The Context of the Research - Many high-profile research papers and syntheses have equated increased vegetation productivity and shifting vegetation types in northern high latitudes with increased net carbon (C) sequestration from the atmosphere. Although logical and intuitive, this largely overlooks the potential fate of pre-existing soil organic carbon (SOC) in these regions. This is a problem because soils at high latitudes are notably C-rich (containing ~570 Pg C in boreal/taiga forest and tundra soils alone; note, 1 Pg (Peta-gram) = 1,000,000,000 tonnes) and this pool is dynamic, intrinsically interacting both with vegetation cover and with climate. Although challenging to investigate, we cannot overlook below-ground processes if we are to understand net C budgets on timescales relevant to the Climate Emergency. Understanding the fundamental mechanisms controlling the accumulation, stability, and loss of soil organic matter (SOM) is as essential for predicting the Earth's future climate as understanding photosynthesis and plant productivity. However, our understanding of, and ability to model, SOM dynamics lags far behind that of primary productivity. Furthermore, rapid warming at high northern latitudes adds urgency to understanding controls on whole-ecosystem C cycling, net fluxes of CO2 between ecosystems and the atmosphere, and the vulnerability of SOM to changes in both climate and management (for example, tree planting for C-sequestration). Aims and Objectives - In MYCONET we focus on the 'mycorrhizosphere' (the soil and organisms directly influenced by roots and their mycorrhizal fungi) of C-rich soils of northern high latitudes and its potential response both to increasing plant productivity and to shifts to woodier shrub and tree communities. We hypothesise that associated changes in the mycorrhizosphere could, paradoxically, result in net losses, rather than gains, of soil C over timescales (i.e. several decades) of relevance to the Climate Emergency. This would represent a 'positive feedback' on climate change (i.e. when the rates of CO2 emission to the atmosphere, due to SOM decomposition, exceed net rates of CO2 uptake via photosynthesis). We will push the frontiers by applying ground-breaking techniques in the use - and innovative experimental deployment - of natural abundance (and depleted) radiocarbon (14C), together with metagenomics, soil and root-tip enzyme assays and SOM chemistry, to quantify and understand the processes and dynamics of the mycorrhizosphere and how these affect SOC stocks. We focus, in detail, on the process of 'priming' (which occurs when material added to soil affects the rate of decomposition of SOM, either positively or negatively), and the specific role of mycorrhizal fungi in this, and related, processes. We will measure these processes both in situ (in the Arctic and the UK uplands) and in controlled experiments (using specific combinations of tree, shrub and mycorrhizal symbionts), as part of an integrated package of mechanistic studies, soil profile analysis and dynamic SOM modelling, to quantify and understand how priming works, and the implications for SOM dynamics, ecosystem C fluxes, and nutrient cycling. Potential applications and benefits - By applying ground-breaking techniques MYCONET will transform our understanding of plant-soil interactions and the role of mycorrhizal fungi in SOM dynamics. The fundamental new knowledge gained will significantly improve regional and global modelling of climate-biogeochemical interactions, with a particular focus on the indirect effects of shifting plant communities. The project has relevance for the pan-Arctic 'shrubification', as well as for ecosystems being managed for C-sequestration or 're-wilding'. This project is especially timely, given the major policy emphasis and public interest in tree planting for C sequestration.

Riparian zones are the dynamic interfaces between terrestrial and aquatic systems, ultimately governing transfers of the macronutrients carbon (C), nitrogen (N) and phosphorus (P) between the land and the oceans, via rivers. The concern is that with a changing climate, the stability of these systems is shifting, and potentially the nutrient cycling rates are accelerating as a consequence. This proposal focuses on our concept of the dynamic riparian reactive interface (RRI) and how it governs the fate of nutrients down the system from the land to the river, perhaps to the atmosphere, and onward to the oceans. The proposal describes an approach that combines data-rich UK research catchments (Scottish Dee, English Eden) with flagship international catchment platforms (in Sweden and Germany). We propose to conduct new biogeochemical research and new modelling across geo-climatic regions to evaluate riparian functions controlling the potential acceleration in nutrient mass transfers across the land to water interface and how these may scale to globally-significant changes in nutrient cycles as our climate changes.

