Soil CO2 efflux in contrasting boreal deciduous and coniferous stands and its contribution to the ecosystem carbon balance

Similar nonsteady‐state automated chamber systems were used to measure and partition soil CO₂ efflux in contrasting deciduous (trembling aspen) and coniferous (black spruce and jack pine) stands located within 100 km of each other near the southern edge of the Boreal forest in Canada. The stands were exposed to similar climate forcing in 2003, including marked seasonal variations in soil water availability, which provided a unique opportunity to investigate the influence of climate and stand characteristics on soil CO₂ efflux and to quantify its contribution to the net ecosystem CO₂ exchange (NEE) as measured with the eddy‐covariance technique. Partitioning of soil CO₂ efflux between soil respiration (including forest‐floor vegetation) and forest‐floor photosynthesis showed that short‐ and long‐term temporal variations of soil CO₂ efflux were related to the influence of (1) soil temperature and water content on soil respiration and (2) below‐canopy light availability, plant water status and forest‐floor plant species composition on forest‐floor photosynthesis. Overall, the three stands were weak to moderate sinks for CO₂ in 2003 (NEE of ?103, ?80 and ?28 g C m^?2 yr^?1 for aspen, black spruce and jack pine, respectively). Forest‐floor respiration accounted for 86%, 73% and 75% of annual ecosystem respiration, in the three respective stands, while forest‐floor photosynthesis contributed to 11% and 14% of annual gross ecosystem photosynthesis in the black spruce and jack pine stands, respectively. The results emphasize the need to perform concomitant measurements of NEE and soil CO₂ efflux at longer time scales in different ecosystems in order to better understand the impacts of future interannual climate variability and vegetation dynamics associated with climate change on each component of the carbon balance.     

The contribution of bryophytes to the carbon exchange for a temperate rainforest

Bryophytes blanket the floor of temperate rainforests in New Zealand and may influence a number of important ecosystem processes, including carbon cycling. Their contribution to forest floor carbon exchange was determined in a mature, undisturbed podocarp‐broadleaved forest in New Zealand, dominated by 100–400‐year‐old rimu (Dacrydium cupressimum) trees. Eight species of mosses and 13 species of liverworts contributed to the 62% cover of the diverse forest floor community. The bryophyte community developed a relatively thin (depth <30 mm), but dense, canopy that experienced elevated CO₂ partial pressures (median 46.6 Pa immediately below the bryophyte canopy) relative to the surrounding air (median 37.6 Pa at 100 mm above the canopy). Light‐saturated rates of net CO₂ exchange from 14 microcosms collected from the forest floor were highly variable; the maximum rate of net uptake (bryophyte photosynthesis – whole‐plant respiration) per unit ground area at saturating irradiance was 1.9 μmol m^?2 s^?1 and in one microcosm, the net rate of CO₂ exchange was negative (respiration). CO₂ exchange for all microcosms was strongly dependent on water content. The average water content in the microcosms ranged from 1375% when fully saturated to 250% when air‐dried. Reduction in water content across this range resulted in an average decrease of 85% in net CO₂ uptake per unit ground area. The results from the microcosms were used in a model to estimate annual carbon exchange for the forest floor. This model incorporated hourly variability in average irradiance reaching the forest floor, water content of the bryophyte layer, and air and soil temperature. The annual net carbon uptake by forest floor bryophytes was 103 g m^?2, compared to annual carbon efflux from the forest floor (bryophyte and soil respiration) of ?1010 g m^?2. To put this in perspective of the magnitude of the components of CO₂ exchange for the forest floor, the bryophyte layer reclaimed an amount of CO₂ equivalent to only about 10% of forest floor respiration (bryophyte plus soil) or ～11% of soil respiration. The contribution of forest floor bryophytes to productivity in this temperate rainforest was much smaller than in boreal forests, possibly because of differences in species composition and environmental limitations to photosynthesis. Because of their close dependence on water table depth, the contribution of the bryophyte community to ecosystem CO₂ exchange may be highly responsive to rapid changes in climate.     

