Root dynamics and demography in shortgrass steppe under elevated CO2, and comments on minirhizotron methodology

The dynamics and demography of roots were followed for 5 years that spanned wet and drought periods in native, semiarid shortgrass steppe grassland exposed to ambient and elevated atmospheric CO₂ treatments. Elevated compared with ambient CO₂ concentrations resulted in greater root‐length growth (+52%), root‐length losses (+37%), and total pool sizes (+41%). The greater standing pool of roots under elevated compared with ambient CO₂ was because of the greater number of roots (+35%), not because individuals were longer. Loss rates increased relatively less than growth rates because life spans were longer (+41%). The diameter of roots was larger under elevated compared with ambient CO₂ only in the upper soil profile. Elevated CO₂ affected root architecture through increased branching. Growth‐to‐loss ratio regressions to time of equilibrium indicate very long turnover times of 5.8, 7.0, and 5.3 years for control, ambient, and elevated CO₂, respectively. Production was greater under elevated compared with ambient CO₂ both below‐ and aboveground, and the above‐ to belowground ratios did not differ between treatments. However, estimates of belowground production differed among methods of calculation using minirhizotron data, as well as between minirhizotron and root‐ingrowth methods. Users of minirhizotrons may need to consider equilibration in terms of both new growth and disappearance, rather than just growth. Large temporal pulses of root initiation and termination rates of entire individuals were observed (analogous to birth–death rates), and precipitation explained more of the variance in root initiation than termination. There was a dampening of the pulsing in root initiation and termination under elevated CO₂ during both wet and dry periods, which may be because of conservation of soil water reducing the suddenness of wet pulses and duration and severity of dry pulses. However, a very low degree of synchrony was observed between growth and disappearance (production and decomposition).     

Asymmetry in above‐ and belowground productivity responses to N addition in a semi‐arid temperate steppe

Nitrogen (N) enrichment often increases aboveground net primary productivity (ANPP) of the ecosystem, but it is unclear if belowground net primary productivity (BNPP) track responses of ANPP. Moreover, the frequency of N inputs may affect primary productivity but is rarely studied. To assess the response patterns of above‐ and belowground productivity to rates of N addition under different addition frequencies, we manipulated the rate (0–50 g N m^?2 year^?1) and frequency (twice vs. monthly additions per year) of NH₄NO₃ inputs for six consecutive years in a temperate grassland in northern China and measured ANPP and BNPP from 2012 to 2014. In the low range of N addition rates, BNPP showed the greatest negative response and ANPP showed the greatest positive responses with increases in N addition (<10 g N m^?2 year^?1). As N addition increased beyond 10 g N m^?2 year^?1, increases in ANPP dampened and decreases in BNPP ceased altogether. The response pattern of net primary productivity (combined above‐ and belowground; NPP) corresponded more closely to ANPP than to BNPP. The N effects on BNPP and BNPP/NPP (f_BNPP) were not dependent on N addition frequency in the range of N additions typically associated with N deposition. BNPP was more sensitive to N addition frequency than ANPP, especially at low rates of N addition. Our findings provide new insights into how plants regulate carbon allocation to different organs with increasing N rates and changing addition frequencies. These root response patterns, if incorporated into Earth system models, may improve the predictive power of C dynamics in dryland ecosystems in the face of global atmospheric N deposition.     

The effects of elevated atmospheric CO<Subscript>2</Subscript> and nitrogen amendments on subsurface CO<Subscript>2</Subscript> production and concentration dynamics in a maturing pine forest

