Belowground positive and negative feedbacks on CO2 growth enhancement

In this paper we present a conceptual model of integrated plant-soil interactions which illustrates the importance of identifying the primary belowground feedbacks, both positive and negative, which can simultaneously affect plant growth responses to elevated CO₂. The primary negative feedbacks share the common feature of reducing the amount of nutrients available to plants. These negative feedbacks include increased litter C/N ratios, and therefore reduced mineralization rates, increased immobilization of available nutrients by a larger soil microbial pool, and increased storage of nutrients in plant biomass and detritus due to increases in net primary productivity (NPP). Most of the primary positive feedbacks share the common feature of being plant mediated feedbacks, the only exception being Zak et al.'s hypothesis that increased microbial biomass will be accompanied by increased mineralization rates. Plant nutrient uptake may be increased through alterations in root architecture, physiology, or mycorrhizal symbioses. Further, the increased C/N ratios of plant tissue mean that a given level of NPP can be achieved with a smaller supply of nitrogen.Identification of the net plant-soil feedbacks to enhanced productivity with elevated CO₂ are a critical first step for any ecosystem. It is necessary, however, that we first identify how universally applicable the results are from one study of one ecosystem before ecosystem models incorporate this information. The effect of elevated CO₂ on plant growth (including NPP, tissue quality, root architecture, mycorrhizal symbioses) can vary greatly for different species and environmental conditions. Therefore it is reasonable to expect that different ecosystems will show different patterns of interacting positive and negative feedbacks within the plant-soil system. This inter-ecosystem variability in the potential for long-term growth responses to rising CO₂ levels implies that we need to parameterize mechanistic models of the impact of elevated CO₂ on ecosystem productivity using a detailed understanding of each ecosystem of interest.     

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.     

The effects of elevated atmospheric CO2 on the amount and depth distribution of plant water uptake in a California annual grassland

Soil moisture profiles can affect species composition and ecosystem processes, but the effects of increased concentrations of atmospheric carbon dioxide ([CO₂]) on the vertical distribution of plant water uptake have not been studied. Because plant species composition affects soil moisture profiles, and is likely to shift under elevated [CO₂], it is also important to test whether the indirect effects of [CO₂] on soil water content may depend on species composition. We examined the effects of elevated [CO₂] and species composition on soil moisture profiles in an annual grassland of California. We grew monocultures and a mixture of Avena barbata and Hemizonia congesta– the dominant species of two phenological groups – in microcosms exposed to ambient (～370 μmol mol^?1) and elevated (～700 μmol mol^?1) [CO₂]. Both species increased intrinsic and yield‐based water use efficiency under elevated [CO₂], but soil moisture increased only in communities with A. barbata, the dominant early‐season annual grass. In A. barbata monocultures, the [CO₂] treatment did not affect the depth distribution of soil water loss. In contrast to communities with A. barbata, monocultures of H. congesta, a late‐season annual forb, did not conserve water under elevated [CO₂], reflecting the increased growth of these plants. In late spring, elevated [CO₂] also increased the efficiency of deep roots in H. congesta monocultures. Under ambient [CO₂], roots below 60 cm accounted for 22% of total root biomass and were associated with 9% of total water loss, whereas in elevated [CO₂], 16% of total belowground biomass was associated with 34% of total water loss. Both soil moisture and isotope data showed that H. congesta monocultures grown under elevated [CO₂] began extracting water from deep soils 2 weeks earlier than plants in ambient [CO₂].     

