Combined effects of atmospheric CO<Subscript>2</Subscript> and N availability on the belowground carbon and nitrogen dynamics of aspen mesocosms

It is uncertain whether elevated atmospheric CO₂ will increase C storage in terrestrial ecosystems without concomitant increases in plant access to N. Elevated CO₂ may alter microbial activities that regulate soil N availability by changing the amount or composition of organic substrates produced by roots. Our objective was to determine the potential for elevated CO₂ to change N availability in an experimental plant-soil system by affecting the acquisition of root-derived C by soil microbes. We grew Populus tremuloides (trembling aspen) cuttings for 2 years under two levels of atmospheric CO₂ (36.7 and 71.5 Pa) and at two levels of soil N (210 and 970 μg N g^–1). Ambient and twice-ambient CO₂ concentrations were applied using open-top chambers, and soil N availability was manipulated by mixing soils differing in organic N content. From June to October of the second growing season, we measured midday rates of soil respiration. In August, we pulse-labeled plants with ¹⁴CO₂ and measured soil ¹⁴CO₂ respiration and the ¹⁴C contents of plants, soils, and microorganisms after a 6-day chase period. In conjunction with the August radio-labeling and again in October, we used ¹⁵N pool dilution techniques to measure in situ rates of gross N mineralization, N immobilization by microbes, and plant N uptake. At both levels of soil N availability, elevated CO₂ significantly increased whole-plant and root biomass, and marginally increased whole-plant N capital. Significant increases in soil respiration were closely linked to increases in root biomass under elevated CO₂. CO₂ enrichment had no significant effect on the allometric distribution of biomass or ¹⁴C among plant components, total ¹⁴C allocation belowground, or cumulative (6-day) ¹⁴CO₂ soil respiration. Elevated CO₂ significantly increased microbial ¹⁴C contents, indicating greater availability of microbial substrates derived from roots. The near doubling of microbial ¹⁴C contents at elevated CO₂ was a relatively small quantitative change in the belowground C cycle of our experimental system, but represents an ecologically significant effect on the dynamics of microbial growth. Rates of plant N uptake during both 6-day periods in August and October were significantly greater at elevated CO₂, and were closely related to fine-root biomass. Gross N mineralization was not affected by elevated CO₂. Despite significantly greater rates of N immobilization under elevated CO₂, standing pools of microbial N were not affected by elevated CO₂, suggesting that N was cycling through microbes more rapidly. Our results contained elements of both positive and negative feedback hypotheses, and may be most relevant to young, aggrading ecosystems, where soil resources are not yet fully exploited by plant roots. If the turnover of microbial N increases, higher rates of N immobilization may not decrease N availability to plants under elevated CO₂. Received: 12 February 1999 / Accepted: 2 March 2000     

Soil carbon and nitrogen cycling and storage throughout the soil profile in a sweetgum plantation after 11 years of CO2‐enrichment

Increased partitioning of carbon (C) to fine roots under elevated [CO₂], especially deep in the soil profile, could alter soil C and nitrogen (N) cycling in forests. After more than 11 years of free‐air CO₂ enrichment in a Liquidambar styraciflua L. (sweetgum) plantation in Oak Ridge, TN, USA, greater inputs of fine roots resulted in the incorporation of new C (i.e., C with a depleted δ¹³C) into root‐derived particulate organic matter (POM) pools to 90‐cm depth. Even though production in the sweetgum stand was limited by soil N availability, soil C and N contents were greater throughout the soil profile under elevated [CO₂] at the conclusion of the experiment. Greater C inputs from fine‐root detritus under elevated [CO₂] did not result in increased net N immobilization or C mineralization rates in long‐term laboratory incubations, possibly because microbial biomass was lower in the CO₂‐enriched plots. Furthermore, the δ¹³CO₂ of the C mineralized from the incubated soil closely tracked the δ¹³C of the labile POM pool in the elevated [CO₂] treatment, especially in shallower soil, and did not indicate significant priming of the decomposition of pre‐experiment soil organic matter (SOM). Although potential C mineralization rates were positively and linearly related to total SOM C content in the top 30 cm of soil, this relationship did not hold in deeper soil. Taken together with an increased mean residence time of C in deeper soil pools, these findings indicate that C inputs from relatively deep roots under elevated [CO₂] may increase the potential for long‐term soil C storage. However, C in deeper soil is likely to take many years to accrue to a significant fraction of total soil C given relatively smaller root inputs at depth. Expanded representation of biogeochemical cycling throughout the soil profile may improve model projections of future forest responses to rising atmospheric [CO₂].     