Profiles of subsurface soil CO₂ concentration, soil temperature, and soil moisture, and throughfall were measured continuously during the years 2005 and 2006 in 16 locations at the free air CO₂ enrichment facility situated within a temperate loblolly pine (Pinus taeda L.) stand. Sampling at these locations followed a 4 by 4 replicated experimental design comprised of two atmospheric CO₂ concentration levels (ambient [CO₂]^a, ambient + 200 ppmv, [CO₂]^e) and two soil nitrogen (N) deposition levels (ambient, ambient + fertilization at 11.2 g_N m⁻² year⁻¹). The combination of these measurements permitted indirect estimation of belowground CO₂ production and flux profiles in the mineral soil. Adjacent to the soil CO₂ profiles, direct (chamber-based) measurements of CO₂ fluxes from the soil–litter complex were simultaneously conducted using the automated carbon efflux system. Based on the measured soil CO₂ profiles, neither [CO₂]^e nor N fertilization had a statistically significant effect on seasonal soil CO₂, CO₂ production, and effluxes from the mineral soil over the study period. Soil moisture and temperature had different effects on CO₂ concentration depending on the depth. Variations in CO₂ were mostly explained by soil temperature at deeper soil layers, while water content was an important driver at the surface (within the first 10 cm), where CO₂ pulses were induced by rainfall events. The soil effluxes were equal to the CO₂ production for most of the time, suggesting that the site reached near steady-state conditions. The fluxes estimated from the CO₂ profiles were highly correlated to the direct measurements when the soil was neither very dry nor very wet. This suggests that a better parameterization of the soil CO₂ diffusivity is required for these soil moisture extremes.     

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.     

Growth,photosynthesis, and metabolism of sugar beet at an early stage of exposure to elevated CO<Subscript>2</Subscript>

The effects of CO₂ concentration (C _a) on growth, photosynthesis, and the activity of enzymes associated with the translocation and assimilation of CO₂ were studied in sugar beet (Beta vulgaris L. subsp. saccharifera, cv. Ramonskaya) plants. The plants were grown in controlled-climate chamber to the stage of 3–4 leaves and then used in experiments. Experimental plants were exposed in boxes to doubled C _a (700 µl/l, 2C plants), whereas control plants were kept in a chamber with ambient atmosphere (350 µl/l, 1C plants). As compared with 1C plants, in 3 and 8 days, the leaf area of 2C plants increased by 14 and 26%, respectively. The rate of their photosynthesis (P _n) measured in 3, 6, and 8 days increased by 85, 47, and 52%, respectively, whereas in normal air, the values of P _n in 2C plants were by 12, 19, and 15% lower than in 1C plants. After 8-day growth, the content of soluble carbohydrates in the leaves of 2C plants attained 7.2%, being by 80% greater than in 1C plants; the content of starch did not exceed 3%. The total content of chlorophylls a and b in the leaves of 2C plants was by 14% greater than in 1C plants, but their ratio was essentially the same. The level of protein in 2C plants was by 13.4% lower than in 1C plants. The activity and content of Rubisco in 1C and 2C plants were similar. As compared with 1C plants, in 2C plants the activity of soluble carbonic anhydrase (sCA) was lower by 34% in 3 days and by 18% in 8 days; the activity of carbonic anhydrase of membrane preparations (mCA), was lower by 24 and 77%, respectively. Catalase activity in 2C plants became by 8% lower than in 1C plants only after 8 days. A reduction in the photosynthetic ability of 2C plants in ambient atmosphere, a decrease in activity of sCA and, especially, of mCA observed together with invariable activity and content of Rubisco in the leaf extracts are interpreted as early symptoms of acclimation of young plants of sugar beet to elevated CO₂.Translated from Fiziologiya Rastenii, Vol. 52, No. 2, 2005, pp. 184–190.Original Russian Text Copyright © 2005 by Ignatova, Novichkova, Mudrik, Lyubimov, Ivanov, Romanova.This revised version was published online in April 2005 with a corrected cover date.     

On-line estimation of O<Subscript>2</Subscript> production,CO<Subscript>2</Subscript> uptake,and growth kinetics of microalgal cultures in a gas-tight photobioreactor

Growth of the green algae Chlamydomonas reinhardtii and Chlorella sp. in batch cultures was investigated in a novel gas-tight photobioreactor, in which CO₂, H₂, and N₂ were titrated into the gas phase to control medium pH, dissolved oxygen partial pressure, and headspace pressure, respectively. The exit gas from the reactor was circulated through a loop of tubing and re-introduced into the culture. CO₂ uptake was estimated from the addition of CO₂ as acidic titrant and O₂ evolution was estimated from titration by H₂, which was used to reduce O₂ over a Pd catalyst. The photosynthetic quotient, PQ, was estimated as the ratio between O₂ evolution and CO₂ up-take rates. NH₄ ⁺, NO₂ ⁻, or NO₃ ⁻ was the final cell density limiting nutrient. Cultures of both algae were, in general, characterised by a nitrogen sufficient growth phase followed by a nitrogen depleted phase in which starch was the major product. The estimated PQ values were dependent on the level of oxidation of the nitrogen source. The PQ was 1 with NH₄ ⁺ as the nitrogen source and 1.3 when NO₃ ⁻ was the nitrogen source. In cultures grown on all nitrogen sources, the PQ value approached 1 when the nitrogen source was depleted and starch synthesis became dominant, to further increase towards 1.3 over a period of 3–4 days. This latter increase in PQ, which was indicative of production of reduced compounds like lipids, correlated with a simultaneous increase in the degree of reduction of the biomass. When using the titrations of CO₂ and H₂ into the reactor headspace to estimate the up-take of CO₂, the production of O₂, and the PQ, the rate of biomass production could be followed, the stoichiometrical composition of the produced algal biomass could be estimated, and different growth phases could be identified.     

