Forest responses to CO2 enrichment and climate warming

Two of the major uncertainties in forecasting future terrestrial sources and sinks of CO₂ are the CO₂-enhanced growth response of forests and soil warming effects on net CO₂ efflux from forests. Carbon dioxide enrichment of tree seedlings over time periods less than 1 yr has generally resulted in enhanced rates of photosynthesis, decreased respiration, and increased growth, with minor increases in leaf area and small changes in C allocation. Exposure of woody species to elevated CO₂ over several years has shown that high rates of photosynthesis may be sustained, but net C accumulation may not necessarily increase if CO₂ release from soil respiration increases. The impact of the 25% rise in atmospheric CO₂ with industrialization has been examined in tree ring chronologies from a range of species and locations. In contrast to the seedling tree results, there is no convincing evidence for CO₂-enhanced stem growth of mature trees during the last several decades. However, if mature trees show a preferential root growth response to CO₂ enrichment, the gain in root mass for an oak-hickory forest in eastern Tennessee is estimated to be only 9% over the last 40 years. Root data bases are inadequate for detecting such an effect. A very small shift in ecosystem nutrients from soil to vegetation could support CO₂-enhanced growth. Climate warming and the accompanying increase in mean soil temperature could have a greater effect than CO₂ enrichment on terrestrial sources and sinks of CO₂. Soil respiration and N mineralization have been shown to increase with soil temperature. If plant growth increases with increased N availability, and more C is fixed in growth than is released by soil respiration, then a negative feedback on climate warming will occur. If warming results in a net increase in CO₂ efflux from forests, then a positive feedback will follow. A 2 to 4°C increase in soil temperature could increase CO₂ efflux from soil by 15 to 32% in eastern deciduous forests. Quantifying C budget responses of forests to future global change scenarios will be speculative until mature tree responses to CO₂ enrichment and the effects of temperature on terrestrial sources and sinks of CO₂ can be determined.     

Spatial Variability of Carbon Dioxide Emission by Soils in the Main Types of Forest Ecosystems at the Zvenigorod Biological Station of Moscow State University

Carbon dioxide (CO₂) emission from the soil surface in forest biogeocenoses of the Zvenigorod Biological Station of Moscow State University in summer varies on average from 120 to 350 mg C–CO₂/(m² h) and depends on the hydrothermal conditions (soil moisture and temperature) and the type of phytocenosis. The intensity of CO₂ emission in the biogeocenosis does not depend on its parcel structure and varies with respect to plant microgroups: it is maximum in oxalis pine–spruce and maple–lime forests and bracken spruce–birch forests and minimum in areas of forest fall without vegetation. The upper (from 0 to 20 cm thick) soil layer provides up to 50% of the total soil CO₂ emission. The role of microbial respiration in the total CO₂ emission from soils is determined by weather conditions and varies from 9–33% in a dry summer to 55–75% in a summer with favorable temperature and moisture.     

Effects of global change and human disturbance on soil carbon cycling in boreal forest: A review

Increasing human demands for Earth’s resources are hastening many environmental changes and creating a need to incorporate the routine monitoring of ecosystem functions into forest management.Under global change and anthropogenic disturbances,soil carbon (C) cycling in terrestrial ecosystems is undergoing substantial changes that result in the transformation between soil C sources and sinks.Therefore,the forest C budget requires an understanding of the underlying soil C dynamic under environment...     

