Loss of plant biodiversity eliminates stimulatory effect of elevated CO2 on earthworm activity in grasslands

Earthworms are among the world’s most important ecosystem engineers because of their effects on soil fertility and plant productivity. Their dependence on plants for carbon, however, means that any changes in plant community structure or function caused by rising atmospheric CO₂ or loss of plant species diversity could affect earthworm activity, which may feed back on plant communities. Production of surface casts measured during three consecutive years in field experimental plots (n = 24, 1.2 m²) planted with local calcareous grassland species that varied in plant species richness (diversity levels: high, 31 species; medium, 12; low, 5) and were exposed to ambient (356 μl CO₂ l^?1) or elevated (600 μl CO₂ l^?1) CO₂ was only consistently stimulated in high diversity plots exposed to elevated CO₂ (+120 %, 31 spp: 603 ± 52 under ambient CO₂ vs. 1,325 ± 204 g cast dwt. m^?2 year^?1 under elevated CO₂ in 1996; +77 %, 940 ± 44 vs. 1,663 ± 204 g cast dwt. m^?2 year^?1 in 1998). Reductions in plant diversity had little effect on cast production in ecosystems maintained at ambient CO₂, but the stimulatory effect of elevated CO₂ on cast production disappeared when plant species diversity was decreased to 12 and 5 species. High diversity plots were also the only communities that included plant species that an earlier field study showed to be among the most responsive to elevated CO₂ and to be most preferred by earthworms to deposit casts near. Further, the +87 % CO₂-induced increase in cast production measured over the 3 years corresponded to a parallel increase in cumulative total nitrogen of 5.7 g N m^?2 and would help explain the large stimulation of aboveground plant biomass production observed in high-diversity communities under elevated CO₂. The results of this study demonstrate how the loss of plant species from communities can alter responses of major soil heterotrophs and consequently ecosystem biogeochemistry.     

Stability of above-ground and below-ground processes to extreme drought in model grassland ecosystems: Interactions with plant species diversity and soil nitrogen availability

Extreme drought events have the potential to cause dramatic changes in ecosystem structure and function, but the controls upon ecosystem stability to drought remain poorly understood. Here we used model systems of two commonly occurring, temperate grassland communities to investigate the short-term interactive effects of a simulated 100-year summer drought event, soil nitrogen (N) availability and plant species diversity (low/high) on key ecosystem processes related to carbon (C) and N cycling. Whole ecosystem CO₂ fluxes and leaching losses were recorded during drought and post-rewetting. Litter decomposition and C/N stocks in vegetation, soil and soil microbes were assessed 4 weeks after the end of drought. Experimental drought caused strong reductions in ecosystem respiration and net ecosystem CO₂ exchange, but ecosystem fluxes recovered rapidly following rewetting irrespective of N and species diversity. As expected, root C stocks and litter decomposition were adversely affected by drought across all N and plant diversity treatments. In contrast, drought increased soil water retention, organic nutrient leaching losses and soil fertility. Drought responses of above-ground vegetation C stocks varied depending on plant diversity, with greater stability of above-ground vegetation C to drought in the high versus low diversity treatment. This positive effect of high plant diversity on above-ground vegetation C stability coincided with a decrease in the stability of microbial biomass C. Unlike species diversity, soil N availability had limited effects on the stability of ecosystem processes to extreme drought. Overall, our findings indicate that extreme drought events promote post-drought soil nutrient retention and soil fertility, with cascading effects on ecosystem C fixation rates. Data on above-ground ecosystem processes underline the importance of species diversity for grassland function in a changing environment. Furthermore, our results suggest that plant–soil interactions play a key role for the short-term stability of above-ground vegetation C storage to extreme drought events.     

