulting from source-sink imbalance. We hypothesized that the increase in MLA benefits plants by increasing the net assimilation rate through maintaining a high leaf nitrogen content per area (NLA). To test this hypothesis, Polygonum cuspidatum was grown at ambient (370 micro mol mol-1) and elevated (700 micro mol mol-1) [CO2] with three levels of N supply. Elevated [CO2] significantly increased MLA with smaller effects on NLA and leaf mass ratio (fLM). The effect of change in MLA on plant growth was investigated by the sensitivity analysis: MLA values observed at ambient and elevated [CO2] were substituted into a steady-state growth model to calculate the relative growth rate (R). At ambient [CO2], substitution of a high MLA (observed at elevated [CO2]) did not increase R, compared with R for a low MLA (observed at ambient [CO2]), whereas at elevated [CO2] the high MLA always increased R compared with R at the low MLA. These results suggest that the increase in MLA contributes to growth enhancement under elevated [CO2]. The optimal combination of fLM and MLA to maximize R was determined for different [CO2] and N availabilities. The optimal fLM was nearly constant, while the optimal MLA increased at elevated [CO2], and decreased at higher N availabilities. The changes in fLM of actual plants may compensate for the limited plasticity of MLA.     

Root system adjustments: regulation of plant nutrient uptake and growth responses to elevated CO2

Nutrients such as nitrogen (N) and phosphorus (P) often limit plant growth rate and production in natural and agricultural ecosystems. Limited availability of these nutrients is also a major factor influencing long-term plant and ecosystem responses to rising atmospheric CO₂ levels, i.e., the commonly observed short-term increase in plant biomass may not be sustained over the long-term. Therefore, it is critical to obtain a mechanistic understanding of whether elevated CO₂ can elicit compensatory adjustments such that acquisition capacity for minerals increases in concert with carbon (C) uptake. Compensatory adjustments such as increases in (a) root mycorrhizal infection, (b) root-to-shoot ratio and changes in root morphology and architecture, (c) root nutrient absorption capacity, and (d) nutrient-use efficiency can enable plants to meet an increased nutrient demand under high CO₂. Here we examine the literature to assess the extent to which these mechanisms have been shown to respond to high CO₂. The literature survey reveals no consistent pattern either in direction or magnitude of responses of these mechanisms to high CO₂. This apparent lack of a pattern may represent variations in experimental protocol and/or interspecific differences. We found that in addressing nutrient uptake responses to high CO₂ most investigators have examined these mechanisms in isolation. Because such mechanisms can potentially counterbalance one another, a more reliable prediction of elevated CO₂ responses requires experimental designs that integrate all mechanisms simultaneously. Finally, we present a functional balance (FB) model as an example of how root system adjustments and nitrogen-use efficiency can be integrated to assess growth responses to high CO₂. The FB model suggests that the mechanisms of increased N uptake highlighted here have different weights in determining overall plant responses to high CO₂. For example, while changes in root-to-shoot biomass allocation, r, have a small effect on growth, adjustments in uptake rate per unit root mass, [`(n)]\bar \nu , and photosynthetic N use efficiency, p*, have a significantly greater leverage on growth responses to elevated CO₂ except when relative growth rate (RGR) reaches its developmental limit, maximum RGR (RGR_max).     

New insights into the cellular mechanisms of plant growth at elevated atmospheric carbon dioxide concentrations

Rising atmospheric carbon dioxide concentration ([CO₂]) significantly influences plant growth, development, and biomass. Increased photosynthesis rate, together with lower stomatal conductance, has been identified as the key factors that stimulate plant growth at elevated [CO₂] (e[CO₂]). However, variations in photosynthesis and stomatal conductance alone cannot fully explain the dynamic changes in plant growth. Stimulation of photosynthesis at e[CO₂] is always associated with post‐photosynthetic secondary metabolic processes that include carbon and nitrogen metabolism, cell cycle functions, and hormonal regulation. Most studies have focused on photosynthesis and stomatal conductance in response to e[CO₂], despite the emerging evidence of e[CO₂]'s role in moderating secondary metabolism in plants. In this review, we briefly discuss the effects of e[CO₂] on photosynthesis and stomatal conductance and then focus on the changes in other cellular mechanisms and growth processes at e[CO₂] in relation to plant growth and development. Finally, knowledge gaps in understanding plant growth responses to e[CO₂] have been identified with the aim of improving crop productivity under a CO₂ rich atmosphere.     