Increased Litter Build Up and Soil Organic Matter Stabilization in a Poplar Plantation After 6 Years of Atmospheric CO2 Enrichment (FACE): Final Results of POP-EuroFACE Compared to Other Forest FACE Experiments

Free air CO₂ enrichment (FACE) experiments in aggrading temperate forests and plantations have been initiated to test whether temperate forest ecosystems act as sinks for anthropogenic emissions of CO₂. These FACE experiments have demonstrated increases in net primary production and carbon (C) storage in forest vegetation due to increased atmospheric CO₂ concentrations. However, the fate of this extra biomass in the forest floor or mineral soil is less clear. After 6 years of FACE treatment in a short-rotation poplar plantation, we observed an additional sink of 32 g C m⁻² y⁻¹ in the forest floor. Mineral soil C content increased equally under ambient and increased CO₂ treatment during the 6-year experiment. However, during the first half of the experiment the increase in soil C was suppressed under FACE due to a priming effect, that is, the additional labile C increased the mineralization of older SOM, whereas during the second half of the experiment the increase in soil C was larger under FACE. An additional sink of 54 g C m⁻² y⁻¹ in the top 10 cm of the mineral soil was created under FACE during the second half of the experiment. Although, this FACE effect was not significant due to a combination of soil spatial variability and the low number of replicates that are inherent to the present generation of forest stand FACE experiments. Physical fractionation by wet sieving revealed an increase in the C and nitrogen (N) content of macro-aggregates due to FACE. Further fractionation by density showed that FACE increased C and N contents of the light iPOM and mineral associated intra-macro-aggregate fractions. Isolation of micro-aggregates from macro-aggregates and subsequent fractionation by density revealed that FACE increased C and N contents of the light iPOM, C content of the fine iPOM and C and N contents of the mineral associated intra-micro-aggregate fractions. From this we infer that the amount of stabilized C and N increased under FACE treatment. We compared our data with published results of other forest FACE experiments and infer that the type of vegetation and soil base saturation, as a proxy for bioturbation, are important factors related to the size of the additional C sinks of the forest floor–soil system under FACE. Author Contribution: MRH conceived of and designed the study, performed research, analyzed data, and wrote the paper; GES conceived of and designed the study and performed research.     

Contrasting effects of low and high nitrogen additions on soil CO2 flux components and ectomycorrhizal fungal sporocarp production in a boreal forest

Nitrogen (N) added through atmospheric deposition or as fertilizer to boreal and temperate forests reduces both soil decomposer activity (heterotrophic respiration) and the activity of roots and mycorrhizal fungi (autotrophic respiration). However, these negative effects have been found in studies that applied relatively high levels of N, whereas the responses to ambient atmospheric N deposition rates are still not clear. Here, we compared an unfertilized control boreal forest with a fertilized forest (100 kg N ha^?1 yr^?1) and a forest subject to N‐deposition rates comparable to those in Central Europe (20 kg N ha^?1 yr^?1) to investigate the effects of N addition rate on different components of forest floor respiration and the production of ectomycorrhizal fungal sporocarps. Soil collars were used to partition heterotrophic (R_h) and autotrophic (R_a) respiration, which was further separated into respiration by tree roots (R_tr) and mycorrhizal hyphae (R_m). Total forest floor respiration was twice as high in the low N plot compared to the control, whereas there were no differences between the control and high N plot. There were no differences in R_h respiration among plots. The enhanced forest floor respiration in the low N plot was, therefore, the result of increased R_a respiration, with an increase in R_tr respiration, and a doubling of R_m respiration. The latter was corroborated by a slightly greater ectomycorrhizal (EM) fungal sporocarp production in the low N plot as compared to the control plot. In contrast, EM fungal sporocarp production was nearly eliminated, and R_m respiration severely reduced, in the high N plot, which resulted in significantly lower R_a respiration. We thus found a nonlinear response of the R_a components to N addition rate, which calls for further studies of the quantitative relations among N addition rate, plant photosynthesis and carbon allocation, and the function of EM fungi.     

Contrasting responses to drought of forest floor CO2 efflux in a Loblolly pine plantation and a nearby Oak-Hickory forest

Forest floor CO₂ efflux (F_ff) depends on vegetation type, climate, and soil physical properties. We assessed the effects of biological factors on F_ff by comparing a maturing pine plantation (PP) and a nearby mature Oak‐Hickory‐type hardwood forest (HW). F_ff was measured continuously with soil chambers connected to an IRGA during 2001–2002. At both sites, F_ff depended on soil temperature at 5 cm (T₅) when soil was moist (soil moisture, θ>0.20 m³ m^?3), and on both T₅ and θ when soil was drier. A model (F_ff (T₅, θ)) explained 92% of the variation in the daily mean F_ff at both sites. Higher radiation reaching the ground during the leafless period, and a thinner litter layer because of faster decomposition, probably caused higher soil temperature at HW compared with PP. The annual F_ff was estimated at 1330 and 1464 g C m^?2 yr^?1 for a year with mild drought (2001) at PP and HW, respectively, and 1231 and 1557 g C m^?2 yr^?1 for a year with severe drought (2002). In the wetter year, higher soil temperature and moisture at HW compared with PP compensated for the negative effect on F_ff of the response to these variables resulting in similar annual F_ff at both stands. In the drier year, however, the response to soil temperature and moisture was more similar at the two stands causing the difference in the state variables to impel a higher F_ff at HW. A simple mass balance indicated that in the wetter year, C in the litter–soil system was at steady state at HW, and was accruing at PP. However, HW was probably losing C from the mineral soil during the severe drought year of 2002, while PP was accumulating C at a lower rate because of a loss of C from the litter layer. Such contrasting behavior of two forest types in close proximity might frustrate attempts to estimate regional carbon (C) fluxes and net C exchange.     

