Genetic components to belowground carbon fluxes in a riparian forest ecosystem: a common garden approach

Soil carbon dioxide (CO(2)) efflux is a major component of terrestrial carbon (C) cycles; yet, the demonstration of covariation between overstory tree genetic-based traits and soil C flux remains a major frontier in understanding biological controls over soil C. Here, we used a common garden with two native tree species, Populus fremontii and P. angustifolia, and their naturally occurring hybrids to test the predictability of belowground C fluxes on the basis of taxonomic identity and genetic marker composition of replicated clones of individual genotypes. Three patterns emerged: soil CO(2) efflux and ratios of belowground flux to aboveground productivity differ by as much as 50-150% as a result of differences in clone identity and cross type; on the basis of Mantel tests of molecular marker matrices, we found that c. 30% of this variation was genetically based, in which genetically similar trees support more similar soil CO(2) efflux under their canopies than do genetically dissimilar trees; and the patterns detected in an experimental garden match observations in the wild, and seem to be unrelated to measured abiotic factors. Our findings suggest that the genetic makeup of the plants growing on soil has a significant influence on the release of C from soils to the atmosphere.     

Total Belowground Carbon Allocation in a Fast-growing Eucalyptus Plantation Estimated Using a Carbon Balance Approach

Trees allocate a large portion of gross primary production belowground for the production and maintenance of roots and mycorrhizae. The difficulty of directly measuring total belowground carbon allocation (TBCA) has limited our understanding of belowground carbon (C) cycling and the factors that control this important flux. We measured TBCA over 4 years using a conservation of mass, C balance approach in replicate stands of fast growing Eucalyptus saligna Smith with different nutrition management and tree density treatments. We measured TBCA as surface carbon dioxide (CO₂) efflux (“soil” respiration) minus C inputs from aboveground litter plus the change in C stored in roots, litter, and soil. We evaluated this C balance approach to measuring TBCA by examining (a) the variance in TBCA across replicate plots; (b) cumulative error associated with summing components to arrive at our estimates of TBCA; (c) potential sources of error in the techniques and assumptions; (d) the magnitude of changes in C stored in soil, litter, and roots compared to TBCA; and (e) the sensitivity of our measures of TBCA to differences in nutrient availability, tree density, and forest age. The C balance method gave precise estimates of TBCA and reflected differences in belowground allocation expected with manipulations of fertility and tree density. Across treatments, TBCA averaged 1.88 kg C m⁻² y⁻¹ and was 18% higher in plots planted with 10⁴ trees/ha compared to plots planted with 1111 trees/ha. TBCA was 12% lower (but not significantly so) in fertilized plots. For all treatments, TBCA declined linearly with stand age. The coefficient of variation (CV) for TBCA for replicate plots averaged 17%. Averaged across treatments and years, annual changes in C stored in soil, the litter layer, and coarse roots (−0.01, 0.06, and 0.21 kg C m⁻² y⁻¹, respectively) were small compared with surface CO₂ efflux (2.03 kg C m⁻² y⁻¹), aboveground litterfall (0.42 kg C m⁻² y⁻¹), and our estimated TBCA (1.88 kg C m⁻² y⁻¹). Based on studies from similar sites, estimates of losses of C through leaching, erosion, or storage of C in deep soil were less than 1% of annual TBCA. Received 6 March 2001; accepted 7 January 2002.     

Physiological girdling of pine trees via phloem chilling: proof of concept

Quantifying below-ground carbon (C) allocation is particularly difficult as methods usually disturb the root-mycorrhizal-soil continuum. We reduced C allocation below ground of loblolly pine trees by: (1) physically girdling trees and (2) physiologically girdling pine trees by chilling the phloem. Chilling reduced cambium temperatures by approximately 18 degrees C. Both methods rapidly reduced soil CO2 efflux, and after approximately 10 days decreased net photosynthesis (P(n)), the latter indicating feedback inhibition. Chilling decreased soil-soluble C, indicating that decreased soil CO2 efflux may have been mediated by a decrease in root C exudation that was rapidly respired by microbes. These effects were only observed in late summer/early autumn when above-ground growth was minimal, and not in the spring when above-ground growth was rapid. All of the effects were rapidly reversed when chilling was ceased. In fertilized plots, both chilling and physical girdling methods reduced soil CO2 efflux by approximately 8%. Physical girdling reduced soil CO2 efflux by 26% in non-fertilized plots. This work demonstrates that phloem chilling provides a non-destructive alternative to reducing the movement of recent photosynthate below the point of chilling to estimate C allocation below ground on large trees.     