色季拉山4种林型土壤呼吸及其影响因子

土壤碳是森林生态系统最大的碳库,是其森林生态系统碳循环的极其重要组分。森林土壤呼吸时陆地生态系统土壤呼吸的重要组成部分,其动态变化对全球碳平衡有着重要的影响,然而目前对藏东南地区森林土壤呼吸的研究还比较薄弱。为探讨不同林型土壤呼吸差异及其影响因子,采用Li-8100便携式土壤呼吸测定仪,研究了藏东南色季拉山4种原始森林生态系统(高山灌丛AS、方枝柏SS、杜鹃RF、急尖长苞冷杉AGSF)的土壤碳动态。结果表明:(1)藏东南色季拉山寒温带森林土壤呼吸具有明显的日变化和季节变化。在日变化方面,CO2的排放通量存在明显的日变化规律,排放通量在白天16:00左右最高,最低值出现在凌晨6:00左右,一天内土壤呼吸作用均呈单峰型曲线变化。季节变化方面,CO2排放的通量的季节变化趋势表现为6月份随着天气转暖和植被生长土壤呼吸作用逐渐增大,7月份气温最高时土壤呼吸作用也达到最大值随后,9月份气温逐渐下降,土壤呼吸作用也逐渐降低。(2)4种森林类型的土壤呼吸速率在植物生长季内与土壤表层(10 cm)土壤温度均呈不同程度的正相关,而与土壤含水量的相关性较弱。土壤温度是决定藏东南色季拉山土壤呼季节变化的主要因子。该研究为明确森林生态系统土壤呼吸变化规律及其影响因素的控制提供参考,同时对估算地区碳平衡、评估区域碳源汇具有重要意义。     

Vegetation redistribution: A possible biosphere source of CO2 during climatic change

A new biogeographic model, MAPSS, predicts changes in vegetation leaf area index (LAI), site water balance and run off, as well as changes in Biome boundaries. Potential scenarios of equilibrium vegetation redistribution under 2 × CO₂ climate from five different General Circulation Models (GCMs) are presented. In general, large spatial shifts in temperate and boreal vegetation are predicted under the different scenarios; while, tropical vegetation boundaries are predicted (with one exception) to experience minor distribution contractions. Maps of predicted changes in forest LAI imply drought-induced losses of biomass over most forested regions, even in the tropics. Regional patterns of forest decline and dieback are surprisingly consistent among the five GCM scenarios, given the general lack of consistency in predicted changes in regional precipitation patterns. Two factors contribute to the consistency among the GCMs of the regional ecological impacts of climatic change: 1) regional, temperature-induced increases in potential evapotranspiration (PET) tend to more than offset regional increases in precipitation; and, 2) the unchanging background interplay between the general circulation and the continental margins and mountain ranges produces a fairly stable pattern of regionally specific sensitivity to climatic change. Two areas exhibiting among the greatest sensitivity to drought-induced forest decline are eastern North America and eastern Europe to western Russia. Drought-induced vegetation decline (losses of LAI), predicted under all GCM scenarios, will release CO₂ to the atmosphere; while, expansion of forests at high latitudes will sequester CO₂. The imbalance in these two rate processes could produce a large, transient pulse of CO₂ to the atmosphere.     

Changes in soil organic matter composition are associated with forest encroachment into grassland with long-term fire history

This study investigates if Araucaria forest (C₃ metabolism) expansion on frequently burnt grassland (C₄ metabolism) in the southern Brazilian highland is linked to the chemical composition of soil organic matter (SOM) in non‐allophanic Andosols. We used the ¹³C/¹²C isotopic signature to group heavy organo‐mineral fractions according to source vegetation and ¹³C NMR spectroscopy, lignin analyses (CuO oxidation) and measurement of soil colour lightness to characterize their chemical compositions. Large proportions of aromatic carbon (C) combined with small contents of lignin‐derived phenols in the heavy fractions of grassland soils and grass‐derived lower horizons of Araucaria forest soils indicate the presence of charred grass residues in SOM. The contribution of this material may have led to the unusual increase in C/N ratios with depth in burnt grassland soils and to the differentiation of C₃‐ and C₄‐derived SOM, because heavy fractions from unburnt Araucaria forest and shrubland soils have smaller proportions of aromatic C, smaller C/N ratios and are paler compared with those with C₄ signatures. We found that lignins are not applicable as biomarkers for plant origin in these soils with small contents of strongly degraded and modified lignins as the plant‐specific lignin patterns are absent in heavy fractions. In contrast, the characteristic contents of alkyl C and O/N‐alkyl C of C₃ trees or shrubs and C₄ grasses are reflected in the heavy fractions. They show consistent changes of the (alkyl C)/(O/N‐alkyl C) ratio and the ¹³C/¹²C isotopic signature with soil depth, indicating their association with C₄ and C₃ vegetation origin. This study demonstrates that soils may preserve organic matter components from earlier vegetation and land‐use, indicating that the knowledge of past vegetation covers is necessary to interpret SOM composition.     

