No overall stimulation of soil respiration under mature deciduous forest trees after 7 years of CO2 enrichment

The anthropogenic rise in atmospheric CO₂ is expected to impact carbon (C) fluxes not only at ecosystem level but also at the global scale by altering C cycle processes in soils. At the Swiss Canopy Crane (SCC), we examined how 7 years of free air CO₂ enrichment (FACE) affected soil CO₂ dynamics in a ca. 100‐year‐old mixed deciduous forest. The use of ¹³C‐depleted CO₂ for canopy enrichment allowed us to trace the flow of recently fixed C. In the 7th year of growth at ～550 ppm CO₂, soil respiratory CO₂ consisted of 39% labelled C. During the growing season, soil air CO₂ concentration was significantly enhanced under CO₂‐exposed trees. However, elevated CO₂ failed to stimulate cumulative soil respiration (R_s) over the growing season. We found periodic reductions as well as increases in instantaneous rates of R_s in response to elevated CO₂, depending on soil temperature and soil volumetric water content (VWC; significant three‐way interaction). During wet periods, soil water savings under CO₂‐enriched trees led to excessive VWC (>45%) that suppressed R_s. Elevated CO₂ stimulated R_s only when VWC was ≤40% and concurrent soil temperature was high (>15 °C). Seasonal Q₁₀ estimates of R_s were significantly lower under elevated (Q₁₀=3.30) compared with ambient CO₂ (Q₁₀=3.97). However, this effect disappeared when three consecutive sampling dates of extremely high VWC were disregarded. This suggests that elevated CO₂ affected Q₁₀ mainly indirectly through changes in VWC. Fine root respiration did not differ significantly between treatments but soil microbial biomass (C_mic) increased by 14% under elevated CO₂ (marginally significant). Our findings do not indicate enhanced soil C emissions in such stands under future atmospheric CO₂. It remains to be shown whether C losses via leaching of dissolved organic or inorganic C (DOC, DIC) help to balance the C budget in this forest.     

Above- and below-ground responses of C3–C4 species mixtures to elevated CO2 and soil water availability

We evaluated the influences of CO₂[Control, ～ 370 µ mol mol ^{? 1}; 200 µ mol mol ^{? 1} above ambient applied by free‐air CO₂ enrichment (FACE)] and soil water (Wet, Dry) on above‐ and below‐ground responses of C₃ (cotton, Gossypium hirsutum) and C₄ (sorghum, Sorghum bicolor) plants in monocultures and two density mixtures. In monocultures, CO₂ enrichment increased height, leaf area, above‐ground biomass and reproductive output of cotton, but not sorghum, and was independent of soil water treatment. In mixtures, cotton, but not sorghum, above‐ground biomass and height were generally reduced compared to monocultures, across both CO₂ and soil water treatments. Density did not affect individual plant responses of either cotton or sorghum across the other treatments. Total (cotton + sorghum) leaf area and above‐ground biomass in low‐density mixtures were similar between CO₂ treatments, but increased by 17–21% with FACE in high‐density mixtures, due to a 121% enhancement of cotton leaf area and a 276% increase in biomass under the FACE treatment. Total root biomass in the upper 1.2 m of the soil was not influenced by CO₂ or by soil water in monoculture or mixtures; however, under dry conditions we observed significantly more roots at lower soil depths ( > 45 cm). Sorghum roots comprised 81–85% of the total roots in the low‐density mixture and 58–73% in the high‐density mixture. CO₂‐enrichment partly offset negative effects of interspecific competition on cotton in both low‐ and high‐density mixtures by increasing above‐ground biomass, with a greater relative increase in the high‐density mixture. As a consequence, CO₂‐enrichment increased total above‐ground yield of the mixture at high density. Individual plant responses to CO₂ enrichment in global change models that evaluate mixed plant communities should be adjusted to incorporate feedbacks for interspecific competition. Future field studies in natural ecosystems should address the role that a CO₂‐mediated increase in C₃ growth may have on subsequent vegetation change.     