Root biomass and nutrient dynamics in a scrub-oak ecosystem under the influence of elevated atmospheric CO2

Elevated CO₂ can increase fine root biomass but responses of fine roots to exposure to increased CO₂ over many years are infrequently reported. We investigated the effect of elevated CO₂ on root biomass and N and P pools of a scrub-oak ecosystem on Merritt Island in Florida, USA, after 7 years of CO₂ treatment. Roots were removed from 1-m deep soil cores in 10-cm increments, sorted into different categories (<0.25 mm, 0.25–1 mm, 1–2 mm, 2 mm to 1 cm, >1 cm, dead roots, and organic matter), weighed, and analyzed for N, P and C concentrations. With the exception of surface roots <0.25 mm diameter, there was no effect of elevated CO₂ on root biomass. There was little effect on C, N, or P concentration or content with the exception of dead roots, and <0.25 mm and 1–2 mm diameter live roots at the surface. Thus, fine root mass and element content appear to be relatively insensitive to elevated CO₂. In the top 10 cm of soil, biomass of roots with a diameter of <0.25 mm was depressed by elevated CO₂. Elevated CO₂ tended to decrease the mass and N content of dead roots compared to ambient CO₂. A decreased N concentration of roots <0.25 mm and 1–2 mm in diameter under elevated CO₂ may indicate reduced N supply in the elevated CO₂ treatment. Our study indicated that elevated CO₂ does not increase fine root biomass or the pool of C in fine roots. In fact, elevated CO₂ tends to reduce biomass and C content of the most responsive root fraction (<0.25 mm roots), a finding that may have more general implications for understanding C input into the soil at higher atmospheric CO₂ concentrations.     

Increased forest carbon storage with increased atmospheric CO2 despite nitrogen limitation: a game‐theoretic allocation model for trees in competition for nitrogen and light

Changes in resource availability often cause competitively driven changes in tree allocation to foliage, wood, and fine roots, either via plastic changes within individuals or through turnover of individuals with differing strategies. Here, we investigate how optimally competitive tree allocation should change in response to elevated atmospheric CO₂ along a gradient of nitrogen and light availability, together with how those changes should affect carbon storage in living biomass. We present a physiologically‐based forest model that includes the primary functions of wood and nitrogen. From a tree's perspective, wood is an offensive and defensive weapon used against neighbors in competition for light. From a biogeochemical perspective, wood is the primary living reservoir of stored carbon. Nitrogen constitutes a tree's photosynthetic machinery and the support systems for that machinery, and its limited availability thus reduces a tree's ability to fix carbon. This model has been previously successful in predicting allocation to foliage, wood, and fine roots along natural productivity gradients. Using game theory, we solve the model for competitively optimal foliage, wood, and fine root allocation strategies for trees in competition for nitrogen and light as a function of CO₂ and nitrogen mineralization rate. Instead of down‐regulating under nitrogen limitation, carbon storage under elevated CO₂ relative to carbon storage at ambient CO₂ is approximately independent of the nitrogen mineralization rate. This surprising prediction is a consequence of both increased competition for nitrogen driving increased fine root biomass and increased competition for light driving increased allocation to wood under elevated CO₂.     

