Effects of elevated atmospheric CO2 on soil microbiota in calcareous grassland

Microbial responses to three years of CO₂ enrichment (600 μL L^–1) in the field were investigated in calcareous grassland. Microbial biomass carbon (C) and soil organic C and nitrogen (N) were not significantly influenced by elevated CO₂. Microbial C:N ratios significantly decreased under elevated CO₂ (– 15%, P = 0.01) and microbial N increased by + 18% (P = 0.04). Soil basal respiration was significantly increased on one out of 7 sampling dates (+ 14%, P = 0.03; December of the third year of treatment), whereas the metabolic quotient for CO₂ (qCO₂ = basal respiration/microbial C) did not exhibit any significant differences between CO₂ treatments. Also no responses of microbial activity and biomass were found in a complementary greenhouse study where intact grassland turfs taken from the field site were factorially treated with elevated CO₂ and phosphorus (P) fertilizer (1 g P m^–2 y^–1). Previously reported C balance calculations showed that in the ecosystem investigated growing season soil C inputs were strongly enhanced under elevated CO₂. It is hypothesized that the absence of microbial responses to these enhanced soil C fluxes originated from mineral nutrient limitations of microbial processes. Laboratory incubations showed that short-term microbial growth (one week) was strongly limited by N availability, whereas P was not limiting in this soil. The absence of large effects of elevated CO₂ on microbial activity or biomass in such nutrient-poor natural ecosystems is in marked contrast to previously published large and short-term microbial responses to CO₂ enrichment which were found in fertilized or disturbed systems. It is speculated that the absence of such responses in undisturbed natural ecosystems in which mineral nutrient cycles have equilibrated over longer periods of time is caused by mineral nutrient limitations which are ineffective in disturbed or fertilized systems and that therefore microbial responses to elevated CO₂ must be studied in natural, undisturbed systems.     

Maintenance of leaf N controls the photosynthetic CO2 response of grassland species exposed to 9 years of free‐air CO2 enrichment

Determining underlying physiological patterns governing plant productivity and diversity in grasslands are critical to evaluate species responses to future environmental conditions of elevated CO₂ and nitrogen (N) deposition. In a 9‐year experiment, N was added to monocultures of seven C₃ grassland species exposed to elevated atmospheric CO₂ (560 μmol CO₂ mol^?1) to evaluate how N addition affects CO₂ responsiveness in species of contrasting functional groups. Functional groups differed in their responses to elevated CO₂ and N treatments. Forb species exhibited strong down‐regulation of leaf N_mass concentrations (?26%) and photosynthetic capacity (?28%) in response to elevated CO₂, especially at high N supply, whereas C₃ grasses did not. Hence, achieved photosynthetic performance was markedly enhanced for C₃ grasses (+68%) in elevated CO₂, but not significantly for forbs. Differences in access to soil resources between forbs and grasses may distinguish their responses to elevated CO₂ and N addition. Forbs had lesser root biomass, a lower distribution of biomass to roots, and lower specific root length than grasses. Maintenance of leaf N, possibly through increased root foraging in this nutrient‐poor grassland, was necessary to sustain stimulation of photosynthesis under long‐term elevated CO₂. Dilution of leaf N and associated photosynthetic down‐regulation in forbs under elevated [CO₂], relative to the C₃ grasses, illustrates the potential for shifts in species composition and diversity in grassland ecosystems that have significant forb and grass components.     

Response of aboveground grassland biomass and soil moisture to moderate long-term CO2 enrichment

