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     

Canopy N and P dynamics of a southeastern US pine forest under elevated CO2

Forest production is strongly nutrient limited throughout the southeastern US. If nutrient limitations constrain plant acquisition of essential resources under elevated CO₂, reductions in the mass or nutrient content of forest canopies could constrain C assimilation from the atmosphere. We tested this idea by quantifying canopy biomass, foliar concentrations of N and P, and the total quantity of N and P in a loblolly pine (Pinus taeda) canopy subject to 4 years of free-air CO₂ enrichment. We also used N:P ratios to detect N versus P limitation to primary production under elevated CO₂. Canopy biomass was significantly higher under elevated CO₂ during the first 4 years of this experiment. Elevated CO₂ significantly reduced the concentration of N in loblolly pine foliage (5% relative to ambient CO₂) but not P. Despite the slight reduction foliage N concentrations, there were significant increases in canopy N and P contents under elevated CO₂. Foliar N:P ratios were not altered by elevated CO₂ and were within a range suggesting forest production is N limited not P limited. Despite the clear limitation of NPP by N under ambient and elevated CO₂ at this site, there is no evidence that the mass of N or P in the canopy is declining through the first 4 years of CO₂ fumigation. As a consequence, whole-canopy C assimilation is strongly stimulated by elevated CO₂ making this forest a larger net C sink under elevated CO₂ than under ambient CO₂. We discuss the potential for future decreases in canopy nutrient content as a result of limited changes in the size of the plant-available pools of N under elevated CO₂.     

CO<Subscript>2</Subscript> exchange in a temperate Japanese cypress forest compared with that in a cool-temperate deciduous broad-leaved forest

To examine the characteristics of carbon exchange in coniferous forests, we analysed the seasonal and diurnal patterns of CO₂ exchange, as measured using the eddy covariance method, in a Japanese cypress forest in the Kiryu Experimental Watershed (KEW) in central Japan. The net CO₂ exchange data during periods of low-friction velocity conditions and during periods of missing data were interpolated. The daily CO₂ uptake was observed throughout the year, with maximum values occurring in early summer. Periods of low carbon uptake were seen in late summer owing to high respiratory CO₂ efflux. The diurnal and seasonal patterns of daytime CO₂ exchange at KEW were compared with those in a cool-temperate deciduous forest of the Tomakomai Experimental Forest (TOEF) in Japan. The environmental differences between evergreen and deciduous forests affected the seasonal patterns of carbon uptake. Although there were great differences in the mean monthly air temperatures between the sites, the mean monthly daytime carbon uptake was almost equal at both sites during the peak growing period. The carbon-uptake values at the same PAR level were greater before noon than after noon, especially at TOEF, suggesting the stomatal regulation of carbon uptake.     

Leaf and canopy responses to elevated CO2 in a pine forest under free-air CO2 enrichment

Physiological responses to elevated CO₂ at the leaf and canopy-level were studied in an intact pine (Pinus taeda) forest ecosystem exposed to elevated CO₂ using a free-air CO₂ enrichment (FACE) technique. Normalized canopy water-use of trees exposed to elevated CO₂ over an 8-day exposure period was similar to that of trees exposed to current ambient CO₂ under sunny conditions. During a portion of the exposure period when sky conditions were cloudy, CO₂-exposed trees showed minor (7%) but significant reductions in relative sap flux density compared to trees under ambient CO₂ conditions. Short-term (minutes) direct stomatal responses to elevated CO₂ were also relatively weak (5% reduction in stomatal aperture in response to high CO₂ concentrations). We observed no evidence of adjustment in stomatal conductance in foliage grown under elevated CO₂ for nearly 80 days compared to foliage grown under current ambient CO₂, so intrinsic leaf water-use efficiency at elevated CO₂ was enhanced primarily by direct responses of photosynthesis to CO₂. We did not detect statistical differences in parameters from photosynthetic responses to intercellular CO₂ (A _net-C _i curves) for Pinus taeda foliage grown under elevated CO₂ (550 mol mol^–1) for 50–80 days compared to those for foliage grown under current ambient CO₂ from similar-sized reference trees nearby. In both cases, leaf net photosynthetic rate at 550 mol mol^–1 CO₂ was enhanced by approximately 65% compared to the rate at ambient CO₂ (350 mol mol^–1). A similar level of enhancement under elevated CO₂ was observed for daily photosynthesis under field conditions on a sunny day. While enhancement of photosynthesis by elevated CO₂ during the study period appears to be primarily attributable to direct photosynthetic responses to CO₂ in the pine forest, longer-term CO₂ responses and feedbacks remain to be evaluated.     