Elevated atmospheric CO2 increases fine root production, respiration, rhizosphere respiration and soil CO2 efflux in Scots pine seedlings

In this study, we investigated the impact of elevated atmospheric CO₂ (ambient + 350 μmol mol^–1) on fine root production and respiration in Scots pine (Pinus sylvestris L.) seedlings. After six months exposure to elevated CO₂, root production measured by root in-growth bags, showed significant increases in mean total root length and biomass, which were more than 100% greater compared to the ambient treatment. This increased root length may have lead to a more intensive soil exploration. Chemical analysis of the roots showed that the roots in the elevated treatment accumulated more starch and had a lower C/N-ratio. Specific root respiration rates were significantly higher in the elevated treatment and this was probably attributed to increased nitrogen concentrations in the roots. Rhizospheric respiration and soil CO₂ efflux were also enhanced in the elevated treatment. These results clearly indicate that under elevated atmospheric CO₂ root production and development in Scots pine seedlings is altered and respiratory carbon losses through the root system are increased.     

Fine root turnover of Japanese white birch (<Emphasis Type="Italic">Betula platyphylla</Emphasis> var. <Emphasis Type="Italic">japonica</Emphasis>) grown under elevated CO<Subscript>2</Subscript> in northern Japan

Key message

Elevated CO ₂ reduced fine root dynamics (production and turnover) of white birch seedlings, especially grown in volcanic ash soil compared with brown forest soil.

Abstract

Increased atmospheric CO₂ usually enhances photosynthetic ability and growth of trees. To understand how increased CO₂ affects below-ground part of trees under varied soil condition, we investigated the responses of the fine root (diameter <2 mm) dynamics of Japanese white birch (Betula platyphylla var. japonica) which was planted in 2010. The three-year-old birch seedlings were grown in four experimental treatments comprising two levels of CO₂, i.e., ambient: 380–390 and elevated: 500 μmol mol^?1, in combination with two kinds of soil: brown forest (BF) soil and volcanic ash (VA) soil which has few nutrients. The growth and turnover of fine roots were measured for 3 years (2011–2013) using the Mini-rhizotron. In the first observation year, live fine root length (standing crop) in BF soil was not affected by CO₂ treatment, but it was reduced by the elevated CO₂ from the second observation year. In VA soil, live fine root length was reduced by elevated CO₂ for all 3 years. Fine root turnover tended to decrease under elevated CO₂ compared with ambient in both soil types during the first and second observation years. Turnover of fine root production and mortality was also affected by the two factors, elevated CO₂ and different soil types. Median longevity of fine root increased under elevated CO₂, especially in VA soil at the beginning, and a shorter fine root lifespan appeared after 2 years of observation (2011–2012). These results suggest that elevated CO₂ does not consistently stimulate fine root turnover, particularly during the plant seedlings stage, as it may depend on the costs and benefits of constructing and retaining roots. Therefore, despite the other uncontrollable environment factors, carbon sequestration to the root system may be varied by CO₂ treatment period, soil type and plant age.

     

Simple additive effects are rare: a quantitative review of plant biomass and soil process responses to combined manipulations of CO2 and temperature