A field study of the effects of elevated CO2 on plant biomass and community structure in a calcareous grassland

The effects of elevated CO₂ on plant biomass and community structure have been studied for four seasons in a calcareous grassland in northwest Switzerland. This highly diverse, semi-natural plant community is dominated by the perennial grass Bromus erectus and is mown twice a year to maintain species composition. Plots of 1.3 m² were exposed to ambient or elevated CO₂ concentrations (n = 8) using a novel CO₂ exposure technique, screen-aided CO₂ control (SACC) starting in March 1994. In the 1st year of treatment, the annual harvested biomass (sum of aboveground biomass from mowings in June and October) was not significantly affected by elevated CO₂. However, biomass increased significantly at elevated CO₂ in the 2nd (+20%, P = 0.05), 3rd (+21%, P = 0.02) and 4th years (+29%, P = 0.02). There were no detectable differences in root biomass in the top 8 cm of soil between CO₂ treatments on eight out of nine sampling dates. There were significant differences in CO₂ responsiveness between functional groups (legumes, non-leguminous forbs, graminoids) in the 2nd (P = 0.07) and 3rd (P < 0.001) years of the study. The order of CO₂ responsiveness among functional groups changed substantially from the 2nd to the 3rd year; for example, non-leguminous forbs had the smallest relative response in the 2nd year and the largest in the 3rd year. By the 3rd year of CO₂ exposure, large species-specific differences in CO₂ response had developed. For five important species or genera the order of responsiveness was Lotus corniculatus (+271%), Carex flacca (+249%), Bromus erectus (+33%), Sanguisorba minor (no significant CO₂ effect), and six Trifolium species (a negative response that was not significant). The positive CO₂ responses in Bromus and Carex were most closely related to increases in tiller number. Species richness was not affected by CO₂ treatment, but species evenness increased under elevated CO₂ (modified Hill ratio; P = 0.03) in June of the 3rd year, resulting in a marginally significant increase in species diversity (Simpson's index; P = 0.09). This and other experiments with calcareous grassland plants show that elevated atmospheric CO₂ concentrations can substantially alter the structure of calcareous grassland communities and may increase plant community biomass. Received: 12 July 1997 / Accepted: 14 September 1998     

Effects of elevated [CO2] at the community level mediated by root symbionts

This review examines the effects of elevated [CO₂] on plant symbioses with mycorrhizal fungi and root nodule bacteria, with emphasis on community and ecosystem processes. The effects of elevated [CO₂] on the relationships between single plant species and root symbionts are considered first. There is some evidence that plant infection by and/or biomass of root symbionts are stimulated by elevated [CO₂], but growth enhancement of the host seemingly depends on its degree of dependence on symbiosis and on soil nutrient availability. Second, the effects of elevated [CO₂] on the relationships between plant multispecies assemblages and soil, and likely impacts on above-ground and belowground diversity, are analysed. Experimental and modelling work have suggested the existence of complex feedbacks in the responses of plants and the rhizosphere to CO₂ enrichment. By modifying C inputs from plants to soil, elevated [CO₂] may affect the biomass, the infectivity, and the species/isolate composition of root symbionts. This has the potential to alter community structure and ecosystem functioning. Finally, the incorporation of type and degree of symbiotic dependence into the definition of plant functional types, and into experimental work within the context of global change research, are discussed. More experimental work on the effects of elevated [CO₂] at the community/ecosystem level, explicitly considering the role of root symbioses, is urgently needed.     

The type of competition modulates the ecophysiological response of grassland species to elevated CO2 and drought

The effects of elevated CO₂ and drought on ecophysiological parameters in grassland species have been examined, but few studies have investigated the effect of competition on those parameters under climate change conditions. The objective of this study was to determine the effect of elevated CO₂ and drought on the response of plant water relations, gas exchange, chlorophyll a fluorescence and aboveground biomass in four grassland species, as well as to assess whether the type of competition modulates that response. Elevated CO₂ in well‐watered conditions increased aboveground biomass by augmenting CO₂ assimilation. Drought reduced biomass by reducing CO₂ assimilation rate via stomatal limitation and, when drought was more severe, also non‐stomatal limitation. When plants were grown under the combined conditions of elevated CO₂ and drought, drought limitation observed under ambient CO₂ was reduced, permitting higher CO₂ assimilation and consequently reducing the observed decrease in aboveground biomass. The response to climate change was species‐specific and dependent on the type of competition. Thus, the response to elevated CO₂ in well‐watered grasses was higher in monoculture than in mixture, while it was higher in mixture compared to monoculture for forbs. On the other hand, forbs were more affected than grasses by drought in monoculture, while in mixture the negative effect of drought was higher in grasses than in forbs, due to a lower capacity to acquire water and mineral nutrients. These differences in species‐level growth responses to CO₂ and drought may lead to changes in the composition and biodiversity of the grassland plant community in future climate conditions.     