Exposure to an enriched CO2 atmosphere alters carbon assimilation and allocation in a pine forest ecosystem

We linked a leaf-level CO₂ assimilation model with a model that accounts for light attenuation in the canopy and measurements of sap-flux-based canopy conductance into a new canopy conductance-constrained carbon assimilation (4C-A) model. We estimated canopy CO₂ uptake (A_nC) at the Duke Forest free-air CO₂ enrichment (FACE) study. Rates of A_nC estimated from the 4C-A model agreed well with leaf gas exchange measurements (A_net) in both CO₂ treatments. Under ambient conditions, monthly sums of net CO₂ uptake by the canopy (A_nC) were 13% higher than estimates based on eddy-covariance and chamber measurements. Annual estimates of A_nC were only 3% higher than carbon (C) accumulations and losses estimated from ground-based measurements for the entire stand. The C budget for the Pinus taeda component was well constrained (within 1% of ground-based measurements). Although the closure of the C budget for the broadleaf species was poorer (within 20%), these species are a minor component of the forest. Under elevated CO₂, the C used annually for growth, turnover, and respiration balanced only 80% of the A_nC. Of the extra 700 g C m⁻² a⁻¹ (1999 and 2000 average), 86% is attributable to surface soil CO₂ efflux. This suggests that the production and turnover of fine roots was underestimated or that mycorrhizae and rhizodeposition became an increasingly important component of the C balance. Under elevated CO₂, net ecosystem production increased by 272 g C m⁻² a⁻¹: 44% greater than under ambient CO₂. The majority (87%) of this C was sequestered in a moderately long-term C pool in wood, with the remainder in the forest floor–soil subsystem.     

Effects of experimental drought on soil respiration and radiocarbon efflux from a temperate forest soil

Soil moisture affects microbial decay of SOM and rhizosphere respiration (RR) in temperate forest soils, but isolating the response of soil respiration (SR) to summer drought and subsequent wetting is difficult because moisture changes are often confounded with temperature variation. We distinguished between temperature and moisture effects by simulation of prolonged soil droughts in a mixed deciduous forest at the Harvard Forest, Massachusetts. Roofs constructed over triplicate 5 × 5 m² plots excluded throughfall water during the summers of 2001 (168 mm) and 2002 (344 mm), while adjacent control plots received ambient throughfall and the same natural temperature regime. In 2003, throughfall was not excluded to assess the response of SR under natural weather conditions after two prolonged summer droughts. Throughfall exclusion significantly decreased mean SR rate by 53 mg C m^?2 h^?1 over 84 days in 2001, and by 68 mg C m^?2 h^?1 over 126 days in 2002, representing 10–30% of annual SR in this forest and 35–75% of annual net ecosystem exchange (NEE) of C. The differences in SR were best explained by differences in gravimetric water content in the Oi horizon (r²=0.69) and the Oe/Oa horizon (r²=0.60). Volumetric water content of the A horizon was not significantly affected by throughfall exclusion. The radiocarbon signature of soil CO₂ efflux and of CO₂ respired during incubations of O horizon, A horizon and living roots allowed partitioning of SR into contributions from young C substrate (including RR) and from decomposition of older SOM. RR (root respiration and microbial respiration of young substrates in the rhizosphere) made up 43–71% of the total C respired in the control plots and 41–80% in the exclusion plots, and tended to increase with drought. An exception to this trend was an interesting increase in CO₂ efflux of radiocarbon‐rich substrates during a period of abundant growth of mushrooms. Our results suggest that prolonged summer droughts decrease primarily heterotrophic respiration in the O horizon, which could cause increases in the storage of soil organic carbon in this forest. However, the C stored during two summers of simulated drought was only partly released as increased respiration during the following summer of natural throughfall. We do not know if this soil C sink during drought is transient or long lasting. In any case, differential decomposition of the O horizon caused by interannual variation of precipitation probably contributes significantly to observed interannual variation of NEE in temperate forests.     