P uptake by arbuscular mycorrhizal hyphae: effect of soil temperature and atmospheric CO2 enrichment

Mycorrhizas are ubiquitous symbioses that may have an important role in the movement of C from air to soil. Studies on the effects of climate change factors on mycorrhizas have been concentrated on the effects of atmospheric [CO₂] whereas temperature effects have been neglected. Based on previous results showing no effect of varying atmospheric [CO₂] on the development and P uptake of the arbuscular mycorrhizal fungi (AMF) colonizing plants growing in controlled conditions, we hypothesized that soil temperature would have a higher impact on AMF development and nutrient uptake than the effects of [CO₂] on the host plant. Pea plants were grown in association with either a single isolate of Glomus caledonium or AMF from field soil in factorial combination with the corresponding current (10 °C) or elevated (15 °C) soil temperatures at current (350 p.p.m) or elevated (700 p.p.m) atmospheric [CO₂]. ³³P uptake by extraradical AMF hyphae was measured independently from root P uptake in a root exclusion compartment. Intraradical colonization developed well at both soil temperatures and almost duplicated from 10 to 15 °C. Extraradical mycelium developed only at 15 °C in the root exclusion compartment and hyphal P uptake could therefore be studied at 15 °C only. Hyphal P uptake differed markedly between inoculum types, but was not altered by growing the host plants at two atmospheric [CO₂] levels. No significant [CO₂] × soil temperature interactions were observed. The results suggested that, in the system tested, AMF development and function is likely more influenced by the temperature component of climate change than by its [CO₂] component. We suggest that much more attention should be paid to temperature effects in future studies.     

The influence of plant species,fertilization and elevated CO2 on soil aggregate stability

We tested the effects of plant species, fertilization and elevated CO₂ on water-stable soil aggregation. Five annual grassland species and a plant community were grown in outdoor mesocosms for 4 years, with and without NPK fertilization, at ambient or elevated atmospheric CO₂ concentrations. Aggregate stability (resistance of aggregates to slaking) in the top 0.15 m of soil differed among plant species. However, the more diverse plant community did not enhance aggregate stability relative to most monocultures. Species differences in aggregate stability were positively correlated with soil active bacterial biomass, but did not correlate with root biomass or fungal length. Plant species did not affect aggregate stability lower in the soil profile (0.15–0.45 m), where soil biological activity is generally decreased. Elevated CO₂ and NPK fertilization altered many of the factors known to influence aggregation, but did not affect water-stable aggregation at either depth, in any of the plant treatments. These results suggest that global changes will alter soil structure primarily due to shifts in vegetation composition.     

Growth of Eastern Cottonwoods (Populus deltoides) in elevated [CO2] stimulates stand-level respiration and rhizodeposition of carbohydrates, accelerates soil nutrient depletion, yet stimulates above- and belowground biomass production

We took advantage of the distinctive system‐level measurement capabilities of the Biosphere 2 Laboratory (B2L) to examine the effects of prolonged exposure to elevated [CO₂] on carbon flux dynamics, above‐ and belowground biomass changes, and soil carbon and nutrient capital in plantation forest stands over 4 years. Annually coppiced stands of eastern cottonwoods (Populus deltoides) were grown under ambient (400 ppm) and two levels of elevated (800 and 1200 ppm) atmospheric [CO₂] in carbon and N‐replete soils of the Intensive Forestry Mesocosm in the B2L. The large semiclosed space of B2L uniquely enabled precise CO₂ exchange measurements at the near ecosystem scale. Highly controllable climatic conditions within B2L also allowed for reproducible examination of CO₂ exchange under different scales in space and time. Elevated [CO₂] significantly stimulated whole‐system maximum net CO₂ influx by an average of 21% and 83% in years 3 and 4 of the experiment. Over the 4‐year experiment, cumulative belowground, foliar, and total aboveground biomass increased in both elevated [CO₂] treatments. After 2 years of growth at elevated [CO₂], early season stand respiration was decoupled from CO₂ influx aboveground, presumably because of accelerated fine root production from stored carbohydrates in the coppiced system prior to canopy development and to the increased soil carbohydrate status under elevated [CO₂] treatments. Soil respiration was stimulated by elevated [CO₂] whether measured at the system level in the undisturbed soil block, by soil collars in situ, or by substrate‐induced respiration in vitro. Elevated [CO₂] accelerated depletion of soil nutrients, phosphorus, calcium and potassium, after 3 years of growth, litter removal, and coppicing, especially in the upper soil profile, although total N showed no change. Enhancement of above‐ and belowground biomass production by elevated [CO₂] accelerated carbon cycling through the coppiced system and did not sequester additional carbon in the soil.     