Daily and seasonal CO2 exchange in Scots pine grown under elevated O3 and CO2: experiment and simulation

Starting in early spring of 1994, naturally regenerated, 30-year-old Scots pine (Pinus sylvestris L.) trees were grown in open-top chambers and exposed in situ to doubled ambient O₃,doubled ambient CO₂ and a combination of O₃ and CO₂ from 15 April to 15 September. To investigate daily and seasonal responses of CO₂ exchange to elevated O₃ and CO₂, the CO₂ exchange of shoots was measured continuously by an automatic system for measuring gas exchange during the course of one year (from 1 Januray to 31 December 1996). A process-based model of shoot photosynthesis was constructed to quantify modifications in the intrinsic capacity of photosynthesis and stomatal conductance by simulating the daily CO₂ exchange data from the field. Results showed that on most days of the year the model simulated well the daily course of shoot photosynthesis. Elevated O₃ significantly decreased photosynthetic capacity and stomatal conductance during the whole photosynthetic period. Elevated O₃ also led to a delay in onset of photosynthetic recovery in early spring and an increase in the sensitivity of photosynthesis to environmental stress conditions. The combination of elevated O₃ and CO₂ had an effect on photosynthesis and stomatal conductance similar to that of elevated O₃ alone, but significantly reduced the O₃-induced depression of photosynthesis. Elevated CO₂ significantly increased the photosynthetic capacity of Scots pine during the main growing season but slightly decreased it in early spring and late autumn. The model calculation showed that, compared to the control treatment, elevated O₃ alone and the combination of elevated O₃ and CO₂ decreased the annual total of net photosynthesis per unit leaf area by 55% and 38%, respectively. Elevated CO₂ increased the annual total of net photosynthesis by 13%.     

Rhizodeposition under ambient and elevated CO2 levels

As global CO₂ levels rise, can soils store more carbon and so buffer atmospheric CO₂ levels? Answering this question requires a knowledge of the rates of C inputs to soil and of CO₂ outputs via decomposition. Below-ground inputs from roots are a major component of the C flow into soils but are still poorly understood. In this article, new techniques for measuring rhizodeposition are reviewed and discussed and the need for cross-comparisons between methods is identified. One component of rhizodeposition, root exudation, is examined in more detail and evidence is presented which suggests that current estimates of exudate flow into soils are incorrect. A mechanistic mathematical model is used to explore how exudate flows might change under elevated CO₂.     

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.     

Effects of elevated CO2 and nitrogen on growth ofPoa pratensis (L.)

The growth responses of a grass,Poa pratensis, to elevated CO₂ and nitrogen were investigated. Light-saturated photosynthetic rate per unit leaf area increased with exposure to elevated CO₂, while dry weight did not respond to increased CO₂. Patterns of biomass allocation within plants, including leaf area, leaf area ratio, specific leaf area, and root to shoot ratios, were not altered by elevated CO₂, but changed considerably with N treatment Shoot and whole-plant tissue N concentrations were significantly diluted by elevated CO₂ (Tukey test, P < 0.05). Total N content did not differ significantly among CO₂ treatments. The absence of a concomitant increase in N uptake under elevated CO₂ may have caused a dilution in plant tissue [N], probably negating the positive effects of increased photosynthesis on biomass accumulation.     

Effects of [CO2] and nitrogen fertilization on soils planted with ponderosa pine

The effects of six years treatment with elevated [CO₂] (350, 525, and 700 μl l^-1) and nitrogen (N) (0, 10, and 20 g N m^-2 yr^-1) on soils, soil solution, and CO₂ efflux in an open-top chamber study with ponderosa pine (Pinus ponderosa Laws.) are described. The clearest [CO₂] effect was in year 6, when a pattern of lower soil N concentration and higher C/N ratio with elevated [CO₂] emerged. Statistically significant effects of elevated [CO₂] on soil total C, extractable P, exchangeable Mg²⁺, exchangeable Ca²⁺, base saturation, and soil solution HCO₃ ^- and NO₃ ^- were also found in various treatment combinations and at various times; however, these effects were inconsistent among treatments and years, and in many cases (P, Mg²⁺, Ca²⁺, base saturation) reflected pre-treatment differences. The use of homogenized buried soil bags did not improve the power to detect changes in soil C and N or help resolve the inconsistencies in soil C patterns. Nitrogen fertilization had the expected negative effects on exchangeable Ca²⁺, K⁺, and Mg²⁺ in year 6, presumably because of increased NO₃ ^- leaching, but had no consistent effect on soil C, N, or extractable P. This revised version was published online in June 2006 with corrections to the Cover Date.     