重庆缙云山两种林分土壤呼吸对模拟氮沉降的季节响应差异性

氮沉降对土壤呼吸的影响仍然存在着争论,需要进一步研究。选择重庆缙云山的马尾松林和柑橘林开展了氮添加实验,分别设置3个氮添加水平(低氮T_5:20 g N m~(-2)a~(-1),中氮T_(10):40 g N m~(-2)a~(-1)和高氮T_(15):60 g N m~(-2)a~(-1))和对照(T_0:0 g N m~(-2)a~(-1))共4个水平的处理,各林分每个处理各9次重复,每个处理量分4次,在每个季度开始各施1次。采用ACE(Automated Soil CO_2 Exchange Station,UK)自动土壤呼吸监测系统测定两林分土壤表层(0—10 cm)的呼吸、温度和湿度,分别在当年的7月、9月、11月、第2年的1月、2月、3月、5月、6月各连续测定4d,每天(8:00—18:00)4次,以揭示两种林分土壤呼吸对模拟氮沉降的季节动态响应及其差异性。结果表明:(1)柑橘林与马尾松林林下土壤表层呼吸表现出一致的季节变化动态趋势:夏季春季秋季冬季,但柑橘林土壤呼吸显著高于马尾松林(P0.05)。(2)总体上氮沉降抑制了2种林分土壤表层呼吸,而N沉降量大抑制程度越高。只在冬季土壤湿度低的马尾松林下氮沉降促进了土壤呼吸。(3)土壤温度与土壤呼吸有极显著的正相关指数关系(P0.01),而土壤水分与土壤呼吸有显著的二次模型拟合关系,但均受到氮沉降量处理的影响。综合分析表明,在亚热带山区2类森林下的典型案例结果支持氮沉降抑制土壤呼吸的认识。     

模拟氮沉降和降雨对华西雨屏区常绿阔叶林土壤呼吸的影响

从2013年12月至2014年11月,通过野外原位试验,对华西雨屏区常绿阔叶林进行了模拟氮沉降和降雨试验,采用LI-8100土壤碳通量分析系统(LI-COR Inc.,USA)测定了对照(CK)、氮沉降(N)、减雨(R)、增雨(W)、氮沉降+减雨(NR)、氮沉降+增雨(NW)6个处理水平的土壤呼吸速率,并通过回归方程分析了温度和湿度与土壤呼吸速率间的关系。结果表明:(1)氮沉降和增雨抑制了常绿阔叶林土壤呼吸速率,减雨促进了常绿阔叶林土壤呼吸速率。(2)减雨使华西雨屏区常绿阔叶林土壤呼吸年通量增加了258 g/m~2,而模拟氮沉降和增雨使华西雨屏区常绿阔叶林土壤呼吸年通量分别减少了321g/m~2和406g/m~2。(3)减雨增加了土壤呼吸的温度敏感性,模拟氮沉降和增雨降低了土壤呼吸的温度敏感性。(4)模拟温度和湿度与土壤呼吸速率间回归方程分析表明,土壤水分对土壤呼吸速率的影响较小。(5)模拟氮沉降和增雨处理减少土壤微生物生物量碳、氮的含量,减雨处理增加了土壤微生物生物量碳、氮的含量。(6)模拟氮沉降和降雨对华西雨屏区土壤CO_2释放的影响未表现出明显的交互作用。     

暖温带麻栎林凋落物调节土壤碳排放通量对降雨脉冲的响应

凋落物既是森林生态系统养分循环的重要构件,又是森林土壤环境和功能的关键调节因子。降雨脉冲导致的土壤碳排放变异是陆地生态系统碳汇能力评价的不确定性来源之一。凋落物在调节土壤碳排放对降雨脉冲的响应中的作用仍缺乏科学的评价。通过在暖温带栎类落叶阔叶林中设置不同凋落物处理(对照、去除凋落物和加倍凋落物)和降雨模拟实验以阐明凋落物数量变化对土壤呼吸脉冲的影响。结果表明：模拟降雨脉冲之前,不同凋落物处理下的土壤呼吸存在显著差异;与对照相比,加倍凋落物导致土壤呼吸速率显著增加57.6%,然而,去除凋落物则对土壤呼吸无显著影响。模拟降雨后52小时内,对照、去除凋落物和加倍凋落物样方的土壤累积碳排放量分别为251.69 gC/m~2,250.93 gC/m~2和409.01 gC/m~2,加倍凋落物处理下的土壤碳排放量显著高于对照和去除凋落物处理;然而,去除凋落物与对照之间无显著差异。此外,不同凋落物处理下土壤呼吸的脉冲持续时间存在显著差异;加倍凋落物显著提高降雨后土壤呼吸脉冲的持续时间,分别比对照和去除凋落物高出262%和158%。多元逐步回归分析表明,土壤总碳排放通量和土壤呼吸的脉冲持续时间与土壤理...     