Effects of Herbivory,Fire and N<Subscript>2</Subscript>-fixation on Nutrient Limitation in a Humid African Savanna

The quantities and spatial distribution of nutrients in savanna ecosystems are affected by many factors, of which fire, herbivory and symbiotic N₂-fixation are particularly important. We measured soil nitrogen (N) pools and the relative abundance of N and phosphorus (P) in herbaceous vegetation in five vegetation types in a humid savanna in Tanzania. We also performed a factorial fertilization experiment to investigate which nutrients most limit herbaceous production. N pools in the top 10 cm of soil were low at sites where fires were frequent, and higher in areas with woody legume encroachment, or high herbivore excretion. Biomass production was co-limited by N and P at sites that were frequently burnt or heavily grazed by native herbivores. In contrast, aboveground production was limited by N in areas receiving large amounts of excreta from livestock. N₂-fixation by woody legumes did not lead to P-limitation, but did increase the availability of N relative to P. We conclude that the effects of fire, herbivory and N₂-fixation upon soil N pools and N:P-stoichiometry in savanna ecosystems are, to a large extent, predictable. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users. Author Contributions  P.C., H.O.V. and P.E. designed the study and wrote the paper. P.C. and T.K. performed the research and analyzed the data.     

Growth,biomass production,and assimilatory characters in <Emphasis Type="Italic">Cenchrus ciliaris</Emphasis> L. Under elevated CO<Subscript>2</Subscript> condition

The effect of elevated carbon dioxide (600±50 cm³ m⁻³; C₆₀₀) on growth performance, biomass production, and photosynthesis of Cenchrus ciliaris L. cv. 3108 was studied. This crop responded significantly by plant height, leaf length and width, and biomass production under C₆₀₀. Leaf area index increased triple fold in the crops grown in the open top chamber with C₆₀₀. The biomass production in term of fresh and dry biomass accumulation increased by 134.35 (fresh) and 193.34 (dry) % over the control (C₃₆₀) condition where the crops were grown for 20 d. The rate of photosynthesis and stomatal conductance increased by 24.51 and 46.33 %, respectively, in C₆₀₀ over C₃₆₀ plants. In comparison with C₃₆₀, the rate of transpiration decreased by 6.8 % under C₆₀₀. Long-term exposure (120 d) to C₆₀₀ enhanced photosynthetic water use efficiency by 34 %. Also the contents of chlorophylls a and b significantly increased in C₆₀₀. Thus C. ciliaris grown in C₆₀₀ throughout the crop season may produce more fodder in terms of green biomass.     

Performance of a generalist grasshopper on a C<Subscript>3</Subscript> and a C<Subscript>4</Subscript> grass: compensation for the effects of elevated CO<Subscript>2</Subscript> on plant nutritional quality