In recent years, increased awareness of the potential interactions between rising atmospheric CO₂ concentrations ([ CO₂ ]) and temperature has illustrated the importance of multifactorial ecosystem manipulation experiments for validating Earth System models. To address the urgent need for increased understanding of responses in multifactorial experiments, this article synthesizes how ecosystem productivity and soil processes respond to combined warming and [ CO₂ ] manipulation, and compares it with those obtained in single factor [ CO₂ ] and temperature manipulation experiments. Across all combined elevated [ CO₂ ] and warming experiments, biomass production and soil respiration were typically enhanced. Responses to the combined treatment were more similar to those in the [ CO₂ ]‐only treatment than to those in the warming‐only treatment. In contrast to warming‐only experiments, both the combined and the [ CO₂ ]‐only treatments elicited larger stimulation of fine root biomass than of aboveground biomass, consistently stimulated soil respiration, and decreased foliar nitrogen (N) concentration. Nonetheless, mineral N availability declined less in the combined treatment than in the [ CO₂ ]‐only treatment, possibly due to the warming‐induced acceleration of decomposition, implying that progressive nitrogen limitation (PNL) may not occur as commonly as anticipated from single factor [ CO₂ ] treatment studies. Responses of total plant biomass, especially of aboveground biomass, revealed antagonistic interactions between elevated [ CO₂ ] and warming, i.e. the response to the combined treatment was usually less‐than‐additive. This implies that productivity projections might be overestimated when models are parameterized based on single factor responses. Our results highlight the need for more (and especially more long‐term) multifactor manipulation experiments. Because single factor CO₂ responses often dominated over warming responses in the combined treatments, our results also suggest that projected responses to future global warming in Earth System models should not be parameterized using single factor warming experiments.     

Soil [N] modulates soil C cycling in CO2‐fumigated tree stands: a meta‐analysis

Under elevated atmospheric CO₂ concentrations, soil carbon (C) inputs are typically enhanced, suggesting larger soil C sequestration potential. However, soil C losses also increase and progressive nitrogen (N) limitation to plant growth may reduce the CO₂ effect on soil C inputs with time. We compiled a data set from 131 manipulation experiments, and used meta‐analysis to test the hypotheses that: (1) elevated atmospheric CO₂ stimulates soil C inputs more than C losses, resulting in increasing soil C stocks; and (2) that these responses are modulated by N. Our results confirm that elevated CO₂ induces a C allocation shift towards below‐ground biomass compartments. However, the increased soil C inputs were offset by increased heterotrophic respiration (Rh), such that soil C content was not affected by elevated CO₂. Soil N concentration strongly interacted with CO₂ fumigation: the effect of elevated CO₂ on fine root biomass and –production and on microbial activity increased with increasing soil N concentration, while the effect on soil C content decreased with increasing soil N concentration. These results suggest that both plant growth and microbial activity responses to elevated CO₂ are modulated by N availability, and that it is essential to account for soil N concentration in C cycling analyses.     

An oxygen-mediated positive feedback between elevated carbon dioxide and soil organic matter decomposition in a simulated anaerobic wetland

We examined the effects of elevated atmospheric CO₂ on soil carbon decomposition in an experimental anaerobic wetland system. Pots containing either bare C₄‐derived soil or the C₃ sedge Scirpus olneyi planted in C₄‐derived soil were incubated in greenhouse chambers at either ambient or twice‐ambient atmospheric CO₂. We measured CO₂ flux from each pot, quantified soil organic matter (SOM) mineralization using δ¹³C, and determined root and shoot biomass. SOM mineralization increased in response to elevated CO₂ by 83–218% (P<0.0001). In addition, soil redox potential was significantly and positively correlated with root biomass (P= 0.003). Our results (1) show that there is a positive feedback between elevated atmospheric CO₂ concentrations and wetland SOM decomposition and (2) suggest that this process is mediated by the release of oxygen from the roots of wetland plants. Because this feedback may occur in any wetland system, including peatlands, these results suggest a limitation on the size of the carbon sink presented by anaerobic wetland soils in a future elevated‐CO₂ atmosphere.     

Net mineralization of N at deeper soil depths as a potential mechanism for sustained forest production under elevated [CO2]