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

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

Effects of rainfall events on soil CO2 flux in a cool temperate deciduous broad-leaved forest

The effects of rainfall events on soil CO₂ fluxes were examined in a cool temperate Quercus/Betula forest in Japan. The soil CO₂ fluxes were measured using an open-flow gas exchange system with an infrared gas analyzer in the snow-free season from August 1999 to November 2000. Soil CO₂ flux showed no significant diurnal trend on days without rain. In contrast, rainfall events caused a significant increase in soil CO₂ flux. To determine the effect of rainfall events and to evaluate more precisely the daily and annual soil carbon flux, we constructed a multiple polynomial regression model that included two variables, soil temperature and soil water content, using the soil CO₂ flux data recorded on sunny days. Daily soil carbon fluxes on sunny days calculated by the model were almost the same as those determined by the field measurements. On the contrary, the fluxes measured on rainy days were significantly higher than those calculated daily from the soil carbon fluxes by the model. Annual soil carbon fluxes in 1999 and 2000 were estimated using models that both do and do not take rainfall effects into consideration. The result indicates that post-rainfall increases in soil CO₂ flux represent approximately 16–21% of the annual soil carbon flux in this cool temperate deciduous forest.     

高大气CO2浓度下氮素对小麦叶片光能利用的影响

关于氮素对高大气CO₂浓度下C₃植物光合作用适应现象的调节机理已有较为深入的研究, 但对其光合作用适应现象的光合能量转化和分配机制缺乏系统分析。该文以大气CO₂浓度和施氮量为处理手段, 通过测定小麦(Triticum aestivum)抽穗期叶片的光合作用-胞间CO₂浓度响应曲线以及荧光动力学参数来测算光合电子传递速率和分配去向, 研究了长期高大气CO₂浓度下小麦叶片光合电子传递和分配对施氮量的响应。结果表明, 与正常大气CO₂浓度处理相比, 高大气CO₂浓度下小麦叶片较多的激发能以热量的形式耗散, 增施氮素可使更多的激发能向光化学反应方向的分配, 降低光合能量的热耗散速率; 大气CO₂浓度升高后小麦叶片光化学淬灭系数无明显变化, 高氮叶片的非光化学猝灭降低而低氮叶片明显升高, 施氮促进PSII反应中心的开放比例, 降低光能的热耗散; 高大气CO₂浓度下高氮叶片通过PSII反应中心的光合电子传递速率(J_F)较高, 而且参与光呼吸的非环式电子流速率(J₀)显著降低, 较正常大气CO₂浓度处理的高氮叶片下降了88.40%, 光合速率增加46.47%; 高大气CO₂浓度下小麦叶片J_F-J₀升高而J₀/J_F显著下降, 光呼吸耗能被抑制, 更多的光合电子分配至光合还原过程。因此, 大气CO₂浓度增高条件下, 小麦叶片激发能的热耗散速率增加, 但增施氮素后小麦叶片PSII反应中心开放比例提高, 光化学速率增加, 进入PSII反应中心的电子流速率明显升高, 光呼吸作用被抑制, 光合电子较多地进入光化学过程, 这可能是高氮条件下光合作用适应性下调被缓解的一个原因。     

Time-dependent responses of soil CO2 efflux components to elevated atmospheric [CO2] and temperature in experimental forest mesocosms

We previously used dual stable isotope techniques to partition soil CO₂ efflux into three source components (rhizosphere respiration, litter decomposition, and soil organic matter (SOM) oxidation) using experimental chambers planted with Douglas-fir [Pseudotsuga menziesii (Mirb.) Franco] seedlings. The components responded differently to elevated CO₂ (ambient + 200 mol mol^–1) and elevated temperature (ambient + 4 °C) treatments during the first year. Rhizosphere respiration increased most under elevated CO₂, and SOM oxidation increased most under elevated temperature. However, many studies show that plants and soil processes can respond to altered climates in a transient way. Herein, we extend our analysis to 2 years to evaluate the stability of the responses of the source components. Total soil CO₂ efflux increased significantly under elevated CO₂ and elevated temperature in both years (1994 and 1995), but the enhancement was much less in 1995. Rhizosphere respiration increased less under elevated temperature in 1995 compared with 1994. Litter decomposition also tended to increase comparatively less in 1995 under elevated CO₂, but was unresponsive to elevated temperature between years. In contrast, SOM oxidation was similar under elevated CO₂ in the 2 years. Less SOM oxidation occurred under elevated temperature in 1995 compared with 1994. Our results indicate that temporal variations can occur in CO₂ production by the sources. The variations likely involve responses to antecedent physical disruption of the soil and physiological processes.     