Wheat genotypes differing in aluminum tolerance differ in their growth response to CO2 enrichment in acid soils

Aluminum (Al) toxicity is a major factor limiting plant growth in acid soils. Elevated atmospheric CO₂ [CO₂] enhances plant growth. However, there is no report on the effect of elevated [CO₂] on growth of plant genotypes differing in Al tolerance grown in acid soils. We investigated the effect of short‐term elevated [CO₂] on growth of Al‐tolerant (ET8) and Al‐sensitive (ES8) wheat plants and malate exudation from root apices by growing them in acid soils under ambient [CO₂] and elevated [CO₂] using open‐top chambers. Exposure of ET8 plants to elevated [CO₂] enhanced root biomass only. In contrast, shoot biomass of ES8 was enhanced by elevated [CO₂]. Given that exudation of malate to detoxify apoplastic Al is a mechanism for Al tolerance in wheat plants, ET8 plants exuded greater amounts of malate from root apices than ES8 plants under both ambient and elevated [CO₂]. These results indicate that elevated [CO₂] has no effect on malate exudation in both ET8 and ES8 plants. These novel findings have important implications for our understanding how plants respond to elevated [CO₂] grown in unfavorable edaphic conditions in general and in acid soils in particular.     

Influence of elevated CO2 and nitrogen nutrition on rice plant growth,soil microbial biomass,dissolved organic carbon and dissolved CH4

Rice (Oryza sativa) was grown in six sunlit, semi-closed growth chambers for two seasons at 350 L L^–1 (ambient) and 650 L L^–1 (elevated) CO₂ and different levels of nitrogen (N) supplement. The objective of this research was to study the influence of CO₂ enrichment and N nutrition on rice plant growth, soil microbial biomass, dissolved organic carbon (DOC) and dissolved CH₄. Elevated CO₂ concentration ([CO₂]) demonstrated a wide range of enhancement to both above- and below-ground plant biomass, in particular to stems and roots (for roots when N was not limiting) in the mid-season (80 days after transplanting) and stems/ears at the final harvest, depending on season and the level of N supplement. Elevated [CO₂] significantly increased microbial biomass carbon in the surface 5 cm soil when N (90 kg ha^–1) was in sufficient supply. Low N supplement (30 kg ha^–1) limited the enhancement of root growth by elevated [CO₂], leading consequently to diminished response of soil microbial biomass carbon to CO₂ enrichment. The concentration of dissolved CH₄ (as well as soil DOC, but to a lesser degree) was observed to be positively related to elevated [CO₂], especially at high rate of N application (120 kg ha^–1) or at 10 cm depth (versus 5 cm depth) in the later half of the growing season (at 80 kg N ha^–1). Root senescence in the late season complicated the assessment of the effect of elevated [CO₂] on root growth and soil organic carbon turnover and thus caution should be taken when interpreting respective high CO₂ results.     

The influence of temperature and labile C substrates on heterotrophic respiration in response to elevated CO2 and nitrogen fertilization

Important effects of elevated [CO₂] on SOM are expected as a consequence of increased labile organic substrates derived from plants. The present study tests the hypotheses that, under elevated [CO₂]: 1) soil heterotrophic respiration will increase due to roots-microbes-soil interactions; 2) the increased labile C will boost soil heterotrophic respiration, depending on N availability; 3) the temperature sensitivity of soil respiration will change, depending on nitrogen inputs and plant activity. To test these hypotheses, we measured the heterotrophic respiration of intact soil cores collected in a poplar plantation exposed to elevated [CO₂] and two nitrogen inputs, at different temperatures. Additional physical (water content, root biomass) and biochemical parameters (microbial biomass, labile C) were determined on the same samples. The soil samples were collected at the POP-EuroFACE experimental site (Italy), where a Populus x euramericana plantation was exposed for 6 years to 550 ppm [CO₂] (Free Air CO₂ Enrichment) at two different nitrogen inputs (none or 290 kg ha^?1). The higher heterotrophic respiration under elevated [CO₂] (+30% on average) was driven by the larger pool of soil labile C (+57% on average). The temperature sensitivity of soil respiration was unaffected by elevated [CO₂], but was positively affected by N fertilization. Our results indicate that only a fraction of the extra carbon fixed by photosynthesis in elevated [CO₂] will contribute to enhanced carbon storage into the soil because of the contemporary stimulation of soil heterotrophic respiration. At the same time, the fraction remaining in the soil will enhance the pool of soil labile C.     