The increasing CO₂ concentration in Earths atmosphere is expected to cause a greater decline in the nutritional quality of C₃ than C₄ plants. As a compensatory response, herbivorous insects may increase their feeding disproportionately on C₃ plants. These hypotheses were tested by growing the grasses Lolium multiflorum C₃) and Bouteloua curtipendula C₄) at ambient (370 ppm) and elevated (740 ppm) CO₂ levels in open top chambers in the field, and comparing the growth and digestive efficiencies of the generalist grasshopper Melanoplus sanguinipes on each of the four plant × CO₂ treatment combinations. As expected, the nutritional quality of the C₃ grass declined to a greater extent than did that of the C₄ grass at elevated CO₂; protein levels declined in the C₃ grass, while levels of carbohydrates (sugar, fructan and starch) increased. However, M. sanguinipes did not significantly increase its consumption rate to compensate for the lower nutritional quality of the C₃ grass grown under elevated CO₂. Instead, these grasshoppers appear to use post-ingestive mechanisms to maintain their growth rates on the C₃ grass under elevated CO₂. Consumption rates of the C₃ and C₄ grasses were also similar, demonstrating a lack of compensatory feeding on the C₄ grass. We also examined the relative efficiencies of nutrient utilization from a C₃ and C₄ grass by M. sanguinipes to test the basis for the C₄ plant avoidance hypothesis. Contrary to this hypothesis, neither protein nor sugar was digested with a lower efficiency from the C₄ grass than from the C₃ grass. A novel finding of this study is that fructan, a potentially large carbohydrate source in C₃ grasses, is utilized by grasshoppers. Based on the higher nutrient levels in the C₃ grass and the better growth performance of M. sanguinipes on this grass at both CO₂ levels, we conclude that C₃ grasses are likely to remain better host plants than C₄ grasses in future CO₂ conditions.     

Linkages of plant stoichiometry to ecosystem production and carbon fluxes with increasing nitrogen inputs in an alpine steppe

Unprecedented levels of nitrogen (N) have entered terrestrial ecosystems over the past century, which substantially influences the carbon (C) exchange between the atmosphere and biosphere. Temperature and moisture are generally regarded as the major controllers over the N effects on ecosystem C uptake and release. N‐phosphorous (P) stoichiometry regulates the growth and metabolisms of plants and soil organisms, thereby affecting many ecosystem C processes. However, it remains unclear how the N‐induced shift in the plant N:P ratio affects ecosystem production and C fluxes and its relative importance. We conducted a field manipulative experiment with eight N addition levels in a Tibetan alpine steppe and assessed the influences of N on aboveground net primary production (ANPP), gross ecosystem productivity (GEP), ecosystem respiration (ER), and net ecosystem exchange (NEE); we used linear mixed‐effects models to further determine the relative contributions of various factors to the N‐induced changes in these parameters. Our results showed that the ANPP, GEP, ER, and NEE all exhibited nonlinear responses to increasing N additions. Further analysis demonstrated that the plant N:P ratio played a dominate role in shaping these C exchange processes. There was a positive relationship between the N‐induced changes in ANPP (ΔANPP) and the plant N:P ratio (ΔN:P), whereas the ΔGEP, ΔER, and ΔNEE exhibited quadratic correlations with the ΔN:P. In contrast, soil temperature and moisture were only secondary predictors for the changes in ecosystem production and C fluxes along the N addition gradient. These findings highlight the importance of plant N:P ratio in regulating ecosystem C exchange, which is crucial for improving our understanding of C cycles under the scenarios of global N enrichment.     

Interactive effects of elevated CO<Subscript>2</Subscript>, N deposition and climate change on plant litter quality in a California annual grassland

Although global changes can alter ecosystem nutrient dynamics indirectly as a result of their effects on plant litter quality, the interactive effects of global changes on plant litter remain largely unexplored in natural communities. We investigated the effects of elevated CO₂, N deposition, warming and increased precipitation on the composition of organic compounds in plant litter in a fully-factorial experiment conducted in a California annual grassland. While lignin increased within functional groups under elevated CO₂, this effect was attenuated by warming in grasses and by water additions in forbs. CO₂-induced increases in lignin within functional groups also were counteracted by an increase in the relative biomass of forbs, which contained less lignin than grasses. Consequently, there was no net change in the overall lignin content of senesced tissue at the plot level under elevated CO₂. Nitrate additions increased N in both grass and forb litter, although this effect was attenuated by water additions. Relative to changes in N within functional groups, changes in functional group dominance had a minor effect on overall litter N at the plot level. Nitrate additions had the strongest effect on decomposition, increasing lignin losses from Avena litter and interacting with water additions to increase decomposition of litter of other grasses. Increases in lignin that resulted from elevated CO₂ had no effect on decomposition but elevated CO₂ increased N losses from Avena litter. Overall, the interactions among elements of global change were as important as single-factor effects in influencing plant litter chemistry. However, with the exception of variation in N, litter quality had little influence on decomposition over the short term.     

