No overall stimulation of soil respiration under mature deciduous forest trees after 7 years of CO2 enrichment

The anthropogenic rise in atmospheric CO₂ is expected to impact carbon (C) fluxes not only at ecosystem level but also at the global scale by altering C cycle processes in soils. At the Swiss Canopy Crane (SCC), we examined how 7 years of free air CO₂ enrichment (FACE) affected soil CO₂ dynamics in a ca. 100‐year‐old mixed deciduous forest. The use of ¹³C‐depleted CO₂ for canopy enrichment allowed us to trace the flow of recently fixed C. In the 7th year of growth at ～550 ppm CO₂, soil respiratory CO₂ consisted of 39% labelled C. During the growing season, soil air CO₂ concentration was significantly enhanced under CO₂‐exposed trees. However, elevated CO₂ failed to stimulate cumulative soil respiration (R_s) over the growing season. We found periodic reductions as well as increases in instantaneous rates of R_s in response to elevated CO₂, depending on soil temperature and soil volumetric water content (VWC; significant three‐way interaction). During wet periods, soil water savings under CO₂‐enriched trees led to excessive VWC (>45%) that suppressed R_s. Elevated CO₂ stimulated R_s only when VWC was ≤40% and concurrent soil temperature was high (>15 °C). Seasonal Q₁₀ estimates of R_s were significantly lower under elevated (Q₁₀=3.30) compared with ambient CO₂ (Q₁₀=3.97). However, this effect disappeared when three consecutive sampling dates of extremely high VWC were disregarded. This suggests that elevated CO₂ affected Q₁₀ mainly indirectly through changes in VWC. Fine root respiration did not differ significantly between treatments but soil microbial biomass (C_mic) increased by 14% under elevated CO₂ (marginally significant). Our findings do not indicate enhanced soil C emissions in such stands under future atmospheric CO₂. It remains to be shown whether C losses via leaching of dissolved organic or inorganic C (DOC, DIC) help to balance the C budget in this forest.     

An Analysis of Respiratory Activity,Q10, and Microbial Community Composition of Soils from High and Low Tussock Sites at Toolik,Alaska

ABSTRACT. High latitude microbial communities, incurring increased global warming, are a potential major source of respiratory CO₂ contributing to an enhanced greenhouse effect. Data on respiration and microbial density are presented for a moist, high tussock site compared with a low, water saturated site. The density of bacteria and eukaryotic microbes was nearly equivalent at both sites and potentially could yield substantial release of respiratory CO₂ with continued warming. Respiratory rates for soil from the high site were greater than the low. The Q₁₀ of 2.4 for the high tussock sample was approximately 1.3 × that of the low site sample (Q₁₀ of 1.7).     

Responses of dryland soil respiration and soil carbon pool size to abrupt vs. gradual and individual vs. combined changes in soil temperature, precipitation, and atmospheric [CO2]: a simulation analysis

With the large extent and great amount of soil carbon (C) storage, drylands play an important role in terrestrial C balance and feedbacks to climate change. Yet, how dryland soils respond to gradual and concomitant changes in multiple global change drivers [e.g., temperature (T_s), precipitation (Ppt), and atmospheric [CO₂] (CO₂)] has rarely been studied. We used a process‐based ecosystem model patch arid land simulator to simulate dryland soil respiration (R_s) and C pool size (C_s) changes to abrupt vs. gradual and single vs. combined alterations in T_s, Ppt and CO₂ at multiple treatment levels. Results showed that abrupt perturbations generally resulted in larger R_s and had longer differentiated impacts than did gradual perturbations. R_s was stimulated by increases in T_s, Ppt, and CO₂ in a nonlinear fashion (e.g., parabolically or asymptotically) but suppressed by Ppt reduction. Warming mainly stimulated heterotrophic R_s (i.e., R_h) whereas Ppt and CO₂ influenced autotrophic R_s (i.e., R_a). The combined effects of warming, Ppt, and CO₂ were nonadditive of primary single‐factor effects as a result of substantial interactions among these factors. Warming amplified the effects of both Ppt addition and CO₂ elevation whereas Ppt addition and CO₂ elevation counteracted with each other. Precipitation reduction either magnified or suppressed warming and CO₂ effects, depending on the magnitude of factor's alteration and the components of R_s (R_a or R_h) being examined. Overall, Ppt had dominant influence on dryland R_s and C_s over T_s and CO₂. Increasing Ppt individually or in combination with T_s and CO₂ benefited soil C sequestration. We therefore suggested that global change experimental studies for dryland ecosystems should focus more on the effects of precipitation regime changes and the combined effects of Ppt with other global change factors (e.g., T_s, CO₂, and N deposition).     