Elevated atmospheric carbon dioxide concentrations [CO₂] is projected to increase forest production, which could increase ecosystem carbon (C) storage. This study contributes to our broad goal of understanding the causes and consequences of increased fine‐root production and mortality under elevated [CO₂] by examining potential gross nitrogen (N) cycling rates throughout the soil profile. Our study was conducted in a CO₂‐enriched sweetgum (Liquidambar styraciflua L.) plantation in Oak Ridge, TN, USA. We used ¹⁵N isotope pool dilution methodology to measure potential gross N cycling rates in laboratory incubations of soil from four depth increments to 60 cm. Our objectives were twofold: (1) to determine whether N is available for root acquisition in deeper soil and (2) to determine whether elevated [CO₂], which has increased inputs of labile C resulting from greater fine‐root mortality at depth, has altered N cycling rates. Although gross N fluxes declined with soil depth, we found that N is potentially available for roots to access, especially below 15 cm depth where rates of microbial consumption of mineral N were reduced relative to production. Overall, up to 60% of potential gross N mineralization and 100% of potential net N mineralization occurred below 15 cm depth at this site. This finding was supported by in situ measurements from ion‐exchange resins, where total inorganic N availability at 55 cm depth was equal to or greater than N availability at 15 cm depth. While it is likely that trees grown under elevated [CO₂] are accessing a larger pool of inorganic N by mining deeper soil, we found no effect of elevated [CO₂] on potential gross or net N cycling rates. Thus, increased root exploration of the soil volume under elevated [CO₂] may be more important than changes in potential gross N cycling rates in sustaining forest responses to rising atmospheric CO₂.     

Response of Norway spruce root system to elevated atmospheric CO2 concentration

Root structure parameters, root biomass and allometric relationships between above- and belowground biomass were investigated in young Norway spruce (Picea abies [L.] Karst.) trees cultivated inside the glass domes with ambient (AC, 375 μmol(CO₂) mol^?1) and elevated (EC, A + 375 μmol(CO₂) mol^?1) atmospheric CO₂ concentrations ([CO₂]). After 8 years of fumigation, a mean EC tree in comparison with AC one exhibited about 37 % higher belowground biomass. The growth of primary root structure was unaffected by elevated [CO₂]; however, the biomass of secondary roots growing on the primary root structure and the biomass of secondary roots growing in the zone between the soil surface and the first primary root ramification were significantly higher in EC comparing with AC treatment about 58 and 70 %, respectively. The finest root’s (diameter up to 1 mm) biomass as well as length and surface area of both primary and secondary root structures showed the highest difference between the treatments; advancing EC to AC by 43 % on average. Therefore, Norway spruce trees cultivated under well-watered and rather nitrogen-poor soil conditions responded to the air elevated [CO₂] environment by the enhancement of the secondary root structure increment, by enlargement of root length and root absorbing area, and also by alternation of root to aboveground organ biomass proportion. Higher root to leaf and root to stem basal area ratios could be beneficial for Norway spruce trees to survive periods with limited soil water availability.     

Production, turnover and mycorrhizal colonization of root systems of three Populus species grown under elevated CO2 (POPFACE)

A fast growing high density Populus plantation located in central Italy was exposed to elevated carbon dioxide for a period of three years. An elevated CO₂ treatment (550 ppm), of 200 ppm over ambient (350 ppm) was provided using a FACE technique. Standing root biomass, fine root turnover and mycorrhizal colonization of the following Populus species was examined: Populus alba L., Populus nigra L., Populus x euramericana Dode (Guinier). Elevated CO₂ increased belowground allocation of biomass in all three species examined, standing root biomass increased by 47–76% as a result of FACE treatment. Similarly, fine root biomass present in the soil increased by 35–84%. The FACE treatment resulted in 55% faster fine root turnover in P. alba and a 27% increase in turnover of roots of P. nigra and P. x euramericana. P. alba and P. nigra invested more root biomass into deeper soil horizon under elevated CO₂. Response of the mycorrhizal community to elevated CO₂ was more varied, the rate of infection increased only in P. alba for both ectomycorrhizal (EM) and arbuscular mycorrhizas (AM). The roots of P. nigra showed greater infection only by AM and the colonization of the root system of P. x euramericana was not affected by FACE treatment. The results suggest that elevated atmospheric CO₂ conditions induce greater belowground biomass investment, which could lead to accumulation of assimilated C in the soil profile. This may have implications for C sequestration and must be taken into account when considering long‐term C storage in the soil.     