Rising atmospheric CO₂ concentrations may alter C cycling and community composition, however, long-term studies in (semi-)natural ecosystems are still rare. In May 1998, the Giessen FACE (Free Air Carbon dioxide Enrichment) experiment started in a grassland ecosystem near Giessen, Germany, consisting of three enrichment (E plots) and three ambient control rings (A plots). Carbon dioxide concentrations were raised to +20% above ambient all-year-round during daylight hours. The wet grassland (Arrhenatheretum elatioris Br.-Bl.; not ploughed for >100 years) has been fertilized with 40 kg ha⁻¹ yr⁻¹ N, and mown two times each year for decades. Since 1993, the biomass has been monitored and since 1997 it was divided into grasses, legumes and non-leguminous forbs.During the 5 years prior to CO₂ enrichment, the annual biomass yield from the A plots was non-significantly higher (3%) than the later E plots yield. Under CO₂ enrichment, the biomass increased significantly from the third enrichment year on by 9.8%, 7.7% and 11.2% in the years 2000–2002, respectively. The increase was surprisingly high considering the moderate CO₂ enrichment regime of only +20% and sub-optimal N supply, possibly suggesting a non-linear response of temperate grassland ecosystems to rising atmospheric CO₂ levels.The leaf area index did not change significantly under elevated CO₂, nor did the soil moisture in the top 15 cm increase. No correlation existed between the magnitude of the yield stimulation under elevated CO₂ and the precipitation sums preceding the respective harvests. The grass biomass increased significantly under FACE, while the forb biomass declined strongly in the fourth and fifth year. The legume fraction was mostly below 1% of the total yield, and did not respond to CO₂ enrichment. These findings are in contrast to other grassland results and possible reasons are discussed.     

Atmospheric CO2 elevation has little effect on nitrifying and denitrifying enzyme activity in four European grasslands

The objective of this study was to determine what patterns, if any, existed in the response of nitrifying enzyme activity (NEA), denitrifying enzyme activity (DEA), soil microbial N and soil inorganic N to elevated CO₂ across a broad range of grassland environments. We studied the response of these N pools and microbial activities in four CO₂‐enrichment sites of the MEGARICH project (Managing European Grasslands as a Sustainable Resource in a Changing Climate). CO₂ treatment was studied in factorial combination with a cutting frequency treatment at two sites and with a temperature treatment at one site. Our study showed that microbial biomass N, NEA, DEA and extractable soil [NH₄⁺] and [NO₃^?] were generally not affected by elevated CO₂ in these grassland ecosystems after several years of treatment, nor by cutting frequency or temperature at the sites that included these treatments. Exceptions to this were that DEA and soil [NO₃^?] decreased by 22% and 45%, respectively, at the French site at elevated CO₂. We discuss the possible explanations for this lack of response.     

Salinity and sea level mediate elevated CO2 effects on C3–C4 plant interactions and tissue nitrogen in a Chesapeake Bay tidal wetland

Elevated atmospheric carbon dioxide concentrations ([CO₂]) generally increase plant photosynthesis in C₃ species, but not in C₄ species, and reduce stomatal conductance in both C₃ and C₄ plants. In addition, tissue nitrogen concentration ([N]) often fails to keep pace with enhanced carbon gain under elevated CO₂, particularly in C₃ species. While these responses are well documented in many species, implications for plant growth and nutrient cycling in native ecosystems are not clear. Here we present data on 18 years of measurement of above and belowground biomass, tissue [N] and total standing crop of N for a Scirpus olneyi‐dominated (C₃ sedge) community, a Spartina patens‐dominated (C₄ grass) community and a C₃–C₄‐mixed species community exposed to ambient and elevated (ambient +340 ppm) atmospheric [CO₂] in natural salinity and sea level conditions of a Chesapeake Bay wetland. Increased biomass production (shoots plus roots) under elevated [CO₂] in the S. olneyi‐dominated community was sustained throughout the study, averaging approximately 35%, while no significant effect of elevated [CO₂] was found for total biomass in the C₄‐dominated community. We found a significant decline in C₄ biomass (correlated with rising sea level) and a concomitant increase in C₃ biomass in the mixed community. This shift from C₄ to C₃ was accelerated by the elevated [CO₂] treatment. The elevated [CO₂] stimulation of total biomass accumulation was greatest during rainy, low salinity years: the average increase above the ambient treatment during the three wettest years (1994, 1996, 2003) was 2.9 t ha⁻¹ but in the three driest years (1995, 1999, 2002), it was 1.2 t ha⁻¹. Elevated [CO₂] depressed tissue [N] in both species, but especially in the S. olneyi where the relative depression was positively correlated with salinity and negatively related with the relative enhancement of total biomass production. Thus, the greatest amount of carbon was added to the S. olneyi‐dominated community during years when shoot [N] was reduced the most, suggesting that the availability of N was not the most or even the main limitation to elevated [CO₂] stimulation of carbon accumulation in this ecosystem.     