Below-ground respiratory responses of sugar maple and red maple saplings to atmospheric CO2 enrichment and elevated air temperature

The research described in this paper represents a part of a much broader research project with the general objective of describing the effects of elevated [CO2] and temperature on tree growth, physiological processes, and ecosystem-level processes. The specific objective of this research was to examine the below-ground respiratory responses of sugar maple (Acer saccharum Marsh.) and red maple (Acer rubrum L.) seedlings to elevated atmospheric [CO2] and temperature. Red maple and sugar maple seedlings were planted in the ground in each of 12 open-top chambers and exposed from 1994 through 1997 to ambient air or air enriched with 30 Pa CO2,< in combination with ambient or elevated (+4 °C) air temperatures. Carbon dioxide efflux was measured around the base of the seedlings and from root-exclusion zones at intervals during 1995 and 1996 and early 1997. The CO2 efflux rates averaged 0.4 μmol CO2 m-2 s-1 in the root-exclusion zones and 0.75 μmol CO2 m-2 s-1 around the base of the seedlings. Mineral soil respiration in root-exclusion zones averaged 12% higher in the high temperature treatments than at ambient temperature, but was not affected by CO2 treatments. The fraction of total efflux attributable to root + rhizosphere respiration ranged from 14 to 61% in measurements made around red maple plants, and from 35 to 62% around sugar maple plants. Root respiration rates ranged from 0 to 0.94 μmol CO2 s-1 m-2 of soil surface in red maple and from 0 to 1.02 in sugar maple. In both 1995 and 1996 root respiration rates of red maple were highest in high-CO2 treatments and lowest in high temperature treatments. Specific red maple root respiration rates of excised roots from near the soil surface in 1996 were also highest under CO2 enrichment and lowest in high temperature treatments. In sugar maple the highest rates of CO2 efflux were from around the base of plants exposed to both high temperature and high-CO2, even though specific respiration rates were< lowest for this species under the high temperature and CO2 enrichment regime. In both species, patterns of response to treatments were similar in root respiration and root mass, indicating that the root respiration responses were due in part to differences in root mass. The results underscore the need for separating the processes occurring in the roots from those in the forest floor and mineral soil in order to increase our understanding of the effects of global climate change on carbon sequestration and cycling in the below-ground systems of forests.     

Clonal variation in above- and below-ground growth responses of Populus tremuloides Michaux: Influence of soil warming and nutrient availability

Trembling aspen (Populus tremuloides Michx.) is the most widely distributed tree species in North America making it important to terrestrial carbon and nutrient cycles. Due to anthropogenic climate change high latitude temperatures are expected to increase, making it necessary to assess the feedback between above- and below-ground carbon pools to increased temperature at sites of both high and low N-availability. We grew four clones of aspen at two levels of soil temperature and two levels of soil N-availability for 98 days and quantified photosynthesis, growth, biomass allocation, and root length production and mortality. High soil temperature increased rates of photosynthesis (65%), resulting in greater whole-plant growth (37%) through increases in roots, stems, and foliage; however these increases generally occurred only in soil of high N-availability. Root:shoot biomass allocation varied between clones but was unaffected by the soil temperature or N-availability treatments. Root length production and mortality increased at elevated soil temperature, but this response was modified by soil N-availability. At high soil temperature, soil N-availability had little effect on root dynamics, while at low soil temperature, high soil N-availability increased both the production and mortality (turnover) of roots. We conclude that trembling aspen has the potential for substantially greater growth and root turnover under conditions of warmer soil at sites of both high and low N-availability, but that allometric patterns of growth are under strong genetic, rather than environmental control. This revised version was published online in June 2006 with corrections to the Cover Date.     

格氏栲和杉木人工林地下碳分配

通过对福建三明36年生的格氏栲人工林和杉木人工林林木地下C分配(TBCA)进行研究,结果表明,由分室累加法直接测定的格氏栲和杉木人工林的TBCA分别为8.426和4.040 t C.hm-2.-a 1。在格氏栲和杉木人工林TBCA组成中,根系净生产量和根系呼吸各约占50%;在根系年净生产量中,细根年净生产量和粗根年净生产量各约占75%和25%。而格氏栲和杉木人工林的细根年C归还量则均约占各自TBCA的1/3(分别为33%和36%)。在假设地下C库处于稳定状态时,由C平衡法计算的格氏栲和杉木人工林的TBCA(分别为6.039t C.hm-2.-a 1和2.987 t C.hm-2.-a 1)低于分室累加法,这与两种人工林地下C库尚未达到稳定状态有关。利用R a ich and N ade lhoffer全球模式方程推算的格氏栲和杉木人工林的TBCA(分别为9.771t C.hm-2.a-1和5.344 t C.hm-2.-a 1)则高于分室累加法,这与全球模式方程只是一种全球尺度规律有关。     

