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Elevated atmospheric CO2 concentration and climate change may substantially alter soil carbon (C) dynamics, which in turn may impact future climate through feedback cycles. However, only very few field experiments worldwide have combined elevated CO2 (eCO2) with both warming and changes in precipitation in order to study the potential combined effects of changes in these fundamental drivers of C cycling in ecosystems. We exposed a temperate heath/grassland to eCO2, warming, and drought, in all combinations for 8 years. At the end of the study, soil C stocks were on average 0.927 kg C/m2 higher across all treatment combinations with eCO2 compared to ambient CO2 treatments (equal to an increase of 0.120 ± 0.043 kg C m−2 year−1), and showed no sign of slowed accumulation over time. However, if observed pretreatment differences in soil C are taken into account, the annual rate of increase caused by eCO2 may be as high as 0.177 ± 0.070 kg C m−2 year−1. Furthermore, the response to eCO2 was not affected by simultaneous exposure to warming and drought. The robust increase in soil C under eCO2 observed here, even when combined with other climate change factors, suggests that there is continued and strong potential for enhanced soil carbon sequestration in some ecosystems to mitigate increasing atmospheric CO2 concentrations under future climate conditions. The feedback between land C and climate remains one of the largest sources of uncertainty in future climate projections, yet experimental data under simulated future climate, and especially including combined changes, are still scarce. Globally coordinated and distributed experiments with long‐term measurements of changes in soil C in response to the three major climate change‐related global changes, eCO2, warming, and changes in precipitation patterns, are, therefore, urgently needed. [ABSTRACT FROM AUTHOR]

A fundamental challenge in experimental ecology is to capture nonlinearities of ecological responses to interacting environmental drivers. Here, we demonstrate that gradient designs outperform replicated designs for detecting and quantifying nonlinear responses. We report the results of (1) multiple computer simulations and (2) two purpose‐designed empirical experiments. The findings consistently revealed that unreplicated sampling at a maximum number of sampling locations maximised prediction success (i.e. the R² to the known truth) irrespective of the amount of stochasticity and the underlying response surfaces, including combinations of two linear, unimodal or saturating drivers. For the two empirical experiments, the same pattern was found, with gradient designs outperforming replicated designs in revealing the response surfaces of underlying drivers. Our findings suggest that a move to gradient designs in ecological experiments could be a major step towards unravelling underlying response patterns to continuous and interacting environmental drivers in a feasible and statistically powerful way. [ABSTRACT FROM AUTHOR]

Manipulation experiments are often used to investigate ecological and environmental causal relationships and to understand and forecast impacts of anthropogenic pressures on ecosystem functioning. Such manipulation experiments often use factorial designs, and the data are analyzed using factorial linear models. Factorial designs build on the fundamental assumption that the treatment factors are independent and orthogonal. This assumption is, however, often violated because of variation within and in particular covariation between the performed experimental manipulations. For example, manipulation of temperature and precipitation in factorial setups has been widely applied in climate experiments, but manipulating soil temperature will likely have a strong impact on soil water content. Such dependency among environmental state variables will violate the assumed orthogonality in a factorial linear model and may lead to erroneous conclusions. Here, we demonstrate the importance of the assumption of orthogonality using simulated ecological responses that act on observed soil state variables from a large climate experiment with an apparent orthogonal design. More specifically, we explore the problematic consequences of analyzing ecological treatments as categorical variables in a linear model. Suitable alternative methods for the statistical analysis of manipulated ecological experiments are suggested. The key recommendation is to use the observed effects of the manipulations on the state variables directly in the analysis instead of the categories of treatments. For example, if soil water content and temperature are manipulated, then it is essential to measure the water content and temperature in the soil of all the manipulated plots. [ABSTRACT FROM AUTHOR]