CO2 enrichment and carbon partitioning to phenolics: do plant responses accord better with the protein competition or the growth differentiation balance models?

Rising levels of atmospheric CO₂ can alter plant growth and partitioning to secondary metabolites. The protein competition model (PCM) and the extended growth/differentiation balance model (GDB_e) are similar but alternative models that address ontogenetic and environmental effects on whole‐plant carbon partitioning to the phenylpropanoid biosynthetic pathway, making many divergent predictions. To test the validity of the models, we compare plant responses to one key prediction: if CO₂ enrichment simultaneously stimulates both photosynthesis and growth, then PCM predicts that partitioning to phenolic compounds will decline, whereas GDB_e generally predicts the opposite. Elevated CO₂ (at 548 ppm) increased the biomass growth (ca 23%) as well as the net photosynthesis (ca 13%) of 1‐year‐old potted paper birch, Betula papyrifera Marsh., in a free air carbon dioxide enrichment study (FACE) in northern Wisconsin. Concomitantly, elevated CO₂ increased carbon partitioning to all measured classes of phenolics (Folin‐Denis phenolics, HPLC low molecular weight phenolics (i.e. cinnamic acid derivatives, flavonol glycosides, and flavon‐3‐ols), condensed tannins, and acid‐detergent lignin) in leaves. In stem tissues, tannins and lignin increased, but F‐D phenolics did not. In root tissues, F‐D phenolics, and tannins increased, but lignin did not. The data suggest that CO₂ enrichment stimulated pathway‐wide increase in carbon partitioning to phenylpropanoids. High CO₂ plants had 11.8% more F‐D phenolics, 19.3% more tannin, and 10% more lignin than ambient plants after adjusting for plant mass via analysis of covariance. In general, the results unequivocally support the predictions of the GDB_e model. By way of contrast, results from many parallel studies on FACE trembling aspen, Populus tremuloides Michx., suggest that although CO₂ enrichment has consistently stimulated both photosynthesis and growth, it apparently did not generally stimulate pathway‐wide increases, or decreases, in carbon partitioning to phenylpropanoids in leaves and wood, but rather has specifically, though not consistently, increased partitioning to foliar phenolic glycosides. Likewise, in this case, GDB_e's predictions better accord with the FACE aspen data than PCM's. If further tests of the two models also support GDB rather than PCM, then PCM's main assumption (whole‐plant N rather than C is limiting partitioning to phenolic synthesis) may be incorrect.     

The mechanisms of plant photosynthetic acclimation to elevated CO2 concentration]

When measured at a same CO(2) concentration, net photosynthetic rate is often significantly lower in long-term high CO(2)-grown plants than the ambient CO(2)-grown ones. This phenomenon is termed photosynthetic acclimation or down-regulation. Although there have been many reports and reviews, the mechanism(s) of the photosynthetic acclimation is not very clear. Combining the work of the authors' group, this paper briefly reviews the progress in studies on the mechanism(s) of the photosynthetic acclimation to elevated CO(2). It is suggested that besides the possible effects of respiration enhancement and excessive photosynthate accumulation, RuBP carboxylation limitation and RuBP regeneration limitation are probably the main factors leading to the photosynthetic acclimation.     