Keeling plots for hummingbirds: a method to estimate carbon isotope ratios of respired CO<Subscript>2</Subscript> in small vertebrates

The carbon isotope composition of an animals breath reveals the composition of the nutrients that it catabolizes for energy. Here we describe the use of Keeling plots, a method widely applied in ecosystem ecology, to measure the ¹³C of respired CO₂ of small vertebrates. We measured the ¹³C of Rufous Hummingbirds (Selasphorus rufus) in the laboratory and of Mourning (Zenaida macroura) and White-winged (Z. asiatica) Doves in the field. In the laboratory, when hummingbirds were fed a sucrose based C3 diet, the ¹³C of respired CO₂ was not significantly different from that of their diet (¹³C_{C3 diet}). The ¹³C of respired CO₂ for C3 fasted birds was slightly, albeit significantly, depleted in ¹³C relative to ¹³C_{C3 diet}. Six hours after birds were shifted to a sucrose based C4 diet, the isotopic composition of their breath revealed that birds were catabolizing a mixture of nutrients derived from both the C3 and the C4 diet. In the field, the ¹³C of respired CO₂ from Mourning and White-winged Doves reflected that of their diets: the CAM saguaro cactus (Carnegeia gigantea) and C3 seeds, respectively. Keeling plots are an easy, effective and inexpensive method to measure ¹³C of respired CO₂ in the lab and the field.     

Fine root chemistry and decomposition in model communities of north-temperate tree species show little response to elevated atmospheric CO<Subscript>2</Subscript> and varying soil resource availability

Rising atmospheric [CO₂] has the potential to alter soil carbon (C) cycling by increasing the content of recalcitrant constituents in plant litter, thereby decreasing rates of decomposition. Because fine root turnover constitutes a large fraction of annual NPP, changes in fine root decomposition are especially important. These responses will likely be affected by soil resource availability and the life history characteristics of the dominant tree species. We evaluated the effects of elevated atmospheric [CO₂] and soil resource availability on the production and chemistry, mycorrhizal colonization, and decomposition of fine roots in an early- and late-successional tree species that are economically and ecologically important in north temperate forests. Open-top chambers were used to expose young trembling aspen (Populus tremuloides) and sugar maple (Acer saccharum) trees to ambient (36 Pa) and elevated (56 Pa) atmospheric CO₂. Soil resource availability was composed of two treatments that bracketed the range found in the Upper Lake States, USA. After 2.5 years of growth, sugar maple had greater fine root standing crop due to relatively greater allocation to fine roots (30% of total root biomass) relative to aspen (7% total root biomass). Relative to the low soil resources treatment, aspen fine root biomass increased 76% with increased soil resource availability, but only under elevated [CO₂]. Sugar maple fine root biomass increased 26% with increased soil resource availability (relative to the low soil resources treatment), and showed little response to elevated [CO₂]. Concentrations of N and soluble phenolics, and C/N ratio in roots were similar for the two species, but aspen had slightly higher lignin and lower condensed tannins contents compared to sugar maple. As predicted by source-sink models of carbon allocation, pooled constituents (C/N ratio, soluble phenolics) increased in response to increased relative carbon availability (elevated [CO₂]/low soil resource availability), however, biosynthetically distinct compounds (lignin, starch, condensed tannins) did not always respond as predicted. We found that mycorrhizal colonization of fine roots was not strongly affected by atmospheric [CO₂] or soil resource availability, as indicated by root ergosterol contents. Overall, absolute changes in root chemical composition in response to increases in C and soil resource availability were small and had no effect on soil fungal biomass or specific rates of fine root decomposition. We conclude that root contributions to soil carbon cycling will mainly be influenced by fine root production and turnover responses to rising atmospheric [CO₂], rather than changes in substrate chemistry.     

Biomass production in a nitrogen-fertilized,tallgrass prairie ecosystem exposed to ambient and elevated levels of CO2