Elevated carbon dioxide increases soil nitrogen and phosphorus availability in a phosphorus‐limited Eucalyptus woodland

Free‐air CO₂ enrichment (FACE) experiments have demonstrated increased plant productivity in response to elevated (e)CO₂, with the magnitude of responses related to soil nutrient status. Whilst understanding nutrient constraints on productivity responses to eCO₂ is crucial for predicting carbon uptake and storage, very little is known about how eCO₂ affects nutrient cycling in phosphorus (P)‐limited ecosystems. Our study investigates eCO₂ effects on soil N and P dynamics at the EucFACE experiment in Western Sydney over an 18‐month period. Three ambient and three eCO₂ (+150 ppm) FACE rings were installed in a P‐limited, mature Cumberland Plain Eucalyptus woodland. Levels of plant accessible nutrients, evaluated using ion exchange resins, were increased under eCO₂, compared to ambient, for nitrate (+93%), ammonium (+12%) and phosphate (+54%). There was a strong seasonality to responses, particularly for phosphate, resulting in a relatively greater stimulation in available P, compared to N, under eCO₂ in spring and summer. eCO₂ was also associated with faster nutrient turnover rates in the first six months of the experiment, with higher N (+175%) and P (+211%) mineralization rates compared to ambient rings, although this difference did not persist. Seasonally dependant effects of eCO₂ were seen for concentrations of dissolved organic carbon in soil solution (+31%), and there was also a reduction in bulk soil pH (‐0.18 units) observed under eCO₂. These results demonstrate that CO₂ fertilization increases nutrient availability – particularly for phosphate – in P‐limited soils, likely via increased plant belowground investment in labile carbon and associated enhancement of microbial turnover of organic matter and mobilization of chemically bound P. Early evidence suggests that there is the potential for the observed increases in P availability to support increased ecosystem C‐accumulation under future predicted CO₂ concentrations.     

How far do experimentally elevated CO2 levels reach into the surrounding? – An example using the 13C label of soil organic matter as an archive

During the last two decades, free air CO₂ enrichment (FACE) studies have been conducted to study the effects of rising atmospheric CO₂ concentrations on ecosystems. The distances between fumigated and control plots differ widely among those projects, but no experimental data are available how far into the surrounding area an effect of CO₂ fumigation can be detected. As the CO₂ gas added to the fumigated plots has a different ¹³C label than ambient atmospheric CO₂, its carbon can be traced into plants and soil organic matter (SOM). The Swiss FACE in Eschikon had been conducted for 10 years on a grassland site. After it had ended, we analysed soil samples from three transects extending from the plots to the surrounding area for their organic carbon (C_org) content and carbon isotopic signature. We determined the maximum spatial extension to which carbon originating from the fumigation was incorporated into SOM. A budget of the fumigation gas‐derived C_org in the upper 10 cm of the soil showed that approximately 50 kg C were stored within the plots, and an additional 31 kg C were stored in their immediate surroundings up to a distance of 9 m from the gas pipes. The presented approach provides us with a method to determine a posteriori the extension to which the CO₂ fumigation treatment contaminated its immediate surroundings during a FACE experiment. In the presented example, this showed that the distances between plots could have been reduced significantly. Although not generalizable to other experimental settings, the finding indicates that optimizing the spatial layout, e.g. by modelling gas dispersion, will be useful when planning future large‐scale FACE infrastructures. Our approach provides a solid basis to test such gas‐dispersion models on existing FACE sites before planning new sites.     

A free-air enrichment system for exposing tall forest vegetation to elevated atmospheric CO2

A free-air CO₂ enrichment (FACE) system was designed to permit the experimental exposure of tall vegetation such as stands of forest trees to elevated atmospheric CO₂ concentrations ([CO₂]_a) without enclosures that alter tree microenvironment. We describe a prototype FACE system currently in operation in forest plots in a maturing loblolly pine (Pinus taeda L.) stand in North Carolina, USA. The system uses feedback control technology to control [CO₂] in a 26 m diameter forest plot that is over 10 m tall, while monitoring the 3D plot volume to characterize the whole-stand CO₂ regime achieved during enrichment. In the second summer season of operation of the FACE system, atmospheric CO₂ enrichment was conducted in the forest during all daylight hours for 96.7% of the scheduled running time from 23 May to 14 October with a preset target [CO₂] of 550 μmol mol^–1, ≈ 200 μmol mol^–1 above ambient [CO₂]. The system provided spatial and temporal control of [CO₂] similar to that reported for open-top chambers over trees, but without enclosing the vegetation. The daily average daytime [CO₂] within the upper forest canopy at the centre of the FACE plot was 552 ± 9 μmol mol^–1 (mean ± SD). The FACE system maintained 1-minute average [CO₂] to within ± 110 μmol mol^–1 of the target [CO₂] for 92% of the operating time. Deviations of [CO₂] outside of this range were short-lived (most lasting < 60 s) and rare, with fewer than 4 excursion events of a minute or longer per day. Acceptable spatial control of [CO₂] by the system was achieved, with over 90% of the entire canopy volume within ± 10% of the target [CO₂] over the exposure season. CO₂ consumption by the FACE system was much higher than for open-top chambers on an absolute basis, but similar to that of open-top chambers and branch bag chambers on a per unit volume basis. CO₂ consumption by the FACE system was strongly related to windspeed, averaging 50 g CO₂ m^–3 h^–1 for the stand for an average windspeed of 1.5 m s^–1 during summer. The [CO₂] control results show that the free-air approach is a tractable way to study long-term and short-term alterations in trace gases, even within entire tall forest ecosystems. The FACE approach permits the study of a wide range of forest stand and ecosystem processes under manipulated [CO₂]_a that were previously impossible or intractable to study in true forest ecosystems.     