Nutrient relations in calcareous grassland under elevated CO2

Plant nutrient responses to 4 years of CO₂ enrichment were investigated in situ in calcareous grassland. Beginning in year 2, plant aboveground C:N ratios were increased by 9% to 22% at elevated CO₂ (P < 0.01), depending on year. Total amounts of N removed in biomass harvests during the first 4 years were not affected by elevated CO₂ (19.9 ± 1.3 and 21.1 ± 1.3 g N m⁻² at ambient and elevated CO₂), indicating that the observed plant biomass increases were solely attained by dilution of nutrients. Total aboveground P and tissue N:P ratios also were not altered by CO₂ enrichment (12.5 ± 2 g N g⁻¹ P in both treatments). In contrast to non-legumes (>98% of community aboveground biomass), legume C/N was not reduced at elevated CO₂ and legume N:P was slightly increased. We attribute the less reduced N concentration in legumes at elevated CO₂ to the fact that virtually all legume N originated from symbiotic N₂ fixation (%N_dfa ≈ 90%), and thus legume growth was not limited by soil N. While total plant N was not affected by elevated CO₂, microbial N pools increased by +18% under CO₂ enrichment (P = 0.04) and plant available soil N decreased. Hence, there was a net increase in the overall biotic N pool, largely due increases in the microbial N pool. In order to assess the effects of legumes for ecosystem CO₂ responses and to estimate the degree to which plant growth was P-limited, two greenhouse experiments were conducted, using firstly undisturbed grassland monoliths from the field site, and secondly designed `microcosm' communities on natural soil. Half the microcosms were planted with legumes and half were planted without. Both monoliths and microcosms were exposed to elevated CO₂ and P fertilization in a factored design. After two seasons, plant N pools in both unfertilized monoliths and microcosm communities were unaffected by CO₂ enrichment, similar to what was found in the field. However, when P was added total plant N pools increased at elevated CO₂. This community-level effect originated almost solely from legume stimulation. The results suggest a complex interaction between atmospheric CO₂ concentrations, N and P supply. Overall ecosystem productivity is N-limited, whereas CO₂ effects on legume growth and their N₂ fixation are limited by P. Received: 12 July 1997 / Accepted: 15 April 1998     

Effects of elevated CO2, nitrogen deposition, and decreased species diversity on foliar fungal plant disease

Three components of global change, elevated CO₂, nitrogen addition, and decreased plant species richness (‘diversity’), increased the percent leaf area infected by fungi (pathogen load) for much to all of the plant community in one year of a factorial grassland experiment. Decreased plant diversity had the broadest effect, increasing pathogen load across the plant community. Decreased diversity increased pathogen load primarily by allowing remaining plant species to increase in abundance, facilitating spread of foliar fungal pathogens specific to each plant species. Changes in plant species composition also strongly influenced community pathogen load, with communities that lost less disease prone plant species increasing more in pathogen load. Elevated CO₂ increased pathogen load of C₃ grasses, perhaps by decreasing water stress, increasing leaf longevity, and increasing photosynthetic rate, all of which can promote foliar fungal disease. Decreased plant diversity further magnified the increase in C₃ grass pathogen load under elevated CO₂. Nitrogen addition increased pathogen load of C₄ grasses by increasing foliar nitrogen concentration, which can enhance pathogen infection, growth, and reproduction. Because changes in foliar fungal pathogen load can strongly influence grassland ecosystem processes, our study suggests that increased pathogen load can be an important mechanism by which global change affects grassland ecosystems.     

