The net ecosystem exchange (NEE) is the difference between ecosystem CO2 assimilation and CO2 losses to the atmosphere. Ecosystem respiration (Reco), the efflux of CO2 from the ecosystem to the atmosphere, includes the soil-to-atmosphere carbon flux (i.e., soil respiration; Rsoil) and aboveground plant respiration. Therefore, Rsoil is a fraction of Reco and theoretically has to be smaller than Reco at daily, seasonal, and annual scales. However, several studies estimating Reco with the eddy covariance technique and measuring Rsoil within the footprint of the tower have reported higher Rsoil than Reco at different time scales. Here, we compare four different and contrasting ecosystems (from forest to grasslands, and from boreal to semiarid) to test if measurements of Reco are consistently higher than Rsoil. In general, both fluxes showed similar temporal patterns, but Reco was not consistently higher than Rsoil from daily to annual scales across sites. We identified several issues that apply for measuring NEE and measuring/upscaling Rsoil that could result in an underestimation of Reco and/or an overestimation of Rsoil. These issues are discussed based on (a) nighttime measurements of NEE, (b) Rsoil measurements, and (c) the interpretation of the functional relationships of these fluxes with temperature (i.e., Q10). We highlight that there is still a need for better integration of Rsoil with eddy covariance measurements to address challenges related to the spatial and temporal variability of Reco and Rsoil.

Aim: The aim was to explore how conversions of primary or secondary forests to plantations or agricultural systems influence soil microbial communities and soil carbon (C) cycling. Location: Global. Time period: 1993–2017. Major taxa studied: Soil microbes. Methods: A meta-analysis was conducted to examine effects of forest degradation on soil properties and microbial attributes related to microbial biomass, activity, community composition and diversity based on 408 cases from 119 studies in the world. Results: Forest degradation decreased the ratios of K-strategists to r-strategists (i.e., ratios of fungi to bacteria, Acidobacteria to Proteobacteria, Actinobacteria to Bacteroidetes and Acidobacteria1 Actinobacteria to Proteobacteria 1 Bacteroidetes). The response ratios (RRs) of the K-strategist to r-strategist ratios to forest degradation decreased and increased with increased RRs of soil pH and soil C to nitrogen ratio (C:N), respectively. Forest degradation increased the bacterial alpha-diversity indexes, of which the RRs increased and decreased as the RRs of soil pH and soil C:N increased, respectively. The overall RRs across all the forest degradation types ranked as microbial C (240.4%)> soil C (233.3%)> microbial respiration (218.9%)> microbial C to soil C ratio (qMBC; 215.9%), leading to the RRs of microbial respiration rate per unit microbial C (qCO2) and soil C decomposition rate (respiration rate per unit soil C), on average, increasing by 143.2 and 125.0%, respectively. Variances of the RRs of qMBC and qCO2 were significantly explained by the soil C, soil C:N and mean annual precipitation. Main conclusions: Forest degradation consistently shifted soil microbial community compositions from K-strategist dominated to r-strategist dominated, altered soil properties and stimulated microbial activity and soil C decomposition. These results are important for modelling the soil C cycling under projected global land-use changes and provide supportive evidence for applying the macroecology theory on ecosystem succession and disturbance in soil microbial ecology

The rate of CO2 diffusion from soils to the atmosphere (soil CO2 efflux, soil respiration; Rsoil) reflects the integrated activity of roots and microbes and is among the largest fluxes of the terrestrial global C cycle. Most experiments have demonstrated that Rsoil increases by 20–35% following the exposure of an ecosystem to an atmosphere enriched in CO2 (i.e., eCO2), but such experiments have largely been performed in young and N-limited ecosystems. Here, we exposed a mature and phosphorus-limited eucalypt woodland to eCO2 and measured Rsoil across three full years with a combination of manual surveys and automated measurements. We also implemented an empirical model describing the dependence of Rsoil on volumetric soil water content (h) and soil temperature (Tsoil) to produce annual Rsoil flux estimates. Rsoil varied strongly with Tsoil, h, and precipitation in complex and interacting ways. The realized long-term (weeks to years) temperature dependence (Q10) of Rsoil increased from * 1.6 at low h up to * 3 at high h. Additionally, Rsoil responded strongly and rapidly to precipitation events in a manner that depended on the conditions of h and Tsoil at the beginning of the rain event; Rsoil increased by up to 300% within 30 min when rain fell on dry soil that had not experience rain in the preceding week, but Rsoil was rapidly reduced by up to 70% when rain fell on wet soil, leading to flooding. Repeated measures analysis of Rsoil observations over 3 years indicated no significant change in response to CO2 enrichment (P = 0.7), and elevated CO2 did not alter the dependence of Rsoil on Tsoil or h. However, eCO2 increased Rsoil observations by * 10% under some constrained and moderate environmental conditions. Annual Rsoil flux sums estimated with an empirical model were * 7% higher in eCO2 plots than in aCO2 plots, but this difference was not statistically significant. The lack of a large eCO2 effect on Rsoil is consistent with recent evidence that aboveground net primary production was not stimulated by eCO2 in this ecosystem. The C budget of this mature woodland may be less affected by eCO2 than Responsible Editor: Egbert Matzner. Electronic supplementary material The online version of this article (https://doi.org/10.1007/s10533-018-0457-7) contains supplementary material, which is available to authorized users. J. E. Drake  C. A. Macdonald  M. G. Tjoelker  P. B. Reich  B. K. Singh  I. C. Anderson  D. S. Ellsworth Hawkesbury Institute for the Environment, Western Sydney University, Locked Bag 1797, Penrith, NSW 2751, Australia J. E. Drake (&) Department of Forest and Natural Resources Management, College of Environmental Science and Forestry, State University of New York, 1 Forestry Dr, Syracuse, NY, USA e-mail: jedrake@esf.edu P. B. Reich Department of Forest Resources, University of Minnesota, St. Paul, MN, USA 123 Biogeochemistry https://doi.org/10.1007/s10533-018-0457-7the young N-limited ecosystems that have been studied previously.

The temperature sensitivity of heterotrophic soil respiration is crucial for modeling carbon dynamics but it is variable. Presently, however, most models employ a fixed value of 1.5 or 2.0 for the increase of soil respiration per 10°C increase in temperature (Q10). Here, we identified the variability of Q10 at a regional scale (Rur catchment, Germany/Belgium/Netherlands). We divided the study catchment into environmental soil classes (ESC), which we define as unique combinations of land use, aggregated soil groups, and texture. We took nine soil samples from each ESC (108 samples) and incubated them at four soil moisture levels and five temperatures (5–25°C). We hypothesized that Q10 variability is controlled by soil organic carbon (SOC) degradability and soil moisture and that ESC can be used as a widely available proxy for Q10, owing to differences in SOC degradability. Measured Q10 values ranged from 1.2 to 2.8 and were correlated with indicators of SOC degradability (e.g., pH, r=-0.52). The effect of soil moisture on Q10 was variable: Q10 increased with moisture in croplands, but decreased in forests. The ESC captured significant parts of Q10 variability under dry (R²=0.44) and intermediate (R²=0.36) moisture conditions, where Q10 increased in the order cropland