Soil CO2 efflux in a boreal pine forest under atmospheric CO2 enrichment and air warming

The response of forest soil CO₂ efflux to the elevation of two climatic factors, the atmospheric concentration of CO₂ (↑CO₂ of 700 μmol mol⁻¹) and air temperature (↑ T with average annual increase of 5°C), and their combination (↑CO₂+↑ T ) was investigated in a 4-year, full-factorial field experiment consisting of closed chambers built around 20-year-old Scots pines ( Pinus sylvestris L.) in the boreal zone of Finland. Mean soil CO₂ efflux in May–October increased with elevated CO₂ by 23–37%, with elevated temperature by 27–43%, and with the combined treatment by 35–59%. Temperature elevation was a significant factor in the combined 4-year efflux data, whereas the effect of elevated CO₂ was not as evident. Elevated temperature had the most pronounced impact early and late in the season, while the influence of elevated CO₂ alone was especially notable late in the season. Needle area was found to be a significant predictor of soil CO₂ efflux, particularly in August, a month of high root growth, thus supporting the assumption of a close link between whole-tree physiology and soil CO₂ emissions. The decrease in the temperature sensitivity of soil CO₂ efflux observed in the elevated temperature treatments in the second year nevertheless suggests the existence of soil response mechanisms that may be independent of the assimilating component of the forest ecosystem. In conclusion, elevated atmospheric CO₂ and air temperature consistently increased forest soil CO₂ efflux over the 4-year period, their combined effect being additive, with no apparent interaction.     

A method for experimental heating of intact soil profiles for application to climate change experiments

A new system for simulating future belowground temperature increases was conceived, simulated, constructed and tested in a temperate deciduous forest in Oak Ridge, TN, USA. The new system uses low‐wattage, 3 m deep heaters installed around the circumference of a defined soil volume. The heaters add the necessary energy to achieve a set soil temperature differential within the treatment area and add exterior energy inputs equal to those, which might be lost from lateral heat conduction. The method, which was designed to work in conjunction with aboveground heated chambers, requires only two control sensor positions one for aboveground air temperatures at 1 m and another for belowground temperatures at 0.8 m. The method is capable of achieving temperature differentials of at least +4.0±0.5 °C for soils to a measured depth of ?2 m. These +4 °C differential soil temperatures were sustained in situ throughout 2009, and both diurnal and seasonal cycles at all soil depths were retained using this simple heating approach. Measured mean energy inputs required to sustain the target heating level of +4 °C over the 7.1 m² target area were substantial for aboveground heating (21.1 kW h day^?1 m^?2), but 16 times lower for belowground heaters (1.3 kW h day^?1 m^?2). Observations of soil CO₂ efflux from the surface of the target soil volumes showed CO₂ losses throughout 2009 that were elevated above the temperature response curve that have been reported in previous near‐surface soil warming studies. Stimulation of biological activity within previously undisturbed deep‐soil carbon stocks is the hypothesized source. Long‐term research programs may be able to apply this new heating method that captures expected future warming and temperature dynamics throughout the soil profile to address uncertainties in process‐level responses of microbial, plant and animal communities in whole, intact ecosystems.     

Does soil CO2 efflux acclimatize to elevated temperature and CO2 during long-term treatment of Douglas-fir seedlings?

We investigated the effects of elevated soil temperature and atmospheric CO2 on soil CO2 efflux (SCE) during the third and fourth years of study. We hypothesized that elevated temperature would stimulate SCE, and elevated CO2 would also stimulate SCE with the stimulation being greater at higher temperatures. The study was conducted in sun-lit controlled-environment chambers using Douglas-fir (Pseudotsuga menziesii) seedlings grown in reconstructed litter-soil systems. We used a randomized design with two soil temperature and two atmospheric CO2 treatments. The SCE was measured every 4 wk for 18 months. Neither elevated temperature nor CO2 stimulated SCE. Elevated CO2 increased the temperature sensitivity of SCE. During the winter, the relationship between SCE and soil moisture was negative but it was positive during the summer. The seasonal patterns in SCE were associated with seasonal changes in photosynthesis and above-ground plant growth. SCE acclimatized in the high-temperature treatment, probably because of a loss of labile soil carbon. Elevated CO2 treatment increased the temperature sensitivity of SCE, probably through an increase in substrate availability.     