Transpiration alters the contribution of autotrophic and heterotrophic components of soil CO2 efflux

? An unbiased partitioning of autotrophic and heterotrophic components of soil CO(2) efflux is important to estimate forest carbon budgets and soil carbon sequestration. The contribution of autotrophic sources to soil CO(2) efflux (F(A)) may be underestimated during the daytime as a result of internal transport of CO(2) produced by root respiration through the transpiration stream. ? Here, we tested the hypothesis that carbon isotope composition of soil CO(2) efflux (δ(FS)) in a Eucalyptus plantation grown on a C(4) soil is enriched during the daytime, which will indicate a decrease in F(A) during the periods of high transpiration. ? Mean δ(FS) of soil CO(2) efflux decreased to -25.7‰ during the night and increased to -24.7‰ between 11:00 and 15:00 h when the xylem sap flux density was at its maximum. ? Our results indicate a decrease in the contribution of root respiration to soil CO(2) efflux during the day that may be interpreted as a departure of root-produced CO(2) in the transpiration stream, leading to a 17% underestimation of autotrophic contribution to soil CO(2) efflux on a daily timescale.     

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.     

Fertilization effects on fineroot biomass, rhizosphere microbes and respiratory fluxes in hardwood forest soils

Fertilizer-induced reductions in CO(2) flux from soil ((F)CO(2)) in forests have previously been attributed to decreased carbon allocation to roots, and decreased decomposition as a result of nitrogen suppression of fungal activity. Here, we present evidence that decreased microbial respiration in the rhizosphere may also contribute to (F)CO(2) reductions in fertilized forest soils. Fertilization reduced (F)CO(2) by 16-19% in 65-yr-old plantations of northern red oak (Quercus rubra) and sugar maple (Acer saccharum), and in a natural 85-yr-old yellow birch (Betula allegheniensis) stand. In oak plots, fertilization had no effects on fine root biomass but reduced mycorrhizal colonization by 18% and microbial respiration by 43%. In maple plots, fertilization reduced root biomass, mycorrhizal colonization and microbial respiration by 22, 16 and 46%, respectively. In birch plots, fertilization reduced microbial respiration by 36%, but had variable effects on root biomass and mycorrhizal colonization. In plots of all three species, fertilization effects on microbial respiration were greater in rhizosphere than in bulk soil, possibly as a result of decreased rhizosphere carbon flux from these species in fertile soils. Because rhizosphere processes may influence nutrient availability and carbon storage in forest ecosystems, future research is needed to better quantify rhizo-microbial contributions to (F)CO(2).     

Adaptation of tree growth to elevated CO2: quantitative trait loci for biomass in Populus

* Information on the genetic variation of plant response to elevated CO(2) (e[CO(2)]) is needed to understand plant adaptation and to pinpoint likely evolutionary response to future high atmospheric CO(2) concentrations. * Here, quantitative trait loci (QTL) for above- and below-ground tree growth were determined in a pedigree - an F(2) hybrid of poplar (Populus trichocarpa and Populus deltoides), following season-long exposure to either current day ambient CO(2) (a[CO(2)]) or e[CO(2)] at 600 microl l(-1), and genotype by environment interactions investigated. * In the F(2) generation, both above- and below-ground growth showed a significant increase in e[CO(2)]. Three areas of the genome on linkage groups I, IX and XII were identified as important in determining above-ground growth response to e[CO(2)], while an additional three areas of the genome on linkage groups IV, XVI and XIX appeared important in determining root growth response to e[CO(2)]. * These results quantify and identify genetic variation in response to e[CO(2)] and provide an insight into genomic response to the changing environment.     

