Impacts of tropospheric ozone and climate change on net primary productivity and net carbon exchange of China's forest ecosystems

Aim We investigated how ozone pollution and climate change/variability have interactively affected net primary productivity (NPP) and net carbon exchange (NCE) across China's forest ecosystem in the past half century. Location Continental China. Methods Using the dynamic land ecosystem model (DLEM) in conjunction with 10‐km‐resolution gridded historical data sets (tropospheric O₃ concentrations, climate variability/change, and other environmental factors such as land‐cover/land‐use change (LCLUC), increasing CO₂ and nitrogen deposition), we conducted nine simulation experiments to: (1) investigate the temporo‐spatial patterns of NPP and NCE in China's forest ecosystems from 1961–2005; and (2) quantify the effects of tropospheric O₃ pollution alone or in combination with climate variability and other environmental stresses on forests' NPP and NCE. Results China's forests acted as a carbon sink during 1961–2005 as a result of the combined effects of O₃, climate, CO₂, nitrogen deposition and LCLUC. However, simulated results indicated that elevated O₃ caused a 7.7% decrease in national carbon storage, with O₃‐induced reductions in NCE (Pg C year^?1) ranging from 0.4–43.1% among different forest types. Sensitivity experiments showed that climate change was the dominant factor in controlling changes in temporo‐spatial patterns of annual NPP. The combined negative effects of O₃ pollution and climate change on NPP and NCE could be largely offset by the positive fertilization effects of nitrogen deposition and CO₂. Main conclusions In the future, tropospheric O₃ should be taken into account in order to fully understand the variations of carbon sequestration capacity of forests and assess the vulnerability of forest ecosystems to climate change and air pollution. Reducing air pollution in China is likely to increase the resilience of forests to climate change. This paper offers the first estimate of how prevention of air pollution can help to increase forest productivity and carbon sequestration in China's forested ecosystems.     

Sources of increased N uptake in forest trees growing under elevated CO2: results of a large‐scale 15N study

Nitrogen availability in terrestrial ecosystems strongly influences plant productivity and nutrient cycling in response to increasing atmospheric carbon dioxide (CO₂). Elevated CO₂ has consistently stimulated forest productivity at the Duke Forest free‐air CO₂ enrichment experiment throughout the decade‐long experiment. It remains unclear how the N cycle has changed with elevated CO₂ to support this increased productivity. Using natural‐abundance measures of N isotopes together with an ecosystem‐scale ¹⁵N tracer experiment, we quantified the cycling of ¹⁵N in plant and soil pools under ambient and elevated CO₂ over three growing seasons to determine how elevated CO₂ changed N cycling between plants, soil, and microorganisms. After measuring natural‐abundance ¹⁵N differences in ambient and CO₂‐fumigated plots, we applied inorganic ¹⁵N tracers and quantified the redistribution of ¹⁵N for three subsequent growing seasons. The natural abundance of leaf litter was enriched under elevated compared to ambient CO₂, consistent with deeper rooting and enhanced N mineralization. After tracer application, ¹⁵N was initially retained in the organic and mineral soil horizons. Recovery of ¹⁵N in plant biomass was 3.5 ± 0.5% in the canopy, 1.7 ± 0.2% in roots and 1.7 ± 0.2% in branches. After two growing seasons, ¹⁵N recoveries in biomass and soil pools were not significantly different between CO₂ treatments, despite greater total N uptake under elevated CO₂. After the third growing season, ¹⁵N recovery in trees was significantly higher in elevated compared to ambient CO₂. Natural‐abundance ¹⁵N and tracer results, taken together, suggest that trees growing under elevated CO₂ acquired additional soil N resources to support increased plant growth. Our study provides an integrated understanding of elevated CO₂ effects on N cycling in the Duke Forest and provides a basis for inferring how C and N cycling in this forest may respond to elevated CO₂ beyond the decadal time scale.     