Leaf litter production and decomposition in a poplar short-rotation coppice exposed to free air CO2 enrichment (POPFACE)

The capacity of forest ecosystems to sequester C in the soil relies on the net balance between litter production above, as well as, below ground, and decomposition processes. Nitrogen mineralization and its availability for plant growth and microbial activity often control the speed of both processes. Litter production, decomposition and N mineralization are strongly interdependent. Thus, their responses to global environmental changes (i.e. elevated CO₂, climate, N deposition, etc.) cannot be fully understood if they are studied in isolation. In the present experiment, we investigated litter fall, litter decomposition and N dynamics in decomposing litter of three Populus spp., in the second and third growing season of a short rotation coppice under FACE. Elevated CO₂ did not affect annual litter production but slightly retarded litter fall in the third growing season. In all species, elevated CO₂ lowered N concentration, resulting in a reduction of N input to the soil via litter fall, but did not affect lignin concentrations. Litter decomposition was studied in bags incubated in situ both in control and FACE plots. Litter lost between 15% and 18% of the original mass during the eight months of field incubation. On average, litter produced under elevated CO₂ attained higher residual mass than control litter. On the other end, when litter was incubated in FACE plots it exhibited higher decay rates. These responses were strongly species‐specific. All litter increased their N content during decomposition, indicating immobilization of N from external sources. Independent of the initial quality, litter incubated on FACE soils immobilized less N, possibly as a result of lower N availability in the soil. Indeed, our results refer to a short‐term decomposition experiment. However, according to a longer‐term model extrapolation of our results, we anticipate that in Mediterranean climate, under elevated atmospheric CO₂, soil organic C pool of forest ecosystems may initially display faster turnover, but soil N availability will eventually limit the process.     

Warming prevents the elevated CO2-induced reduction in available soil nitrogen in a temperate, perennial grassland

Rising atmospheric carbon dioxide concentration ([CO₂]) has the potential to stimulate ecosystem productivity and sink strength, reducing the effects of carbon (C) emissions on climate. In terrestrial ecosystems, increasing [CO₂] can reduce soil nitrogen (N) availability to plants, preventing the stimulation of ecosystem C assimilation; a process known as progressive N limitation. Using ion exchange membranes to assess the availability of dissolved organic N, ammonium and nitrate, we found that CO₂ enrichment in an Australian, temperate, perennial grassland did not increase plant productivity, but did reduce soil N availability, mostly by reducing nitrate availability. Importantly, the addition of 2 °C warming prevented this effect while warming without CO₂ enrichment did not significantly affect N availability. These findings indicate that warming could play an important role in the impact of [CO₂] on ecosystem N cycling, potentially overturning CO₂‐induced effects in some ecosystems.     

Net soil carbon input under ambient and elevated CO2 concentrations: isotopic evidence after 4 years

Elevation of atmospheric CO₂ concentration is predicted to increase net primary production, which could lead to additional C sequestration in terrestrial ecosystems. Soil C input was determined under ambient and Free Atmospheric Carbon dioxide Enrichment (FACE) conditions for Lolium perenne L. and Trifolium repens L. grown for four years in a sandy‐loam soil. The ¹³C content of the soil organic matter C had been increased by 5‰ compared to the native soil by prior cropping to corn (Zea mays) for > 20 years. Both species received low or high amounts of N fertilizer in separate plots. The total accumulated above‐ground biomass produced by L. perenne during the 4‐year period was strongly dependent on the amount of N fertilizer applied but did not respond to increased CO₂. In contrast, the total accumulated above‐ground biomass of T. repens doubled under elevated CO₂ but remained independent of N fertilizer rate. The C:N ratio of above‐ground biomass for both species increased under elevated CO₂ whereas only the C:N ratio of L. perenne roots increased under elevated CO₂. Root biomass of L. perenne doubled under elevated CO₂ and again under high N fertilization. Total soil C was unaffected by CO₂ treatment but dependent on species. After 4 years and for both crops, the fraction of new C (F‐value) under ambient conditions was higher (P= 0.076) than under FACE conditions: 0.43 vs. 0.38. Soil under L. perenne showed an increase in total soil organic matter whereas N fertilization or elevated CO₂ had no effect on total soil organic matter content for both systems. The net amount of C sequestered in 4 years was unaffected by the CO₂ concentration (overall average of 8.5 g C kg^?1 soil). There was a significant species effect and more new C was sequestered under highly fertilized L. perenne. The amount of new C sequestered in the soil was primarily dependent on plant species and the response of root biomass to CO₂ and N fertilization. Therefore, in this FACE study net soil C sequestration was largely depended on how the species responded to N rather than to elevated CO₂.     