A global comparison of grassland biomass responses to CO2 and nitrogen enrichment

Grassland ecosystems cover vast areas of the Earth''s surface and provide many ecosystem services including carbon (C) storage, biodiversity preservation and the production of livestock forage. Predicting the future delivery of these services is difficult, because widespread changes in atmospheric CO₂ concentration, climate and nitrogen (N) inputs are expected. We compiled published data from global change driver manipulation experiments and combined these with climate data to assess grassland biomass responses to CO₂ and N enrichment across a range of climates. CO₂ and N enrichment generally increased aboveground biomass (AGB) but effects of CO₂ enrichment were weaker than those of N. The response to N was also dependent on the amount of N added and rainfall, with a greater response in high precipitation regions. No relationship between response to CO₂ and climate was detected within our dataset, thus suggesting that other site characteristics, e.g. soils and plant community composition, are more important regulators of grassland responses to CO₂. A statistical model of AGB response to N was used in conjunction with projected N deposition data to estimate changes to future biomass stocks. This highlighted several potential hotspots (e.g. in some regions of China and India) of grassland AGB gain. Possible benefits for C sequestration and forage production in these regions may be offset by declines in plant biodiversity caused by these biomass gains, thus necessitating careful management if ecosystem service delivery is to be maximized. An approach such as ours, in which meta-analysis is combined with global scale model outputs to make large-scale predictions, may complement the results of dynamic global vegetation models, thus allowing us to form better predictions of biosphere responses to environmental change.     

Neutron radiography as a tool for revealing root development in soil: capabilities and limitations

Chronic atmospheric nitrogen deposition affects the cycling of carbon (C) and nitrogen (N) in forest ecosystems, and thereby alters the stable C isotopic abundance of plant and soil. Three successional stages, disturbed, rehabilitated and mature forests were studied for their responses to different nitrogen input levels. N-addition manipulative experiments were conducted at low, medium and high N levels. To study the responses of C cycling to N addition, the C concentration and ¹³C natural abundances for leaf, litter and soil were measured. Labile organic carbon fractions in mineral soils were measured to quantify the dynamics of soil organic C (SOC). Results showed that three-year continuous N addition did not significantly increase foliar C and N concentration, but decreased C/N ratio and enriched ¹³C in N-rich forests. In addition, N addition significantly decreased microbial biomass C, and increased water soluble organic C in surface soils of N-rich forests. This study suggests that N addition enhances the water consumption per unit C assimilation of dominant plant species, restricts SOC turnover in N-poor forests at early and medium successional stages (thus favored SOC sequestration), and vice versa for N-rich mature forests.     

A 3-year exposure to CO2 and O3 induced minor changes in soil N cycling in a meadow ecosystem

We investigated the individual and interactive effects of moderately elevated CO₂ (ambient air + 100 ppm) and/or O₃ (40–50 ppb) on soil N cycling and microbial biomass N in a 3-year open-top chamber experiment conducted in meadow mesocosms. The results show that elevated O₃ decreased the concentrations of mineral N and NH₄⁺-N in the mesocosm soil in the last growing season (2004). Total N, NO₃⁻-N, microbial biomass N, decomposition rate, potential nitrification and denitrification were not affected by elevated O₃ and/or CO₂. It is thus concluded that the proposed future ambient O₃ and CO₂ levels, such as used in this experiment, may not induce major changes in the below-ground N processes in N-poor northern European hay meadow ecosystems.     

The impact of elevated CO2, increased nitrogen availability and biodiversity on plant tissue quality and decomposition