Soil respiration under prolonged soil warming: are rate reductions caused by acclimation or substrate loss?

The world's soils contain a large amount of carbon so that even a fractionally small loss or gain could have a quantitatively important feedback effect on net CO₂ emissions to the atmosphere. It is therefore important to fully understand the temperature dependence of soil‐carbon decomposition. Evidence from various observations can be used to quantify the temperature dependence of carbon efflux, but it is important to ensure that confounding factors, such as changing water relations or availability of readily decomposable substrate, are fully considered in inferring an underlying temperature response from observed response patterns. A number of recent findings from soil‐warming experiments have led to the suggestion that stimulation of soil‐carbon efflux by increasing temperature is only transitory before acclimation takes place and carbon efflux rates return to similar rates as before the increase in temperature. It is shown here that this response pattern can be explained through a simple two‐pool soil‐carbon model with no acclimation response needing to be invoked. The temporal pattern is, instead, due to depletion of readily decomposable substrate. It shows that findings of reduced respiration rate in soil‐warming experiments are consistent with unchanged high temperature sensitivity of organic carbon decomposition and affirms that there is, indeed, a danger of positive feedback between global warming and the release of soil organic carbon that can lead to further warming.     

全球变暖背景下土壤微生物呼吸的热适应性:证据、机理和争议

土壤微生物呼吸的热适应性被认为是决定陆地生态系统对全球变暖反馈作用的潜在重要机制,可能显著改变未来的气候变化趋势,然而学术界对于这一机制是否真实存在尚有分歧。阐述了土壤微生物呼吸的热适应性概念,从证据、机理和争议3方面对已有研究进展进行了综述和分析。土壤微生物呼吸的热适应性是微生物在群落尺度上对温度变化的适应性,具有坚实的生物学与生态学理论基础,研究者们运用各类指标已在许多实验中证实土壤微生物物种及群落的呼吸过程能够在高温环境产生适应性变化。土壤微生物呼吸的热适应性机理涉及生物膜结构变化、酶活性变化、微生物碳分配比例变化和微生物群落结构变化等方面。关于土壤微生物呼吸热适应性的争议可能是由研究方法、微生物物种及环境条件的差异引起的。根据对已有研究的分析,认为土壤微生物呼吸的热适应性是真实存在的,未来的研究可进一步探索土壤微生物呼吸的热适应性机理,深入研究环境和全球变化对土壤微生物呼吸的热适应性影响,定量评估土壤微生物呼吸的热适应性对陆地生态系统反馈过程的影响。     

Does elevated atmospheric CO2 concentration inhibit mitochondrial respiration in green plants?

ATP, adenosine triphosphate
K_m, Michaelis-Menton coefficient
C_a, concentration of CO₂ in the air (μmol mol^–1)
NAD, oxidized nicotin adenine dinucleotide
NADH, reduced nicotin adenine dinucleotide
NADP, oxidized nicotin adenine phosphate dinucleotide
NADPH, reduced nicotine adenine phosphate dinucleotide
R, rate of respiration per unit DW [μmol g
DW^–1], Rubisco, ribulose-1,5-bisphosphate carboxylase/oxygenase
V_c,_max, maximum in vivo rate of carboxylation at Rubisco (μmol m^–2 s^–1)

There is abundant evidence that a reduction in mitochondrial respiration of plants occurs when atmospheric CO₂ (C_a) is increased. Recent reviews suggest that doubling the present C_a will reduce the respiration rate [per unit dry weight (DW)] by 15 to 18%. The effect has two components: an immediate, reversible effect observed in leaves, stems, and roots of plants as well as soil microbes, and an irreversible effect which occurs as a consequence of growth in elevated C_a and appears to be specific to C₃ species. The direct effect has been correlated with inhibition of certain respiratory enzymes, namely cytochrome-c-oxidase and succinate dehydrogenase, and the indirect or acclimation effect may be related to changes in tissue composition. Although no satisfactory mechanisms to explain these effects have been demonstrated, plausible mechanisms have been proposed and await experimental testing. These are carbamylation of proteins and direct inhibition of enzymes of respiration. A reduction of foliar respiration of 15% by doubling present ambient C_a would represent 3 Gt of carbon per annum in the global carbon budget.