Why do trees in different tropical forests grow at different rates? Why do some trees within a site grow faster than others? At first impression, It seems a reasonable assumption that the 'visible productivity' (e.g. wood production and canopy litterfall) is somehow related to how much carbon and energy the forest or the individual tree captures from photosynthesis, the Gross Primary Productivity (GPP); this assumption is implicit in much of the forest ecology literature, as well as in many biosphere models. When we see explanations as why forests are increasing growth rates in response to global change, or increased productivity after disturbance, we tend to frame these explanations in the context of increased photosynthesis (either because of increased abiotic drivers - e.g. increased light or carbon dioxide, or because of increased photosyntheric capacity, e.g leaf nitrogen content) However, our recent work in Amazonia has indicated that the site-to-site variability in net primary productivity (NPP) in lowland rainforests is not related to how much carbon and energy the forest captures through photosynthesis, but much more determined by how much of that captured carbon used by plants for their internal metabolism (Malhi et al., submitted to Nature), the autotrophic respiration, Ra. This tentative finding has consequences for much of tropical forest research, and global change vegetation models. Moreover, our early results suggest that disturbance is the main determinant of how much an ecosystem allocates to autotrophic respiration, with less autotrophic respiration in disturbed systems. We would now like to explore this topic further in five ways: (i) by exploring in greater detail the spatial and temporal variation of autotrophic respiration; (ii) by greatly increasing the number of sites investigated; (iii) by assessing the extent to which results from Amazonia are generalisable in another biogeographical realm, namely equatorial Africa; (iv) by explicitly exploring how disturbance affects carbon use and allocation by tracking these before and after selective logging; (v) by exploring how much interspecific variation in NPP is determined by autotrophic respiration. The underlying hypotheses we are exploring are that (i) there is no significant site-to-site variation in the GPP of moist tropical lowland forests (within Africa and in comparison to Amazonia), despite variation is soil properties, climate and tree species composition; (ii) there is substantial site-to-site variation in net primary productivity (NPP), and this is mainly driven by shifts in carbon use efficiency (CUE, the proportion of photosynthetic carbon converted to biomass), and (iii) forest CUE increases substantially after disturbance (logging) and subsequently declines over time, and (iv) this shift is driven by differing plastic variation in CUE within surviving individuals, rather than by community replacement. In the process, we will pioneer comprehensive carbon cycle assessment in intact and disturbed African tropical forests, replicated across two contrasting countries, Ghana (West Africa) and Gabon (Central Africa). Our sampling strategy will encompass plots in (i) wet primary forests (2 countries x 2 plots), (ii) moist primary forests (2 countries x 2 plots),(iii) tracking sites before, during and after logging disturbance (2 countries x 2 plots), and (iv) plots recovering from logging disturbance 10, 15 and 20 years ago (2 countries x 2 plots). At all sites we will collect 2.0-2.5 years of data. Our project will provide substantial scientific capacity building in Ghana and Gabon,we will train and utilise 6 student field researchers (3 full time, 3 part-time) in each country, and hold wider-reach training workshops in carbon cycle science in each country at the start and end of the project. this event.

Plants are currently reducing the rate of 21st Century climate change by absorbing a substantial amount of the carbon dioxide that Humankind releases to the atmosphere through the burning of fossil fuels. However, the rate of carbon dioxide production by soils as plant material decomposes (known as soil respiration) increases at higher temperatures. Therefore, as global temperatures rise, it is feared that ecosystems which are currently absorbing carbon dioxide may begin to release it, with models predicting that this could increase the rate of climate change by 40 %. This prediction is based largely on knowledge of how soil respiration responds to short-term changes in temperature. However, in long-term warming experiments, following the initial stimulation of activity, rates of respiration tend to decline back towards pre-warming levels. This has led to the suggestion that the micro-organisms responsible for breaking down organic matter may be acclimating to compensate for the warmer temperatures, and that this phenomenon may preserve carbon stocks in the world's soils. There is an alternative explanation for the patterns observed in long-term warming experiments. The initial stimulation of activity may result in the depletion of soil carbon stores, leaving microbes with less to break down, and so reducing rates of respiration. While acclimation could preserve stocks, the carbon depletion explanation implies that the reduction in respiration rates is simply a consequence of the continuing loss of carbon from soils to the atmosphere. Therefore, it is critical to distinguish between these two possible explanations. Previously, methodological limitations have prevented us from determining which explanation is correct. The problem was that when soils are warmed up, acclimation and carbon loss are both expected to reduce respiration rates, making it impossible to distinguish between them. We have shown that this problem can be overcome by using soil cooling. When soils are cooled, initially activity will decline