Insects affect relationships between plant species richness and ecosystem processes

This study examined whether insects can alter relationships between plant species diversity and ecosystem function in grassland communities, by (i) altering biomass across a plant diversity gradient, (ii) altering relative abundances of plant species, or (iii) altering ecosystem function directly. We measured herbivore damage on seminatural grassland plots planted with 1, 2, 4, 8, or 12 plant species, and compared plant biomass in a subset of these plots with replicates in which insect levels were reduced. Plant biomass and herbivore damage increased with species richness. Reducing insect populations resulted in greater evenness of relative plant species abundances and revealed a strong positive relationship between plant species richness and above-ground biomass. Reducing insects also changed the relationship between plant species richness and decomposition. Plant species mixtures and their relative abundances partially explained plant biomass results, but not decomposition results. These results suggest that insects can alter relationships between plant diversity and ecosystem processes through all three mechanisms.     

Interactive Effects of Time, CO2, N, and Diversity on Total Belowground Carbon Allocation and Ecosystem Carbon Storage in a Grassland Community

Predicting if ecosystems will mitigate or exacerbate rising CO₂ requires understanding how elevated CO₂ will interact with coincident changes in diversity and nitrogen (N) availability to affect ecosystem carbon (C) storage. Yet achieving such understanding has been hampered by the difficulty of quantifying belowground C pools and fluxes. Thus, we used mass balance calculations to quantify the effects of diversity, CO₂, and N on both the total amount of C allocated belowground by plants (total belowground C allocation, TBCA) and ecosystem C storage in a periodically burned, 8-year Minnesota grassland biodiversity, CO₂, and N experiment (BioCON). Annual TBCA increased in response to elevated CO₂, enriched N, and increasing diversity. TBCA was positively related to standing root biomass. After removing the influence of root biomass, the effect of elevated CO₂ remained positive, suggesting additional drivers of TBCA apart from those that maintain high root biomass. Removing root biomass effects resulted in the effects of N and diversity becoming neutral or negative (depending on year), suggesting that the positive effects of diversity and N on TBCA were related to treatment-driven differences in root biomass. Greater litter production in high diversity, elevated CO₂, and enhanced N treatments increased annual ecosystem C loss in fire years and C gain in non-fire years, resulting in overall neutral C storage rates. Our results suggest that frequently burned grasslands are unlikely to exhibit enhanced C sequestration with increasing atmospheric CO₂ levels or N deposition.     

Effects of Warming, Summer Drought, and CO2 Enrichment on Aboveground Biomass Production, Flowering Phenology, and Community Structure in an Upland Grassland Ecosystem

Future climate scenarios predict simultaneous changes in environmental conditions, but the impacts of multiple climate change drivers on ecosystem structure and function remain unclear. We used a novel experimental approach to examine the responses of an upland grassland ecosystem to the 2080 climate scenario predicted for the study area (3.5°C temperature increase, 20% reduction in summer precipitation, atmospheric CO₂ levels of 600 ppm) over three growing seasons. We also assessed whether patterns of grassland response to a combination of climate change treatments could be forecast by ecosystem responses to single climate change drivers. Effects of climate change on aboveground production showed considerable seasonal and interannual variation; April biomass increased in response to both warming and the simultaneous application of warming, summer drought, and CO₂ enrichment, whereas October biomass responses were either non-significant or negative depending on the year. Negative impacts of summer drought on production were only observed in combination with a below-average rainfall regime, and showed lagged effects on spring biomass. Elevated CO₂ had no significant effect on aboveground biomass during this study. Both warming and the 2080 climate change scenario were associated with a significant advance in flowering time for the dominant grass species studied. However, flowering phenology showed no significant response to either summer drought or elevated CO₂. Species diversity and equitability showed no response to climate change treatments throughout this study. Overall, our data suggest that single-factor warming experiments may provide valuable information for projections of future ecosystem changes in cool temperate grasslands.     