杉木林年龄序列地下碳分配变化

  森林地下碳分配在森林碳平衡和碳吸存中具有重要作用, 而揭示人工林生长过程中地下碳分配变化对于人工林碳汇估算和碳汇管理等有重要意义。通过采用年龄序列方法研究了杉木(Cunninghamia lanceolata)林生长过程中地下碳分配变化特点。年龄序列为福建省南平7 a生(幼龄林)、16 a生(中龄林)、21 a生(近熟林)、41 a生(成熟林)和88 a生(老龄林)的杉木林。细根净生产力测定采用连续土芯法, 根系呼吸测定采用壕沟法, 生物量增量测定采用异速生长方程, 地上年凋落物量采用凋落物收集框测定。结果表明: 杉木林细根净生产力在中龄林前没有显著差异, 维持在较高水平; 但此后则显著下降。细根净生产力/地上凋落物量比值随林龄增加而显著下降。老龄林的根系呼吸显著低于其它林龄林分, 根系呼吸与细根生物量间呈显著线性相关。中龄林和近成熟林的地下碳分配(Total belouground carbon allocation, TBCA)显著高于幼龄林和成熟林, 而老龄林的则最低。中龄林、近成熟林和成熟林的地上部分净生产力/TBCA比值显著高于幼龄林和老龄林, 而杉木林的根系碳利用效率(RCUE)则呈现出随林龄增加而降低的趋势。     

Shifts in fine root traits within and among species along a fine-scale hydrological gradient

Background and AimsLessons from above-ground trait ecology and resource economics theory may not be directly translatable to below-ground traits due to differences in function, trade-offs and environmental constraints. Here we examine root functional traits within and across species along a fine-scale hydrological gradient. We ask two related questions: (1) What is the relative magnitude of trait variation across the gradient for within- versus among-species variation? (2) Do correlations among below-ground plant traits conform with predictions from resource-economic spectrum theory?MethodsWe sampled four below-ground fine-root traits (specific root length, branching intensity, root tissue density and root dry matter content) and four above-ground traits (specific leaf area, leaf size, plant height and leaf dry matter content) in vascular plants along a fine-scale hydrological gradient within a wet heathland community in south-eastern Australia. Below-ground and above-ground traits were sampled both within and among species.Key ResultsRoot traits shifted both within and among species across the hydrological gradient. Within- and among-species patterns for root tissue density showed similar declines towards the wetter end of the gradient. Other root traits showed a variety of patterns with respect to within- and among-species variation. Filtering of species has a stronger effect compared with the average within-species shift: the slopes of the relationships between soil moisture and traits were steeper across species than slopes of within species. Between species, below-ground traits were only weakly linked to each other and to above-ground traits, but these weak links did in some cases correspond with predictions from economic theory.ConclusionsOne of the challenges of research on root traits has been considerable intraspecific variation. Here we show that part of intraspecific root trait variation is structured by a fine-scale hydrological gradient, and that the variation aligns with among-species trends in some cases. Patterns in root tissue density are especially intriguing and may play an important role in species and individual response to moisture conditions. Given the importance of roots in the uptake of resources, and in carbon and nutrient turnover, it is vital that we establish patterns of root trait variation across environmental gradients.     

On the multi-temporal correlation between photosynthesis and soil CO2 efflux: reconciling lags and observations

Although there is increasing evidence of the temporal correlation between photosynthesis and soil CO(2) efflux, no study has so far tested its generality across the growing season at multiple study sites and across several time scales. Here, we used continuous (hourly) data and applied time series analysis (wavelet coherence analysis) to identify temporal correlations and time lags between photosynthesis and soil CO(2) efflux for three forests from different climates and a grassland. Results showed the existence of multi-temporal correlations at time periods that varied between 1 and 16 d during the growing seasons at all study sites. Temporal correlations were strongest at the 1 d time period, with longer time lags for forests relative to the grassland. The multi-temporal correlations were not continuous throughout the growing season, and were weakened when the effect of variations in soil temperature and CO(2) diffusivity on soil CO(2) efflux was taken into account. Multi-temporal correlations between photosynthesis and soil CO(2) efflux exist, and suggest that multiple biophysical drivers (i.e. photosynthesis, soil CO(2) diffusion, temperature) are likely to coexist for the regulation of allocation and transport speed of carbon during a growing season. Future studies should consider the multi-temporal influence of these biophysical drivers to investigate their effect on the transport of carbon through the soil-plant-atmosphere continuum.     