Effects of elevated CO2 on growth and carbon/nutrient balance in the deciduous woody shrub Lindera benzoin (L.) Blume (Lauraceae)

We examined the effects of elevated CO₂ on growth and carbon/nutrient balance in a natural population of the deciduous temperate zone shrub Lindera benzoin. Our data concern whole plant, leaf, and stem growth for the first two seasons of a long-term field experiment in which CO₂ levels were manipulated in situ. In addition to growth parameters, we evaluated changes in leaf and stem chemistry, including total nitrogen, nonstructural carbohydrates, and total phenolics. Over the course of this study, L. benzoin appeared to respond to elevated CO₂ primarily by physiological and biochemical changes, with only a slight enhancement in aboveground growth (ramet height). Positive effects on aboveground growth were primarily evident in young (nonreproductive) ramets. Our results suggest that nitrogen limitation may have constrained plants to allocate carbohydrates produced in response to elevated CO₂ primarily to storage and belowground growth, and perhaps to increased secondary chemical production, rather than to increased stem and leaf growth. We discuss our results in terms of changes in carbon/nutrient balance induced by elevated CO₂, and provide predictions for future changes in this system based upon constraints imposed by intrinsic and extrinsic factors and their potential effects on the reallocation of stored reserves.     

Response of a Sphagnum bog plant community to elevated CO2 and N supply

The factors determining herbaceous canopy architecture are poorlyunderstood, especially in natural and semi-natural plant communities. Inthis study, we tested three main hypotheses: (1) the structure of herbaceouscanopies can be explained by the vertical distribution of functional groupsdefined by leaf width and the presence/absence of leaves on upright stem;(2) the degree of canopy stratification is greater in habitats that experiencelower spatial heterogeneity in the supply of light (i.e., grasslands as opposedto forest herb layers); and (3) there is significant variation among specieswithin a growth-form, with respect to their vertical position in thecanopy. We used plant foliage height distribution data from 14 grassland and 13forest herbaceous communities to test these hypotheses. A general linear mixedmodel was applied to specify the proportions of total variance in the foliageheight, accounted for by the fixed effects of plants' basicgrowth-form properties (growth-form) and community type(forest/grassland), and by the random effects of sampling site, samplingpoint, and individual species. We were also interested in the correlation ofthedegree of the stratification with various community characteristics(productivity, other canopy properties, species richness, variation ofspecies' traits) and light availability. There was some evidence ofoverall canopy stratification according toplant growth-form, since plants with leafy stem were locatedsignificantly higher. However, such a pattern of two more or less distinctlayers (grasses + upright forbs and rosette forbs) occurred withconsistency only in grasslands (greater homogeneity in light). Thebetween-species variation within a growth-form was a highlysignificant predictor of canopy vertical structure in the 27 communities. Theproportion of total observed variance, explainable throughspecies-specific effects, was comparable to that caused bybetween-site differences. The effect of community horizontal pattern wasless obvious, but still significant. The site by site analysis revealed thatthe degree to which horizontalpatchiness explained variation in vertical canopy structure was negativelyrelated to the relative importance of species-specific effects, showingthat small between-species differences lead to a more obviouswithin-community horizontal pattern, and vice versa. The upper bound ofthe degree of foliage stratification, according to growth-form, wasrelated to the variability of species light requirements and to relative (tocommunity pool size) richness, indicating that certain aspects of canopyarchitecture might be explained through community species composition anddiversity pattern.     