The response of soil CO2 flux to changes in atmospheric CO2, nitrogen supply and plant diversity

We measured soil CO₂ flux over 19 sampling periods that spanned two growing seasons in a grassland Free Air Carbon dioxide Enrichment (FACE) experiment that factorially manipulated three major anthropogenic global changes: atmospheric carbon dioxide (CO₂) concentration, nitrogen (N) supply, and plant species richness. On average, over two growing seasons, elevated atmospheric CO₂ and N fertilization increased soil CO₂ flux by 0.57 µmol m^?2 s^?1 (13% increase) and 0.37 µmol m^?2 s^?1 (8% increase) above average control soil CO₂ flux, respectively. Decreases in planted diversity from 16 to 9, 4 and 1 species decreased soil CO₂ flux by 0.23, 0.41 and 1.09 µmol m^?2 s^?1 (5%, 8% and 21% decreases), respectively. There were no statistically significant pairwise interactions among the three treatments. During 19 sampling periods that spanned two growing seasons, elevated atmospheric CO₂ increased soil CO₂ flux most when soil moisture was low and soils were warm. Effects on soil CO₂ flux due to fertilization with N and decreases in diversity were greatest at the times of the year when soils were warm, although there were no significant correlations between these effects and soil moisture. Of the treatments, only the N and diversity treatments were correlated over time; neither were correlated with the CO₂ effect. Models of soil CO₂ flux will need to incorporate ecosystem CO₂ and N availability, as well as ecosystem plant diversity, and incorporate different environmental factors when determining the magnitude of the CO₂, N and diversity effects on soil CO₂ flux.     

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.     

Transpiration of a 64-year old maritime pine stand in Portugal

Alpine plant species have been shown to exhibit a more pronounced increase in leaf photosynthesis under elevated CO₂ than lowland plants. In order to test whether this higher carbon fixation efficiency will translate into increased biomass production under CO₂ enrichment we exposed plots of narrow alpine grassland (Swiss Central Alps, 2470 m) to ambient (355 μl l^-1) and elevated (680 μl l^-1) CO₂ concentration using open top chambers. Part of the plost received moderate mineral nutrient additions (40 kg ha^-1 year^-1 of nitrogen in a complete fertilizer mix). Under natural nutrient supply CO₂ enrichment had no effect on biomass production per unit land area during any of the three seasons studied so far. Correspondingly, the dominant species Carex curvula and Leontodon helveticus as well as Trifolium alpinum did not show a growth response either at the population level or at the shoot level. However, the subdominant generalistic species Poa alpina strongly increased shoot growth (+47%). Annual root production (in ingrowth cores) was significantly enhanced in C. curvula in the 2nd and 3rd year of investigation (+43%) but was not altered in the bulk samples for all species. Fertilizer addition generally stimulated above-ground (+48%) and below-ground (+26%) biomass production right from the beginning. Annual variations in weather conditions during summer also strongly influenced above-ground biomass production (19–27% more biomass in warm seasons compared to cool seasons). However, neither nutrient availability nor climate had a significant effect on the CO₂ response of the plants. Our results do not support the hypothesis that alpine plants, due to their higher carbon uptake efficiency, will increase biomass production under future atmospheric CO₂ enrichment, at least not in such late successional communities. However, as indicated by the response of P. alpina, species-specific responses occur which may lead to altered community structure and perhaps ecosystem functioning in the long-term. Our findings further suggest that possible climatic changes are likely to have a greater impact on plant growth in alpine environments than the direct stimulation of photosynthesis by CO₂. Counter-intuitively, our results suggest that even under moderate climate warming or enhanced atmospheric nitrogen deposition positive biomass responses to CO₂ enrichment of the currently dominating species are unlikely.     