Long-term effects of elevated atmospheric CO2 on below-ground biomass and transformations to soil organic matter in grassland

We determined the effects of elevated [CO₂] on the quantity and quality of below-ground biomass and several soil organic matter pools at the conclusion of an eight-year CO₂ enrichment experiment on native tallgrass prairie. Plots in open-top chambers were exposed continuously to ambient and twice-ambient [CO₂] from early April through late October of each year. Soil was sampled to a depth of 30 cm beneath and next to the crowns of C4 grasses in these plots and in unchambered plots. Elevated [CO₂] increased the standing crops of rhizomes (87%), coarse roots (46%), and fibrous roots (40%) but had no effect on root litter (mostly fine root fragments and sloughed cortex material >500 μm). Soil C and N stocks also increased under elevated [CO₂], with accumulations in the silt/clay fraction over twice that of particulate organic matter (POM; >53 μm). The mostly root-like, light POM (density ≤1.8 Mg m^-3) appeared to turn over more rapidly, while the more amorphous and rendered heavy POM (density >1.8 Mg m^-3) accumulated under elevated [CO₂]. Overall, rhizome and root C:N ratios were not greatly affected by CO₂ enrichment. However, elevated [CO₂] increased the C:N ratios of root litter and POM in the surface 5 cm and induced a small but significant increase in the C:N ratio of the silt/clay fraction to a depth of 15 cm. Our data suggest that 8 years of CO₂ enrichment may have affected elements of the N cycle (including mineralization, immobilization, and asymbiotic fixation) but that any changes in N dynamics were insufficient to prevent significant plant growth responses. This revised version was published online in June 2006 with corrections to the Cover Date.     

Carbon use in root respiration as affected by elevated atmospheric O2

The use of fossil fuel is predicted to cause an increase of the atmospheric CO₂ concentration, which will affect the global pattern of temperature and precipitation. It is therefore essential to incorporate effects of temperature and water supply on the carbon requirement for root respiration of plants to predict effects of elevated [CO₂] on the carbon budget of natural and managed systems.There is insufficient information to support the contentention that an increase in the concentration of CO₂ in the atmosphere will enhance the CO₂ concentration in the soil to an extent that is likely to affect root respiration. Moreover, there is no convincing evidence for a direct effect of elevated atmospheric [CO₂] on the rate of root respiration per unit root mass or the fraction of carbon required for root respiration. However, there are likely to be indirect effects of elevated [CO₂] on the carbon requirement of plants in natural systems.Firstly, it is very likely that the carbon requirement of root respiration relative to that fixed in photosynthesis will increase when elevated [CO₂] induces a decrease in nutrient status of the plants. Although earlier papers have emphasized that elevated [CO₂] favours investment of biomass in roots relative to that in leaves, these are in fact indirect effects. The increase in root weight ratio is due to the more rapid depletion of nutrients in the root environment as a consequence of enhanced growth. This will decrease the specific rate of root respiration, but increase the carbon requirement as a fraction of the carbon fixed in photosynthesis. It is likely that these effects will be minor in systems where the nutrient supply is very high, e.g. in many managed arable systems, and increase with decreasing soil fertility, i.e. in many natural systems.Secondly, a decrease in rainfall in some parts of the world may cause a shortage in water supply which favours the carbon partitioning to roots. Water stress is likely to reduce rates of root respiration per unit root mass, but enhance the fraction of total assimilates required for root respiration, due to greater allocation of biomass to roots.Increased temperatures are unlikely to affect the specific rate of root respiration in all species. Broadly generalized, the effect of temperature on biomass allocation is that the relative investment of biomass in roots is lowest at a certain optimum temperature and increases at both higher and lower temperatures. The root respiration of some species acclimates to growth temperature, so that the effect of global temperature rise is entirely accounted for by the effect of temperature on biomass allocation. The specific rate of root respiration of other species will increase with global warming. In response to global warming the carbon requirement of roots is likely to decrease in temperate regions, when temperatures are suboptimal for the roots' capacity to acquire water. Here global warming will induce a smaller biomass allocation to the roots. Conversely, the carbon requirements are more likely to increase in mediterranean environments, where temperatures are often supraoptimal and a rise in temperature will induce greater allocation of biomass to the roots.     