Carbon balance in tussock tundra under ambient and elevated atmospheric CO2

Summary Whole ecosystem CO₂ flux under ambient (340 l/l) and elevated (680 l/l) CO₂ was measured in situ in Eriophorum tussock tundra on the North Slope of Alaska. Elevated CO₂ resulted in greater carbon acquisition than control treatments and there was a net loss of CO₂ under ambient conditions at this upland tundra site. These measurements indicate a current loss of carbon from upland tundra, possibly the result of recent climatic changes. Elevated CO₂ for the duration of one growing season appeared to delay the onset of dormancy and resulted in approximately 10 additional days of positive ecosystem flux. Homeostatic adjustment of ecosystem CO₂ flux (sum of species' response) was apparent by the third week of exposure to elevated CO₂. Ecosystem dark respiration rates were not significantly higher at elevated CO₂ levels. Rapid homeostatic adjustment to elevated CO₂ may limit carbon uptake in upland tundra. Abiotic factors were evaluated as predictors of ecosystem CO₂ flux. For chambers exposed to ambient and elevated CO₂ levels for the duration of the growing season, seasonality (Julian day) was the best predictor of ecosystem CO₂ flux at both ambient and elevated CO₂ levels. Light (PAR), soil temperature, and air temperature were also predictive of seasonal ecosystem flux, but only at elevated CO₂ levels. At any combination of physical conditions, flux of the elevated CO₂ treatment was greater than that at ambient. In short-term manipulations of CO₂, tundra exposed to elevated CO₂ had threefold greater carbon gain, and had one half the ecosystem level, light compensation point when compared to ambient CO₂ treatments. Elevated CO₂-acclimated tundra had twofold greater carbon gain compared to ambient treatments, but there was no difference in ecosystem level, light compensation point between elevated and ambient CO₂ treatments. The predicted future increases in cloudiness could substantially decrease the effect of elevated atmospheric CO₂ on net ecosystem carbon budget. These analyses suggest little if any long-term stimulation of ecosystem carbon acquisition by increases in atmospheric CO₂.     

Influence of elevated CO2 and nitrogen nutrition on rice plant growth,soil microbial biomass,dissolved organic carbon and dissolved CH4

Rice (Oryza sativa) was grown in six sunlit, semi-closed growth chambers for two seasons at 350 L L^–1 (ambient) and 650 L L^–1 (elevated) CO₂ and different levels of nitrogen (N) supplement. The objective of this research was to study the influence of CO₂ enrichment and N nutrition on rice plant growth, soil microbial biomass, dissolved organic carbon (DOC) and dissolved CH₄. Elevated CO₂ concentration ([CO₂]) demonstrated a wide range of enhancement to both above- and below-ground plant biomass, in particular to stems and roots (for roots when N was not limiting) in the mid-season (80 days after transplanting) and stems/ears at the final harvest, depending on season and the level of N supplement. Elevated [CO₂] significantly increased microbial biomass carbon in the surface 5 cm soil when N (90 kg ha^–1) was in sufficient supply. Low N supplement (30 kg ha^–1) limited the enhancement of root growth by elevated [CO₂], leading consequently to diminished response of soil microbial biomass carbon to CO₂ enrichment. The concentration of dissolved CH₄ (as well as soil DOC, but to a lesser degree) was observed to be positively related to elevated [CO₂], especially at high rate of N application (120 kg ha^–1) or at 10 cm depth (versus 5 cm depth) in the later half of the growing season (at 80 kg N ha^–1). Root senescence in the late season complicated the assessment of the effect of elevated [CO₂] on root growth and soil organic carbon turnover and thus caution should be taken when interpreting respective high CO₂ results.     

Insights into mycorrhizal colonisation at elevated CO2: a simple carbon partitioning model

A simulation model was used to investigate the effect of an increased rate of plant photosynthesis at enhanced atmospheric CO2 concentration on a non-leguminous plant-mycorrhizal fungus association. The model allowed the user to modify carbon allocation patterns at three levels: (1) within the plant (shoot–root), (2) between the plant and the mycorrhizal fungus and (3) within the mycorrhizal fungus (intraradical–extraradical structures). Belowground (root and fungus) carbon losses via respiration (and turnover) could also be manipulated. The specific objectives were to investigate the dynamic nature of the potential effects of elevated CO2 on mycorrhizal colonisation and to elucidate some of the various mechanisms by which these effects may be negated. Many of the simulations showed that time (i.e. plant age) had a more significant effect on the observed stimulation of mycorrhizal colonisation by elevated CO2 than changes in carbon allocation patterns or belowground carbon losses. There were two main mechanisms which negated a stimulatory effect of elevated CO2 on internal mycorrhizal colonisation: an increased mycorrhizal carbon allocation to the external hyphal network and an increased rate of mycorrhizal respiration. The results are discussed in relation to real experiments. The need for studies consisting of multiple harvests is emphasised, as is the use of allometric analysis. Implications at the ecosystem level are discussed and key areas for future research are presented.     