Seasonal changes in the effects of free-air CO2 enrichment (FACE) on growth, morphology and physiology of rice root at three levels of nitrogen fertilization

Over time, the relative effects of elevated [CO₂] on the aboveground photosynthesis, growth and development of rice (Oryza sativa L.) are likely to be changed with increasing duration of CO₂ exposure, but the resultant effects on rice belowground responses remain to be evaluated. To investigate the impacts of elevated [CO₂] on seasonal changes in root growth, morphology and physiology of rice, a free‐air CO₂ enrichment (FACE) experiment was performed at Wuxi, Jiangsu, China, in 2002–2003. A japonica cultivar with large panicle was exposed to two [CO₂] (ambient [CO₂], 370 μmol mol⁻¹; elevated [CO₂], 570 μmol mol⁻¹) at three levels of nitrogen (N): low (LN, 15 g N m⁻²), medium (MN, 25 g N m⁻²) and high N (HN, 35 g N m⁻²). Elevated [CO₂] increased cumulative root volume, root dry weight, adventitious root length and adventitious root number at all developmental stages by 25–71%, which was mainly associated with increased root growth rate during early growth period (EGP) and lower rate of root senescence during late growth period (LGP), while a slight inhibition of root growth rate occurred during middle growth period (MGP). For individual adventitious roots, elevated [CO₂] increased average length, volume, diameter and dry weight early in the season, but the effects gradually disappeared in subsequent stages. Total surface area and active adsorption area per unit root dry weight reached their maxima 10 days earlier in FACE vs. ambient plants, but both of them together with root oxidation ability per unit root dry weight declined with elevated [CO₂] during MGP and LGP, the decline being larger during MGP than LGP. The CO₂‐induced decreases in specific root activities during MGP and LGP were associated with a larger amount of root accumulation during EGP and lower N concentration and higher C/N ratio in roots during MGP and LGP in FACE vs. ambient plants. The results suggest that most of the CO₂‐induced increases in shoot growth of rice are similarly associated with increased root growth.     

Widespread foliage δ15N depletion under elevated CO2: inferences for the nitrogen cycle

Leaf ¹⁵N signature is a powerful tool that can provide an integrated assessment of the nitrogen (N) cycle and whether it is influenced by rising atmospheric CO₂ concentration. We tested the hypothesis that elevated CO₂ significantly changes foliage δ¹⁵N in a wide range of plant species and ecosystem types. This objective was achieved by determining the δ¹⁵N of foliage of 27 field‐grown plant species from six free‐air CO₂ enrichment (FACE) experiments representing desert, temperate forest, Mediterranean‐type, grassland prairie, and agricultural ecosystems. We found that within species, the δ¹⁵N of foliage produced under elevated CO₂ was significantly lower (P<0.038) compared with that of foliage grown under ambient conditions. Further analysis of foliage δ¹⁵N by life form and growth habit revealed that the CO₂ effect was consistent across all functional groups tested. The examination of two chaparral shrubs grown for 6 years under a wide range of CO₂ concentrations (25–75 Pa) also showed a significant and negative correlation between growth CO₂ and leaf δ¹⁵N. In a select number of species, we measured bulk soil δ¹⁵N at a depth of 10 cm, and found that the observed depletion of foliage δ¹⁵N in response to elevated CO₂ was unrelated to changes in the soil δ¹⁵N. While the data suggest a strong influence of elevated CO₂ on the N cycle in diverse ecosystems, the exact site(s) at which elevated CO₂ alters fractionating processes of the N cycle remains unclear. We cannot rule out the fact that the pattern of foliage δ¹⁵N responses to elevated CO₂ reported here resulted from a general drop in δ¹⁵N of the source N, caused by soil‐driven processes. There is a stronger possibility, however, that the general depletion of foliage δ¹⁵N under high CO₂ may have resulted from changes in the fractionating processes within the plant/mycorrhizal system.     