Helianthus maximiliani and species fine‐scale spatial pattern affect diversity interactions in reconstructed tallgrass prairies

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Rising sea level,temperature, and precipitation impact plant and ecosystem responses to elevated CO2 on a Chesapeake Bay wetland: review of a 28‐year study

An ongoing field study of the effects of elevated atmospheric CO₂ on a brackish wetland on Chesapeake Bay, started in 1987, is unique as the longest continually running investigation of the effects of elevated CO₂ on an ecosystem. Since the beginning of the study, atmospheric CO₂ increased 18%, sea level rose 20 cm, and growing season temperature varied with approximately the same range as predicted for global warming in the 21st century. This review looks back at this study for clues about how the effects of rising sea level, temperature, and precipitation interact with high atmospheric CO₂ to alter the physiology of C3 and C4 photosynthetic species, carbon assimilation, evapotranspiration, plant and ecosystem nitrogen, and distribution of plant communities in this brackish wetland. Rising sea level caused a shift to higher elevations in the Scirpus olneyi C3 populations on the wetland, displacing the Spartina patens C4 populations. Elevated CO₂ stimulated carbon assimilation in the Scirpus C3 species measured by increased shoot and root density and biomass, net ecosystem production, dissolved organic and inorganic carbon, and methane production. But elevated CO₂ also decreased biomass of the grass, S. patens C4. The elevated CO₂ treatment reduced tissue nitrogen concentration in shoots, roots, and total canopy nitrogen, which was associated with reduced ecosystem respiration. Net ecosystem production was mediated by precipitation through soil salinity: high salinity reduced the CO₂ effect on net ecosystem production, which was zero in years of severe drought. The elevated CO₂ stimulation of shoot density in the Scirpus C3 species was sustained throughout the 28 years of the study. Results from this study suggest that rising CO₂ can add substantial amounts of carbon to ecosystems through stimulation of carbon assimilation, increased root exudates to supply nitrogen fixation, reduced dark respiration, and improved water and nitrogen use efficiency.     

No cumulative effect of 10 years of elevated [CO2] on perennial plant biomass components in the Mojave Desert

Elevated atmospheric CO₂ concentrations ([CO₂]) generally increase primary production of terrestrial ecosystems. Production responses to elevated [CO₂] may be particularly large in deserts, but information on their long‐term response is unknown. We evaluated the cumulative effects of elevated [CO₂] on primary production at the Nevada Desert FACE (free‐air carbon dioxide enrichment) Facility. Aboveground and belowground perennial plant biomass was harvested in an intact Mojave Desert ecosystem at the end of a 10‐year elevated [CO₂] experiment. We measured community standing biomass, biomass allocation, canopy cover, leaf area index (LAI), carbon and nitrogen content, and isotopic composition of plant tissues for five to eight dominant species. We provide the first long‐term results of elevated [CO₂] on biomass components of a desert ecosystem and offer information on understudied Mojave Desert species. In contrast to initial expectations, 10 years of elevated [CO₂] had no significant effect on standing biomass, biomass allocation, canopy cover, and C : N ratios of above‐ and belowground components. However, elevated [CO₂] increased short‐term responses, including leaf water‐use efficiency (WUE) as measured by carbon isotope discrimination and increased plot‐level LAI. Standing biomass, biomass allocation, canopy cover, and C : N ratios of above‐ and belowground pools significantly differed among dominant species, but responses to elevated [CO₂] did not vary among species, photosynthetic pathway (C₃ vs. C₄), or growth form (drought‐deciduous shrub vs. evergreen shrub vs. grass). Thus, even though previous and current results occasionally show increased leaf‐level photosynthetic rates, WUE, LAI, and plant growth under elevated [CO₂] during the 10‐year experiment, most responses were in wet years and did not lead to sustained increases in community biomass. We presume that the lack of sustained biomass responses to elevated [CO₂] is explained by inter‐annual differences in water availability. Therefore, the high frequency of low precipitation years may constrain cumulative biomass responses to elevated [CO₂] in desert environments.     

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.     