Soil respiration (Rs) is an important component of ecosystem carbon balance and accurate quantification of the diurnal and seasonal variation of Rs is crucial for correct interpretation of the response of Rs to biotic and abiotic factors, as well as for estimating annual soil CO2 efflux rates. In this study, we measured Rs hourly for one year by automated closed chambers in a temperate Danish beech forest. The data showed a clear diurnal pattern of Rs across all seasons with higher rates during night-time than during day-time. However, further analysis showed a clear negative relationship between flux rates and friction velocity (u∗) above the canopy, suggesting that Rs was overestimated at low atmospheric turbulence throughout the year due to non-steady state conditions during measurements. Filtering out data at low u∗ values removed or even inverted the observed diurnal pattern, such that the highest effluxes were now observed during day-time, and also led to a substantial decrease in the estimated annual soil CO2 efflux. By installing fans to produce continuous turbulent mixing of air around the soil chambers, we tested the hypothesis that overestimation of soil CO2 effluxes during low u∗ can be eliminated if proper mixing of air is ensured, and indeed the use of fans removed the overestimation of Rs rates during low u∗. Artificial turbulent air mixing may thus provide a method to overcome the problems of using closed chamber gas exchange measurement techniques during naturally occurring low atmospheric turbulence conditions. Other possible effects from using fans during soil CO2 efflux measurements are discussed. In conclusion, periods with low atmospheric turbulence may provide a significant source of error in Rs rates estimated by the use of closed chamber techniques and erroneous data must be filtered out to obtain unbiased diurnal patterns, accurate relationships to biotic and abiotic factors, and before estimating Rs fluxes over longer time scales. [ABSTRACT FROM AUTHOR]

Well-defined productivity-precipitation relationships of ecosystems are needed as benchmarks for the validation of land models used for future projections. The productivity-precipitation relationship may be studied in two ways: the spatial approach relates differences in productivity to those in precipitation among sites along a precipitation gradient (the spatial fit, with a steeper slope); the temporal approach relates interannual productivity changes to variation in precipitation within sites (the temporal fits, with flatter slopes). Precipitation-reduction experiments in natural ecosystems represent a complement to the fits, because they can reduce precipitation below the natural range and are thus well suited to study potential effects of climate drying. Here, we analyse the effects of dry treatments in eleven multiyear precipitation-manipulation experiments, focusing on changes in the temporal fit. We expected that structural changes in the dry treatments would occur in some experiments, thereby reducing the intercept of the temporal fit and displacing the productivity-precipitation relationship downward the spatial fit. The majority of experiments (72%) showed that dry treatments did not alter the temporal fit. This implies that current temporal fits are to be preferred over the spatial fit to benchmark land-model projections of productivity under future climate within the precipitation ranges covered by the experiments. Moreover, in two experiments, the intercept of the temporal fit unexpectedly increased due to mechanisms that reduced either water loss or nutrient loss. The expected decrease of the intercept was observed in only one experiment, and only when distinguishing between the late and the early phases of the experiment. This implies that we currently do not know at which precipitation-reduction level or at which experimental duration structural changes will start to alter ecosystem productivity. Our study highlights the need for experiments with multiple, including more extreme, dry treatments, to identify the precipitation boundaries within which the current temporal fits remain valid. [ABSTRACT FROM AUTHOR]

Estimates of carbon leaching losses from different land use systems are few and their contribution to the net ecosystem carbon balance is uncertain. We investigated leaching of dissolved organic carbon (DOC), dissolved inorganic carbon (DIC), and dissolved methane (CH), at forests, grasslands, and croplands across Europe. Biogenic contributions to DIC were estimated by means of its δC signature. Leaching of biogenic DIC was 8.3±4.9 g m yr for forests, 24.1±7.2 g m yr for grasslands, and 14.6±4.8 g m yr for croplands. DOC leaching equalled 3.5±1.3 g m yr for forests, 5.3±2.0 g m yr for grasslands, and 4.1±1.3 g m yr for croplands. The average flux of total biogenic carbon across land use systems was 19.4±4.0 g C m yr. Production of DOC in topsoils was positively related to their C/N ratio and DOC retention in subsoils was inversely related to the ratio of organic carbon to iron plus aluminium (hydr)oxides. Partial pressures of CO in soil air and soil pH determined DIC concentrations and fluxes, but soil solutions were often supersaturated with DIC relative to soil air CO. Leaching losses of biogenic carbon (DOC plus biogenic DIC) from grasslands equalled 5-98% (median: 22%) of net ecosystem exchange (NEE) plus carbon inputs with fertilization minus carbon removal with harvest. Carbon leaching increased the net losses from cropland soils by 24-105% (median: 25%). For the majority of forest sites, leaching hardly affected actual net ecosystem carbon balances because of the small solubility of CO in acidic forest soil solutions and large NEE. Leaching of CH proved to be insignificant compared with other fluxes of carbon. Overall, our results show that leaching losses are particularly important for the carbon balance of agricultural systems. [ABSTRACT FROM AUTHOR]