二氧化碳储存通量对森林生态系统碳收支的影响

涡度相关系统观测高度以下的CO₂储存通量对准确评价森林生态系统与大气间净CO₂交换量（NEE）有着重要的影响.本研究以长白山阔叶红松林为研究对象,利用2003年的涡度相关观测数据以及CO₂浓度廓线数据,分析了CO₂储存通量的变化规律及其对碳收支过程的影响.结果表明：涡度相关观测高度以下的CO₂储存通量具有典型的日变化特征,其最大变化量出现在大气稳定与不稳定层结转换期.利用涡度相关系统观测的单点CO₂浓度变化方法与利用CO₂浓度廓线方法计算的CO₂储存通量差异不显著.忽略CO₂储存通量,在半小时尺度上会造成对夜间和白天的NEE分别低估25%和19%,在日和年尺度上,会对NEE低估10%和25%;忽略CO₂储存通量,会低估Michaelis-Menten光响应方程及Lloyd-Taylor呼吸方程的参数,并且对表观初始量子效率α和参考呼吸R_ref的低估最大;忽略CO₂储存通量,在半小时、日及年尺度上,均会对总光合作用（GPP）和生态系统呼吸（R_e）低估约20%.     

Stoichiometric response of nitrogen-fixing and non-fixing dicots to manipulations of CO2, nitrogen, and diversity

Human activities have resulted in increased nitrogen deposition and atmospheric CO₂ concentrations in the biosphere, potentially causing significant changes in many ecological processes. In addition to these ongoing perturbations of the abiotic environment, human-induced losses of biodiversity are also of major concern and may interact in important ways with biogeochemical perturbations to affect ecosystem structure and function. We have evaluated the effects of these perturbations on plant biomass stoichiometric composition (C:N:P ratios) within the framework of the BioCON experimental setup (biodiversity, CO₂, N) conducted at the Cedar Creek Natural History Area, Minnesota. Here we present data for five plant species: Solidago rigida, Achillea millefolium, Amorpha canescens, Lespedeza capitata, and Lupinus perennis. We found significantly higher C:N and C:P ratios under elevated CO₂ treatments, but species responded idiosyncratically to the treatment. Nitrogen addition decreased C:N ratios, but this response was greater in the ambient CO₂ treatments than under elevated CO₂. Higher plant species diversity generally lowered both C:N and C:P ratios. Importantly, increased diversity also led to a more modest increase in the C:N ratio with elevated CO₂ levels. In addition, legumes exhibited lower C:N and higher C:P and N:P ratios than non-legumes, highlighting the effect of physiological characteristics defining plant functional types. These data suggest that atmospheric CO₂ levels, N availability, and plant species diversity interact to affect both aboveground and belowground processes by altering plant elemental composition.     

Increased nitrate availability in the soil of a mixed mature temperate forest subjected to elevated CO2 concentration (canopy FACE)

In a mature temperate forest in Hofstetten, Switzerland, deciduous tree canopies were subjected to a free‐air CO₂ enrichment (FACE) for a period of 8 years. The effect of this treatment on the availability of nitrogen (N) in the soil was assessed along three transects across the experimental area, one under Fagus sylvatica, one under Quercus robur and Q. petraea and one under Carpinus betulus. Nitrate, ammonium and dissolved organic N (DON) were analysed in soil solution obtained with suction cups. Nitrate and ammonium were also captured in buried ion‐exchange resin bags. These parameters were related to the local intensity of the FACE treatment as measured from the ¹³C depletion of dissolved inorganic carbon in the soil solution. Over the 8 years of experiment, the CO₂ enrichment reduced DON concentrations, did not affect ammonium, but induced higher nitrate concentrations, both in soil solution and resin bags. In the nitrate captured in the resin bags, the natural abundance of the isotope ¹⁵N increased strongly. This indicates that the CO₂ enrichment accelerated net nitrification, probably as an effect of the higher soil moisture resulting from the reduced transpiration of the CO₂‐enriched trees. It is also possible that N mineralization was enhanced by root exudates (priming effect) or that the uptake of inorganic N by these trees decreased slightly as the result of a reduced N demand for fine‐root growth. In this mature deciduous forest, we did not observe any progressive N limitation due to elevated atmospheric CO₂ concentrations; on the contrary, we observed an enhanced N availability over the 8 years of our measurements. This may, together with the global warming projected, exacerbate problems related to N saturation and nitrate leaching, although it is uncertain how long the observed trends will last in the future.     