Subalpine grassland carbon dioxide fluxes indicate substantial carbon losses under increased nitrogen deposition,but not at elevated ozone concentration

Ozone (O₃) and nitrogen (N) deposition affect plant carbon (C) dynamics and may change ecosystem C‐sink/‐source properties. We studied effects of increased background [O₃] (up to [ambient] × 2) and increased N deposition (up to +50 kg ha^?1 a^?1) on mature, subalpine grassland during the third treatment year. During 10 days and 13 nights, distributed evenly over the growth period of 2006, we measured ecosystem‐level CO₂ exchange using a static cuvette. Light dependency of gross primary production (GPP) and temperature dependency of ecosystem respiration rates (R_eco) were established. Soil temperature, soil water content, and solar radiation were monitored. Using R_eco and GPP values, we calculated seasonal net ecosystem production (NEP), based on hourly averages of global radiation and soil temperature. Differences in NEP were compared with differences in soil organic C after 5 years of treatment. The high [O₃] had no effect on aboveground dry matter productivity (DM), but seasonal mean rates of both R_eco and GPP decreased ca. 8%. NEP indicated an unaltered growing season CO₂–C balance. High N treatment, with a +31% increase in DM, mean R_eco increased ca. 3%, but GPP decreased ca. 4%. Consequently, seasonal NEP yielded a 53.9 g C m^?2 (±22.05) C loss compared with control. Independent of treatment, we observed a negative NEP of 146.4 g C m^?2 (±15.3). Carbon loss was likely due to a transient management effect, equivalent to a shift from pasture to hay meadow and a drought effect, specific to the 2006 summer climate. We argue that this resulted from strongly intensified soil microbial respiration, following mitigation of nutrient limitation. There was no interaction between O₃ and N treatments. Thus, during the 2006 growing season, the subalpine grassland lost >2% of total topsoil organic C as respired CO₂, with increased N deposition responsible for one‐third of that loss.     

Evaluation of terrestrial carbon cycle models for their response to climate variability and to CO2 trends

The purpose of this study was to evaluate 10 process‐based terrestrial biosphere models that were used for the IPCC fifth Assessment Report. The simulated gross primary productivity (GPP) is compared with flux‐tower‐based estimates by Jung et al. [Journal of Geophysical Research 116 (2011) G00J07] (JU11). The net primary productivity (NPP) apparent sensitivity to climate variability and atmospheric CO₂ trends is diagnosed from each model output, using statistical functions. The temperature sensitivity is compared against ecosystem field warming experiments results. The CO₂ sensitivity of NPP is compared to the results from four Free‐Air CO₂ Enrichment (FACE) experiments. The simulated global net biome productivity (NBP) is compared with the residual land sink (RLS) of the global carbon budget from Friedlingstein et al. [Nature Geoscience 3 (2010) 811] (FR10). We found that models produce a higher GPP (133 ± 15 Pg C yr^?1) than JU11 (118 ± 6 Pg C yr^?1). In response to rising atmospheric CO₂ concentration, modeled NPP increases on average by 16% (5–20%) per 100 ppm, a slightly larger apparent sensitivity of NPP to CO₂ than that measured at the FACE experiment locations (13% per 100 ppm). Global NBP differs markedly among individual models, although the mean value of 2.0 ± 0.8 Pg C yr^?1 is remarkably close to the mean value of RLS (2.1 ± 1.2 Pg C yr^?1). The interannual variability in modeled NBP is significantly correlated with that of RLS for the period 1980–2009. Both model‐to‐model and interannual variation in model GPP is larger than that in model NBP due to the strong coupling causing a positive correlation between ecosystem respiration and GPP in the model. The average linear regression slope of global NBP vs. temperature across the 10 models is ?3.0 ± 1.5 Pg C yr^?1 °C^?1, within the uncertainty of what derived from RLS (?3.9 ± 1.1 Pg C yr^?1 °C^?1). However, 9 of 10 models overestimate the regression slope of NBP vs. precipitation, compared with the slope of the observed RLS vs. precipitation. With most models lacking processes that control GPP and NBP in addition to CO₂ and climate, the agreement between modeled and observation‐based GPP and NBP can be fortuitous. Carbon–nitrogen interactions (only separable in one model) significantly influence the simulated response of carbon cycle to temperature and atmospheric CO₂ concentration, suggesting that nutrients limitations should be included in the next generation of terrestrial biosphere models.     

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.     