Carbon balance of a tropical savanna of northern Australia

Through estimations of above- and below-ground standing biomass, annual biomass increment, fine root production and turnover, litterfall, canopy respiration and total soil CO₂ efflux, a carbon balance on seasonal and yearly time-scales is developed for a Eucalypt open-forest savanna in northern Australia. This carbon balance is compared to estimates of carbon fluxes derived from eddy covariance measurements conducted at the same site. The total carbon (C) stock of the savanna was 204±53 ton C ha^–1, with approximately 84% below-ground and 16% above-ground. Soil organic carbon content (0–1 m) was 151±33 ton C ha^–1, accounting for about 74% of the total carbon content in the ecosystem. Vegetation biomass was 53±20 ton C ha^–1, 39% of which was found in the root component and 61% in above-ground components (trees, shrubs, grasses). Annual gross primary production was 20.8 ton C ha^–1, of which 27% occurred in above-ground components and 73% below-ground components. Net primary production was 11 ton C ha^–1 year^–1, of which 8.0 ton C ha^–1 (73%) was contributed by below-ground net primary production and 3.0 ton C ha^–1 (27%) by above-ground net primary production. Annual soil carbon efflux was 14.3 ton C ha^–1 year^–1. Approximately three-quarters of the carbon flux (above-ground, below-ground and total ecosystem) occur during the 5–6 months of the wet season. This savanna site is a carbon sink during the wet season, but becomes a weak source during the dry season. Annual net ecosystem production was 3.8 ton C ha^–1 year^–1.     

喀斯特地区土壤表层CO2释放通量的影响因素Ⅰ:规律

测定了贵州喀斯特地区土壤表层CO2释放通量,同时还测定了土壤微生物生物量碳以及土壤可溶性有机质含量和土壤湿度。研究表明,贵州喀斯特地区全年土壤表层CO2释放通量与温度变化呈正相关关系,与土壤微生物生物量碳呈负相关关系;当温度＞20℃时,土壤表层CO2释放通量与土壤湿度呈正相关,与土壤可溶性有机碳含量呈负相关。     

Soil Respiration along Environmental Gradients in Olympic National Park

Although mountainous landscapes dominate large areas of the Earth, our understanding of how elevation and aspect influence soil respiration in complex mountainous terrain is very limited. Therefore, we measured soil respiration throughout the growing season in 1999 and 2000 at 11 forested sites in Olympic National Park, Washington, USA along elevation-climatic gradients. The study sites ranged from temperate rain forest to alpine forests near tree line. Soil temperature was a significant predictor of soil respiration at all sites, and soil moisture explained additional variability at three sites (R2 from 0.42 to 0.90, P ≤ 0.01). Soil temperatures at the highest-elevation sites were 4.5°C cooler than those at the lowest elevation, but there were no relationships between soil respiration rates at a given temperature and elevation or mean annual temperature that would indicate acclimation of soil respiration to the cooler temperatures at high-elevation sites. Experimental urea additions (1.0 and 2.0 g N m-2 y-1) made at seven of the sites had no consistent effect on soil respiration. Total soil carbon dioxide (CO2) efflux during the growing season (May-September) varied from 0.34 to 0.75 kg C/m2 and was greater at low-elevation sites with warmer soil temperatures and longer growing seasons. Elevation and the length of the frost-free season could both be used to predict growing season (r2 = 0.53) and annual (r2 = 0.81) soil CO2 efflux for the 10 sites located in steep mountainous terrain. Significant correlations also existed with mean annual temperature. These results suggest that warmer soils and a longer snow-free season associated with climatic warming could cause the mountainous ecosystems of the Olympic peninsula to evolve increasing amounts of CO2 from all elevations and aspects.     

Contrasting seasonal patterns of fine root production for blue oaks (Quercus douglasii) and annual grasses in California oak woodland

In a blue oak woodland in NE California, we used root ingrowth cores to study seasonal patterns of fine root (< 2 mm diameter) production (FRP) for annual grasses and blue oaks (Quercus douglasiiHook and Arn.). At each of three sites (River, Upland and Hilltop) there were three studies: a short-term seasonal study, a long-term cumulative study and a core nutrient enrichment study. In the short-term study, ingrowth cores were installed and harvested in 3-month intervals. Grass and oak FRP dominated in different seasons. Grass FRP was greater in fall (Oct–Dec, 26.0 g m^–2 month^–1(gmm)) and winter (Jan–Mar, 18.1 gmm), lower in spring (Apr–Jun, 2.9 gmm) and negligible in summer (July–Sep). In contrast, oak FRP was greater in spring (6.1 gmm) with lower, but significant summer growth (2.9 gmm). Grass FRP declined steadily with soil depth (0–40 cm) in all seasons. Oak FRP changed little with soil depth, except in the summer, when production was greatest at lower soil depths. In the long-term study, ingrowth cores were installed and harvested after 3, 6, 9 and 12 months. Oak annual FRP was greater in the long-term study than in the short-term study at the River and Upland sites. Thus production in the short-term study may be an underestimate, due to disturbance effects. Nutrient enrichment of cores with N-P-K increased grass FRP, but surprisingly, decreased oak FRP, suggesting that annual grasses may have competitive advantages in nutrient enriched soil.     