but if acclimation occurs to compensate for the lowering of temperature, rates of respiration should subsequently increase. On the other hand, as carbon losses continue at the lower temperature, albeit at a reduced rate, they cannot be implicated in any recovery of respiration rates. So carbon loss and thermal acclimation are now working in opposite directions, allowing us to distinguish between them. This logic was applied to determine whether microbial activity in soils taken from arctic Sweden acclimates to changes in temperature. After cooling, respiration rates showed no signs of recovery. Rather, many days after temperatures were reduced, respiration rates in the cooled soils continued to decline steeply, with no such response being observed in soils maintained at a warmer temperature. So the effect of cooling was amplified over time. It appears that the soil microbes were responding to the colder temperatures by further reducing activity. Looking at this in reverse, a more active microbial community survived at higher temperatures; so microbial community responses enhanced the effect of temperature on decomposition rates. This phenomenon has not been observed before, and we do not know how prevalent it might be. By extending our work to soils sampled from different ecosystems and at sites ranging from the high Arctic to the Mediterranean, our grant proposal aims to investigate how important soil microbial community responses to temperature are in controlling decomposition rates in European soils. We will determine whether acclimation occurs or whether microbial community responses generally enhance respiratory responses to temperature. We will also investigate how the overall response is controlled. Our project will improve understanding of how global warming will affect decomposition rates in soils, and allow more accurate predictions of rates of 21st century climate change to be made.

Biodiversity is declining at an alarming rate. Multiple stressors are driving many of these declines with freshwater (FW) ecosystems particularly impacted. Ephemeral FWs (e.g. marshes, ponds) are exceptionally biodiverse and highly exposed to varied environmental stressors but are generally overlooked within academia and regulation. Amphibians have been a major faunal component of these habitats for at least 350 million years, being highly evolved to these ecosystems. Amphibians and wetlands are some of the most highly threatened Phyla/ecosystems globally, with wetland health key to the climate crisis, due to the high methane levels emitted from human impacted systems. Using both field and laboratory approaches, here we will investigate the environmental stressor combinations driving negative impacts in amphibians (common frog, Rana temporaria) and seek to develop a biomonitoring approach to assess the health of these vital ecosystems. As amphibians are the most highly threatened vertebrate Phyla, this project is highly relevant to conservation priorities. General health, disease status, stress markers and global gene expression in wild and caged tadpoles will be measured. The use of toxicogenomics and alterations to physiology to assess impacts on tadpoles allows both the anchoring of molecular initiating events to downstream physiological endpoints and resulting adversity, as well as mapping these responses to stressor combinations. This mapping presents a highly novel approach, allowing the identification of specific stressors and their combinations that are driving negative impacts, and is widely applicable across biota. Catchment-scale eco-epidemiological studies between wild taxa and the presence/severity of stressors often rank pollution as amongst the most important variables driving negative effects in FWs. However, studies on effects of pollution at environmentally relevant levels and mixture combinations are scarce, particularly in the context of multiple stressors. Here pollutant mixture formulations will be based directly on measured levels in ephemeral FWs and combined with other ubiquitous stressors (salinity, heat wave and/or invasive crayfish - Pacifasticus leniusculus cue), all at environmentally relevant levels and combinations. These laboratory exposures will be highly novel and of vital importance to understand the true impacts of multiple stressors on iconic amphibian biota that inhabit vital ephemeral FWs. It will be tested how best to utilise data from single stressor exposures, to predict effects using theoretical models. For this, we will apply novel theoretical paradigms to the data - dominance (few stressors contribute disproportionately to observed effects) and burden (total stressor load determines effects) - which have huge potential for wide applicability for multi-stressor science. In contrast to the single-endpoint approach, here we propose to use ecological modelling to investigate effects on whole organisms and their populations in order to drastically improve the utility of these data for conservation. Finally, by transplanting spawn and sampling both caged and native tadpoles, the utility of naïve/locally adapted tadpoles as a biomonitoring tool to assess the health of FW wetlands will be assessed. This work will address an important gap in the literature between field-based catchment-level evidence demonstrating the importance of multiple stressors and the current limited laboratory-based evidence/understanding; as well as developing a new testing paradigm with practical application for conservation. The research team combines excellence in FW ecotoxicology, multiple stressors/mixture effect biology, FW ecology, ecological modelling, bioinformatics and chemistry needed for this project. In addition, the project partners and supporting organisations comprise a range of stakeholders that are focused on the health of FW ecosystems and reducing the impacts of pollution.