Increased biomass production in terrestrial ecosystems with elevated atmospheric CO₂ may be constrained by nutrient limitations as a result of increased requirement or reduced availability caused by reduced turnover rates of nutrients. To determine the short-term impact of nitrogen (N) fertilization on plant biomass production under elevated CO₂, we compared the response of N-fertilized tallgrass prairie at ambient and twice-ambient CO₂ levels over a 2-year period. Native tallgrass prairie plots (4.5 m diameter) were exposed continuously (24 h) to ambient and twice-ambient CO₂ from 1 April to 26 October. We compared our results to an unfertilized companion experiment on the same research site. Above- and belowground biomass production and leaf area of fertilized plots were greater with elevated than ambient CO₂ in both years. The increase in biomass at high CO₂ occurred mainly aboveground in 1991, a dry year, and belowground in 1990, a wet year. Nitrogen concentration was lower in plants exposed to elevated CO₂, but total standing crop N was greater at high CO₂. Increased root biomass under elevated CO₂ apparently increased N uptake. The biomass production response to elevated CO₂ was much greater on N-fertilized than unfertilized prairie, particularly in the dry year. We conclude that biomass production response to elevated CO₂ was suppressed by N limitation in years with below-normal precipitation. Reduced N concentration in above- and belowground biomass could slow microbial degradation of soil organic matter and surface litter, thereby exacerbating N limitation in the long term.     

Predicting responses of photosynthesis and root fraction to elevated [CO2]a: interactions among carbon,nitrogen, and growth*

At elevated atmospheric CO₂ concentrations ([CO₂]_a), photosynthetic capacity (A_max) and root fraction (η_R, the ratio of root to plant dry mass) increased in some studies and decreased in others. Here, we have explored possible causes of this, focusing on the relative magnitudes of the effects of elevated [CO₂]_a on specific leaf (n_m) and plant (n_p) nitrogen concentrations, leaf mass per unit area (h), and plant nitrogen productivity (α). In our survey of 39 studies with 35 species, we found that elevated [CO₂]_a led to decreased n_m and n_p in all the studies and to increased h and α in most of the studies. The magnitudes of these changes varied with species and with experimental conditions. Based on a model that integrated [CO₂]_a-induced changes in leaf nitrogen into a biochemically based model of leaf photosynthesis, we predicted that, to a first approximation, photosynthesis will be upregulated (A_max will increase) when growth at increased [CO₂]_a leads to increases in h that are larger than decreases in n_m. Photosynthesis will be downregulated (A_max will decrease) when increases in h are smaller than decreases in n_m. The model suggests that photosynthetic capacity increases at elevated [CO₂]_a only when additional leaf mesophyll more than compensates the effects of nitrogen dilution. We considered two kinds of regulatory paradigms that could lead to varying responses of η_R to elevated [CO₂]_a, and compared the predictions of each with the data. A simple static model based on the functional balance concept predicts that η_R should increase when neither n_p nor h is very responsive to elevated [CO₂]_a. The quantitative and qualitative agreement of the predictions with data from the literature, however, is poor. A model that predicts η_R from the relative sensitivities of photosynthesis and relative growth rate to elevated [CO₂]_a corresponds much more closely to the observations. In general, root fraction increases if the response of photosynthesis to [CO₂]_a is greater than that of relative growth rate.     

Effects of elevated [CO2] on forest growth and carbon storage: a modelling analysis of the consequences of changes in litter quality/quantity and root exudation

Many researchers have proposed that the stimulus of plant growth under elevated [CO₂] observed in short-term experiments will be moderated in the longer term by a reduction in soil nitrogen (N) availability linked to decreased litter quality and/or increased litter production. However, these negative feedbacks may be offset to some extent by a stimulus in N fixation linked to increased root exudation. The aim of this modelling study is to examine how changes in litter quality/quantity and root exudation –- if they occur –- will affect the CO₂ responses of net primary productivity and ecosystem carbon (C) storage on different timescales. We apply a model of C and N cycling in forest ecosystems (G’DAY) to stands of Norway spruce (Picea abies, L. Cast) growing at a N-limited experimental site at Flakaliden, Sweden, and draw the following conclusions: (1) in the absence of changes in litter quality and root exudation, the short-term CO₂ stimulus of litter quantity leads to only a minimal CO₂ stimulus of productivity or C storage in the medium term (≈ 20 years) and long term (≈ 200 years), because of constraints on soil N availability; (2) increasing plant nitrogen use efficiency (via a decrease in the N:C ratio of new litter) makes little impact on these results; (3) a significant CO₂ response in the medium term requires a substantial decrease in the N:C ratio of older litter, when it is approaching stabilisation as soil organic matter, although the long-term CO₂ response remains small; and (4) an increase in N fixation leads to a small effect on productivity in the short term, but a very large effect on both productivity and C storage in the long term. These results suggest that soil N constraints on the long-term CO₂-fertilisation effect can be overcome to a significant extent only by increases in N acquisition, although only modest increases may be required. This revised version was published online in June 2006 with corrections to the Cover Date.     