Drivers of increased soil respiration in a poplar coppice exposed to elevated CO2

Background and aims

The response of soil respiration (SR) to elevated CO₂ is driven by a number of processes and feedbacks. This work aims to i) detect the effect of elevated CO₂ on soil respiration during the second rotation of a short rotation forest, at two levels of N availability; and ii) identify the main drivers behind any changes in soil respiration.

Methods

A poplar plantation (POP-EUROFACE) was grown for two rotations of 3 years under elevated CO₂ maintained by a FACE (Free Air CO₂ Enrichment) technique. Root biomass, litter production and soil respiration were followed for two consecutive years after coppice.

Results

In the plantation, the stimulation of fine root and litter production under elevated CO₂ observed at the beginning of the rotation declined over time. Soil respiration (SR) was continuously stimulated by elevated CO₂, with a much larger enhancement during the growing (up to 111 %) than in the dormant season (40 %). The SR increase at first appeared to be due to the increase in fine root biomass, but at the end of the 2nd rotation was supported by litter decomposition and the availability of labile C. Soil respiration increase under elevated CO₂ was not affected by N availability.

Conclusions

The stimulation of SR by elevated CO₂ was sustained by the decomposition of above and belowground litter and by the greater availability of easily decomposable substrates into the soil. In the final year as elevated CO₂ did not increase C allocation to roots, the higher SR suggests greater C losses from the soil, thus reducing the potential for C accumulation.     

Correlation of foliage and litter chemistry of sugar maple, Acer saccharum, as affected by elevated CO2 and varying N availability, and effects on decomposition

Rising atmospheric carbon dioxide has the potential to alter leaf litter chemistry, potentially affecting decomposition and rates of carbon and nitrogen cycling in forest ecosystems. This study was conducted to determine whether growth under elevated atmospheric CO₂ altered the quality and microbial decomposition of leaf litter of a widely distributed northern hardwood species at sites of low and high soil nitrogen availability. In addition, we assessed whether the carbon–nutrient balance (CNB) and growth differentiation balance (GDB) hypotheses could be extended to predict changes in litter quality in response to resource availability. Sugar maple (Acer saccharum) was grown in the field in open‐top chambers at 36 and 55 Pa partial pressure CO₂, and initial soil mineralization rates of 45 and 348 μg N g^?1 d^?1. Naturally senesced leaf litter was assessed for chemical composition and incubated in the laboratory for 111 d. Microbial respiration and the production of dissolved organic carbon (DOC) were quantified as estimates of decomposition. Elevated CO₂ and low soil nitrogen resulted in higher litter concentrations of nonstructural carbohydrates and condensed tannins, higher C/N ratios and lower N concentrations. Soil N availability appears to have had a greater effect on litter quality than did atmospheric CO₂, although the treatments were additive, with highest concentrations of nonstructural carbohydrates and condensed tannins occurring under elevated CO₂–low soil N. Rates of microbial respiration and the production of DOC were insensitive to differences in litter quality. In general, concentrations of litter constituents, except for starch, were highly correlated to those in live foliage, and the CNB/GDB hypotheses proved useful in predicting changes in litter quality. We conclude the chemical composition of sugar maple litter will change in the future in response to rising atmospheric CO₂, and that soil N availability will exert a major control. It appears that microbial metabolism will not be directly affected by changes in litter quality, although conclusions regarding decomposition as a whole must consider the entire soil food web.     