川西亚高山森林土壤呼吸和微生物生物量碳氮对施氮的响应

随着全球大气氮沉降的明显增加,将有可能显著影响我国西部地区受氮限制的亚高山森林生态系统。土壤微生物是生态系统的重要组成部分,是土壤物质循环和能量流动的重要参与者。由于生态系统类型、土壤养分、氮沉降背景值等的差异,土壤呼吸和土壤生物量碳氮对施氮的响应存在许多不确定性。而施氮会不会促进亚高山森林生态系统中土壤呼吸和微生物对土壤碳氮的固定?基于此假设,选择了川西60年生的四川红杉(Larix mastersiana)亚高山针叶林为研究对象,通过4个水平的土壤施氮控制试验(CK:0 g m~(-2) a~(-1)、N1:2 g m~(-2)a~(-1)、N2:5 g m~(-2) a~(-1)、N3:10 g m~(-2)a~(-1)),监测了土壤呼吸及土壤微生物生物量碳氮在一个生长季的动态情况。结果表明:施氮对土壤呼吸各指标和土壤微生物碳氮都有极显著的影响,施氮能促进土壤全呼吸、自养呼吸、异养呼吸通量和土壤微生物生物量碳氮的增长,施氮使土壤呼吸通量提高了11%—15%,土壤微生物量碳提高了5%—9%,土壤微生物量氮提高了23%—34%。在中氮水平下(5 g m~(-2) a~(-1))对土壤呼吸的促进最显著。相关分析发现,土壤呼吸与微生物生物量碳氮和微生物代谢商极呈显著正相关,微生物量碳氮与土壤温度呈极显著的正相关,与土壤湿度呈极显著负相关。通过一般线性回归拟合土壤呼吸速率与土壤10 cm温湿度的关系,发现土壤呼吸速率与土壤温度呈极显著的正相关,与土壤湿度极显著负相关(P0.001),中氮水平下土壤温度敏感性系数Q_(10)值(7.10)明显高于对照(4.26)。     

An initial intercomparison of micrometeorological and ecological inventory estimates of carbon exchange in a mid-latitude deciduous forest

The role of mid‐latitude forests in the sequestration of carbon (C) is of interest to an increasing number of scientists and policy‐makers alike. Net CO₂ exchange can be estimated on an annual basis, using eddy‐covariance techniques or from ecological inventories of C fluxes to and from a forest. Here we present an intercomparison of annual estimates of C exchange in a mixed hardwood forest in the Morgan‐Monroe State Forest, Indiana, USA for two years, 1998 and 1999. Based on eddy‐covariance measurements made at 1.8 times canopy height from a tower, C uptake by the forest was 237 and 287 g C m^?2 y^?1 for 1998 and 1999, respectively. For the same time period, biometric and ecophysiological measures and modelled estimates of all significant carbon fluxes within deciduous forests were made, including: change in living biomass, aboveground and belowground detritus production, foliage consumption, and forest floor and soil respiration. Using this ecological inventory method for these same two time periods, C uptake was estimated to be 271 and 377 g C m^?2 y^?1, which are 14.3% and 31.4% larger, respectively, than the tower‐based values. The relative change between this method's annual estimates is consistent with that of the eddy‐covariance based values. Our results indicate that the difference in annual C exchange rates was due to reduced heterotrophic soil respiration in 1999.     

Modelling night‐time ecosystem respiration by a constrained source optimization method