Reduced N cycling in response to elevated CO2, warming,and drought in a Danish heathland: Synthesizing results of the CLIMAITE project after two years of treatments

Field‐scale experiments simulating realistic future climate scenarios are important tools for investigating the effects of current and future climate changes on ecosystem functioning and biogeochemical cycling. We exposed a seminatural Danish heathland ecosystem to elevated atmospheric carbon dioxide (CO₂), warming, and extended summer drought in all combinations. Here, we report on the short‐term responses of the nitrogen (N) cycle after 2 years of treatments. Elevated CO₂ significantly affected aboveground stoichiometry by increasing the carbon to nitrogen (C/N) ratios in the leaves of both co‐dominant species (Calluna vulgaris and Deschampsia flexuosa), as well as the C/N ratios of Calluna flowers and by reducing the N concentration of Deschampsia litter. Belowground, elevated CO₂ had only minor effects, whereas warming increased N turnover, as indicated by increased rates of microbial NH₄⁺ consumption, gross mineralization, potential nitrification, denitrification and N₂O emissions. Drought reduced belowground gross N mineralization and decreased fauna N mass and fauna N mineralization. Leaching was unaffected by treatments but was significantly higher across all treatments in the second year than in the much drier first year indicating that ecosystem N loss is highly sensitive to changes and variability in amount and timing of precipitation. Interactions between treatments were common and although some synergistic effects were observed, antagonism dominated the interactive responses in treatment combinations, i.e. responses were smaller in combinations than in single treatments. Nonetheless, increased C/N ratios of photosynthetic tissue in response to elevated CO₂, as well as drought‐induced decreases in litter N production and fauna N mineralization prevailed in the full treatment combination. Overall, the simulated future climate scenario therefore lead to reduced N turnover, which could act to reduce the potential growth response of plants to elevated atmospheric CO₂ concentration.     

Community metabolism on rocky-shore assemblages in a mesocosm: A. Fluctuations in production,respiration, chlorophyll a content and C:N ratios of grazed and non-grazed assemblages

Studies were undertaken with the aim of developing a standardized method for assessing environmental pollution in sediments by utilization of life-history data of freshwater tubificids. Similar bioassay methods have long been used for Daphnia magna, species of Ceriodaphnia and Nitocra, etc. in accordance with guidelines from the International Organization for Standardization (ISO). Tubifex tubifex was found to be the most likely candidate for such bioassays, since the species is readily kept in culture and reproduces more or less consistantly.The culturing method is slightly modified from Kosiorek (1974). This paper provides an example of the particular sensitivity of this kind of bioassay method in the detection of heavy metal contamination of lake sediments. Sediments from the oligotrophic Lake Runn were considered suitable for the purpose, since the lake receives waste water from a major mining industry in Sweden. Metal analyses of the sediments had revealed the agents likely to be causing the decreased biological activity measured in the lake; rough amplitudes for mercury: 800–3600 ng · g^-1 dw, copper: 800–1800 g · g^-1 dw, zinc: 3.3 – 8.1 mg · g g^-1 dw have been estimated for surficial sediments.Young tubificids exposed to Lake Runn sediments did not grow much and died off within a short period of time. No reproduction occurred. Sediments from Lake Runn, when mixed with sediments from the eutrophic Lake Hjälmaren, made reproduction of T. tubifex occur only in mixtures containing less than 50% L. Runn sediments. The growth rate, reproductive success and the very timing of consecutive reproductive events of cohort individuals were found to be highly indicative of toxic effects. When additional food sources were available, however, these effects were largely masked. Therefore, extra food rations were excluded from the original method.     