Soil processes and global change

 Contributors to the Intergovernmental Panel on Climate Change (IPCC) generally agree that increases in the atmospheric concentration of greenhouse trace gases (i.e., CO₂, CH₄, N₂O, O₃) since preindustrial times, about the year 1750, have led to changes in the earth's climate. During the past 250 years the atmospheric concentrations of CO₂, CH₄, and N₂O have increased by 30, 145, and 15%, respectively. A doubling of preindustrial CO₂ concentrations by the end of the twenty-first century is expected to raise global mean surface temperature by about 2  °C and increase the frequency of severe weather events. These increases are attributed mainly to fossil fuel use, land-use change, and agriculture. Soils and climate changes are related by bidirectional interactions. Soil processes directly affect climatic changes through the production and consumption of CO₂, CH₄, and N₂O and, indirectly, through the production and consumption of NH₃, NO_x, and CO. Although CO₂ is primarily produced through fossil fuel combustion, land-use changes, conversion of forest and grasslands to agriculture, have contributed significantly to atmospheric increase of CO₂. Changes in land use and management can also result in the net uptake, sequestration, of atmospheric CO₂. CH₄ and N₂O are produced (30% and 70%, respectively) in the soil, and soil processes will likely regulate future changes in the atmospheric concentration of these gases. The soil-atmosphere exchange of CO₂, CH₄, and N₂O are interrelated, and changes in one cycle can impart changes in the N cycle and resulting soil-atmosphere exchange of N₂O. Conversely, N addition increases C sequestration. On the other hand, soil processes are influenced by climatic change through imposed changes in soil temperature, soil water, and nutrient competition. Increasing concentrations of atmospheric CO₂ alters plant response to environmental parameters and frequently results in increased efficiency in use of N and water. In annual crops increased CO₂ generally leads to increased crop productivity. In natural systems, the long-term impact of increased CO₂ on ecosystem sustainability is not known. These changes may also result in altered CO₂, CH₄, and N₂O exchange with the soil. Because of large temporal and spatial variability in the soil-atmosphere exchange of trace gases, the measurement of the absolute amount and prediction of the changes of these fluxes, as they are impacted by global change on regional and global scales, is still difficult. In recent years, however, much progress has been made in decreasing the uncertainty of field scale flux measurements, and efforts are being directed to large scale field and modeling programs. This paper briefly relates soil process and issues akin to the soil-atmosphere exchange of CO₂, CH₄, and N₂O. The impact of climate change, particularly increasing atmospheric CO₂ concentrations, on soil processes is also briefly discussed. Received: 1 December 1997     

Interannual variation in net ecosystem productivity of Canadian forests as affected by regional weather patterns – A Fluxnet-Canada synthesis

Large-scale weather events such as the El Niño Southern Oscillation (ENSO), Pacific Decadal Oscillation (PDO), and droughts are known to cause substantial interannual variation in the net ecosystem productivity (NEP) of tropical, temperate and boreal forests. Hypotheses for the impacts on NEP of changes in air temperature (T_a) and precipitation associated with these events were tested at diurnal, seasonal and annual time scales using the terrestrial ecosystem model ecosys with measurements of CO₂ and energy exchange from 1998 to 2006 at eddy covariance (EC) flux towers along a transcontinental transect of forest stands in the Fluxnet-Canada Research Network (FCRN).¹ These tests were supported at seasonal time scales by remotely-sensed vegetation indices, and at decadal time scales by wood growth increments from tree-ring and inventory studies. Collectively, results from this testing indicate that large-scale weather events during the study period caused spatially coherent changes in NEP, although these changes may vary with climate zone, species and topography. High T_a episodes, such as occurred with greater frequency during ENSO/PDO events, adversely affected diurnal CO₂ exchange of temperate and boreal conifers, but had little effect on that of a boreal deciduous forest. These contrasting responses of CO₂ exchange to T_a were attributed in the model to greater xylem resistance to water uptake in coniferous vs. deciduous trees. Sustained warming such as occurred during ENSO/PDO events extended the period of net C uptake and thus raised annual NEP at boreal coniferous and deciduous sites, but did not do so at a temperate coniferous site where annual NEP was reduced. However the rise in NEP of boreal conifers with warming was partially offset by the adverse effects of high T_a on diurnal CO₂ exchange, so that the rise in NEP with warming remained smaller than that at a boreal deciduous site. A 3-year drought during the study period adversely affected annual NEP of well-drained boreal deciduous forests but did not affect that of poorly-drained boreal conifers. This lack of effect was attributed in the model to low coniferous evapotranspiration rates and to subsurface water recharge. Drought effects on NEP were therefore largely determined by topography. These contrasting responses of different forest stands to warming and drought indicate divergent changes in forest growth with interannual changes in weather. Such divergent changes are consistent with the complex changes in forest NDVI and net C uptake observed over time in several large-scale remote-sensing studies.     