Soil carbon dioxide partial pressure and dissolved inorganic carbonate chemistry under elevated carbon dioxide and ozone

Global emissions of atmospheric CO₂ and tropospheric O₃ are rising and expected to impact large areas of the Earths forests. While CO₂ stimulates net primary production, O₃ reduces photosynthesis, altering plant C allocation and reducing ecosystem C storage. The effects of multiple air pollutants can alter belowground C allocation, leading to changes in the partial pressure of CO₂ (pCO₂) in the soil , chemistry of dissolved inorganic carbonate (DIC) and the rate of mineral weathering. As this system represents a linkage between the long- and short-term C cycles and sequestration of atmospheric CO₂, changes in atmospheric chemistry that affect net primary production may alter the fate of C in these ecosystems. To date, little is known about the combined effects of elevated CO₂ and O₃ on the inorganic C cycle in forest systems. Free air CO₂ and O₃ enrichment (FACE) technology was used at the Aspen FACE project in Rhinelander, Wisconsin to understand how elevated atmospheric CO₂ and O₃ interact to alter pCO₂ and DIC concentrations in the soil. Ambient and elevated CO₂ levels were 360±16 and 542±81 l l^–1, respectively; ambient and elevated O₃ levels were 33±14 and 49±24 nl l^–1, respectively. Measured concentrations of soil CO₂ and calculated concentrations of DIC increased over the growing season by 14 and 22%, respectively, under elevated atmospheric CO₂ and were unaffected by elevated tropospheric O₃. The increased concentration of DIC altered inorganic carbonate chemistry by increasing system total alkalinity by 210%, likely due to enhanced chemical weathering. The study also demonstrated the close coupling between the seasonal ¹³C of soil pCO₂ and DIC, as a mixing model showed that new atmospheric CO₂ accounted for approximately 90% of the C leaving the system as DIC. This study illustrates the potential of using stable isotopic techniques and FACE technology to examine long- and short-term ecosystem C sequestration.     

Elevated CO2 increases microbial carbon substrate use and nitrogen cycling in Mojave Desert soils

Identifying soil microbial responses to anthropogenically driven environmental changes is critically important as concerns intensify over the potential degradation of ecosystem function. We assessed the effects of elevated atmospheric CO₂ on microbial carbon (C) and nitrogen (N) cycling in Mojave Desert soils using extracellular enzyme activities (EEAs), community‐level physiological profiles (CLPPs), and gross N transformation rates. Soils were collected from unvegetated interspaces between plants and under the dominant shrub (Larrea tridentata) during the 2004–2005 growing season, an above‐average rainfall year. Because most measured variables responded strongly to soil water availability, all significant effects of soil water content were used as covariates to remove potential confounding effects of water availability on microbial responses to experimental treatment effects of cover type, CO₂, and sampling date. Microbial C and N activities were lower in interspace soils compared with soils under Larrea, and responses to date and CO₂ treatments were cover specific. Over the growing season, EEAs involved in cellulose (cellobiohydrolase) and orthophosphate (alkaline phosphatase) degradation decreased under ambient CO₂, but increased under elevated CO₂. Microbial C use and substrate use diversity in CLPPs decreased over time, and elevated CO₂ positively affected both. Elevated CO₂ also altered microbial C use patterns, suggesting changes in the quantity and/or quality of soil C inputs. In contrast, microbial biomass N was higher in interspace soils than soils under Larrea, and was lower in soils exposed to elevated CO₂. Gross rates of NH₄⁺ transformations increased over the growing season, and late‐season NH₄⁺ fluxes were negatively affected by elevated CO₂. Gross NO₃⁻ fluxes decreased over time, with early season interspace soils positively affected by elevated CO₂. General increases in microbial activities under elevated CO₂ are likely attributable to greater microbial biomass in interspace soils, and to increased microbial turnover rates and/or metabolic levels rather than pool size in soils under Larrea. Because soil water content and plant cover type dominates microbial C and N responses to CO₂, the ability of desert landscapes to mitigate or intensify the impacts of global change will ultimately depend on how changes in precipitation and increasing atmospheric CO₂ shift the spatial distribution of Mojave Desert plant communities.     