Dynamics of daily height growth in Scots pine trees at elevated temperature and CO2

The aim of this study was to analyse and model the effects of elevated temperature and carbon dioxide concentration on daily height growth of 20-year-old Scots pines (Pinus sylvestris L.). The trees were grown with a low nitrogen supply in closed chambers with a factorial combination of two temperature regimes (ambient and elevated) and two carbon dioxide concentrations (ambient and twice ambient). The temperature elevation corresponded to the predicted increase at the site after a doubling in atmospheric CO₂. The height growth of Scots pines was first empirically studied in terms of its onset, cessation and duration, and the allocation of daily height growth within the growing period in 2000 and 2001, and then a model predicting daily height growth as a function of daily temperature and temperature sum was developed. The empirical results showed elevated temperature to be the dominant variable explaining variation in daily height growth. Elevated temperature also hastened both the onset and cessation of height growth, and the temperature sums for both of them were higher in the elevated than in the ambient temperature treatments. The daily variation in height growth could also be explained by the daily mean temperature in the model. Elevated CO₂ concentration had no effect on the onset, cessation or duration of height growth. The amount of height growth was not affected by any of the treatments.     

Photosynthetic and growth response to fumigation with SO2 at elevated CO2 for C3 and C4 plants

Summary Six early successional plant species with differing photosynthetic pathways (3 C₃ species and 3 C₄ species) were grown at either 300, 600, or 1,200 ppm CO₂ and at either 0.0 or 0.25 ppm SO₂. Total plant growth increased with CO₂ concentration for the C₃ species and varied only slightly with CO₂ for the C₄ species. Fumigation with SO₂ caused reduced growth of the C₃ species at 300 ppm CO₂ but not at the higher concentrations of CO₂. Fumigation with SO₂ reduced growth of the C₄ species at high CO₂ and increased growth at 300 ppm CO₂. Leaf area increased with increasing CO₂ for all plant species. Fumigation with SO₂ reduced leaf area of C₃ plants more at low CO₂ than at high CO₂ while leaf area of C₄ plants was reduced more at high CO₂ than at low CO₂. These results support the notion that C₃ species are more sensitive to SO₂ fumigation than are C₄ species at concentrations of CO₂ equal to that found in normal ambient air. However, the difference in sensitivity to SO₂ between C₃ and C₄ species was found to be reversed at higher concentrations of CO₂. A possible explanation for this reversal based upon differences in stomatal response to elevated CO₂ between C₃ and C₄ species is discussed.     

Alteration of forest succession and carbon cycling under elevated CO2

Regenerating forests influence the global carbon (C) cycle, and understanding how climate change will affect patterns of regeneration and C storage is necessary to predict the rate of atmospheric carbon dioxide (CO₂) increase in future decades. While experimental elevation of CO₂ has revealed that young forests respond with increased productivity, there remains considerable uncertainty as to how the long‐term dynamics of forest regrowth are shaped by elevated CO₂ (eCO₂). Here, we use the mechanistic size‐ and age‐ structured Ecosystem Demography model to investigate the effects of CO₂ enrichment on forest regeneration, using data from the Duke Forest Free‐Air Carbon dioxide Enrichment (FACE) experiment, a forest chronosequence, and an eddy‐covariance tower for model parameterization and evaluation. We find that the dynamics of forest regeneration are accelerated, and stands consistently hit a variety of developmental benchmarks earlier under eCO₂. Because responses to eCO₂ varied by plant functional type, successional pathways, and mature forest composition differed under eCO₂, with mid‐ and late‐successional hardwood functional types experiencing greater increases in biomass compared to early‐successional functional types and the pine canopy. Over the simulation period, eCO₂ led to an increase in total ecosystem C storage of 9.7 Mg C ha^‐1. Model predictions of mature forest biomass and ecosystem–atmosphere exchange of CO₂ and H₂O were sensitive to assumptions about nitrogen limitation; both the magnitude and persistence of the ecosystem response to eCO₂ were reduced under N limitation. In summary, our simulations demonstrate that eCO₂ can result in a general acceleration of forest regeneration while altering the course of successional change and having a lasting impact on forest ecosystems.     

