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Physical diffusion of isotopic tracers into and out of soil pores causes considerable uncertainty for the timing and magnitude of plant belowground allocation in pulse-labelling experiments. Here, we partitioned soil CO₂ isotopic fluxes into abiotic tracer flux (physical return), heterotrophic flux, and autotrophic flux contributions following ¹³CO₂ labelling of a Swedish Pinus sylvestris forest. Soil CO₂ efflux and its isotopic composition from a combination of deep and surface soil collars was monitored using a field-deployed mass spectrometer. Additionally, ¹³CO₂ within the soil profile was monitored. Physical (abiotic) efflux of ¹³CO₂ from soil pore spaces was found to be significant for up to 48 h after pulse labelling, and equalled the amount of biotic label flux over 6 d. Measured and modelled changes in ¹³CO₂ concentration throughout the soil profile corroborated these results. Tracer return via soil CO₂ efflux correlated significantly with the proximity of collars to trees, while daily amplitudes of total flux (including heterotrophic and autotrophic sources) showed surprising time shifts compared with heterotrophic fluxes. The results show for the first time the significance of the confounding influence of physical isotopic CO₂-tracer return from the soil matrix, calling for the inclusion of meaningful control treatments in future pulse-chase experiments.

Arbuscular mycorrhizal fungi (AMF) are widespread in tropical forests and represent a major sink of photosynthate, yet their contribution to soil respiration in such ecosystems remains unknown. Using in-growth mesocosms we measured AMF mycelial respiration in two separate experiments: (1) an experiment in a semi-evergreen moist tropical forest, and (2) an experiment with 6-m-tall Pseudobombax septenatum in 4.5-m³ containers, for which we also determined the dependence of AMF mycelial respiration on the supply of carbon from the plant using girdling and root-cutting treatments. In the forest, AMF mycelia respired carbon at a rate of 1.4 t ha⁻¹ yr⁻¹, which accounted for 14 ± 6% of total soil respiration and 26 ± 12% of root-derived respiration. For P. septenatum, 40 ± 6% of root-derived respiration originated from AMF mycelia and carbon was respired < 4 h after its supply from roots. We conclude that arbuscular mycorrhizal mycelial respiration can be substantial in lowland tropical forests. As it is highly dependent on the recent supply of carbon from roots, a function of aboveground fixation, AMF mycelial respiration is therefore an important pathway of carbon flux from tropical forest trees to the atmosphere.

Temporal variations of carbon isotope composition of soil CO2 efflux (FS and δ13 CFS) at different time scales should reflect both temporal variations of the climate conditions that affect canopy functioning and temporal changes in the relative contribution of autotrophic respiration to total FS. A tunable diode laser spectrophotometer (TDLS) was installed in the Hesse forest (northeast of France) early during the 2007 growing season to determine the seasonal and daily variability in δ13 CFS. This method, based on the measurement of the absorption of an infrared laser emission at specific wave lengths of the 13 CO2 and 12 CO2, allows the continuous monitoring of the two isotopologues. The concentrations of the two isotopologues in FS were continuously monitored from June to November 2007 using chamber method and Keeling plots drawn from nocturnal accumulation of CO2 below the canopy. These TDLS measurements and isotope ratio mass spectrometer based Keeling plots gave very similar values of δ13 CFS, showing the reliability of the TDLS system in this context. Results were analysed with regard to seasonal and daily changes in climatic and edaphic variables and compared with the δ13 CofCO2 respired by roots, litter and soil incubated under controlled conditions. Pronounced daily as well as seasonal variations in δ13 CFS were recorded (up to 1.5‰). The range of variation of δ13 CFS was of the same order of magnitude at both diurnal and seasonal scales. δ13 CFS observed in the field fluctuated between values of litter and of root respiration recorded during incubation, suggesting that temporal (and probably spatial) variations were associated with changes in the relative contribution of the two compartments during the day and during the season.