Soil carbon sequestration in a pine forest after 9 years of atmospheric CO2 enrichment

The impact of anthropogenic CO₂ emissions on climate change may be mitigated in part by C sequestration in terrestrial ecosystems as rising atmospheric CO₂ concentrations stimulate primary productivity and ecosystem C storage. Carbon will be sequestered in forest soils if organic matter inputs to soil profiles increase without a matching increase in decomposition or leaching losses from the soil profile, or if the rate of decomposition decreases because of increased production of resistant humic substances or greater physical protection of organic matter in soil aggregates. To examine the response of a forest ecosystem to elevated atmospheric CO₂ concentrations, the Duke Forest Free‐Air CO₂ Enrichment (FACE) experiment in North Carolina, USA, has maintained atmospheric CO₂ concentrations 200 μL L^?1 above ambient in an aggrading loblolly pine (Pinus taeda) plantation over a 9‐year period (1996–2005). During the first 6 years of the experiment, forest‐floor C and N pools increased linearly under both elevated and ambient CO₂ conditions, with significantly greater accumulations under the elevated CO₂ treatment. Between the sixth and ninth year, forest‐floor organic matter accumulation stabilized and C and N pools appeared to reach their respective steady states. An additional C sink of ～30 g C m^?2 yr^?1 was sequestered in the forest floor of the elevated CO₂ treatment plots relative to the control plots maintained at ambient CO₂ owing to increased litterfall and root turnover during the first 9 years of the study. Because we did not detect any significant elevated CO₂ effects on the rate of decomposition or on the chemical composition of forest‐floor organic matter, this additional C sink was likely related to enhanced litterfall C inputs. We also failed to detect any statistically significant treatment effects on the C and N pools of surface and deep mineral soil horizons. However, a significant widening of the C : N ratio of soil organic matter (SOM) in the upper mineral soil under both elevated and ambient CO₂ suggests that N is being transferred from soil to plants in this aggrading forest. A significant treatment × time interaction indicates that N is being transferred at a higher rate under elevated CO₂ (P=0.037), suggesting that enhanced rates of SOM decomposition are increasing mineralization and uptake to provide the extra N required to support the observed increase in primary productivity under elevated CO₂.     

Influence of rhizodeposition under elevated CO2 on plant nutrition and soil organic matter

Atmospheric CO₂ concentrations can influence ecosystem carbon storage through net primary production (NPP), soil carbon storage, or both. In assessing the potential for carbon storage in terrestrial ecosystems under elevated CO₂, both NPP and processing of soil organic matter (SOM), as well as the multiple links between them, must be examined. Within this context, both the quantity and quality of carbon flux from roots to soil are important, since roots produce specialized compounds that enhance nutrient acquisition (affecting NPP), and since the flux of organic compounds from roots to soil fuels soil microbial activity (affecting processing of SOM).From the perspective of root physiology, a technique is described which uses genetically engineered bacteria to detect the distribution and amount of flux of particular compounds from single roots to non-sterile soils. Other experiments from several labs are noted which explore effects of elevated CO₂ on root acid phosphatase, phosphomonoesterase, and citrate production, all associated with phosphorus nutrition. From a soil perspective, effects of elevated CO₂ on the processing of SOM developed under a C4 grassland but planted with C3 California grassland species were examined under low (unamended) and high (amended with 20 g m^–2 NPK) nutrients; measurements of soil atmosphere 13C combined with soil respiration rates show that during vegetative growth in February, elevated CO₂ decreased respiration of carbon from C4 SOM in high nutrient soils but not in unamended soils.This emphasis on the impacts of carbon loss from roots on both NPP and SOM processing will be essential to understanding terrestrial ecosystem carbon storage under changing atmospheric CO₂ concentrations.Abbreviations SOM soil organic matter - NPP net primary productivity - NEP net ecosystem productivity - PNPP p-nitrophenyl phosphate     

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.     