The fate of photosynthetically-fixed carbon in Lolium perenne grassland as modified by elevated CO2 and sward management

Prediction of the impact of climate change requires the response of carbon (C) flow in plant-soil systems to increased CO(2) to be understood. A mechanism by which grassland C sequestration might be altered was investigated by pulse-labelling Lolium perenne swards, which had been subject to CO(2) enrichment and two levels of nitrogen (N) fertilization for 10 yr, with (14)CO(2). Over a 6-d period 40-80% of the (14)C pulse was exported from mature leaves, 1-2% remained in roots, 2-7% was lost as below-ground respiration, 0.1% was recovered in soil solution, and 0.2-1.5% in soil. Swards under elevated CO(2) with the lower N supply fixed more (14)C than swards grown in ambient CO(2), exported more fixed (14)C below ground and respired less than their high-N counterparts. Sward cutting reduced root (14)C, but plants in elevated CO(2) still retained 80% more (14)C below ground than those in ambient CO(2). The potential for below-ground C sequestration in grasslands is enhanced under elevated CO(2), but any increase is likely to be small and dependent upon grassland management.     

Forest soil CO2 flux: uncovering the contribution and environmental responses of ectomycorrhizas

Forests play a critical role in the global carbon cycle, being considered an important and continuing carbon sink. However, the response of carbon sequestration in forests to global climate change remains a major uncertainty, with a particularly poor understanding of the origins and environmental responses of soil CO₂ efflux. For example, despite their large biomass, the contribution of ectomycorrhizal (EM) fungi to forest soil CO₂ efflux and responses to changes in environmental drivers has, to date, not been quantified in the field. Their activity is often simplistically included in the ‘autotrophic’ root respiration term. We set up a multiplexed continuous soil respiration measurement system in a young Lodgepole pine forest, using a mycorrhizal mesh collar design, to monitor the three main soil CO₂ efflux components: root, extraradical mycorrhizal hyphal, and soil heterotrophic respiration. Mycorrhizal hyphal respiration increased during the first month after collar insertion and thereafter remained remarkably stable. During autumn the soil CO₂ flux components could be divided into ∼60% soil heterotrophic, ∼25% EM hyphal, and ∼15% root fluxes. Thus the extraradical EM mycelium can contribute substantially more to soil CO₂ flux than do roots. While EM hyphal respiration responded strongly to reductions in soil moisture and appeared to be highly dependent on assimilate supply, it did not responded directly to changes in soil temperature. It was mainly the soil heterotrophic flux component that caused the commonly observed exponential relationship with temperature. Our results strongly suggest that accurate modelling of soil respiration, particularly in forest ecosystems, needs to explicitly consider the mycorrhizal mycelium and its dynamic response to specific environmental factors. Moreover, we propose that in forest ecosystems the mycorrhizal CO₂ flux component represents an overflow ‘CO₂ tap’ through which surplus plant carbon may be returned directly to the atmosphere, thus limiting expected carbon sequestration from trees under elevated CO₂.     

黄土丘陵区不同演替阶段生物结皮对土壤CO2通量的影响

生物结皮是土壤表面具有光合活性的致密复合层,是土 气界面CO₂通量的影响因子之一.本文采用改进的Li-8100土壤碳通量测量系统,研究了黄土丘陵区退耕地上不同演替阶段生物结皮对土壤CO₂通量的影响.结果表明: 光照条件下,生物结皮土壤CO₂通量较除去生物结皮显著下降,其中藻结皮和藓结皮分别下降了92%和305%;生物结皮对土壤CO₂通量的降低程度与其生物组成和生物量有关,深色藻结皮和藓结皮土壤CO₂通量较裸地分别降低了141%和484%.生物结皮土壤CO₂通量的日变化呈降低-升高-降低的趋势,而裸地CO₂通量日变化趋势为单峰曲线,藻结皮、藓结皮的碳吸收峰值分别出现在8:00和9:00前后,其CO₂通量分别为0.13和-1.02 μmol CO₂·m^-2·s^-1;藻结皮24 h CO₂通量排放总量较裸地增加7.7%,而藓结皮减少了29.6%.生物结皮对土壤CO₂通量的影响显著,在评价退耕地土壤碳循环时,应考虑生物结皮的影响.