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.     

Nitrate and ammonium uptake for single-and mixed-species communities grown at elevated CO2

Sustained increases in plant production in elevated CO₂ depend on adequate belowground resources. Mechanisms for acquiring additional soil resources include increased root allocation and changes in root morphology or physiology. CO₂ research to date has focused almost exclusively on changes in biomass and allocation. We examined physiological changes in nitrate and ammonium uptake in elevated CO₂, hypothesizing that uptake rates would increase with the amount of available CO₂. We combined our physiological estimates of nitrogen uptake with measurements of root biomass to assess whole root-system rates of nitrogen uptake. Surprisingly, physiological rates of ammonium uptake were unchanged with CO₂, and rates of nitrate uptake actually decreased significantly (P<0.005). Root boomass increased 23% in elevated CO₂ (P<0.005), but almost all of this increase came in fertilized replicates. Rates of root-system nitrogen uptake in elevated CO₂ increased for ammonium in nutrient-rich soil (P<0.05) and were unchanged for nitrate (P>0.80). Root-system rates of nitrogen uptake were more strongly correlated with physiological uptake rates than with root biomass in unamended soil, but the reverse was true in fertilized replicates. We discuss nitrogen uptake and changes in root biomass in the context of root nutrient concentrations (which were generally unchanged with CO₂) and standing pools of belowground plant nitrogen. In research to date, there appears to be a fairly general increase in root biomass with elevated CO₂, and little evidence of up-regulation in root physiology.     

Root Biomass of Individual Species, and Root Size Characteristics After Five Years of CO2 Enrichment on Native Shortgrass Steppe

Information from field studies investigating the responses of roots to increasing atmospheric CO₂ is limited and somewhat inconsistent, due partly to the difficulty in studying root systems in situ. In this report, we present standing root biomass of species and root length and diameter after five years of CO₂ enrichment (∽720 μmol mol⁻¹) in large (16 m² ground area) open-top chambers placed over a native shortgrass steppe in Colorado, USA. Total root biomass in 100 cm long×20 cm wide×75 cm depth soil monoliths and root biomass of the three dominant grass species of the site were not significantly affected by elevated CO₂. Root biomass of Stipa comata in the 0–20 cm soil depth was nearly 100% greater in elevated vs. ambient CO₂ chambers, but this was not statistically significant (P=0.14). However, there was a significant 37% increase in fine root length under elevated CO₂ in the 0–10 cm soil depth layer. Other reports from this study suggest that the increase in fine roots is primarily from improved seedling recruitment of S. comata under elevated CO₂. Few treatment differences in root length or diameter were detected in lower 10 cm depth increments, to 80 cm. These results reflect the root status integrated over two wet, two dry and one normal precipitation years and approximately one complete cycle of root turn-over on the shortgrass steppe. We conclude that increasing atmospheric CO₂ will have only small effects on standing root biomass and root length and diameter of most shortgrasss steppe species. However, the potential increased competitive ability of Stipa comata, a low forage quality species, could alter the ecosystem from the current dominant, high forage quality species, Bouteloua gracilis. B. gracilis is very well adapted to the frequent droughts of the shortgrass steppe. Increased competitive ability of less desirable plant species under increasing atmospheric CO₂ will have large implications for long-term sustainability of grassland ecosystems.     

