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In this research we consider the response of soil respiration under elevated CO2 (eCO2) in an oak-dominated temperate forest. We hypothesised that under elevated CO2 (550 ppm) soil moisture would increase as a result of reduced stomatal conductance, which would in turn lead to higher soil respiration. Continuous measurements were performed on three pairs of plots near Stafford (United Kingdom). Respiration was measured diurnally for 2 minutes each time, using the LI-COR 8100A set-up, and the rate of respiration (flux rate) was calculated SoilFluxPro software. Next, an empirical model was fitted to the dataset based on hourly averages of the flux rates, soil temperature, and soil moisture. Three respiration collars per plot were averaged, thus accounting for spatial variability within the site. Model parameterization and gap filling were conducted on individual plots to calculate annual rates for 2019-2021. Cross-validation was performed by using 80% (randomly selected) of each dataset for training and the remaining 20% for testing the data against the parameters obtained by the empirical models. Preliminary results suggest that annual respiration rates were significantly higher for the eCO2 across all pairs in 2019. However, 2 out of 3 pairs in 2020 and 2021 showed significantly higher respiration for the aCO2 plots compared to eCO2, which is not in line with our hypothesis. Relationships with soil moisture and temperature help to explain what drives the difference in these fluxes. Our findings show that the relationship between higher CO2 concentrations in the atmosphere and soil respiration is not a straightforward one, which is of interest when considering the role of forest C-cycling on a global scale.

Anthropogenic CO2 emissions have resulted in elevated CO2 (eCO2) in the atmosphere, and this rise is predicted to continue1. Increases in CO2 have fertilised forest ecosystems and led to an uptake of CO2 into plant and soil biomass. Early findings at BIFoR FACE (Free-Air Carbon Dioxide Enrichment) showed increased photosynthetic uptake2, fine root net primary productivity3 and soil respiration4, indicating increased carbon (C) allocation belowground and mirroring previous forest FACE experiments. Roots play a key role in whole-plant functions, biogeochemical cycling and interactions with biotic factors, thus based on the early findings we expect that the increased C allocation belowground will have an impact on root biomass and architecture. Root biomass combined with root architecture (such as root diameter and length) are of high importance to elucidate the impacts of eCO2 on primary productivity, interactions in the rhizosphere, carbon sequestration and nutrient cycling5,6. This study assesses the impact of elevated CO2 on root biomass and architecture at the BIFoR FACE the first 5 years of operation (2017-2022). Changes in root biomass and architecture were monitored via soil coring three times a year (spring, summer and autumn) to 30 cm (per horizon). The root biomass in assessed as per dry weight in four different root diameter classes (5 mm) and the root architecture was assessed via fresh root scanning. Root biomass exhibited a prompt and sustained increase under eCO2 during the first 5 years of CO2 fumigation, with the increase being more pronounced for the three smaller diameter classes (5 mm). Increases in root biomass could suggest increases in total root length, root diameter and tissue density, enhancing trees’ capacity to acquire more soil resources such as water and nutrients, or resource storage. References1Intergovernmental Panel on Climate Change; Core Writing Team; Pachauri, R.K.; Meyer, L.A. (Eds.) Climate Change 2014: Synthesis Report, Contribution of Working Groups I, II and III to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change; Intergovernmental Panel on Climate Change: Geneva, Switzerland, 2014; 151p.2Gardner, A., Ellsworth, D., Crous, K., Pritchard, J., Mackenzie, A.R. (2021). Is photosynthetic enhancement sustained through three years of elevated CO2 exposure in 175-year-old Quercus robur? Tree Physiology, 42 (1), 130-1443Ziegler, C., Kulawska, A., Kourmouli, A., Hamilton, L., Shi, Z., MacKenzie, A.R., Dyson, R.J., Johnston, I.G. (2022). Quantification and uncertainty of root growth stimulation by elevated CO2 in mature temperate deciduous forest. Science of the Total Environment, 854,4Kourmouli, A., Hamilton, L., Pihlblad, J., Barba, J., Bartlett, R., MacKenzie, AR., Hartley, I., Shi, Z. (2023). Initial carbon and nutrient responses to free air CO2 enrichment in a mature deciduous woodland. (submitted) 5Norby, R. J., & Jackson, R. B. (2000). Root dynamics and global change: Seeking an ecosystem perspective. New Phytologist, 147, 3–12.6Wilson, S. D. (2014). Below-ground opportunities in vegetation science. Journal of Vegetation Science, 25, 1117–1125.