Disentangling species and functional group richness effects on soil N cycling in a grassland ecosystem

Species richness (SR) and functional group richness (FGR) are often confounded in both observational and experimental field studies of biodiversity and ecosystem function. This precludes discernment of their separate influences on ecosystem processes, including nitrogen (N) cycling, and how those influences might be moderated by global change factors. In a 17‐year field study of grassland species, we used two full factorial experiments to independently vary SR (one or four species, with FGR = 1) and FGR (1–4 groups, with SR = 4) to assess SR and FGR effects on ecosystem N cycling and its response to elevated carbon dioxide (CO₂) and N addition. We hypothesized that increased plant diversity (either SR or FGR) and elevated CO₂ would enhance plant N pools because of greater plant N uptake, but decrease soil N cycling rates because of greater soil carbon inputs and microbial N immobilization. In partial support of these hypotheses, increasing SR or FGR (holding the other constant) enhanced total plant N pools and decreased soil nitrate pools, largely through higher root biomass, and increasing FGR strongly reduced mineralization rates, because of lower root N concentrations. In contrast, increasing SR (holding FGR constant and despite increasing total plant C and N pools) did not alter root N concentrations or net N mineralization rates. Elevated CO₂ had minimal effects on plant and soil N metrics and their responses to plant diversity, whereas enriched N increased plant and soil N pools, but not soil N fluxes. These results show that functional diversity had additional effects on both plant N pools and rates of soil N cycling that were independent of those of species richness.     

The role of plant species in biomass production and response to elevated CO2 and N

In an experiment that factorially manipulated plant diversity, CO₂, and N, we quantified the effects of the presence of species on assemblage biomass over 10 time points distributed over 5 years. Thirteen of the 16 species planted had statistically significant effects on aboveground and/or belowground biomass. Species differed dramatically in their effects on biomass without any relationship between aboveground and below‐ground effects. Temporal complementarity among species in their effects seasonally, successionally, and in response to a dry summer maintained the diversity–biomass relationships over time and may be the cause behind higher diversity plots having less variation in biomass over time. The response of plant biomass to elevated N, but not CO₂, was at times entirely dependent on the presence of a single species.     

Transpiration of a 64-year-old maritime pine stand in Portugal

Alpine plant species have been shown to exhibit a more pronounced increase in leaf photosynthesis under elevated CO₂ than lowland plants. In order to test whether this higher carbon fixation efficiency will translate into increased biomass production under CO₂ enrichment we exposed plots of narrow alpine grassland (Swiss Central Alps, 2470 m) to ambient (355 l l^-1) and elevated (680 l l^-1) CO₂ concentration using open top chambers. Part of the plost received moderate mineral nutrient additions (40 kg ha^-1 year^-1 of nitrogen in a complete fertilizer mix). Under natural nutrient supply CO₂ enrichment had no effect on biomass production per unit land area during any of the three seasons studied so far. Correspondingly, the dominant species Carex curvula and Leontodon helveticus as well as Trifolium alpinum did not show a growth response either at the population level or at the shoot level. However, the subdominant generalistic species Poa alpina strongly increased shoot growth (+47%). Annual root production (in ingrowth cores) was significantly enhanced in C. curvula in the 2nd and 3rd year of investigation (+43%) but was not altered in the bulk samples for all species. Fertilizer addition generally stimulated above-ground (+48%) and below-ground (+26%) biomass production right from the beginning. Annual variations in weather conditions during summer also strongly influenced above-ground biomass production (19–27% more biomass in warm seasons compared to cool seasons). However, neither nutrient availability nor climate had a significant effect on the CO₂ response of the plants. Our results do not support the hypothesis that alpine plants, due to their higher carbon uptake efficiency, will increase biomass production under future atmospheric CO₂ enrichment, at least not in such late successional communities. However, as indicated by the response of P. alpina, species-specific responses occur which may lead to altered community structure and perhaps ecosystem functioning in the long-term. Our findings further suggest that possible climatic changes are likely to have a greater impact on plant growth in alpine environments than the direct stimulation of photosynthesis by CO₂. Counter-intuitively, our results suggest that even under moderate climate warming or enhanced atmospheric nitrogen deposition positive biomass responses to CO₂ enrichment of the currently dominating species are unlikely.