Low soil temperature inhibits the stimulatory effect of elevated [CO2] on height and biomass accumulation of white birch seedlings grown under three non‐limiting phosphorus conditions

White birch (Betula papyrifera Marsh.) seedlings were exposed to ambient or doubled ambient carbon dioxide concentration ([CO₂]), three soil temperatures (T_soil) (low, intermediate, high), and three phosphorus (P) regimes (low, medium, high) in environment‐controlled greenhouses. Height (H), root‐collar diameter (RCD), biomass, and leaf phosphorus concentration (leaf P) were determined four months after initiation of treatments. The low T_soil reduced H, RCD, shoot biomass, root biomass and total seedling biomass whereas the high‐P level and the [CO₂] elevation increased all the growth and biomass parameters. Elevated [CO₂] significantly reduced leaf P. There were significant two‐factor interactions suggesting that the effect of elevated [CO₂] on (1) H, total biomass, biomass of plant components, and leaf P was dependent on T_soil, (2) total biomass was contingent on P regime. For instance, the positive response of H and total biomass to elevated [CO₂] was limited to seedlings raised under the intermediate and high T_soil, respectively. In addition, [CO₂] elevation increased total biomass only at the high‐P regime but not at the low‐ or medium‐P level where the effect of [CO₂] was statistically insignificant. No significant main effect of treatment or interaction was observed for root to shoot biomass ratio.     

Soil Fungi Respond More Strongly than Fine Roots to Elevated CO2 in a Model Regenerating Longleaf Pine-Wiregrass Ecosystem

Rising atmospheric CO₂ concentration will affect belowground processes and forest function. However, the direction and magnitude of change for many soil processes are unknown. We used minirhizotrons to observe fine root and fungal dynamics in response to elevated CO₂ in a model regenerating longleaf pine community in open-top chambers. The model community consisted of five plant species common to xeric sandhills longleaf pine stands: Pinus palustris, Quercus margaretta, Aristida stricta, Crotalaria rotundifolia, and Asclepias tuberosa. Elevated CO₂ significantly increased both fine root and mycorrhizal tip standing crop by more than 50% in the deeper soil horizon (17–34 cm). Rhizomorph standing crop was nearly doubled in both deep and shallow soil (P = 0.04). Survivorship results for fine roots and rhizomorphs varied between soil depths. Fine root survivorship was likely influenced more by changes in community composition and species interactions driven by elevated CO₂ rather than by direct effects of elevated CO₂ on the fine roots of individual species. In this system, it appears that elevated CO₂ led to a greater reliance on fungal symbionts to meet additional nutrient requirements rather than substantially increased root growth.     

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.     

Changes in the microbial community structure of bacteria,archaea and fungi in response to elevated CO2 and warming in an Australian native grassland soil

The microbial community structure of bacteria, archaea and fungi is described in an Australian native grassland soil after more than 5 years exposure to different atmospheric CO₂ concentrations ([CO₂]) (ambient, + 550 ppm) and temperatures (ambient, + 2°C) under different plant functional types (C ₃ and C ₄ grasses) and at two soil depths (0–5 cm and 5–10 cm). Archaeal community diversity was influenced by elevated [CO₂], while under warming archaeal 16S rRNA gene copy numbers increased for C ₄ plant Themeda triandra and decreased for the C ₃ plant community (P < 0.05). Fungal community diversity resulted in three groups based upon elevated [CO₂], elevated [CO₂] plus warming and ambient [CO₂]. Overall bacterial community diversity was influenced primarily by depth. Specific bacterial taxa changed in richness and relative abundance in response to climate change factors when assessed by a high‐resolution 16S rRNA microarray (PhyloChip). Operational taxonomic unit signal intensities increased under elevated [CO₂] for both Firmicutes and Bacteroidetes, and increased under warming for Actinobacteria and Alphaproteobacteria. For the interaction of elevated [CO₂] and warming there were 103 significant operational taxonomic units (P < 0.01) representing 15 phyla and 30 classes. The majority of these operational taxonomic units increased in abundance for elevated [CO₂] plus warming plots, while abundance declined in warmed or elevated [CO₂] plots. Bacterial abundance (16S rRNA gene copy number) was significantly different for the interaction of elevated [CO₂] and depth (P < 0.05) with decreased abundance under elevated [CO₂] at 5–10 cm, and for Firmicutes under elevated [CO₂] (P < 0.05). Bacteria, archaea and fungi in soil responded differently to elevated [CO₂], warming and their interaction. Taxa identified as significantly climate‐responsive could show differing trends in the direction of response (‘+’ or ‘?’) under elevated CO₂ or warming, which could then not be used to predict their interactive effects supporting the need to investigate interactive effects for climate change. The approach of focusing on specific taxonomic groups provides greater potential for understanding complex microbial community changes in ecosystems under climate change.     