Global forest systems: An uncertain response to atmospheric pollutants and global climate change?

Forest systems cover more than 4.1×10⁹ ha of the Earth's land area. The future response and feedbacks of forest systems to atmospheric pollutants and projected climate change may be significant. Boreal, temperate and tropical forest systems play a prominent role in carbon (C), nitrogen (N) and sulfur (S) biogeochemical cycles at regional and global scales. The timing and magnitude of future changes in forest systems will depend on environmental factors such as a changing global climate, an accumulation of CO₂ in the atmosphere, and increase global mineralization of nutrients such as N and S. The interactive effects of all these factors on the world's forest regions are complex and not intuitively obvious and are likely to differ among geographic regions. Although the potential effects of some atmospheric pollutants on forest systems have been observed or simulated, large uncertainty exists in our ability to project future forest distribution, composition and productivity under transient or nontransient global climate change scenarios. The potential to manage and adapt forests to future global environmental conditions varies widely among nations. Mitigation practices, such as liming or fertilization to ameliorate excess NO_x or SO_x or forest management to sequester CO₂ are now being applied in selected nations worldwide.The U.S. Government's right to a non-exclusive, royalty free licence in and to any copyright is acknowledged.     

The impact of fertilizer management on the oxidation status of terrestrial organic matter

The oxidative ratio (the ratio of moles of O₂ produced per mole CO₂ sequestered – OR) of the organic matter in the terrestrial biosphere governs the ability of the terrestrial biosphere to uptake CO₂. The value of OR is known to vary between environments, but it would also be expected to vary with management. This study measured the OR of plant and soil samples from the long‐term grassland plots on the Park Grass experiment at Rothamsted (SE England). The selected plots included those with different fertilizer inputs, including farmyard manure or inorganic fertilizers and an unfertilized control, each with and without lime. The measurements show that: (i) Use of inorganic fertilizer caused the OR of soil organic matter to increase. (ii) Farmyard manure (FYM) caused OR of the soil to increase but that of the vegetation decreased. (iii) Liming had the effect of decreasing OR and counteracting effects of fertilizer. (iv) The OR of the ecosystem increased with FYM application but decreased with inorganic fertilizer application. The global pattern in the use of organic amendments and inorganic fertilizers suggests that the likely impact of the predicted increase in global inorganic fertilizer use will result in a net decrease in the OR of the organic matter of the terrestrial biosphere, and an increase in its ability to act as a carbon sink. Corresponding increases in global FYM use and its impact upon global OR are unlikely to be large enough to counteract this effect.     