One of the main challenges to quantifying ecosystem carbon budgets is properly quantifying the magnitude of night‐time ecosystem respiration. Inverse Lagrangian dispersion analysis provides a promising approach to addressing such a problem when measured mean CO₂ concentration profiles and nocturnal velocity statistics are available. An inverse method, termed ‘Constrained Source Optimization’ or CSO, which couples a localized near‐field theory (LNF) of turbulent dispersion to respiratory sources, is developed to estimate seasonal and annual components of ecosystem respiration. A key advantage to the proposed method is that the effects of variable leaf area density on flow statistics are explicitly resolved via higher‐order closure principles. In CSO, the source distribution was computed after optimizing key physiological parameters to recover the measured mean concentration profile in a least‐square fashion. The proposed method was field‐tested using 1 year of 30‐min mean CO₂ concentration and CO₂ flux measurements collected within a 17‐year‐old (in 1999) even‐aged loblolly pine (Pinus taeda L.) stand in central North Carolina. Eddy‐covariance flux measurements conditioned on large friction velocity, leaf‐level porometry and forest‐floor respiration chamber measurements were used to assess the performance of the CSO model. The CSO approach produced reasonable estimates of ecosystem respiration, which permits estimation of ecosystem gross primary production when combined with daytime net ecosystem exchange (NEE) measurements. We employed the CSO approach in modelling annual respiration of above‐ground plant components (c. 214 g C m^?2 year^?1) and forest floor (c. 989 g C m^?2 year^?1) for estimating gross primary production (c. 1800 g C m^?2 year^?1) with a NEE of c. 605 g C m^?2 year^?1 for this pine forest ecosystem. We conclude that the CSO approach can utilise routine CO₂ concentration profile measurements to corroborate forest carbon balance estimates from eddy‐covariance NEE and chamber‐based component flux measurements.     

Soil carbon sequestration in a pine forest after 9 years of atmospheric CO2 enrichment

The impact of anthropogenic CO₂ emissions on climate change may be mitigated in part by C sequestration in terrestrial ecosystems as rising atmospheric CO₂ concentrations stimulate primary productivity and ecosystem C storage. Carbon will be sequestered in forest soils if organic matter inputs to soil profiles increase without a matching increase in decomposition or leaching losses from the soil profile, or if the rate of decomposition decreases because of increased production of resistant humic substances or greater physical protection of organic matter in soil aggregates. To examine the response of a forest ecosystem to elevated atmospheric CO₂ concentrations, the Duke Forest Free‐Air CO₂ Enrichment (FACE) experiment in North Carolina, USA, has maintained atmospheric CO₂ concentrations 200 μL L^?1 above ambient in an aggrading loblolly pine (Pinus taeda) plantation over a 9‐year period (1996–2005). During the first 6 years of the experiment, forest‐floor C and N pools increased linearly under both elevated and ambient CO₂ conditions, with significantly greater accumulations under the elevated CO₂ treatment. Between the sixth and ninth year, forest‐floor organic matter accumulation stabilized and C and N pools appeared to reach their respective steady states. An additional C sink of ～30 g C m^?2 yr^?1 was sequestered in the forest floor of the elevated CO₂ treatment plots relative to the control plots maintained at ambient CO₂ owing to increased litterfall and root turnover during the first 9 years of the study. Because we did not detect any significant elevated CO₂ effects on the rate of decomposition or on the chemical composition of forest‐floor organic matter, this additional C sink was likely related to enhanced litterfall C inputs. We also failed to detect any statistically significant treatment effects on the C and N pools of surface and deep mineral soil horizons. However, a significant widening of the C : N ratio of soil organic matter (SOM) in the upper mineral soil under both elevated and ambient CO₂ suggests that N is being transferred from soil to plants in this aggrading forest. A significant treatment × time interaction indicates that N is being transferred at a higher rate under elevated CO₂ (P=0.037), suggesting that enhanced rates of SOM decomposition are increasing mineralization and uptake to provide the extra N required to support the observed increase in primary productivity under elevated CO₂.     

A multiyear synthesis of soil respiration responses to elevated atmospheric CO2 from four forest FACE experiments

The rapidly rising concentration of atmospheric CO₂ has the potential to alter forest and global carbon cycles by altering important processes that occur in soil. Forest soils contain the largest and longest lived carbon pools in terrestrial ecosystems and are therefore extremely important to the land–atmosphere exchange of carbon and future climate. Soil respiration is a sensitive integrator of many soil processes that control carbon storage in soil, and is therefore a good metric of changes to soil carbon cycling. Here, we summarize soil respiration data from four forest free‐air carbon dioxide enrichment (FACE) experiments in developing and established forests that have been exposed to elevated atmospheric [CO₂] (168 μL L^?1 average enrichment) for 2–6 years. The sites have similar experimental design and use similar methodology (closed‐path infrared gas analyzers) to measure soil respiration, but differ in species composition of the respective forest communities. We found that elevated atmospheric [CO₂] stimulated soil respiration at all sites, and this response persisted for up to 6 years. Young developing stands experienced greater stimulation than did more established stands, increasing 39% and 16%, respectively, averaged over all years and communities. Further, at sites that had more than one community, we found that species composition of the dominant trees was a major controller of the absolute soil CO₂ efflux and the degree of stimulation from CO₂ enrichment. Interestingly, we found that the temperature sensitivity of bulk soil respiration appeared to be unaffected by elevated atmospheric CO₂. These findings suggest that stage of stand development and species composition should be explicitly accounted for when extrapolating results from elevated CO₂ experiments or modeling forest and global carbon cycles.     