Carbon and nitrogen pools and mineralization in a grassland gley soil under elevated carbon dioxide at a natural CO2 spring

The growth and chemical composition of most plants are influenced by elevated CO₂, but accompanying effects on soil organic matter pools and mineralization are less clearly defined, partly because of the short‐term nature of most studies. Herein we describe soil properties from a naturally occurring cold CO₂ spring (Hakanoa) in Northland, New Zealand, at which the surrounding vegetation has been exposed to elevated CO₂ for at least several decades. The mean annual temperature at this site is ≈ 15.5 °C and rainfall ≈ 1550 mm. The site was unfertilized and ungrazed, with a vegetation of mainly C3 and C4 grasses, and had moderate levels of ‘available’ P. Two soils were present ? a gley soil and an organic soil – but only the gley soil is examined here. Average atmospheric CO₂ concentrations at 17 sampling locations in the gley soil area ranged from 372 to 670 ppmv. In samples at 0–5 cm depth, pH averaged 5.4; average values for organic C were 150 g, total N 11 g, microbial C 3.50 g, and microbial N 0.65 g kg^?1, respectively. Under standardized moisture conditions at 25 °C, average rates of CO₂‐C production (7–14 days) were 5.4 mg kg^?1 h^?1 and of net mineral‐N production (14 ?42 days) 0.40 mg kg^?1 h^?1. These properties were all correlated positively and significantly (P < 0.10) with atmospheric CO₂ concentrations, but not with soil moisture (except for CO₂‐C production) or with clay content; they were, however, correlated negatively and mainly significantly with soil pH. In spite of uncertainties associated with the uncontrolled environment of naturally occurring springs, we conclude that storage of C and N can increase under prolonged exposure to elevated CO₂, and may include an appreciable labile fraction in mineral soil with an adequate nutrient supply.     

Elevated CO2 alters carbon fluxes in early successional Mediterranean ecosystems

Annual carbon budgets of ecosystems are central to our understanding of the biotic control of atmospheric composition, but they are not available under elevated CO₂ for most vegetation types. Using gas exchange techniques, we assessed carbon fluxes of four early successional Mediterranean model communities, consisting of grasses, legumes and composites. The assemblages were grown on the same monoliths for three consecutive years in greenhouses tracking field conditions except for CO₂ maintained at ambient (370 μmol mol^?1) or elevated (700 μmol mol^?1) concentration. During the third year of study, CO₂ enrichment consistently shifted the annual carbon balance towards lower efflux, with displacements between 4.3 and 26.2 mol m^?2 y^?1 (one assemblage became a net CO₂ sink, another just reached equilibrium, and the remaining two remained as a CO₂ source). At least 50% of the shift under elevated CO₂ originated from a decrease in belowground respiration. This indicates that, during this year, CO₂ enrichment did not predominantly enhance C‐cycling, but on the contrary inhibited root respiration or microbial C‐utilization. Although elevated‐CO₂‐grown systems acted as a net CO₂ sink during a longer period of the year (4–7 months) compared with ambient‐CO₂‐grown systems (3–3.5 months), gross canopy photosynthesis was modified only to a limited extent (between ?5.9 and + 14.8%). Interaction between the carbon and the water cycle was apparently responsible for this weak stimulation. In particular, reduced evapotranspiration under elevated CO₂ coincided with inhibited canopy photosynthesis in early spring, most likely resulting from water saturation of the soil. In addition, only the earliest‐planted assemblages had an increased gross canopy photosynthesis during late autumn and early winter. This suggests that a longer summer drought, by delaying the establishment of such an annual type of vegetation, would reduce the positive impact of elevated CO₂ on productivity. Water regime appears to strongly govern the influence of CO₂ on the carbon fluxes in Mediterranean ecosystems with annual herbaceous vegetation.     

Belowground competition and the response of developing forest communities to atmospheric CO2 and O3