Elevated CO₂, increased nitrogen (N) deposition and increasing species richness can increase net primary productivity (NPP). However, unless there are comparable changes in decomposition, increases in productivity will most likely be unsustainable. Without comparable increases in decomposition nutrients would accumulate in dead organic matter leading to nutrient limitations that could eventually prohibit additional increases in productivity. To address this issue, we measured aboveground plant and litter quality and belowground root quality, as well as decomposition of aboveground litter for one and 2‐year periods using in situ litterbags in response to a three‐way factorial manipulation of CO₂ (ambient vs. 560 ppm), N deposition (ambient vs. the addition of 4 g N m⁻² yr⁻¹) and plant species richness (one, four, nine and 16 species) in experimental grassland plots. Litter chemistry responded to the CO₂, N and plant diversity treatments, but decomposition was much less responsive. Elevated CO₂ induced decreases in % N and % lignin in plant tissues. N addition led to increases in % N and decreases in % lignin. Increasing plant diversity led to decreases in % N and % lignin and an increase in % cellulose. In contrast to the litter chemistry changes, elevated CO₂ had a much lower impact on decomposition and resulted in only a 2.5% decrease in carbon (C) loss. Detectable responses were not observed either to N addition or to species richness. These results suggest that global change factors such as biodiversity loss, elevated CO₂ and N deposition lead to significant changes in tissue quality; however, the response of decomposition is modest. Thus, the observed increases in productivity at higher diversity levels and with elevated CO₂ and N fertilization are not matched by an increase in decomposition rates. This lack of coupled responses between production and decomposition is likely to result in an accumulation of nutrients in the litter pool which will dampen the response of NPP to these factors over time.     

Water relations in grassland and desert ecosystems exposed to elevated atmospheric CO2

Atmospheric CO₂ enrichment may stimulate plant growth directly through (1) enhanced photosynthesis or indirectly, through (2) reduced plant water consumption and hence slower soil moisture depletion, or the combination of both. Herein we describe gas exchange, plant biomass and species responses of five native or semi-native temperate and Mediterranean grasslands and three semi-arid systems to CO₂ enrichment, with an emphasis on water relations. Increasing CO₂ led to decreased leaf conductance for water vapor, improved plant water status, altered seasonal evapotranspiration dynamics, and in most cases, periodic increases in soil water content. The extent, timing and duration of these responses varied among ecosystems, species and years. Across the grasslands of the Kansas tallgrass prairie, Colorado shortgrass steppe and Swiss calcareous grassland, increases in aboveground biomass from CO₂ enrichment were relatively greater in dry years. In contrast, CO₂-induced aboveground biomass increases in the Texas C₃/C₄ grassland and the New Zealand pasture seemed little or only marginally influenced by yearly variation in soil water, while plant growth in the Mojave Desert was stimulated by CO₂ in a relatively wet year. Mediterranean grasslands sometimes failed to respond to CO₂-related increased late-season water, whereas semiarid Negev grassland assemblages profited. Vegetative and reproductive responses to CO₂ were highly varied among species and ecosystems, and did not generally follow any predictable pattern in regard to functional groups. Results suggest that the indirect effects of CO₂ on plant and soil water relations may contribute substantially to experimentally induced CO₂-effects, and also reflect local humidity conditions. For landscape scale predictions, this analysis calls for a clear distinction between biomass responses due to direct CO₂ effects on photosynthesis and those indirect CO₂ effects via soil moisture as documented here.     

Root dynamics in a semi-natural grassland in relation to atmospheric carbon dioxide enrichment,soil water and shoot biomass

Plant responses to increasing atmospheric CO₂ concentrations have been studied intensively. However, the effects of elevated CO₂ on root dynamics, which is important for global carbon budgets as well as for nutrient cycling in ecosystems, has received much less attention. We used minirhizotrons inside open-top chambers to study the effects of elevated atmospheric carbon dioxide concentration on root dynamics in a nutrient-poor semi-natural grassland in central Sweden. We conducted our investigation over three consecutive growing seasons during which three treatments were applied at the site: Elevated (≈ 700 μmol mol^-1) and ambient (≈ 360 μmol mol^-1) chamber levels of CO₂ and a control, without a chamber. During 1997, a summer with two dry periods, the elevated treatment compared with ambient had 25% greater mean root counts, 65% greater above-ground biomass and 15% greater soil moisture. The chambers seemed responsible for changes in root dynamics, whereas the elevated CO₂ treatment in general increased the absolute sum of root counts compared with the ambient chamber. In 1998, a wet growing season, there were no significant differences in shoot biomass or root dynamics and both chamber treatments had lower soil moisture than the control. We found that as seasonal dryness increased, the ratio of elevated – ambient shoot biomass production increased while the root to shoot ratio decreased. We conclude that this grasslands response to elevated CO₂ is dependent on seasonal weather conditions and that CO₂ enrichment will most significantly increase production in such a grassland when under water stress. This revised version was published online in June 2006 with corrections to the Cover Date.     