Sequestration and turnover of bacterial- and fungal-derived carbon in a temperate grassland soil under long-term elevated atmospheric pCO2

Temperate grasslands contribute about 20% to the global C budget. Elevation of atmospheric CO₂ concentration (pCO₂) could lead to additional C sequestration into these ecosystems. Microbial‐derived C in the soil comprising about 1–5% of total soil organic carbon may be an important ‘pool’ for long‐term storage of C under future increased atmospheric CO₂ concentrations. In our study, the impact of elevated pCO₂ on bacterial‐ and fungal‐derived C in the soil of Lolium perenne pastures was investigated under free air carbon dioxide enrichment (FACE) conditions. For 7 years, L. perenne swards were exposed to ambient and elevated pCO₂ (36 and 60 Pa pCO₂, respectively). The additional CO₂ in the FACE plots was depleted in ¹³C compared with ambient plots, so that ‘new’ (<7 years) C inputs in the form of microbial‐derived residues could be determined by means of stable C isotope analysis. Amino sugars in soil are reliable organic biomarkers for indicating the presence of microbial‐derived residues, with particular amino sugars indicative of either bacterial or fungal origin. It is assumed that amino sugars are stabilized to a significant extent in soil, and so may play an important role in long‐term C storage. In our study, we were also able to discriminate between ‘old’ (> 7 years) and ‘new’ microbial‐derived C using compound‐specific δ¹³C analysis of individual amino sugars. This new tool was very useful in investigating the potential for C storage in microbial‐derived residues and the turnover of this C in soil under increased atmospheric pCO₂. The ¹³C signature of individual amino sugars varied between ?17.4‰ and ?39.6‰, and was up to 11.5% depleted in ¹³C in the FACE plots when compared with the bulk δ¹³C value of the native C3 L. perenne soil. New amino sugars in the bulk soil contributed up to 16% to the overall amino sugar pool after the first year and between 62% and 125% after 7 years of exposure to elevated pCO₂. Amounts of new glucosamine increased by the greatest amount (16–125%) during the experiment, followed by mannosamine (?9% to 107%), muramic acid (?11% to 97%), and galactosamine (15–62%). Proportions of new amino sugars in particle size fractions varied between 38% for muramic acid in the clay fraction and 100% for glucosamine and galactosamine in the coarse sand fraction. Summarizing, during the 7‐year period, amino sugars constituted only between 0.9% and 1.6% of the total SOC content. Therefore, their absolute significance for long‐term C sequestration is limited. Additionally new amino sugars were only sequestered in the silt fraction upon elevated pCO₂ exposure while amino sugar concentrations in the clay fraction decreased. Overall, amino sugar concentrations in bulk soil did not change significantly upon exposure to elevated pCO₂. The calculated mean residence time of amino sugars was surprisingly low varying between 6 and 90 years in the bulk soil, and between 3 and 30 years in the particle size fractions, representing soil organic matter pools with different but relatively low turnover times. Therefore, compound‐specific δ¹³C analysis of individual amino sugars clearly revealed a high amino sugar turnover despite more or less constant amino sugar concentrations over a 7 years period of exposure to elevated pCO₂.     

Effects of elevated CO2 on fine root dynamics in a Mojave Desert community: a FACE study

Fine roots (≤1 mm diameter) are critical in plant water and nutrient absorption, and it is important to understand how rising atmospheric CO₂ will affect them as part of terrestrial ecosystem responses to global change. This study's objective was to determine the effects of elevated CO₂ on production, mortality, and standing crops of fine root length over 2 years in a free‐air CO₂ enrichment (FACE) facility in the Mojave Desert of southern Nevada, USA. Three replicate 25 m diameter FACE rings were maintained at ambient (～370 μmol mol^?1) and elevated CO₂ (～550 μmol mol^?1) atmospheric concentrations. Twenty‐eight minirhizotron tubes were placed in each ring to sample three microsite locations: evergreen Larrea shrubs, drought‐deciduous Ambrosia shrubs, and along systematic community transects (primarily in shrub interspaces which account for ～85% of the area). Seasonal dynamics were similar for ambient and elevated CO₂: fine root production peaked in April–June, with peak standing crop occurring about 1 month later, and peak mortality occurring during the hot summer months, with higher values for all three measures in a wet year compared with a dry year. Fine root standing crop, production, and mortality were not significantly different between treatments except standing crop along community transects, where fine root length was significantly lower in elevated CO₂. Fine root turnover (annual cumulative mortality/mean standing crop) ranged from 2.33 to 3.17 year^?1, and was not significantly different among CO₂ treatments, except for community transect tubes where it was significantly lower for elevated CO₂. There were no differences in fine root responses to CO₂ between evergreen (Larrea) and drought‐deciduous (Ambrosia) shrubs. Combined with observations of increased leaf‐level water‐use efficiency and lack of soil moisture differences, these results suggest that under elevated CO₂ conditions, reduced root systems (compared with ambient CO₂) appear sufficient to provide resources for modest aboveground production increases across the community, but in more fertile shrub microsites, fine root systems of comparable size with those in ambient CO₂ were required to support the greater aboveground production increases. For community transects, development of the difference in fine root standing crops occurred primarily through lower stimulation of fine root production in the elevated CO₂ treatment during periods of high water availability.     