Priming depletes soil carbon and releases nitrogen in a scrub-oak ecosystem exposed to elevated CO2

Elevated atmospheric CO₂ tends to stimulate plant productivity, which could either stimulate or suppress the processing of soil carbon, thereby feeding back to atmospheric CO₂ concentrations. We employed an acid-hydrolysis-incubation method and a net nitrogen-mineralization assay to assess stability of soil carbon pools and short-term nitrogen dynamics in a Florida scrub-oak ecosystem after six years of exposure to elevated CO₂. We found that soil carbon concentration in the slow pool was 27% lower in elevated than ambient CO₂ plots at 0-10 cm depth. The difference in carbon mass was equivalent to roughly one-third of the increase in plant biomass that occurred in the same experiment. These results concur with previous reports from this ecosystem that elevated CO₂ stimulates microbial degradation of relatively stable soil organic carbon pools. Accordingly, elevated CO₂ increased net N mineralization in the 10-30 cm depth, which may increase N availability, thereby allowing for continued stimulation of plant productivity by elevated CO₂. Our findings suggest that soil texture and climate may explain the differential response of soil carbon among various long-term, field-based CO₂ studies. Increased mineralization of stable soil organic carbon by a CO₂-induced priming effect may diminish the terrestrial carbon sink globally.     

Autumn growth and cold hardening of winter wheat under simulated climate change

Abstract

Plant responses to elevated CO₂ are governed by temperature, and at low temperatures the beneficial effects of CO₂ may be lost. To document the responses of winter cereals grown under cold conditions at northern latitudes, autumn growth of winter wheat exposed to ambient and elevated levels of temperature (+2.5°C), CO₂ (+150 µmol mol^?1), and shade (?30%) was studied in open-top chambers under low light and at low temperatures. Throughout the experiment, temperature dominated plant responses, while the effects of CO₂ were marginal, except for a positive effect on root biomass. Increased temperature resulted in increased leaf area, total biomass, total root biomass, total stem biomass, and number of tillers, but also a lower content of total sugars and a weaker tolerance to frost. The loss of frost tolerance was related to the larger size of plants grown at elevated temperature. The 30% light reduction under shading did not affect the growth, sugar content, or frost tolerance of winter wheat. At the low temperatures found at high latitudes during autumn, the atmospheric CO₂ increase is unlikely to enhance autumn growth of winter wheat to any significant extent, while a temperature increase may have important and major effects on its development and growth.     

Responses of Bougainvillea spectabilis to elevated atmospheric CO2 under galaxolide (HHCB) pollution and the mechanisms of its rhizosphere metabolism

Purpose

Due to the discovery of synthetic musks in soil and the gradual increase in atmospheric carbon dioxide (CO₂), it is important to reveal the potential implications of these compounds for bioremediation systems. Hence, this study was conducted to investigate the combined influence of galaxolide (HHCB) and elevated CO₂ on an ornamental remediation plant.

Materials and methods

We conducted pot experiments with Bougainvillea spectabilis, an ornamental remediation plant, in which the biomass, HHCB and chlorophyll contents, and rhizosphere metabolism of the plants were analyzed.

Results and discussion

We showed that B. spectabilis exhibited high tolerance under combined HHCB and elevated CO₂ stresses. The addition of HHCB alone to the soil did not significantly reduce the biomass components of B. spectabilis, whereas the presence of elevated CO₂ (750 μL L^?1) alone showed a relatively strong ability to increase plant biomass, especially that of the leaves. An elevated CO₂ concentration stimulated the absorption of low doses of HHCB by the roots. Regarding the root metabolites of B. spectabilis, carbohydrates and organic acids were highly correlated with HHCB concentration, and amino acids were well correlated with CO₂ concentration.

Conclusions

Our study indicates that B. spectabilis may be well suited to remove HHCB from contaminated soil under elevated CO₂ levels, and the root metabolism of this plant provides information about HHCB contamination and elevated CO₂ conditions.

     

Influence of forest disturbance on CO2, CH4 and N2O fluxes from larch forest soil in the permafrost taiga region of eastern Siberia