Nitrogen response efficiency increased monotonically with decreasing soil resource availability: a case study from a semiarid grassland in northern China

The concept of nutrient use efficiency is central to understanding ecosystem functioning because it is the step in which plants can influence the return of nutrients to the soil pool and the quality of the litter. Theory suggests that nutrient efficiency increases unimodally with declining soil resources, but this has not been tested empirically for N and water in grassland ecosystems, where plant growth in these ecosystems is generally thought to be limited by soil N and moisture. In this paper, we tested the N uptake and the N use efficiency (NUE) of two Stipa species (S. grandis and S. krylovii) from 20 sites in the Inner Mongolia grassland by measuring the N content of net primary productivity (NPP). NUE is defined as the total net primary production per unit N absorbed. We further distinguished NUE from N response efficiency (NRE; production per unit N available). We found that NPP increased with soil N and water availability. Efficiency of whole-plant N use, uptake, and response increased monotonically with decreasing soil N and water, being higher on infertile (dry) habitats than on fertile (wet) habitats. We further considered NUE as the product of the N productivity (NP the rate of biomass increase per unit N in the plant) and the mean residence time (MRT; the ratio between the average N pool and the annual N uptake or loss). The NP and NUE of S. grandis growing usually in dry and N-poor habitats exceeded those of S. krylovii abundant in wet and N-rich habitats. NUE differed among sites, and was often affected by the evolutionary trade-off between NP and MRT, where plants and communities had adapted in a way to maximize either NP or MRT, but not both concurrently. Soil N availability and moisture influenced the community-level N uptake efficiency and ultimately the NRE, though the response to N was dependent on the plant community examined. These results show that soil N and water had exerted a great impact on the N efficiency in Stipa species. The intraspecific differences in N efficiency within both Stipa species along soil resource availability gradient may explain the differences in plant productivity on various soils, which will be conducive to our general understanding of the N cycling and vegetation dynamics in northern Chinese grasslands.     

Due to symbiotic N2 fixation,five years of elevated atmospheric pCO2 had no effect on the N concentration of plant litter in fertile,mixed grassland

Experimental findings indicate that, in terrestrial ecosystems, nitrogen cycling changes under elevated partial pressure of atmospheric CO₂ (pCO₂). It was suggested that the concentration of N in plant litter as well as the amount of litter are responsible for these changes. However, for grassland ecosystems, there have been no relevant data available to support this hypothesis. Data from five years of the Swiss FACE experiment show that, under fertile soil conditions in a binary plant community consisting of Lolium perenne L. and Trifolium repens L., the concentration of litter N does not change under elevated atmospheric pCO₂; this applies to harvest losses, stubble, stolons and roots as the sources of litter. This is in strong contrast to the CO₂ response of L. perenne swards without associated legumes; in this case the above-ground concentration of biomass N decreased substantially. Increased symbiotic N₂ fixation in T. repens nodules and a greater proportion of the N-rich T. repens in the community are regarded as the main mechanisms that buffer the increased C introduction into the ecosystem under elevated atmospheric pCO₂. Our data also suggest that elevated atmospheric pCO₂ results in greater amounts of litter, mainly due to increased root biomass production. This study indicates that, in a fertile grassland ecosystem with legumes, the concentration of N in plant litter is not affected by elevated atmospheric pCO₂ and, thus, cannot explain CO₂-induced changes in the cycling of N. This revised version was published online in June 2006 with corrections to the Cover Date.     

Glycine uptake in heath plants and soil microbes responds to elevated temperature, CO2 and drought

Temperate terrestrial ecosystems are currently exposed to climatic and air quality changes with increased atmospheric CO₂, increased temperature and prolonged droughts. The responses of natural ecosystems to these changes are focus for research, due to the potential feedbacks to the climate. We here present results from a field experiment in which the effects of these three climate change factors are investigated solely and in all combinations at a temperate heath dominated by heather (Calluna vulgaris) and wavy hair-grass (Deschampsia flexuosa).Climate induced increases in plant production may increase plant root exudation of dissolved organic compounds such as amino acids, and the release of amino acids during decomposition of organic matter. Such free amino acids in soil serve as substrates for soil microorganisms and are also acquired as nutrients directly by plants. We investigated the magnitude of the response to the potential climate change treatments on uptake of organic nitrogen in an in situ pulse labelling experiment with ¹⁵N¹³C₂-labelled glycine (amino acid) injected into the soil.In situ root nitrogen acquisition by grasses responded significantly to the climate change treatments, with larger ¹⁵N uptake in response to warming and elevated CO₂ but not additively when the treatments were combined. Also, a larger grass leaf biomass in the combined T and CO₂ treatment than in individual treatments suggest that responses to combined climate change factors cannot be predicted from the responses to single factors treatments.The soil microbes were superior to plants in the short-term competition for the added glycine, as indicated by an 18 times larger ¹⁵N recovery in the microbial biomass compared to the plant biomass. The soil microbes acquired glycine largely as an intact compound (87%), with no effects of the multi factorial climate change treatment through one year.     