SHORT‐ AND LONG‐TERM EFFECTS OF ELEVATED CO2 ON PHOTOSYNTHESIS AND RESPIRATION IN THE MARINE MACROALGA HIZIKIA FUSIFORMIS (SARGASSACEAE,PHAEOPHYTA) GROWN AT LOW AND HIGH N SUPPLIES1

The short‐term and long‐term effects of elevated CO₂ on photosynthesis and respiration were examined in cultures of the marine brown macroalga Hizikia fusiformis (Harv.) Okamura grown under ambient (375 μL · L^?1) and elevated (700 μL · L^?1) CO₂ concentrations and at low and high N availability. Short‐term exposure to CO₂ enrichment stimulated photosynthesis, and this stimulation was maintained with prolonged growth at elevated CO₂, regardless of the N levels in culture, indicating no down‐regulation of photosynthesis with prolonged growth at elevated CO₂. However, the photosynthetic rate of low‐N‐grown H. fusiformis was more responsive to CO₂ enrichment than that of high‐N‐grown algae. Elevation of CO₂ concentration increased the value of K_1/2(Ci) (the half‐saturation constant) for photosynthesis, whereas high N supply lowered it. Neither short‐term nor long‐term CO₂ enrichment had inhibitory effects on respiration rate, irrespective of the N supply, under which the algae were grown. Under high‐N growth, the Q₁₀ value of respiration was higher in the elevated‐CO₂‐grown algae than the ambient‐CO₂‐grown algae. Either short‐ or long‐term exposure to CO₂ enrichment decreased respiration as a proportion of gross photosynthesis (Pg) in low‐N‐grown H. fusiformis. It was proposed that in a future world of higher atmospheric CO₂ concentration and simultaneous coastal eutrophication, the respiratory carbon flux would be more sensitive to changing temperature.     

Decomposition of Betula papyrifera leaf litter under the independent and interactive effects of elevated CO2 and O3

Litter decay dynamics of paper birch (Betula papyrifera) were assessed at the Aspen free‐air CO₂ enrichment (FACE) facility in northern Wisconsin, USA. Leaf litter was decomposed for 12 months under factorial combinations of 360 vs. 560 μL CO₂ L^?1, crossed with 36 vs. 55 nL O₃ L^?1. To differentiate between substrate quality and environment effects, litterbags were placed in their Native Plots of origin or transplanted into the other treatments. CO₂ enrichment, regardless of O₃ concentration, produced poorer quality litter (high C/N, lignin/N and condensed tannins) than did ambient CO₂ (low C/N, lignin/N and condensed tannins). Substrate quality differences were reflected in the mass loss rates (k‐values), which were high for litter generated under ambient CO₂ (0.887 year^?1) and low for litter generated under elevated CO₂ (0.674 year^?1). The rate‐retarding effects of CO₂ enrichment were neither alleviated nor exacerbated by O₃ exposure. Decay rates varied, however, depending on whether litter was placed back into its plot of origin or transplanted to Common Gardens. The results of this study are species specific, but they have important implications for understanding the processes regulating storage of fixed C and the release of CO₂ from northern forest ecosystems.     

Responses of N fluxes and pools to elevated atmospheric CO2 in model forest ecosystems with acidic and calcareous soils