Effects of inoculation of EPS-producing Pantoea agglomerans on wheat rhizosphere aggregation

We investigated plant and soil nitrogen pools and soil processes in monospecific stands of the C₃ sedge Scirpus olneyi and the C₄ grass Spartina patens grown in the field in open top chambers in a brackish marsh on the Chesapeake Bay. Stands of S. olneyi responded to eight years of elevated CO₂, by increased rates of net ecosystem gas exchange and a large stimulation of net ecosystem production. We conducted our study in the summer of 1994 and 1995 when soil cores were collected and aboveground biomass was estimated. Nitrogen concentration in elevated CO₂ treatments was reduced 15% in stems of S. olneyi and 8% in the upper 10 cm of the soil profile. While total plant nitrogen per unit of land area remained the same between treatments, total soil nitrogen showed a non-significant tendency to decrease in the upper 10 cm of the soil profile in elevated CO₂ both years of study. A significant decrease in soil bulk density largely contributed to the observed decrease in soil nitrogen. Exchangeable nitrogen and potential denitrification rates were also reduced in elevated CO₂, but net nitrogen mineralization was unchanged by elevated CO₂ treatment in S. olneyi both years. Plants and soils in a pure stand of the C₄ grass, S. patens, showed none of these effects of elevated CO₂ treatment. Our data provides evidence of changes in nitrogen dynamics of an ecosystem exposed to elevated CO₂ for eight years; however due to the variability in these data, we cannot say if or how these changes are likely to impact the effect of rising CO₂ on primary production or carbon accumulation in this ecosystem in the future.     

A link between plant diversity, elevated CO2 and soil nitrate

Interactive effects of reductions in plant species diversity and increases in atmospheric CO₂ were investigated in a long-term study in nutrient-poor calcareous grassland. Throughout the experiment, soil nitrate was persistently increased at low plant species diversity, and CO₂ enrichment reduced soil [NO₃^-] at all levels of plant species diversity. In our study, soil [NO₃^-] was unrelated to root length density, microbial biomass N, community legume contents, and experimental plant communities differed only little in total N pools. However, potential nitrification revealed exactly the same treatment effects as soil [NO₃^-], providing circumstantial evidence that nitrification rates drove the observed changes in [NO₃^-]. One possible explanation for plant diversity effects on nitrification lies in spatial and temporal interspecific differences in plant N uptake, which would more often allow accumulation of NH₄⁺ in part of the soil profile at low diversity than in more species-rich plant communities. Consequently, nitrification rates and soil [NO₃^-] would increase. Elevated CO₂ increased soil water contents, which may have improved NO₃^- diffusion to the root surface thereby reducing soil [NO₃^-]. Higher soil moisture at elevated CO₂ might also reduce nitrification rates due to less aerobic conditions. The accordance of the diversity effect on soil [NO₃^-] with previous experiments suggests that increased soil [NO₃^-] at low species diversity is a fairly general phenomenon, although the mechanisms causing high [NO₃^-] may vary. In contrast, experimental evidence for effects of CO₂ enrichment on soil [NO₃^-] is ambiguous, and the antagonistic interaction of plant species reductions and elevated CO₂ we have observed is thus probably less universal.