Interactions between CO2 enhancement and N addition on net primary productivity and water‐use efficiency in a mesocosm with multiple subtropical tree species

Carbon dioxide (CO₂) enhancement (eCO₂) and N addition (aN) have been shown to increase net primary production (NPP) and to affect water‐use efficiency (WUE) for many temperate ecosystems, but few studies have been made on subtropical tree species. This study compared the responses of NPP and WUE from a mesocosm composing five subtropical tree species to eCO₂ (700 ppm), aN (10 g N m^?2 yr^?1) and eCO₂ × aN using open‐top chambers. Our results showed that mean annual ecosystem NPP did not changed significantly under eCO₂, increased by 56% under aN and 64% under eCO₂ × aN. Ecosystem WUE increased by 14%, 55%, and 61% under eCO₂, aN and eCO₂ × aN, respectively. We found that the observed responses of ecosystem WUE were largely driven by the responses of ecosystem NPP. Statistical analysis showed that there was no significant interactions between eCO₂ and aN on ecosystem NPP (P = 0.731) or WUE (P = 0.442). Our results showed that increasing N deposition was likely to have much stronger effects on ecosystem NPP and WUE than increasing CO₂ concentration for the subtropical forests. However, different tree species responded quite differently. aN significantly increased annual NPP of the fast‐growing species (Schima superba). Nitrogen‐fixing species (Ormosia pinnata) grew significantly faster only under eCO₂ × aN. eCO₂ had no effects on annual NPP of those two species but significantly increased annual NPP of other two species (Castanopsis hystrix and Acmena acuminatissima). Differential responses of the NPP among different tree species to eCO₂ and aN will likely have significant implications on the species composition of subtropical forests under future global change.     

Long-term effects of elevated atmospheric CO<Subscript>2</Subscript> on species composition and productivity of a southern African C<Subscript>4</Subscript> dominated grassland in the vicinity of a CO<Subscript>2</Subscript> exhalation

We describe the long-term effects of a CO₂ exhalation, created more than 70 years ago, on a natural C₄ dominated sub-tropical grassland in terms of ecosystem structure and functioning. We tested whether long-term CO₂ enrichment changes the competitive balance between plants with C₃ and C₄ photosynthetic pathways and how CO₂ enrichment has affected species composition, plant growth responses, leaf properties and soil nutrient, carbon and water dynamics. Long-term effects of elevated CO₂ on plant community composition and system processes in this sub-tropical grassland indicate very subtle changes in ecosystem functioning and no changes in species composition and dominance which could be ascribed to elevated CO₂ alone. Species compositional data and soil δ¹³C isotopic evidence suggest no detectable effect of CO₂ enrichment on C₃:C₄ plant mixtures and individual species dominance. Contrary to many general predictions C₃ grasses did not become more abundant and C₃ shrubs and trees did not invade the site. No season length stimulation of plant growth was found even after 5 years of exposure to CO₂ concentrations averaging 610 μmol mol⁻¹. Leaf properties such as total N decreased in the C₃ but not C₄ grass under elevated CO₂ while total non-structural carbohydrate accumulation was not affected. Elevated CO₂ possibly lead to increased end-of-season soil water contents and this result agrees with earlier studies despite the topographic water gradient being a confounding problem at our research site. Long-term CO₂ enrichment also had little effect on soil carbon storage with no detectable changes in soil organic matter found. There were indications that potential soil respiration and N mineralization rates could be higher in soils close to the CO₂ source. The conservative response of this grassland suggests that many of the reported effects of elevated CO₂ on similar ecosystems could be short duration experimental artefacts that disappear under long-term elevated CO₂ conditions.     