The turnover of carbon pools contributing to soil CO2 and soil respiration in a temperate forest exposed to elevated CO2 concentration

Soil carbon is returned to the atmosphere through the process of soil respiration, which represents one of the largest fluxes in the terrestrial C cycle. The effects of climate change on the components of soil respiration can affect the sink or source capacity of ecosystems for atmospheric carbon, but no current techniques can unambiguously separate soil respiration into its components. Long‐term free air CO₂ enrichment (FACE) experiments provide a unique opportunity to study soil C dynamics because the CO₂ used for fumigation has a distinct isotopic signature and serves as a continuous label at the ecosystem level. We used the ¹³C tracer at the Duke Forest FACE site to follow the disappearance of C fixed before fumigation began in 1996 (pretreatment C) from soil CO₂ and soil‐respired CO₂, as an index of belowground C dynamics during the first 8 years of the experiment. The decay of pretreatment C as detected in the isotopic composition of soil‐respired CO₂ and soil CO₂ at 15, 30, 70, and 200 cm soil depth was best described by a model having one to three exponential pools within the soil system. The majority of soil‐respired CO₂ (71%) originated in soil C pools with a turnover time of about 35 days. About 55%, 50%, and 68% of soil CO₂ at 15, 30, and 70 cm, respectively, originated in soil pools with turnover times of less than 1 year. The rest of soil CO₂ and soil‐respired CO₂ originated in soil pools that turn over at decadal time scales. Our results suggest that a large fraction of the C returned to the atmosphere through soil respiration results from dynamic soil C pools that cannot be easily detected in traditionally defined soil organic matter standing stocks. Fast oxidation of labile C substrates may prevent increases in soil C accumulation in forests exposed to elevated [CO₂] and may consequently result in shorter ecosystem C residence times.     

Wood CO2 efflux in a primary tropical rain forest

The balance between photosynthesis and plant respiration in tropical forests may substantially affect the global carbon cycle. Woody tissue CO₂ efflux is a major component of total plant respiration, but estimates of ecosystem‐scale rates are uncertain because of poor sampling in the upper canopy and across landscapes. To overcome these problems, we used a portable scaffolding tower to measure woody tissue CO₂ efflux from ground level to the canopy top across a range of sites of varying slope and soil phosphorus content in a primary tropical rain forest in Costa Rica. The objectives of this study were to: (1) determine whether to use surface area, volume, or biomass for modeling and extrapolating wood CO₂ efflux, (2) determine if wood CO₂ efflux varied seasonally, (3) identify if wood CO₂ efflux varied by functional group, height in canopy, soil fertility, or slope, and (4) extrapolate wood CO₂ efflux to the forest. CO₂ efflux from small diameter woody tissue (<10 cm) was related to surface area, while CO₂ efflux from stems >10 cm was related to both surface area and volume. Wood CO₂ efflux showed no evidence of seasonality over 2 years. CO₂ efflux per unit wood surface area at 25° (F_A) was highest for the N‐fixing dominant tree species Pentaclethra macroloba, followed by other tree species, lianas, then palms. Small diameter F_A increased steeply with increasing height, and large diameter F_A increased with diameter. Soil phosphorus and slope had slight, but complex effects on F_A. Wood CO₂ efflux per unit ground area was 1.34±0.36 μmol m^?2 s^?1, or 508±135 g C m^?2 yr^?1. Small diameter wood, only 15% of total woody biomass, accounted for 70% of total woody tissue CO₂ efflux from the forest; while lianas, only 3% of total woody biomass, contributed one‐fourth of the total wood CO₂ efflux.     