Here, soil CO₂ efflux, minirhizotron fine root production (FRP), and estimated total below-ground carbon allocation (TBCA) were examined along an elevation and hybridization gradient between two cottonwood species. FRP was 72% greater under high-elevation Populus angustifolia, but soil CO₂ efflux and TBCA were 62% and 94% greater, respectively, under low-elevation stands dominated by Populus fremontii, with a hybrid stand showing intermediate values. Differences between the responses of FRP, soil CO₂ efflux and TBCA may potentially be explained in terms of genetic controls; while plant species and hybridization explained variance in carbon flux, we found only weak correlations of FRP and TBCA with soil moisture, and no correlations with soil temperature or nitrogen availability. Soil CO₂ efflux and TBCA were uncorrelated with FRP, suggesting that, although below-ground carbon fluxes may change along environmental and genetic gradients, major components of below-ground carbon flux may be decoupled.

Short rotation forests can serve as sources of renewable energy and possibly for soil C storage. However, the high frequency of management practices and the fertilisation could reduce C storage into the soil, by increasing CO₂ emissions and annulling the potential of C sequestration. The objectives of this work were to evaluate the impacts of coppicing and fertilisation on total soil CO₂ efflux, soil heterotrophic processes and consequent changes of soil C storage in a short rotation poplar plantation. Field soil CO₂ efflux, heterotrophic soil CO₂ efflux and soil organic C were compared before and after coppicing. Temporal dynamics of fine root biomass and water-soluble carbon after coppicing were also analysed. Coppicing increased total soil CO₂ efflux by more than 50%, while heterotrophic soil CO₂ efflux remained unchanged. Nevertheless, an increase in total organic carbon was observed as a result of above and belowground litter inputs, as well as root re-growth and exudation. This trend was more evident in fertilised soils due to lower heterotrophic and autotrophic soil CO₂ effluxes. Fertilisation can reduce the increase of CO₂ emissions after coppicing. Although soil organic C storage increased, the accumulation of labile fractions may trigger microbial respiration in the following years.

We measured the soil surface CO₂ efflux (R S) from January 2005 to December 2006 in two neighboring stands in Gwangneung Forest, central Korea: evergreen coniferous forest (Abies holophylla, stand A) and broad-leaved deciduous forest (Quercus-dominated, stand Q). Regarding seasonal variation, R S rate was low during the winter and early spring months in each stand and peaked in late July [1170 (stand A) and 1130 (stand Q) in 2005, and 1000 (stand A) and 740 (stand Q) mg CO₂ m⁻² h⁻¹ in 2006]. R S rate was higher in stand A than in stand Q during most of the growing season. The pattern of summer rainfall differed between 2005 and 2006. R S rate for both stands was suppressed significantly by the droughts in June 2005 and September 2006. After the heavy rainfall of July 2006, R S rate was lower than in July 2005 in both stands, but this decrement was much greater in stand Q than in stand A. In midsummer (August) 2006, under higher soil temperature (ST) and lower soil water content (SWC) conditions than in August 2005, R S rate of stand A was lower than that in August 2005, whereas stand Q showed no marked change. The exponential relationship between ST and R S accounted for approximately 91-97% of the R S variability in each stand and in each year. In stand A, the application of a second-order polynomial function indicated a significant correlation between SWC and R S when the soil was warm (ST > 15°C). Our results suggest that the seasonality of R S is strongly affected by the pattern of summer rainfall even in an Asia monsoon climate regime. In addition, the vegetation type (i.e., evergreen coniferous forest vs. broad-leaved deciduous forest) plays a significant role in response of R S to various environmental fluctuations such as drought, heavy rainfall, and hot-dry condition.

Summary Intensifying forest disturbance regimes are likely to impact heavily on future carbon (C) budgets of forest ecosystems. Our understanding of how forest disturbance affects the sources of soil CO2 efflux (Fs) is, however, poor. This may lead to uncertainties over future C sink estimates of forest ecosystems and associated feedbacks to the atmosphere. We investigated the impact of forest windthrow on the heterotrophic and autotrophic sources of Fs, underlying biotic and abiotic drivers (i.e. plant community composition, soil organic matter (SOM) properties and soil microclimate), and consequences for soil organic carbon (SOC) stocks in situ along a disturbance chronosequence in the European Alps. This chronosequence facilitated the study of temporal changes in the above parameters between the third and sixth years after windthrow. Along the chronosequence, structural equation modelling revealed that soil temperature, soil moisture, SOM properties and plant community composition explained 90% of the variation in Fs. While no direct interactions among plants and SOM properties could be determined, plants significantly affected soil microclimate. Windthrow had no obvious effect on Fs because reduced autotrophic soil respiration (Ra) was offset by a ∼60% increase in heterotrophic soil respiration (Rh), principally due to increased soil temperatures. Ra after wind‐throw was dominated by grasses and herbs rather than trees; however, a high abundance of ectomycorrhizal fungi suggests an important indirect tree contribution to post‐windthrow Ra. SOC stocks significantly declined over the post‐windthrow period. Our results show that Rh was by far the dominant source of Fs after forest windthrow. As C losses from Rh and SOC stocks were in the same order of magnitude, this study demonstrates that post‐windthrow declines in SOC stocks were mainly driven by a temperature‐related increase in Rh. A lay summary is available for this article. Lay Summary