Increasing CO2 levels are a major global challenge, and the potential mitigation of anthropogenic CO2 emissions by natural carbon sinks remains poorly understood. The uptake of elevated CO2 (eCO2) by the terrestrial biosphere, and subsequent sequestration as biomass in ecosystems, remain hard to quantify in natural ecosystems. Here, we combine field observations of fine root stocks and flows, derived from belowground imaging and soil cores, with image analysis, stochastic modelling, and statistical inference, to elucidate belowground root dynamics in a mature temperate deciduous forest under free-air eCO2 to 150 ppm above ambient levels. eCO2 led to relatively faster root production (a peak volume fold change of 4.52 ± 0.44 eCO2 versus 2.58 ± 0.21 control), with increased root elongation relative to decay the likely causal mechanism for this acceleration. Physical analysis of 552 root systems from soil cores support this picture, with lengths and widths of fine roots significantly increasing under eCO2. Estimated fine root contributions to belowground net primary productivity increase under eCO2 (mean annual 204 ± 93 g dw m−2 yr−1 eCO2 versus 140 ± 60 g dw m−2 yr−1 control). This multi-faceted approach thus sheds quantitative light on the challenging characterisation of the eCO2 response of root biomass in mature temperate forests. publishedVersion

Increasing CO

This research investigates the cascading effects of elevated carbon dioxide (eCO2) fumigation of a mature temperature forest, with a particular focus on the fluxes of greenhouse gases (GHG) nitrous oxide (N2O), methane (CH4) and carbon dioxide (CO2). A field experiment was performed at the Birmingham Institute of Forest Research Free Air Carbon dioxide Enrichment facility (BIFoR FACE), where an oak dominated mixed mature woodland has been under eCO2 since 2017. Fluxes were quantified in situ using the Licor 8100A – an infrared gas analyser measuring total soil respiration (Rs) as CO2, and a Picarro greenhouse gas analyser (G2508), measuringN2O and CH4. Preliminary data from 2019 – 2021 have been analysed and are built on an earlier dataset from 2017-2018, and the role of soil temperature and soil moisture is considered. With more carbon allocation belowground, we expect an increase in microbial activity and consequently larger Rs. Overall, Rs was higher under eCO2 in 2017-2018; however, in years 2019 to 2021, the absolute difference in respiration between eCO2 and control plots gradually decreased and even switched in 2021, with a slight increase in Rs for control plots compared to eCO2 plots. Moreover, annual fluxes of N2O and CH4 were detectable and in general we observed N2O emission and CH4 consumption. My presentation will discuss Rs and N2O and CH4 fluxes and highlight the role of eCO2 as well as environmental and soil conditions that regulate the GHG fluxes, allowing us to compute the net global warming potential of forests under future climates.