As human activity continues to increase CO₂ and O₃, broad expanses of north temperate forests will be simultaneously exposed to elevated concentrations of these trace gases. Although both CO₂ and O₃ are potent modifiers of plant growth, we do not understand the extent to which they alter competition for limiting soil nutrients, like nitrogen (N). We quantified the acquisition of soil N in two 8‐year‐old communities composed of trembling aspen genotypes (n= 5) and trembling aspen–paper birch which were exposed to factorial combinations of CO₂ (ambient and 560 μL L⁻¹) and O₃ (ambient = 30–40 vs. 50–60 nL L⁻¹). Tracer amount of ¹⁵NH₄⁺ were applied to soil to determine how these trace gases altered the competitive ability of genotypes and species to acquire soil N. One year after isotope addition, we assessed N acquisition by measuring the amount of ¹⁵N tracer contained in the plant canopy (i.e. recent N acquisition), as well as the total amount of canopy N (i.e. cumulative N acquisition). Exposure to elevated CO₂ differentially altered recent and cumulative N acquisition among aspen genotypes, changing the rank order in which they obtained soil N. Elevated O₃ also altered the rank order in which aspen genotypes obtained soil N by eliciting increases, decreases and no response among genotypes. If aspen genotypes respond similarly under field conditions, then rising concentrations of CO₂ and O₃ could alter the structure of aspen populations. In the aspen–birch community, elevated CO₂ increased recent N (i.e. ¹⁵N) acquisition in birch (68%) to a greater extent than aspen (19%), suggesting that, over the course of this experiment, birch had gained a competitive advantage over aspen. The response of genotypes and species to rising CO₂ and O₃ concentrations, and how these responses are modified by competitive interactions, has the potential to change the future composition and productivity of northern temperate forests.     

Future carbon dioxide concentration decreases canopy evapotranspiration and soil water depletion by field‐grown maize

Maize, in rotation with soybean, forms the largest continuous ecosystem in temperate North America, therefore changes to the biosphere‐atmosphere exchange of water vapor and energy of these crops are likely to have an impact on the Midwestern US climate and hydrological cycle. As a C₄ crop, maize photosynthesis is already CO₂‐saturated at current CO₂ concentrations ([CO₂]) and the primary response of maize to elevated [CO₂] is decreased stomatal conductance (g_s). If maize photosynthesis is not stimulated in elevated [CO₂], then reduced g_s is not offset by greater canopy leaf area, which could potentially result in a greater ET reduction relative to that previously reported in soybean, a C₃ species. The objective of this study is to quantify the impact of elevated [CO₂] on canopy energy and water fluxes of maize (Zea mays). Maize was grown under ambient and elevated [CO₂] (550 μmol mol^?1 during 2004 and 2006 and 585 μmol mol^?1 during 2010) using Free Air Concentration Enrichment (FACE) technology at the SoyFACE facility in Urbana, Illinois. Maize ET was determined using a residual energy balance approach based on measurements of sensible (H) and soil heat fluxes, and net radiation. Relative to control, elevated [CO₂] decreased maize ET (7–11%; P < 0.01) along with lesser soil moisture depletion, while H increased (25–30 W m^?2; P < 0.01) along with higher canopy temperature (0.5–0.6 °C). This reduction in maize ET in elevated [CO₂] is approximately half that previously reported for soybean. A partitioning analysis showed that transpiration contributed less to total ET for maize compared to soybean, indicating a smaller role of stomata in dictating the ET response to elevated [CO₂]. Nonetheless, both maize and soybean had significantly decreased ET and increased H, highlighting the critical role of elevated [CO₂] in altering future hydrology and climate of the region that is extensively cropped with these species.     

Long‐term dynamics of mycorrhizal root tips in a loblolly pine forest grown with free‐air CO2 enrichment and soil N fertilization for 6 years

Large‐scale, long‐term FACE (Free‐Air CO₂ enrichment) experiments indicate that increases in atmospheric CO₂ concentrations will influence forest C cycling in unpredictable ways. It has been recently suggested that responses of mycorrhizal fungi could determine whether forest net primary productivity (NPP) is increased by elevated CO₂ over long time periods and if forests soils will function as sources or sinks of C in the future. We studied the dynamic responses of ectomycorrhizae to N fertilization and atmospheric CO₂ enrichment at the Duke FACE experiment using minirhizotrons over a 6 year period (2005–2010). Stimulation of mycorrhizal production by elevated CO₂ was observed during only 1 (2007) of 6 years. This increased the standing crop of mycorrhizal tips during 2007 and 2008; during 2008, significantly higher mortality returned standing crop to ambient levels for the remainder of the experiment. It is therefore unlikely that increased production of mycorrhizal tips can explain the lack of progressive nitrogen limitations and associated increases in N uptake observed in CO₂‐enriched plots at this site. Fertilization generally decreased tree reliance on mycorrhizae as tip production declined with the addition of nitrogen as has been shown in many other studies. Annual NPP of mycorrhizal tips was greatest during years with warm January temperatures and during years with cool spring temperatures. A 2 °C increase in average late spring temperatures (May and June) decreased annual production of mycorrhizal root tip length by 50%. This has important implications for ecosystem function in a warmer world in addition to potential for forest soils to sequester atmospheric C.     