The flux of carbon from tree photosynthesis through roots to ectomycorrhizal (ECM) fungi and other soil organisms is assumed to vary with season and with edaphic factors such as nitrogen availability, but these effects have not been quantified directly in the field. To address this deficiency, we conducted high temporal-resolution tracing of ¹³C from canopy photosynthesis to different groups of soil organisms in a young boreal Pinus sylvestris forest. There was a 500% higher below-ground allocation of plant in the late (August) season compared with the early season (June). Labelled was primarily found in fungal fatty acid biomarkers (and rarely in bacterial biomarkers), and in Collembola, but not in Acari and Enchytraeidae. The production of sporocarps of ECM fungi was totally dependent on allocation of recent photosynthate in the late season. There was no short-term (2 wk) effect of additions of N to the soil, but after 1 yr, there was a 60% reduction of below-ground allocation to soil biota. Thus, organisms in forest soils, and their roles in ecosystem functions, appear highly sensitive to plant physiological responses to two major aspects of global change: changes in seasonal weather patterns and N eutrophication.

• In arctic ecosystems, climate change has increased plant productivity. As arctic carbon (C) stocks predominantly are located belowground, the effects of greater plant productivity on soil C storage will significantly determine the net sink/source potential of these ecosystems, but vegetation controls on soil CO₂ efflux remain poorly resolved. • In order to identify the role of canopy-forming species in belowground C dynamics, we conducted a girdling experiment with plots distributed across 1 km₂ of treeline birch (Betula pubescens) forest and willow (Salix lapponum) patches in northern Sweden and quantified the contribution of canopy vegetation to soil CO₂ fluxes and belowground productivity. • Girdling birches reduced total soil CO₂ efflux in the peak growing season by 53%, which is double the expected amount, given that trees contribute only half of the total leaf area in the forest. Root and mycorrhizal mycelial production also decreased substantially. At peak season, willow shrubs contributed 38% to soil CO₂ efflux in their patches. • Our findings indicate that C, recently fixed by trees and tall shrubs, makes a substantial contribution to soil respiration. It is critically important that these processes are taken into consideration in the context of a greening arctic because productivity and ecosystem C sequestration are not synonymous.

We seek to understand how biophysical factors such as soil temperature ( T s ), soil moisture (θ), and gross primary production (GPP) influence CO 2 fluxes across terrestrial ecosystems. Recent advancements in automated measurements and remote-sensing approaches have provided time series in which lags and relationships among variables can be explored. The purpose of this study is to present new applications of continuous measurements of soil CO 2 efflux ( F 0 ) and soil CO 2 concentrations measurements. Here we explore how variation in T s , θ, and GPP (derived from NASA's moderate-resolution imaging spectroradiometer [MODIS]) influence F 0 and soil CO 2 production ( P s ). We focused on seasonal variation and used continuous measurements at a daily timescale across four vegetation types at 13 study sites to quantify: (1) differences in seasonal lags between soil CO 2 fluxes and T s , θ, and GPP and (2) interactions and relationships between CO 2 fluxes with T s , θ, and GPP. Mean annual T s did not explain annual F 0 and P s among vegetation types, but GPP explained 73% and 30% of the variation, respectively. We found evidence that lags between soil CO 2 fluxes and T s or GPP provide insights into the role of plant phenology and information relevant about possible timing of controls of autotrophic and heterotrophic processes. The influences of biophysical factors that regulate daily F 0 and P s are different among vegetation types, but GPP is a dominant variable for explaining soil CO 2 fluxes. The emergence of long-term automated soil CO 2 flux measurement networks provides a unique opportunity for extended investigations into F 0 and P s processes in the near future.