Abstract

To evaluate the effect of increasing forest disturbances on greenhouse gas budgets in a taiga forest in eastern Siberia, CO₂, CH₄ and N₂O fluxes from the soils were measured during the growing season in intact, burnt and clear-felled larch forests (4–5 years after the disturbance). Soil temperature and moisture were higher at the two disturbed sites than at the forest site. A 64–72% decrease in the Q ₁₀ value of soil CO₂ flux from the disturbed sites compared with the forest site (5.92) suggested a reduction in root respiration and a dominance of organic matter decomposition at the disturbed sites. However, the cumulative CO₂ emissions (May–August) were not significantly different among the sites (2.81–2.90 Mg C ha^?1 per 3 months). This might be because decreased larch root respiration was compensated for by increased organic matter decomposition resulting from an increase in the temperature and root respiration of invading vegetation at the disturbed sites. The CH₄ uptake (kg C ha^?1 per 4 months [May–September]) at the burnt site was significantly higher (–0.15) than the uptake at the forest (–0.045) and clear-felled sites (0.0027). Although there were no significant differences among the sites, N₂O emission (kg N ha^?1 per 4 months) was slightly lower at the burnt site (0.013) and higher at the clear-felled site (0.068) than at the forest site (0.038). This different influence of burning and tree felling on CH₄ and N₂O fluxes might result from changes in the physical and chemical properties of the soil with respect to forest fire.     

Global climate changes and the soil cover

The relationships between climate changes and the soil cover are analyzed. The greenhouse effect induced by the rising concentrations of CO₂, CH₄, N₂O, and many other trace gases in the air has been one of the main factors of the global climate warming in the past 30–40 years. The response of soils to climate changes is considered by the example of factual data on soil evolution in the dry steppe zone of Russia. Probable changes in the carbon cycle under the impact of rising CO₂ concentrations are discussed. It is argued that this rise may have an effect of an atmospheric fertilizer and lead to a higher productivity of vegetation, additional input of organic residues into the soils, and activation of soil microflora. Soil temperature and water regimes, composition of soil gases, soil biotic parameters, and other dynamic soil characteristics are most sensitive to climate changes. For the territory of Russia, in which permafrost occupies more than 50% of the territory, the response of this highly sensitive natural phenomenon to climate changes is particularly important. Long-term data on soil temperatures at a depth of 40 cm are analyzed for four large regions of Russia. In all of them, except for the eastern sector of Russian Arctic, a stable trend toward the rise in the mean annual soil temperature. In the eastern sector (the Verkhoyansk weather station), the soil temperature remains stable.     

Interaction of acid rain and global changes: Effects on terrestrial and aquatic ecosystems

Both acid deposition and changes in the global atmosphere and climate affect terrestrial and aquatic ecosystems. In the atmosphere sulphate aerosols tend to increase haze, altering the global radiation balance. Increased nitrogen deposition to N-limited systems such as boreal forests results in increased growth and increased sequestration of atmospheric CO₂, slowing the increase in CO₂ levels in the atmosphere. Future reduction in S and N emissions may result in a trade-off -- better with respect to some effects of acid deposition and greenhouse warming, but worse with respect to others. Global warming may cause the incidence and severity of drought to increase. Mineralisation of N and oxidation of organic S compounds release pulses of SO₄, acid and Al to surface waters. Effects in lakes may include reduced deep water refugia for cold stenotherms, lower nutrient concentrations, and greater penetration of harmful UV radiation. Longer water renewal times cause declines in SO₄ and NO₃, due to increased in situ removal, but increases in base cations. The net result is increased internal alkalinity production. In areas characterised by cold winters, global warming may result in a major shift in hydrologic cycle, with snowmelt episodes occurring during the winter rather than the typical pattern of accumulation in the winter and melting in the spring. Increased storm frequency predicted for the future will cause increased frequency and severity of sea salt episodes in coastal regions. Predicting the interactions of regional and global environmental factors in the coming decades poses new challenges to scientists, managers and policy-makers.     

Emission of CO2 from the surface of oligotrophic bogs with due account for their microrelief in the southern taiga of European Russia

The results of studying the carbon dioxide fluxes from the soil’s surface during three years taking into account the microrelief are summarized. More precise estimates were obtained for the annual CO₂ emission from the oligotrophic peat bogs differing in vegetation and waterlogging in the southern taiga of European Russia. The maximum differences in the rates of the CO₂ emission related to the microrelief elements are characteristic of the treeless ridge-pool complex, where the hollows (without vegetation) emitted CO₂ twice less than the flat areas and thrice less than the hummocks. In the forest bogs, the differences related to the microrelief were significantly lower. In the areas with the ridge-pool microrelief, the weighted average (for 3 years) CO₂ emission was 436 g C/m² per year; in the better drained natural dwarf shrub-cotton grass-sphagnum pine forest, 930; and in the drained pine forest, 1292 g C/m² per year. The share of the CO₂ amount emitted in the cold period (November–April) amounted to 10% of its annual flux from the peat soils of the ridge-pool complex and 17 and 24%, respectively, in the natural and drained pine forests.     