Direct and indirect effects of elevated atmospheric CO2 on net ecosystem production in a Chesapeake Bay tidal wetland

The rapid increase in atmospheric CO₂ concentrations (C_a) has resulted in extensive research efforts to understand its impact on terrestrial ecosystems, especially carbon balance. Despite these efforts, there are relatively few data comparing net ecosystem exchange of CO₂ between the atmosphere and the biosphere (NEE), under both ambient and elevated C_a. Here we report data on annual sums of CO₂ (NEE_net) for 19 years on a Chesapeake Bay tidal wetland for Scirpus olneyi (C₃ photosynthetic pathway)‐ and Spartina patens (C₄ photosynthetic pathway)‐dominated high marsh communities exposed to ambient and elevated C_a (ambient + 340 ppm). Our objectives were to (i) quantify effects of elevated C_a on seasonally integrated CO₂ assimilation (NEE_net = NEE_day + NEE_night, kg C m^?2 y^?1) for the two communities; and (ii) quantify effects of altered canopy N content on ecosystem photosynthesis and respiration. Across all years, NEE_net averaged 1.9 kg m^?2 y^?1 in ambient C_a and 2.5 kg m^?2 y^?1 in elevated C_a, for the C₃‐dominated community. Similarly, elevated C_a significantly (P < 0.01) increased carbon uptake in the C₄‐dominated community, as NEE_net averaged 1.5 kg m^?2 y^?1 in ambient C_a and 1.7 kg m^?2 y^?1 in elevated C_a. This resulted in an average CO₂ stimulation of 32% and 13% of seasonally integrated NEE_net for the C₃‐ and C₄‐dominated communities, respectively. Increased NEE_day was correlated with increased efficiencies of light and nitrogen use for net carbon assimilation under elevated C_a, while decreased NEE_night was associated with lower canopy nitrogen content. These results suggest that rising C_a may increase carbon assimilation in both C₃‐ and C₄‐dominated wetland communities. The challenge remains to identify the fate of the assimilated carbon.     

Reconstruction and attribution of the carbon sink of European forests between 1950 and 2000

European forests are an important carbon sink; however, the relative contributions to this sink of climate, atmospheric CO₂ concentration ([CO₂]), nitrogen deposition and forest management are under debate. We attributed the European carbon sink in forests using ORCHIDEE‐FM, a process‐based vegetation model that differs from earlier versions of ORCHIDEE by its explicit representation of stand growth and idealized forest management. The model was applied on a grid across Europe to simulate changes in the net ecosystem productivity (NEP) of forests with and without changes in climate, [CO₂] and age structure, the three drivers represented in ORCHIDEE‐FM. The model simulates carbon stocks and volume increment that are comparable – root mean square error of 2 m³ ha^?1 yr^?1 and 1.7 kg C m^?2 respectively – with inventory‐derived estimates at country level for 20 European countries. Our simulations estimate a mean European forest NEP of 175 ± 52 g C m^?2 yr^?1 in the 1990s. The model simulation that is most consistent with inventory records provides an upwards trend of forest NEP of 1 ± 0.5 g C m^?2 yr^?2 between 1950 and 2000 across the EU 25. Furthermore, the method used for reconstructing past age structure was found to dominate its contribution to temporal trends in NEP. The potentially large fertilizing effect of nitrogen deposition cannot be told apart, as the model does not explicitly simulate the nitrogen cycle. Among the three drivers that were considered in this study, the fertilizing effect of increasing [CO₂] explains about 61% of the simulated trend, against 26% to changes in climate and 13% only to changes in forest age structure. The major role of [CO₂] at the continental scale is due to its homogeneous impact on net primary productivity (NPP). At the local scale, however, changes in climate and forest age structure often dominate trends in NEP by affecting NPP and heterotrophic respiration.     