While substantial cold-season respiration has been documented in most arctic and alpine ecosystems in recent years, the significance of cold-season photosynthesis in these biomes is still believed to be small. In a mesic, subartic heath during both the cold and warm season, we measured in situ ecosystem respiration and photosynthesis with a chamber technique at ambient conditions and at artificially increased frequency of freeze–thaw (FT) cycles during fall and spring. We fitted the measured ecosystem exchange rates to respiration and photosynthesis models with R2-values ranging from 0.81 to 0.85. As expected, estimated cold-season (October, November, April and May) respiration was significant and accounted for at least 22% of the annual respiratory CO2 flux. More surprisingly, estimated photosynthesis during this period accounted for up to 19% of the annual gross CO2 uptake, suggesting that cold-season photosynthesis partly balanced the cold-season respiratory carbon losses and can be significant for the annual cycle of carbon. Still, during the full year the ecosystem was a significant net source of 120 ± 12 g C m−2 to the atmosphere. Neither respiration nor photosynthetic rates were much affected by the extra FT cycles, although the mean rate of net ecosystem loss decreased slightly, but significantly, in May. The results suggest only a small response of net carbon fluxes to increased frequency of FT cycles in this ecosystem. [ABSTRACT FROM AUTHOR]

This article is a response to Korell et al., 26, 325–327; See also the Commentary on this article by Muller et al., 26, e4–e5; See also the response to this Letter to the Editor by Korell et al., 26, 328–329. [ABSTRACT FROM AUTHOR]

Recent advances in laser spectroscopy have allowed for real-time measurements of the 13C/12C isotopic ratio in CO2, thereby providing new ways to investigate carbon cycling in natural ecosystems. In this study, we combined an Aerodyne quantum cascade laser spectrometer for CO2 isotopes with a LI-COR LI-8100A/8150 automated chamber system to measure the δ13C of CO2 during automated closed-chamber measurements. The isotopic composition of the CO2 flux was determined for each chamber measurement by applying the Keeling plot method. We found that the δ13C measured by the laser spectrometer was influenced by water vapour and CO2 concentration of the sample air and we developed a method to correct for these effects to yield accurate measurements of δ13C. Overall, correcting for the CO2 concentration increased the δ13C determined from the Keeling plots by 3.4‰ compared to 2.1‰ for the water vapour correction. We used the combined system to measure δ13C of the CO2 fluxes automatically every two hours from intact soil, trenched soil, tree stems and coarse roots during a two-month campaign in a Danish beech forest. The mean δ13C was −29.8 ± 0.32‰ for the intact soil plots, which was similar to the mean δ13C of −29.8 ± 1.2‰ for the trenched soil plots. The lowest δ13C was found for the root plots with a mean of −32.6 ± 0.78‰. The mean δ13C of the stems was −30.2 ± 0.74‰, similar to the mean δ13C of the soil plots. In conclusion, the study showed the potential of using a quantum cascade laser spectrometer to measure δ13C of CO2 during automated closed-chamber measurements, thereby allowing for measurements of isotopic ecosystem CO2 fluxes at a high temporal resolution. It also highlighted the importance of proper correction for cross-sensitivity with water vapour and CO2 concentration of the sample air to get accurate measurements of δ13C. [ABSTRACT FROM AUTHOR]