Accelerated belowground C cycling in a managed agriforest ecosystem exposed to elevated carbon dioxide concentrations

We investigated the effects of three elevated atmospheric CO₂ levels on a Populus deltoides plantation at Biosphere 2 Laboratory in Oracle Arizona. Stable isotopes of carbon have been used as tracers to separate the carbon present before the CO₂ treatments started (old C), from that fixed after CO₂ treatments began (new C). Tree growth at elevated [CO₂] increased inputs to soil organic matter (SOM) by increasing the production of fine roots and accelerating the rate of root C turnover. However, soil carbon content decreased as [CO₂] in the atmosphere increased and inputs of new C were not found in SOM. Consequently, the rates of soil respiration increased by 141% and 176% in the 800 and 1200 μL L^?1 plantations, respectively, when compared with ambient [CO₂] after 4 years of exposure. However, the increase in decomposition of old SOM (i.e. already present when CO₂ treatments began) accounted for 72% and 69% of the increase in soil respiration seen under elevated [CO₂]. This resulted in a net loss of soil C at a rate that was between 10 and 20 times faster at elevated [CO₂] than at ambient conditions. The inability to retain new and old C in the soil may stem from the lack of stabilization of SOM, allowing for its rapid decomposition by soil heterotrophs.     

Response of soil biota to elevated atmospheric CO2 in poplar model systems

We tested the hypotheses that increased belowground allocation of carbon by hybrid poplar saplings grown under elevated atmospheric CO₂ would increase mass or turnover of soil biota in bulk but not in rhizosphere soil. Hybrid poplar saplings (Populus×euramericana cv. Eugenei) were grown for 5 months in open-bottom root boxes at the University of Michigan Biological Station in northern, lower Michigan. The experimental design was a randomized-block design with factorial combinations of high or low soil N and ambient (34 Pa) or elevated (69 Pa) CO₂ in five blocks. Rhizosphere microbial biomass carbon was 1.7 times greater in high-than in low-N soil, and did not respond to elevated CO₂. The density of protozoa did not respond to soil N but increased marginally (P < 0.06) under elevated CO₂. Only in high-N soil did arbuscular mycorrhizal fungi and microarthropods respond to CO₂. In high-N soil, arbuscular mycorrhizal root mass was twice as great, and extramatrical hyphae were 11% longer in elevated than in ambient CO₂ treatments. Microarthropod density and activity were determined in situ using minirhizotrons. Microarthropod density did not change in response to elevated CO₂, but in high-N soil, microarthropods were more strongly associated with fine roots under elevated than ambient treatments. Overall, in contrast to the hypotheses, the strongest response to elevated atmospheric CO₂ was in the rhizosphere where (1) unchanged microbial biomass and greater numbers of protozoa (P < 0.06) suggested faster bacterial turnover, (2) arbuscular mycorrhizal root length increased, and (3) the number of microarthropods observed on fine roots rose. Received: 18 March 1997 / Accepted: 5 August 1997     

Regenerating temperate forest mesocosms in elevated CO2: belowground growth and nitrogen cycling

The response of temperate forest ecosystems to elevated atmospheric CO₂ concentrations is important because these ecosystems represent a significant component of the global carbon cycle. Two important but not well understood processes which elevated CO₂ may substantially alter in these systems are regeneration and nitrogen cycling. If elevated CO₂ leads to changes in species composition in regenerating forest communities then the structure and function of these ecosystems may be affected. In most temperate forests, nitrogen appears to be a limiting nutrient. If elevated CO₂ leads to reductions in nitrogen cycling through increased sequestration of nitrogen in plant biomass or reductions in mineralization rates, long-term forest productivity may be constrained. To study these processes, we established mesocosms of regenerating forest communities in controlled environments maintained at either ambient (375 ppm) or elevated (700 ppm) CO₂ concentrations. Mesocosms were constructed from intact monoliths of organic forest soil. We maintained these mesocosms for 2 years without any external inputs of nitrogen and allowed the plants naturally present as seeds and rhizomes to regenerate. We used ¹⁵N pool dilution techniques to quantify nitrogen fluxes within the mesocosms at the end of the 2 years. Elevated atmospheric CO₂ concentration significantly affected a number of plant and soil processes in the experimental regenerating forest mesocosms. These changes included increases in total plant biomass production, plant C/N ratios, ectomycorrhizal colonization of tree fine roots, changes in tree fine root architecture, and decreases in plant NH₄ ⁺ uptake rates, gross NH₄ ⁺ mineralization rates, and gross NH₄ ⁺ consumption rates. In addition, there was a shift in the relative biomass contribution of the two dominant regenerating tree species; the proportion of total biomass contributed by white birch (Betula papyrifera) decreased and the proportion of total biomass contributed by yellow birch (B. alleghaniensis) increased. However, elevated CO₂ had no significant effect on the total amount of nitrogen in plant and soil microbial biomass. In this study we observed a suite of effects due to elevated CO₂, some of which could lead to increases in potential long term growth responses to elevated CO₂, other to decreases. The reduced plant NH₄ ⁺ uptake rates we observed are consistent with reduced NH₄ ⁺ availability due to reduced gross mineralization rates. Reduced NH₄ ⁺ mineralization rates are consistent with the increases in C/N ratios we observed for leaf and fine root material. Together, these data suggest the positive increases in plant root architectural parameters and mycorrhizal colonization may not be as important as the potential negative effects of reduced nitrogen availability through decreased decomposition rates in a future atmosphere with elevated CO₂. Received: 10 January 1997 / Accepted: 25 July 1997     