Genetic variation in plant below-ground response to elevated CO2 and two herbivore species

Aims

It is unclear how changing atmospheric conditions, including rising carbon dioxide concentration, influence interactions between above and below-ground systems and if intraspecific variation exists in this response.

Methods

We assessed interactive effects of atmospheric CO₂ concentration, above-ground herbivory, and plant genotype on root traits and mycorrhizal associations. Plants from five families of Asclepias syriaca, a perennial forb, were grown under ambient and elevated atmospheric CO₂ concentrations. Foliar herbivory by either lepidopteran caterpillars or phloem-feeding aphids was imposed. Mycorrhizal colonization, below-ground biomass, root biomass, and secondary defensive chemistry in roots were quantified.

Results

We observed substantial genetic variation among A. syriaca families in their mycorrhizal colonization levels in response to elevated CO₂ and herbivory treatments. Elevated CO₂ treatment increased root biomass in all genetic families, whereas foliar herbivory tended to decrease root biomass. Root cardenolide concentration and composition varied greatly among plant families, and elevated CO₂ treatment increased root cardenolides in two of the five plant families. Moreover, herbivores differentially affected the composition of cardenolides expressed below ground.

Conclusions

Increased atmospheric CO₂ has the potential to influence interactions among plants, herbivores and mycorrhizal fungi and intraspecific variation suggests that such interactions can evolve.     

Growth and maintenance components of leaf respiration of cotton grown in elevated carbon dioxide partial pressure

Elevated atmospheric carbon dioxide partial pressures have been shown to have variable direct and indirect effects on plant respiration rates. In this study, growth, leaf respiration, and leaf nitrogen and carbohydrate partitioning were measured in Gossypium hirsutum L. grown in 35 and 65 Pa CO₂ for 30d. Growth and maintenance coefficients of leaf respiration were estimated using gas exchange techniques both at night and during the day. Elevated CO₂ stimulated biomass production (107%) and net photo-synthetic rates (35–50%). Total day-time respiration (R_d) was not significantly affected by growth CO₂ partial pressure. However, night respiration (R_n) of leaves grown in 65 Pa CO₂ was significantly greater than that of plants grown in 35 Pa CO₂. Correlation of R_d and R_n with leaf expansion rates indicated that plants in both CO₂ treatments had equivalent growth respiration coefficients but maintenance respiration was significantly greater in elevated CO₂. Increased maintenance coefficients in elevated CO₂ appeared to be related to increased starch accumulation rather than to changes in leaf nitrogen.     

Time-dependent responses of soil CO2 efflux components to elevated atmospheric [CO2] and temperature in experimental forest mesocosms

We previously used dual stable isotope techniques to partition soil CO₂ efflux into three source components (rhizosphere respiration, litter decomposition, and soil organic matter (SOM) oxidation) using experimental chambers planted with Douglas-fir [Pseudotsuga menziesii (Mirb.) Franco] seedlings. The components responded differently to elevated CO₂ (ambient + 200 mol mol^–1) and elevated temperature (ambient + 4 °C) treatments during the first year. Rhizosphere respiration increased most under elevated CO₂, and SOM oxidation increased most under elevated temperature. However, many studies show that plants and soil processes can respond to altered climates in a transient way. Herein, we extend our analysis to 2 years to evaluate the stability of the responses of the source components. Total soil CO₂ efflux increased significantly under elevated CO₂ and elevated temperature in both years (1994 and 1995), but the enhancement was much less in 1995. Rhizosphere respiration increased less under elevated temperature in 1995 compared with 1994. Litter decomposition also tended to increase comparatively less in 1995 under elevated CO₂, but was unresponsive to elevated temperature between years. In contrast, SOM oxidation was similar under elevated CO₂ in the 2 years. Less SOM oxidation occurred under elevated temperature in 1995 compared with 1994. Our results indicate that temporal variations can occur in CO₂ production by the sources. The variations likely involve responses to antecedent physical disruption of the soil and physiological processes.