The eddy covariance measurement technique is broadly applied in terrestrial ecosystems forecosystem-level measurements of CO2, CH4 and H2O exchange between ecosystem andatmosphere. However, in ecological experiments with smaller plot sizes than typicalfootprints of eddy covariance towers, such as in field-scale climate change experiments, theeddy covariance technique cannot be applied. In these experiments, measurements of gasexchange with the atmosphere rely on chamber techniques. However, chamberslarge enough to measure at the ecosystem scale are currently not commerciallyavailable.We built a novel automatic chamber for measurement of gas fluxes at the ecosystemscale in short-stature ecosystems. The chamber covers an area of 2827 cm2 (60cm diameter), is 80 cm tall (volume: 225 L). The unique feature of the chamberis that it is capable of switching automatically between light and darkened modeenabling separation of light-sensitive and light-indifferent processes. For CO2 fluxes,net exchange (NEE) is measured in the light mode, while ecosystem respiration(RE) is measured in the darkened mode and Gross Primary Productivity (GPP) isestimated as NEE – RE. The chamber is controlled by a LI-COR 8100/8150 multiplexersystem.We measured hourly fluxes of CO2, H2O and CH4 continuously for 3 years in DanishCalluna vulgaris (common heather) heathland thus generating more than 40,000 individualchamber measurements. We will present an analysis of the novel, high-frequency data setincluding new insights into the diel fluctuation in RE rates in response to diel, weeklyand seasonal changes in GPP, temperature, soil water availability and rain events.Novel findings include the observation of increased RE at similar temperaturesduring daytime compared to nighttime, thus questioning if RE during daytime can beestimated based on temperature responses from nighttime NEE measurements. Over thestudy period the ecosystem was a net sink for both CO2 and CH4. Finally, we alsoestimated the evapotranspiration rates of the ecosystems by fitting non-linear modelsto the H2O concentration development inside the chamber. The presentation willdiscuss the potential of using closed chambers for estimating evapotranspirationin experiments with plot sizes too small for application of the eddy-covariancetechnique. In summary, the ability of the chamber to be both transparent and darkenedenables high-frequency, direct measurements of the processes of photosynthesisand ecosystem respiration during daytime. This is an advancement compared totransparent-only chambers as well as the eddy covariance technique which bothrely on modelling daytime GPP and RE from the NEE measurements obtained. [ABSTRACT FROM AUTHOR]

Elevated atmospheric CO2 concentration (eCO2) and climate change may significantly alter soil carbon (C) dynamics and thus feedback to future climate. However, only very few field experiments world-wide have combined eCO2 with both warming and changes in precipitation in order to study the potential combined effects of changes in these fundamental drivers of carbon cycling in ecosystems. We exposed a temperate heath/grassland to eCO2, warming, and drought, in all combinations for 8 years. Soil C under ambient CO2 remained constant over time, whereas soil C stocks increased by 0.167 kg C m-2 yr-1 on average across all treatment combinations with eCO2 and showed no sign of slowed accumulation over time. Further, the response to eCO2 was not affected by simultaneous exposure to warming and drought. The robust increase in soil C under eCO2 observed here, even when combined with other climate change factors, suggests that there is continued and strong potential for enhanced soil carbon sequestration in some ecosystems to mitigate increasing atmospheric CO2 concentrations under future climate conditions. The feedback between land C and climate remains one of the largest sources of uncertainty in future climate projections, yet experimental data under simulated future climate, and especially including combined changes, are still scarce. Globally coordinated and distributed experiments with long-term measurements of changes in soil C in response to the three major climate change-related global changes, eCO2, warming, and changes in precipitation patterns, are therefore urgently needed. [ABSTRACT FROM AUTHOR]

Climate change is increasing the frequency and severity of short-term (~1 y) drought events-the most common duration of drought-globally. Yet the impact of this intensification of drought on ecosystem functioning remains poorly resolved. This is due in part to the widely disparate approaches ecologists have employed to study drought, variation in the severity and duration of drought studied, and differences among ecosystems in vegetation, edaphic and climatic attributes that can mediate drought impacts. To overcome these problems and better identify the factors that modulate drought responses, we used a coordinated distributed experiment to quantify the impact of short-term drought on grassland and shrubland ecosystems. With a standardized approach, we imposed ~a single year of drought at 100 sites on six continents. Here we show that loss of a foundational ecosystem function-aboveground net primary production (ANPP)-was 60% greater at sites that experienced statistically extreme drought (1-in-100-y event) vs. those sites where drought was nominal (historically more common) in magnitude (35% vs. 21%, respectively). This reduction in a key carbon cycle process with a single year of extreme drought greatly exceeds previously reported losses for grasslands and shrublands. Our global experiment also revealed high variability in drought response but that relative reductions in ANPP were greater in drier ecosystems and those with fewer plant species. Overall, our results demonstrate with unprecedented rigor that the global impacts of projected increases in drought severity have been significantly underestimated and that drier and less diverse sites are likely to be most vulnerable to extreme drought., Competing Interests: Competing interests statement:The authors declare no competing interest.