The objectives of this study were to estimate how soil type, elevated N deposition (0.7 vs. 7 g N m^–2y^–1) and tree species influence the potential effects of elevated CO₂ (370 vs. 570 mol CO₂ mol^–1) on N pools and fluxes in forest soils. Model spruce-beech forest ecosystems were established on a nutrient-rich calcareous sand and on a nutrient-poor acidic loam in large open-top chambers. In the fourth year of treatment, we measured N concentrations in the soil solution at different depths, estimated N accumulation by ion exchange resin (IER) bags, and quantified N export in drainage water, denitrification, and net N uptake by trees. Under elevated CO₂, concentrations of N in the soil solution were significantly reduced. In the nutrient-rich calcareous sand, CO₂ enrichment decreased N concentrations in the soil solution at all depths (–45 to –100%). In the nutrient-poor acidic loam, the negative CO₂ effect was restricted to the uppermost 5 cm of the soil. Increasing the N deposition stimulated the negative impact of CO₂ enrichment on soil solution N in the acidic loam at 5 cm depth from –20% at low N inputs to –70% at high N inputs. In the nutrient-rich calcareous sand, N additions did not influence the CO₂ effect on soil solution N. Accumulation of N by IER bags, which were installed under individual trees, was decreased at high CO₂ levels under spruce in both soil types. Under beech, this decrease occurred only in the calcareous sand. N accumulation by IER bags was negatively correlated with current-years foliage biomass, suggesting that the reduction of soil N availability indices was related to a CO₂-induced growth enhancement. However, the net N uptake by trees was not significantly increased by elevated CO₂. Thus, we suppose that the reduced N concentrations in the soil solution at elevated CO₂ concentrations were rather caused by an increased N immobilisation in the soil. Denitrification was not influenced by atmospheric CO₂ concentrations. CO₂ enrichment decreased nitrate leaching in drainage by 65%, which suggests that rising atmospheric CO₂ potentially increases the N retention capacity of forest ecosystems.     

No photosynthetic down-regulation in sweetgum trees (Liquidambar styraciflua L.) after three years of CO2 enrichment at the Duke Forest FACE experiment

Photosynthetic capacity and leaf properties of sun and shade leaves of overstorey sweetgum trees (Liquidambar styraciflua L.) were compared over the first 3 years of growth in ambient or ambient + 200 μL L^?1 CO₂ at the Duke Forest Free Air CO₂ Enrichment (FACE) experiment. We were interested in whether photosynthetic down‐regulation to CO₂ occurred in sweetgum trees growing in a forest ecosystem, whether shade leaves down‐regulated to a greater extent than sun leaves, and if there was a seasonal component to photosynthetic down‐regulation. During June and September of each year, we measured net photosynthesis (A) versus the calculated intercellular CO₂ concentration (C_i) in situ and analysed these response curves using a biochemical model that described the limitations imposed by the amount and activity of ribulose‐1,5‐bisphosphate carboxylase/oxygenase (Vc_max) and by the rate of ribulose‐1,5‐bisphosphate (RuBP) regeneration mediated by electron transport (J_max). There was no evidence of photosynthetic down‐regulation to CO₂ in either sun or shade leaves of sweetgum trees over the 3 years of measurements. Elevated CO₂ did not significantly affect Vc_max or J_max. The ratio of Vc_max to J_max was relatively constant, averaging 2·12, and was not affected by CO₂ treatment, position in the canopy, or measurement period. Furthermore, CO₂ enrichment did not affect leaf nitrogen per unit leaf area (N_a), chlorophyll or total non‐structural carbohydrates of sun or shade leaves. We did, however, find a strong relationship between N_a and the modelled components of photosynthetic capacity, Vc_max and J_max. Our data over the first 3 years of this experiment corroborate observations that trees rooted in the ground may not exhibit symptoms of photosynthetic down‐regulation as quickly as tree seedlings growing in pots. There was a strong sustained enhancement of photosynthesis by CO₂ enrichment whereby light‐saturated net photosynthesis of sun leaves was stimulated by 63% and light‐saturated net photosynthesis of shade leaves was stimulated by 48% when averaged over the 3 years. This study suggests that this CO₂ enhancement of photosynthesis will be sustained in the Duke Forest FACE experiment as long as soil N availability keeps pace with photosynthetic and growth processes.     

Interactions between plant growth and soil nutrient cycling under elevated CO2: a meta-analysis

free air carbon dioxide enrichment (FACE) and open top chamber (OTC) studies are valuable tools for evaluating the impact of elevated atmospheric CO₂ on nutrient cycling in terrestrial ecosystems. Using meta‐analytic techniques, we summarized the results of 117 studies on plant biomass production, soil organic matter dynamics and biological N₂ fixation in FACE and OTC experiments. The objective of the analysis was to determine whether elevated CO₂ alters nutrient cycling between plants and soil and if so, what the implications are for soil carbon (C) sequestration. Elevated CO₂ stimulated gross N immobilization by 22%, whereas gross and net N mineralization rates remained unaffected. In addition, the soil C : N ratio and microbial N contents increased under elevated CO₂ by 3.8% and 5.8%, respectively. Microbial C contents and soil respiration increased by 7.1% and 17.7%, respectively. Despite the stimulation of microbial activity, soil C input still caused soil C contents to increase by 1.2% yr^?1. Namely, elevated CO₂ stimulated overall above‐ and belowground plant biomass by 21.5% and 28.3%, respectively, thereby outweighing the increase in CO₂ respiration. In addition, when comparing experiments under both low and high N availability, soil C contents (+2.2% yr^?1) and above‐ and belowground plant growth (+20.1% and+33.7%) only increased under elevated CO₂ in experiments receiving the high N treatments. Under low N availability, above‐ and belowground plant growth increased by only 8.8% and 14.6%, and soil C contents did not increase. Nitrogen fixation was stimulated by elevated CO₂ only when additional nutrients were supplied. These results suggest that the main driver of soil C sequestration is soil C input through plant growth, which is strongly controlled by nutrient availability. In unfertilized ecosystems, microbial N immobilization enhances acclimation of plant growth to elevated CO₂ in the long‐term. Therefore, increased soil C input and soil C sequestration under elevated CO₂ can only be sustained in the long‐term when additional nutrients are supplied.     