Influence of carbon dioxide enrichment, ozone and nitrogen fertilization on cotton (Gossypium hirsutum L.) leaf and root composition

The objective of this study was to test whether elevated [CO₂], [O₃] and nitrogen (N) fertility altered leaf mass per area (LMPA), non‐structural carbohydrate (TNC), N, lignin (LTGA) and proanthocyanidin (PA) concentrations in cotton (Gossypium hirsutum L.) leaves and roots. Cotton was grown in 14 dm³ pots with either sufficient (0·8 g N dm ^{? 3}) or deficient (0·4 and 0·2 g N dm ^{? 3}) N fertilization, and treated in open‐top chambers with either ambient or elevated ( + 175 and + 350 μ mol mol ^{? 1}) [CO₂] in combination with either charcoal‐filtered air (CF) or non‐filtered air plus 1·5 times ambient [O₃]. At about 50 d after planting, LMPA, starch and PA concentrations in canopy leaves were as much as 51–72% higher in plants treated with elevated [CO₂] compared with plants treated with ambient [CO₂], whereas leaf N concentration was 29% lower in elevated [CO₂]‐treated plants compared with controls. None of the treatments had a major effect on LTGA concentrations on a TNC‐free mass basis. LMPA and starch levels were up to 48% lower in plants treated with elevated [O₃] and ambient [CO₂] compared with CF controls, although the elevated [O₃] effect was diminished when plants were treated concurrently with elevated [CO₂]. On a total mass basis, leaf N and PA concentrations were higher in samples treated with elevated [O₃] in ambient [CO₂], but the difference was much reduced by elevated [CO₂]. On a TNC‐free basis, however, elevated [O₃] had little effect on tissue N and PA concentrations. Fertilization treatments resulted in higher PA and lower N concentrations in tissues from the deficient N fertility treatments. The experiment showed that suppression by elevated [O₃] of LMPA and starch was largely prevented by elevated [CO₂], and that interpretation of [CO₂] and [O₃] effects should include comparisons on a TNC‐free basis. Overall, the experiment indicated that allocation to starch and PA may be related to how environmental factors affect source–sink relationships in plants, although the effects of elevated [O₃] on secondary metabolites differed in this respect.     

The turnover of carbon pools contributing to soil CO2 and soil respiration in a temperate forest exposed to elevated CO2 concentration

Soil carbon is returned to the atmosphere through the process of soil respiration, which represents one of the largest fluxes in the terrestrial C cycle. The effects of climate change on the components of soil respiration can affect the sink or source capacity of ecosystems for atmospheric carbon, but no current techniques can unambiguously separate soil respiration into its components. Long‐term free air CO₂ enrichment (FACE) experiments provide a unique opportunity to study soil C dynamics because the CO₂ used for fumigation has a distinct isotopic signature and serves as a continuous label at the ecosystem level. We used the ¹³C tracer at the Duke Forest FACE site to follow the disappearance of C fixed before fumigation began in 1996 (pretreatment C) from soil CO₂ and soil‐respired CO₂, as an index of belowground C dynamics during the first 8 years of the experiment. The decay of pretreatment C as detected in the isotopic composition of soil‐respired CO₂ and soil CO₂ at 15, 30, 70, and 200 cm soil depth was best described by a model having one to three exponential pools within the soil system. The majority of soil‐respired CO₂ (71%) originated in soil C pools with a turnover time of about 35 days. About 55%, 50%, and 68% of soil CO₂ at 15, 30, and 70 cm, respectively, originated in soil pools with turnover times of less than 1 year. The rest of soil CO₂ and soil‐respired CO₂ originated in soil pools that turn over at decadal time scales. Our results suggest that a large fraction of the C returned to the atmosphere through soil respiration results from dynamic soil C pools that cannot be easily detected in traditionally defined soil organic matter standing stocks. Fast oxidation of labile C substrates may prevent increases in soil C accumulation in forests exposed to elevated [CO₂] and may consequently result in shorter ecosystem C residence times.     

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.