Responses of soil microbiota of a late successional alpine grassland to long term CO2 enrichment

We investigated microbial responses in a late successional sedge-dominated alpine grassland to four seasons of CO₂ enrichment. Part of the plots received fertilizer equivalent to 4.5g N m⁻² a⁻¹. Soil basal respiration (R _mic ), the metabolic quotient for CO₂ (qCO₂=R _mic /C _mic ), microbial C and N (C _mic and N _mic ) as well as total soil organic C and N showed no response to CO₂ enrichment alone. However, when the CO₂ treatment was combined with fertilizer addition R _mic and qCO₂ were statistically significantly higher under elevated CO₂ than under ambient conditions (+57% and +71%, respectively). Fertilizer addition increased microbial N pools by 17%, but this was not influenced by elevated CO₂. Microbial C was neither affected by elevated CO₂ nor fertilizer. The lack of a CO₂-effect in unfertilized plots was suprising in the light of our evidence (based on C balance) that enhanced soil C inputs must have occurred under elevated CO₂ regardless of fertilizer treatment. Based on these data and other published work we suggest that microbial responses to elevated CO₂ in such stable, late-successional ecosystems are limited by the availability of mineral nutrients and that results obtained with fertile or heavily disturbed substrates are unsuitable to predict future microbial responses to elevated CO₂ in natural systems. However, when nutrient limitation is removed (e.g. by wet nitrogen deposition) microbes make use of the additional carbon introduced into the soil system. We believe that the response of natural ecosystems to elevated CO₂ must be studied in situ in natural, undisturbed systems.     

Modeled responses of terrestrial ecosystems to elevated atmospheric CO2: a comparison of simulations by the biogeochemistry models of the Vegetation/Ecosystem Modeling and Analysis Project (VEMAP)

Although there is a great deal of information concerning responses to increases in atmospheric CO₂ at the tissue and plant levels, there are substantially fewer studies that have investigated ecosystem-level responses in the context of integrated carbon, water, and nutrient cycles. Because our understanding of ecosystem responses to elevated CO₂ is incomplete, modeling is a tool that can be used to investigate the role of plant and soil interactions in the response of terrestrial ecosystems to elevated CO₂. In this study, we analyze the responses of net primary production (NPP) to doubled CO₂ from 355 to 710 ppmv among three biogeochemistry models in the Vegetation/Ecosystem Modeling and Analysis Project (VEMAP): BIOME-BGC (BioGeochemical Cycles), Century, and the Terrestrial Ecosystem Model (TEM). For the conterminous United States, doubled atmospheric CO₂ causes NPP to increase by 5% in Century, 8% in TEM, and 11% in BIOME-BGC. Multiple regression analyses between the NPP response to doubled CO₂ and the mean annual temperature and annual precipitation of biomes or grid cells indicate that there are negative relationships between precipitation and the response of NPP to doubled CO₂ for all three models. In contrast, there are different relationships between temperature and the response of NPP to doubled CO₂ for the three models: there is a negative relationship in the responses of BIOME-BGC, no relationship in the responses of Century, and a positive relationship in the responses of TEM. In BIOME-BGC, the NPP response to doubled CO₂ is controlled by the change in transpiration associated with reduced leaf conductance to water vapor. This change affects soil water, then leaf area development and, finally, NPP. In Century, the response of NPP to doubled CO₂ is controlled by changes in decomposition rates associated with increased soil moisture that results from reduced evapotranspiration. This change affects nitrogen availability for plants, which influences NPP. In TEM, the NPP response to doubled CO₂ is controlled by increased carboxylation which is modified by canopy conductance and the degree to which nitrogen constraints cause down-regulation of photosynthesis. The implementation of these different mechanisms has consequences for the spatial pattern of NPP responses, and represents, in part, conceptual uncertainty about controls over NPP responses. Progress in reducing these uncertainties requires research focused at the ecosystem level to understand how interactions between the carbon, nitrogen, and water cycles influence the response of NPP to elevated atmospheric CO₂. Received: 13 December 1996 / Accepted: 20 November 1997     

Contrasting growth response of an N2-fixing and non-fixing forb to elevated CO2: dependence on soil N supply