Disentangling root responses to climate change in a semiarid grassland

Future ecosystem properties of grasslands will be driven largely by belowground biomass responses to climate change, which are challenging to understand due to experimental and technical constraints. We used a multi-faceted approach to explore single and combined impacts of elevated CO₂ and warming on root carbon (C) and nitrogen (N) dynamics in a temperate, semiarid, native grassland at the Prairie Heating and CO₂ Enrichment experiment. To investigate the indirect, moisture mediated effects of elevated CO₂, we included an irrigation treatment. We assessed root standing mass, morphology, residence time and seasonal appearance/disappearance of community-aggregated roots, as well as mass and N losses during decomposition of two dominant grass species (a C₃ and a C₄). In contrast to what is common in mesic grasslands, greater root standing mass under elevated CO₂ resulted from increased production, unmatched by disappearance. Elevated CO₂ plus warming produced roots that were longer, thinner and had greater surface area, which, together with greater standing biomass, could potentially alter root function and dynamics. Decomposition increased under environmental conditions generated by elevated CO₂, but not those generated by warming, likely due to soil desiccation with warming. Elevated CO₂, particularly under warming, slowed N release from C₄—but not C₃—roots, and consequently could indirectly affect N availability through treatment effects on species composition. Elevated CO₂ and warming effects on root morphology and decomposition could offset increased C inputs from greater root biomass, thereby limiting future net C accrual in this semiarid grassland.     

Root to shoot ratio of crops as influenced by CO2

Crops of tomorrow are likely to grow under higher levels of atmospheric CO₂. Fundamental crop growth processes will be affected and chief among these is carbon allocation. The root to shoot ratio (R:S, defined as dry weight of root biomass divided by dry weight of shoot biomass) depends upon the partitioning of photosynthate which may be influenced by environmental stimuli. Exposure of plant canopies to high CO₂ concentration often stimulates the growth of both shoot and root, but the question remains whether elevated atmospheric CO₂ concentration will affect roots and shoots of crop plants proportionally. Since elevated CO₂ can induce changes in plant structure and function, there may be differences in allocation between root and shoot, at least under some conditions. The effect of elevated atmospheric CO₂ on carbon allocation has yet to be fully elucidated, especially in the context of changing resource availability. Herein we review root to shoot allocation as affected by increased concentrations of atmospheric CO₂ and provide recommendations for further research. Review of the available literature shows substantial variation in R:S response for crop plants. In many cases (59.5%) R:S increased, in a very few (3.0%) remained unchanged, and in others (37.5%) decreased. The explanation for these differences probably resides in crop type, resource supply, and other experimental factors. Efforts to understand allocation under CO₂ enrichment will add substantially to the global change response data base.Abbreviations R:S root to shoot ratio, dry weight basis