Invasion of <Emphasis Type="Italic">Spartina alterniflora</Emphasis> Enhanced Ecosystem Carbon and Nitrogen Stocks in the Yangtze Estuary,China

Whether plant invasion increases ecosystem carbon (C) stocks is controversial largely due to the lack of knowledge about differences in ecophysiological properties between invasive and native species. We conducted a field experiment in which we measured ecophysiological properties to explore the response of the ecosystem C stocks to the invasion of Spartina alterniflora (Spartina) in wetlands dominated by native Scirpus mariqueter (Scirpus) and Phragmites australis (Phragmites) in the Yangtze Estuary, China. We measured growing season length, leaf area index (LAI), net photosynthetic rate (Pn), root biomass, net primary production (NPP), litter quality and litter decomposition, plant and soil C and nitrogen (N) stocks in ecosystems dominated by the three species. Our results showed that Spartina had a longer growing season, higher LAI, higher Pn, and greater root biomass than Scirpus and Phragmites. Net primary production (NPP) was 2.16 kg C m⁻² y⁻¹ in Spartina ecosystems, which was, on average, 1.44 and 0.47 kg C m⁻² y⁻¹ greater than that in Scirpus and Phragmites ecosystems, respectively. The litter decomposition rate, particularly the belowground decomposition rate, was lower for Spartina than Scirpus and Phragmites due to the lower litter quality of Spartina. The ecosystem C stock (20.94 kg m⁻²) for Spartina was greater than that for Scirpus (17.07 kg m⁻²), Phragmites (19.51 kg m⁻²) and the mudflats (15.12 kg m⁻²). Additionally, Spartina ecosystems had a significantly greater N stock (698.8 g m⁻²) than Scirpus (597.1 g m⁻²), Phragmites ecosystems (578.2 g m⁻²) and the mudflats (375.1 g m⁻²). Our results suggest that Spartina invasion altered ecophysiological processes, resulted in changes in NPP and litter decomposition, and ultimately led to enhanced ecosystem C and N stocks in the invaded ecosystems in comparison to the ecosystems with native species.     

In situ litter decomposition and litter quality in a Mojave Desert ecosystem: effects of elevated atmospheric CO2 and interannual climate variability

Rising atmospheric CO₂ has been predicted to reduce litter decomposition as a result of CO₂‐induced reductions in litter quality. However, available data have not supported this hypothesis in mesic ecosystems, and no data are available for desert or semi‐arid ecosystems, which account for more than 35% of the Earth's land area. The objective of our study was to explore controls on litter decomposition in the Mojave Desert using elevated CO₂ and interannual climate variability as driving environmental factors. In particular, we sought to evaluate the extent to which decomposition is modulated by litter chemistry (C:N) and litter species and tissue composition. Naturally senesced litter was collected from each of nine 25 m diameter experimental plots, with six plots exposed to ambient [CO₂] or 367 μL CO₂ L^?1 and three plots continuously fumigated with elevated [CO₂] (550 μL CO₂ L^?1) using FACE technology beginning in April 1997. All litter collected in 1998 (a wet, or El Niño year; 306 mm precipitation) was pooled as was litter collected in 1999 (a dry year; 94 mm). Samples were allowed to decompose for 4 and 12 months starting in May 2001 in mesh litterbags in the locations from which litter was collected. Decomposition of litter produced under elevated CO₂ and ambient CO₂ did not differ. Litter produced in the wetter year showed more rapid initial decomposition (over the first 4 months) than that produced in the drier year (27±2% yr^?1 or 7.8±0.7 g m^?2 yr^?1 for 1998 litter; 18±3% yr^?1 or 2.2±0.4 g m^?2 yr^?1 for 1999 litter). C:N ratios of litter produced under elevated CO₂ (wet year: 37±0.5; dry year: 42±2.5) were higher than those of litter produced under ambient CO₂ (wet year: 34±1.1; dry year: 35±1.4). Litter production in the wet year (amb. CO₂: 25.1±1.1 g m^?2 yr^?1; elev. CO₂: 35.0±1.1 g m^?2 yr^?1) was more than twice as high as that in the dry year (amb. CO₂: 11.6±1.7 g m^?2, elev. CO₂: 13.3±3.4 g m^?2), and contained a greater proportion of Lycium pallidum and a lower proportion of Larrea tridentata than litter produced in the dry year. Decomposition, viewed across all treatments, decreased with increasing C:N ratios, decreased with increasing proportions of Larrea tridentata and increased with increasing proportions of Lycium pallidum and Lycium andersonii. Because litter C:N did not vary by litter production year, and CO₂ did not alter decomposition or litter species/tissue composition, it is likely that the impact of year‐to‐year variation in precipitation on the proportion of key plant species in the litter may be the most important way in which litter decomposition will be modulated in the Mojave Desert under future rising atmospheric CO₂.