Impact of elevated CO2 concentration on ultrastructure of pericarp and composition of grain in three Triticum species of different ploidy levels

Triticum species of three ploidies were grown under ambient (375 μl/l) or elevated (FACE, 550 μl/l) CO₂ concentration [CO₂] to evaluate their response to CO₂ enrichment. The consistent effect of elevated CO₂ was an increase in concentration of starch and decrease in concentration of protein in the grain. Transmission electron micrographs revealed an increase in width and area of chloroplasts, and change in shape from elliptical in ambient to round in elevated [CO₂]. There was a corresponding increase in starch grain size and number in chloroplasts. The large starch grains distributed among thylakoids resulted in separation and distortion of internal membrane system in chloroplasts. The level of response was different in species of different ploidy levels. Maximum increase in starch concentration, and least decrease in protein concentration, was observed in Triticum dicoccoides, which also proved the most suitable species in terms of C:N ratio.     

Biomass and compositional responses of ectomycorrhizal fungal hyphae to elevated CO2 and nitrogen fertilization

The extramatrical mycelia (EMM) of ectomycorrhizal fungi make up a large proportion of the microbial diversity and biomass in temperate forest soils. Thus, their response to elevated CO(2) can have large effects on plant nutrient acquisition and carbon movement through forests. Here, the effects of CO(2) and nitrogen (N) fertilization on EMM biomass and community structure in Pinus taeda forest plots were examined using sand-filled mesh bags buried in the field, the contents of which were analyzed by phospholipid fatty acid (PLFA) and DNA sequencing. A total of 2138 sequences comprising 295 taxa were recovered; most (83.5%) were from ectomycorrhizal fungal taxa. No biomass increase was detected in elevated CO(2) plots relative to control plots, but individual taxa responded to both CO(2) and N fertilization, four of the six most abundant taxa were less frequent in N-fertilized plots. Thelephoroid and athelioid taxa were both frequent and abundant as EMM, and thelephoroid richness was extremely high. Russula and Cortinariaceae taxa were less abundant and boletoid taxa were more abundant as EMM relative to ectomycorrhizas. The EMM community, sampled across seasons and years, was dynamic with a high degree of interspecific variation in response to CO(2) enrichment and N fertilization.     

Quantitative evaluation of atmospheric CO2 sink into forest soils from the tropics to the boreal zone during the past three decades

A model of soil carbon cycling in forest ecosystems was applied to predict the soil carbon balance in nine forest ecosystems from the tropics to the boreal zone during the past three decades (1965–95). The parameters of carbon flows and initial conditions of carbon pools were decided based on data obtained in each forest stand. Assumptions for model calculation were: (i) primary production (i.e. litterfall and root turnover rates) increased with increasing CO₂ concentrations in the atmosphere (10% per 40 p.p.m. CO₂); and (ii) temperature increased by 0.6°C per 100 years, but precipitation changed little. The simulation employed a daily time step and used daily air temperature and precipitation observed near each forest stand over an average year during the last decade. The model calculations suggest that the accumulation of total soil carbon increased 8.5–10.4 tC (ton of carbon) ha^–1 in broad-leaved forests from the tropics to the cool-temperate zone during the past three decades, but the amount of soil carbon (3.0–8.4 tC ha^–1) increased much less in needle forests from the subtropical to boreal zones during the same period. There is a linear relationship between the increasing rate of soil carbon stock during the past three decades (1965–95) in forest stands concerned (R_MS, % per 30 years) and annual mean temperature of their soils (T₀,°C), as: R_MS = 0.34T₀ + 4.1. Based on the data of carbon stock in forest soil in each climate zone reported, the global sink of atmospheric CO₂ into forest soil was roughly estimated to be 42 GtC (billion tons of carbon) per 30 years, which was 1.4 GtC year^–1 on average over the past three decades.