Changes in C storage by terrestrial ecosystems: How C-N interactions restrict responses to CO2 and temperature

A general model of ecosystem biogeochemistry was used to examine the responses of arctic tundra and temperate hardwood forests to a doubling of CO₂ concentration and to a 5°C increase in average growing season temperature. The amount of C stored in both ecosystems increased with both increased CO₂ and temperature. Under increased CO₂, the increase in C storage was due to increases in the C∶N ratio of both vegetation and soils. Under increased temperature, the increased C storage in the forest was due to a shift in N from soils (with low C∶N ratios) to vegetation (with high C∶N ratios). In the tundra, both a shift in N from soils to vegetation and an increase in C∶N ratios contributed to increased C storage under higher temperatures. Neither ecosystem sequestered N from external sources because the supply rate was low.     

Impact of elevated CO2 on mycorrhizal associations and implications for plant growth

The impact of increasing concentrations of atmospheric CO₂ upon plant physiology has been widely investigated. Plant, and in particular root, growth is nearly always enhanced as a direct consequence of CO₂ enrichment, with C₃ species generally more responsive than C₄ species. Such alterations in plant productivity will have consequence for below-ground processes and increased carbon allocation to the roots may favour symbiotic relationships. This paper discusses the current information available for the consequences of these changes upon mycorrhizal relationships. Generally mycorrhizal plants grown under CO₂ enrichment show enhanced phosphorus uptake but nitrogen uptake is unaffected. This increased nutrient uptake is not correlated with increased mycorrhizal colonization of the roots. Similarly root exudation does not increase under CO₂ enrichment but qualitative differences have yet to be assessed. However, it is predicted that total rhizodeposition of materials will increase as will litter inputs, although mineral and biochemical alterations to these plant derived inputs may occur. The consequences of such changes within the rhizosphere are discussed and future research     

A call to investigate drivers of soil organic matter retention vs. mineralization in a high CO2 world

Understanding how elevated atmospheric CO₂ alters the formation and decomposition of soil organic carbon (SOC) is important but challenging. If elevated CO₂ induces even small changes in rates of formation or decay of SOC, there could be substantial feedbacks on the atmosphere's concentration of CO₂. However, the long turnover times of many SOC pools - decades to centuries - make the detection of changes in the soil's pool size difficult. Long-term CO₂ enrichment experiments have offered unprecedented opportunities to explore these issues in intact ecosystems for more than a decade. Increased NPP with elevated CO₂ has prompted the hypothesis that SOC may increase at the same time that increased vegetation nitrogen (N) uptake and accumulation indicates probable declines in SON. Varying investigators thus have hypothesized that SOC will increase and SON will decline to explain increased NPP with elevated CO₂; researchers also invoke biogeochemical theory and stoichiometric constraints to argue for strong limitations on the co-occurrence of these phenomena. We call for researchers to investigate two broad research questions to elucidate the drivers of these processes. First, we ask how elevated CO₂ influences compound structure and stoichiometry of that proportion of NPP retained by soil profiles for relatively long time periods. We also call for investigations of the mechanisms underlying the decomposition of mineralizable organic matter with elevated CO₂. Specifically, we need to understand how elevated CO₂ influences microbial priming (driven by enhanced microbial energy needs associated with increases in biomass or activity) and microbial mining of N (driven by enhanced microbial N demand associated with greater vegetative N uptake), two processes that necessarily will be constrained by the stoichiometry of both substrates and microbial demands. Applying technologies such as nuclear magnetic resonance and the detection of biomarkers that reveal organic matter structure and origins, and studying microbial stoichiometric constraints, will dramatically improve our ability to predict future patterns of ecosystem C and N cycling.