With the ability to symbiotically fix atmospheric N₂, legumes may lack the N-limitations thought to constrain plant response to elevated concentrations of atmospheric CO₂. The growth and photosynthetic responses of two perennial grassland species were compared to test the hypotheses that (1) the CO₂ response of wild species is limited at low N availability, (2) legumes respond to a greater extent than non-fixing forbs to elevated CO₂, and (3) elevated CO₂ stimulates symbiotic N₂ fixation, resulting in an increased amount of N derived from the atmosphere. This study investigated the effects of atmospheric CO₂ concentration (365 and 700 mol mol^–1) and N addition on whole plant growth and C and N acquisition in an N₂-fixing legume (Lupinus perennis) and a non-fixing forb (Achillea millefolium) in controlled-chamber environments. To evaluate the effects of a wide range of N availability on the CO₂ response, we incorporated six levels of soil N addition starting with native field soil inherently low in N (field soil + 0, 4, 8, 12, 16, or 20 g N m^–2 yr^–1). Whole plant growth, leaf net photosynthetic rates (A), and the proportion of N derived from N₂ fixation were determined in plants grown from seed over one growing season. Both species increased growth with CO₂enrichment, but this response was mediated by N supply only for the non-fixer, Achillea. Its response depended on mineral N supply as growth enhancements under elevated CO₂ increased from 0% in low N soil to +25% at the higher levels of N addition. In contrast, Lupinus plants had 80% greater biomass under elevated CO₂ regardless of N treatment. Although partial photosynthetic acclimation to CO₂ enrichment occurred, both species maintained comparably higher A in elevated compared to ambient CO₂ (+38%). N addition facilitated increased A in Achillea, however, in neither species did additional N availability affect the acclimation response of A to CO₂. Elevated CO₂ increased plant total N yield by 57% in Lupinus but had no effect on Achillea. The increased N in Lupinus came from symbiotic N₂ fixation, which resulted in a 47% greater proportion of N derived from fixation relative to other sources of N. These results suggest that compared to non-fixing forbs, N₂-fixers exhibit positive photosynthetic and growth responses to increased atmospheric CO₂ that are independent of soil N supply. The enhanced amount of N derived from N₂ fixation under elevated CO₂ presumably helps meet the increased N demand in N₂-fixing species. This response may lead to modified roles of N₂-fixers and N₂-fixer/non-fixer species interactions in grassland communities, especially those that are inherently N-poor, under projected rising atmospheric CO₂.     

Increasing CO2 from subambient to elevated concentrations increases grassland respiration per unit of net carbon fixation

Respiration (carbon efflux) by terrestrial ecosystems is a major component of the global carbon (C) cycle, but the response of C efflux to atmospheric CO₂ enrichment remains uncertain. Respiration may respond directly to an increase in the availability of C substrates at high CO₂, but also may be affected indirectly by a CO₂‐mediated alteration in the amount by which respiration changes per unit of change in temperature or C uptake (sensitivity of respiration to temperature or C uptake). We measured CO₂ fluxes continuously during the final 2 years of a 4‐year experiment on C₃/C₄ grassland that was exposed to a 200–560 μmol mol^?1 CO₂ gradient. Flux measurements were used to determine whether CO₂ treatment affected nighttime respiration rates and the response of ecosystem respiration to seasonal changes in net C uptake and air temperature. Increasing CO₂ from subambient to elevated concentrations stimulated grassland respiration at night by increasing the net amount of C fixed during daylight and by increasing either the sensitivity of C efflux to daily changes in C fixation or the respiration rate in the absence of C uptake (basal ecosystem respiration rate). These latter two changes contributed to a 30–47% increase in the ratio of nighttime respiration to daytime net C influx as CO₂ increased from subamient to elevated concentrations. Daily changes in net C uptake were highly correlated with variation in temperature, meaning that the shared contribution of C uptake and temperature in explaining variance in respiration rates was large. Statistically controlling for collinearity between temperature and C uptake reduced the effect of a given change in C influx on respiration. Conversely, CO₂ treatment did not affect the response of grassland respiration to seasonal variation in temperature. Elevating CO₂ concentration increased grassland respiration rates by increasing both net C input and respiration per unit of C input. A better understanding of how C efflux varies with substrate supply thus may be required to accurately assess the C balance of terrestrial ecosystems.