华西雨屏区苦竹林土壤呼吸对模拟氮沉降的响应

2007年11月至2008年11月, 对华西雨屏区苦竹(Pleioblastus amarus)人工林进行了模拟氮沉降试验, 氮沉降水平分别为对照(CK, 0 g N·m^-2·a^-1)、低氮(5 g N·m^-2·a^-1)、中氮(15 g N·m^-2·a^-1)和高氮(30 g N·m^-2·a^-1)。每月下旬, 采用红外CO₂分析法测定土壤呼吸速率, 并定量地对各处理施氮(NH₄NO₃)。结果表明: 2008年试验地氮沉降量为8.241 g·m^-2, 超出该地区氮沉降临界负荷。在生长季节, 苦竹林根呼吸占总土壤呼吸的60%左右。模拟氮沉降促进了苦竹林土壤呼吸速率, 使苦竹林土壤每年向大气释放的CO₂增加了9.4%~28.6%。在大时间尺度上(如1 a), 土壤呼吸主要受温度的影响。2008年6~10月, 土壤呼吸速率24 h平均值均表现为: 对照<低氮<中氮<高氮。氮沉降处理1 a后, 土壤微生物呼吸速率和土壤微生物生物量碳、氮增加, 并且均与氮沉降量具有相同趋势。各处理土壤呼吸速率与10 cm土壤温度、月平均气温呈极显著指数正相关关系, 利用温度单因素模型可以解释土壤呼吸速率的大部分。模拟氮沉降使得土壤呼吸Q₁₀值增大, 表明氮沉降可能增强了土壤呼吸的温度敏感性。在氮沉降持续增加和全球气候变暖的背景下, 氮沉降和温度的共同作用可能使得苦竹林向大气中排放的CO₂增加。     

Estimates of soil respiration and net primary production of three forests at different succession stages in South China

Soil respiration (heterotropic and autotropic respiration, R_g) and aboveground litter fall carbon were measured at three forests at different succession (early, middle and advanced) stages in Dinghushan Biosphere Reserve, Southern China. It was found that the soil respiration increases exponentially with soil temperature at 5 cm depth (T_s) according to the relation R_g=a exp(bT_s), and the more advanced forest community during succession has a higher value of a because of higher litter carbon input than the forests at early or middle succession stages. It was also found that the monthly soil respiration is linearly correlated with the aboveground litter carbon input of the previous month. Using measurements of aboveground litter and soil respiration, the net primary productions (NPPs) of three forests were estimated using nonlinear inversion. They are 475, 678 and 1148 g C m^?2 yr^?1 for the Masson pine forest (MPF), coniferous and broad‐leaf mixed forest (MF) and subtropical monsoon evergreen broad‐leaf forest (MEBF), respectively, in year 2003/2004, of which 54%, 37% and 62% are belowground NPP for those three respective forests if no change in live plant biomass is assumed. After taking account of the decrease in live plant biomass, we estimated the NPP of the subtropical MEBF is 970 g C m^?2 yr^?1 in year 2003/2004. Total amount of carbon allocated below ground for plant roots is 388 g C m^?2 yr^?1 for the MPF, 504 g C m^?2 yr^?1 for the coniferous and broad‐leaf MF and 1254 g C m^?2 yr^?1 for the subtropical MEBF in 2003/2004. Our results support the hypothesis that the amount of carbon allocation belowground increases during forest succession.     

Woody-tissue respiration for Simarouba amara and Minquartia guianensis,two tropical wet forest trees with different growth habits

We measured CO₂ efflux from stems of two tropical wet forest trees, both found in the canopy, but with very different growth habits. The species were Simarouba amara, a fast-growing species associated with gaps in old-growth forest and abundant in secondary forest, and Minquartia guianensis, a slow-growing species tolerant of low-light conditions in old-growth forest. Per unit of bole surface, CO₂ efflux averaged 1.24 mol m^–2 s^–1 for Simarouba and 0.83 mol m^–2s^–1 for Minquartia. CO₂ efflux was highly correlated with annual wood production (r ²=0.65), but only weakly correlated with stem diameter (r ²=0.22). We also partitioned the CO₂ efflux into the functional components of construction and maintenance respiration. Construction respiration was estimated from annual stem dry matter production and maintenance respiration by subtracting construction respiration from the instantaneous CO₂ flux. Estimated maintenance respiration was linearly related to sapwood volume (39.6 mol m^–3s^–1 at 24.6° C, r ²=0.58), with no difference in the rate for the two species. Maintenance respiration per unit of sapwood volume for these tropical wet forest trees was roughly twice that of temperate conifers. A model combining construction and maintenance respiration estimated CO₂ very well for these species (r ²=0.85). For our sample, maintenance respiration was 54% of the total CO₂ efflux for Simarouba and 82% for Minquartia. For our sample, sapwood volume averaged 23% of stem volume when weighted by tree size, or 40% with no size weighting. Using these fractions, and a published estimate of aboveground dry-matter production, we estimate the annual cost of woody tissue respiration for primary forest at La Selva to be 220 or 350 g C m^–2 year^–1, depending on the assumed sapwood volume. These costs are estimated to be less than 13% of the gross production for the forest.