Soil 13C–15N dynamics in an N2-fixing clover system under long-term exposure to elevated atmospheric CO2

Reduced soil N availability under elevated CO₂ may limit the plant's capacity to increase photosynthesis and thus the potential for increased soil C input. Plant productivity and soil C input should be less constrained by available soil N in an N₂‐fixing system. We studied the effects of Trifolium repens (an N₂‐fixing legume) and Lolium perenne on soil N and C sequestration in response to 9 years of elevated CO₂ under FACE conditions. ¹⁵N‐labeled fertilizer was applied at a rate of 140 and 560 kg N ha^?1 yr^?1 and the CO₂ concentration was increased to 60 Pa pCO₂ using ¹³C‐depleted CO₂. The total soil C content was unaffected by elevated CO₂, species and rate of ¹⁵N fertilization. However, under elevated CO₂, the total amount of newly sequestered soil C was significantly higher under T. repens than under L. perenne. The fraction of fertilizer‐N (f_N) of the total soil N pool was significantly lower under T. repens than under L. perenne. The rate of N fertilization, but not elevated CO₂, had a significant effect on f_N values of the total soil N pool. The fractions of newly sequestered C (f_C) differed strongly among intra‐aggregate soil organic matter fractions, but were unaffected by plant species and the rate of N fertilization. Under elevated CO₂, the ratio of fertilizer‐N per unit of new C decreased under T. repens compared with L. perenne. The L. perenne system sequestered more ¹⁵N fertilizer than T. repens: 179 vs. 101 kg N ha^?1 for the low rate of N fertilization and 393 vs. 319 kg N ha^?1 for the high N‐fertilization rate. As the loss of fertilizer‐¹⁵N contributed to the ¹⁵N‐isotope dilution under T. repens, the input of fixed N into the soil could not be estimated. Although N₂ fixation was an important source of N in the T. repens system, there was no significant increase in total soil C compared with a non‐N₂‐fixing L. perenne system. This suggests that N₂ fixation and the availability of N are not the main factors controlling soil C sequestration in a T. repens system.     

Effects of freeze–thaw on C and N release from soils below different vegetation in a montane system: a laboratory experiment

Unstable snow cover and more frequent freeze–thaw events have been predicted for montane areas in southern Norway, where stable winters are common today. These systems are important contributors to the flux of carbon (C) and nitrogen (N) to air and water. Here we quantify and compare the effects of freeze–thaw on C and N release from soils collected below Calluna, Molinia or Sphagnum. Intact organic soil cores were subjected to four different freeze–thaw regimes for four consecutive 2‐week periods: (1) slow cycling (SC) with one long freezing event during each 2‐week period, (2) fast cycling (FC) with four short freezing events during each 2‐week period, (3) permanent frost (PF) and (4) permanent thaw (PT). The freezing temperature was −5 °C and the thawing temperature was 5 °C. Before start of treatment, at the end of each 2‐week period, and during postincubation periods, carbon dioxide (CO₂) emission as well as leachable dissolved organic C (DOC), dissolved organic N (DON), ammonium (NH₄), nitrate (NO₃) and absorbance at 254 nm were measured. In soils from all three vegetations, PF increased the release of CO₂, DOC, DON and NH₄ compared with PT. SC caused some scattered effects whereas FC only resulted in some increase in NO₃ release below Molinia. Generally, the emission of CO₂ and leaching of DOC, DON and NH₄ increased in the following order: Sphagnum < Calluna < Molinia. The release of NO₃ was greatest below Calluna. Our data suggest that vegetation cover and composition seem at least as important as increased soil frost for future winter fluxes of CO₂, DOC, DON and dissolved inorganic N (DIN) from the soil to air and water. The freezing period needs to be sufficiently long to give significant effects.