The carbon fertilization effect under increasing atmospheric carbon dioxide (CO2) may contribute to removing 30% of anthropogenic CO2, with mature forests central to this. However, the ability of mature forests to continue to act as a long-term sink of carbon (C) is dependent on the availability of essential nutrients; nitrogen, and phosphorus. It has been suggested root exudates may increase under elevated CO2 (eCO2) as a mechanism to acquire these nutrients from soil, via priming of the soil microbial community to increase nutrient turnover, or abiotic release. However, this is yet to be tested in a mature forest. Furthermore, it is unknown if root exudate composition also changes in response to eCO2, as has been observed for drought. Given the role of root exudates in nutrient acquisition, their response to elevated CO2 in a mature temperate forest may be a key mechanism for nutrient acquisition, supporting their ability to act as a long-term sink of CO2.We used the unique Birmingham Institute of Forest Research (BIFoR) free air carbon enrichment experiment (FACE), where a mature temperate deciduous forest dominated by English Oaks (Q. robur) is fumigated with eCO2 at +150 ppm above the ambient atmospheric CO2 concentration during the growing season, since 2017. Root exudates were collected quarterly from summer 2020 to summer 2021 from in-situ fine (Carbon exuded by fine roots was 40% higher under elevated CO2 across the year, with a clear seasonal trend whereas nitrogen exudation rate did not significantly differ between elevated CO2 and control plots with no seasonal trend. Enhancement of C exudation resulted in a trend of a relatively larger C:N ratio, indicating a compositional change under eCO2, despite no differences in root C:N. Untargeted metabolomic analysis of root exudates collected in Summer 2020 confirmed significant changes in composition of root exudates. Compounds associated with the metabolism of amino acids, carbohydrates and cofactors and vitamins, and biosynthesis of secondary metabolites were upregulated under eCO2, and this was also reflected in the metabolome of the roots.The increased carbon exudation rates reflected higher photosynthetic rates observed in oaks leaves under eCO2, and compositional changes indicated by lower nitrogen exudation rates, relative to carbon. Furthermore, compositional changes investigated via metabolomics revealed significant changes in the metabolome, pointing to potential eCO2 cascading impacts on nutrient acquisition strategies of mature oaks. These must be accounted for to be able to fully account for nutrient constraints of C uptake by forests under future climates, including within CNP-coupled and ESM models.

Increasing CO2levels are a major global challenge, and the extent to which increasing anthropogenic CO2emissions can be mitigated by natural carbon sinks remains poorly understood. The uptake of elevated CO2(eCO2) by the terrestrial biosphere, and subsequent sequestration as biomass in ecosystems, may act as a negative feedback in the carbon budget, but remains hard to quantify in natural ecosystems. Here, we combine large-scale field observations of fine root stocks and flows, derived from belowground imaging and soil cores, with image analysis, stochastic modelling, and statistical inference, to elucidate belowground root dynamics in a mature temperate deciduous forest under free-air CO2enrichment to 150ppm above ambient levels. Using over 67kframes of belowground observation, we observe that eCO2leads to relatively faster root production (a peak volume fold change of 4.52 ± 0.44 eCO2versus 2.58 ± 0.21 control). We identify an increase in existing root elongation relative to root mass decay as the likely causal mechanism for this acceleration. Direct physical analysis of biomass and width measurements from 552 root systems recovered from soil cores support this picture, with lengths and widths of fine roots significantly increasing under eCO2. We use dynamic measurements to estimate fine root contributions to net primary productivity, finding an increase under eCO2, with an estimated mean annual 204 ± 93 g dw m−2yr−1eCO2versus 140 ± 60 g dw m−2yr−1control. We also quantify and discuss the uncertainties in such productivity measurements. This multi-faceted approach thus sheds quantitative light on the challenging characterisation of the eCO2response of root biomass in mature temperate forests.

The use of disc diffusion susceptibility tests to determine the antibacterial activity of engineered nanoparticles (ENPs) is questionable because their low diffusivity practically prevents them from penetrating through the culture media. In this study, we investigate the ability of such a test, namely the Kirby-Bauer disc diffusion test, to determine the antimicrobial activity of Au and Ag ENPs having diameters from 10 to 40 nm on Escherichia coli cultures. As anticipated, the tests did not show any antibacterial effects of Au nanoparticles (NPs) as a result of their negligible diffusivity through the culture media. Ag NPs on the other hand exhibited a strong antimicrobial activity that was independent of their size. Considering that Ag, in contrast to Au, dissolves upon oxidation and dilution in aqueous solutions, the apparent antibacterial behavior of Ag NPs is attributed to the ions they release. The Kirby-Bauer method, and other similar tests, can therefore be employed to probe the antimicrobial activity of ENPs related to their ability to release ions rather than to their unique size-dependent properties. [Figure not available: see fulltext.].