MEGAN L. PURCHASE
Environmental Researcher
Welcome
I'm a final year PhD candidate at the University of Warwick. I'm interested in soil health and the effects of land use change and human input on the biogeochemistry of soils. Please enjoy having a look at some of my work and feel free to contact me if you have any questions - I'm always happy to talk about soil.
Research Themes
BIOGEOCHEMISTRY
MOLECULAR BIOLOGY
SOIL SCIENCE
Current Projects
Resolving ROS and N cycle mechanisms of soil-sourced reactive N oxides
Connecting cicada emergence to forest emissions of greenhouse gases
Connecting root morphology to soil-sourced gas emissions in agricultural soils
Our lab studies environmental processes that occur within the terrestrial biosphere. We link plants and microbes to the flow of carbon and nutrients in natural and managed ecosystems.
Our research focuses on the structure and ecosystem functions of microbial communities.
Conferences & Symposiums
CENTA Conference 2024
National Exhibition Centre, Birmingham - September 2024
Long Talk
Abstract
Heterotrophic bacteria and fungi are responsible for the decomposition of soil organic matter (SOM), and the subsequent input of nutrients including nitrogen (N) and carbon (C) to the soil. Complex chemical and biological dynamics at the biosphere-atmosphere interface as part of the N cycle lead to the formation and emission of volatile reactive nitrogen oxides (NOy), a group that includes NOx (nitric oxide [NO] + nitrogen dioxide [NO2]) and NOz (nitrous acid [HONO] + nitric acid [HNO3] + nitrogen trioxide [NO3] + …). NOy are climate-relevant gases that contribute to atmospheric pollution and negatively affect human health. However, despite being a significant contributor to global NOy emissions, many mechanisms of soil NOy production are still yet to be discovered and evaluated, resulting in vital information being missed from climate models. Reactive oxygen species (ROS) are also produced as heterotrophic bacteria and fungi decompose organic matter. Physiologically, reactions between ROS and reactive N species are part of inflammation, vasodilation, and aging processes, but here we utilise metagenomic and metatranscriptomic sequencing data alongside continuous gas flux measurements and analysis of soil properties to resolve this chemistry as a pathway to soil-sourced NOy 1-3.
References –
1. Chaki, M. et al. Involvement of Reactive Nitrogen and Oxygen Species (RNS and ROS) in Sunflower-Mildew Interaction. Plant and Cell Physiology 2009, 50 (3), 665–679.
2. Del Río, L. A. ROS and RNS in Plant Physiology: An Overview. Journal of Experimental Botany 2015, 66 (10), 2827–2837.
3. Squadrito, G. L.; Pryor, W. A. Oxidative Chemistry of Nitric Oxide: The Roles of Superoxide, Peroxynitrite, and Carbon Dioxide. Free Radical Biology and Medicine 1998, 25 (4–5), 392–403.
PGR Symposium 2024
University of Warwick - March 2024
School of Life Sciences - Best Talk Award
Abstract
The periodic emergence of cicadas from soil is a unique ecosystem disturbance with potential significance for soil biogeochemistry. Cicadas are insects that spend much of their lives underground as nymphs, undergoing a synchronised mass emergence after 13-17 years. Cicada emergence causes physical disturbance to the soil as the insects emerge but can also cause chemical disturbance in the form of a large input of nutrient-rich organic matter when the adult cicadas die and decompose. In situ measurements of gas fluxes from forest soils in Indiana revealed “hotspots” of nitrous oxide (N2O), a significant greenhouse gas, from chambers where carcasses were present, although 16S rRNA amplicon sequencing of the soil microbial community revealed no differences in N2O producing pathways compared to chambers without carcasses. However, we found the cicada necrobiome is enriched with nitrate-reducing and other denitrifying bacteria, indicating the necrobiome is a likely source of N2O hotspots. With a lab-based time-series study of changes to gas fluxes as carcasses decompose, we were able to replicate the ~18 day N2O pulse, but also measured significantly increased ammonia gas fluxes at this timepoint. Future work will involve examination of temporal changes to the soil microbiome and cicada necrobiome as carcasses decompose to contextualise the quantified gas fluxes.
AGU Annual Meeting 2023
San Francisco, CA - December 2023
eLightning Presentation
British Society of Soil Science Early Career Travel Grant
Abstract
Volatile reactive nitrogen oxides (NOy) are significant atmospheric pollutants, including NOx (nitric oxide [NO] + nitrogen dioxide [NO2]), and NOz (nitrous acid [HONO] + nitric acid [HNO3] + nitrogen trioxide [NO3] + ...). NOy species are products of nitrogen (N) cycle processes such as nitrification and denitrification. Biogenic sources, including soil, account for over 50% of natural NOy emissions to the atmosphere, yet emissions from soils are generally not included in atmospheric models due to a lack of mechanistic data. This work is a unique investigation of NOy fluxes on a landscape scale, taking a comprehensive set of land-use types, human influence, and seasonality into account to determine large-scale heterogeneity to provide a basis for future modelling and hypothesis generation. By coupling 16S rRNA amplicon sequencing and quantitative PCR, we have linked significant differences in functional potential and activity of nitrifying and denitrifying soil microbes across a gradient of urbanisation and land-use to NOy emissions from soil. We have assessed soil physicochemical properties characteristic of different land-use types, and as a result of anthropogenic influence. Further, we have identified the response of soil NOy emissions from varying land-use types to N deposition and found soils subject to more human influence to be less microbially active despite increased available N, potentially as a result of poor soil health from anthropogenic pollution. Structural equation modelling suggests human influence on soils to be a more significant effector of soil NOy emissions than land-use type. Results show significant seasonal variation of rates of N cycle processes and reactive N fluxes, demonstrating a need for temporal scale to be considered in future investigations.
Abstract
Nitrous oxide (N2O) is a potent greenhouse gas with more warming potential than carbon dioxide (CO2) and soils are known to be the dominant source of atmospheric N2O. N2O is produced and emitted during microbial transformations of nitrogen (N) species that make up the N cycle. The synchronous emergence of periodical cicadas represents one of the largest insect emergence events. The year 2021 saw the emergence of the 17-year “Brood X” cicadas (Magicicada septendecim, M. cassinii, and M.Septendecula). The exoskeletons of cicadas are primarily made up of chitin, a polymer of N-acetylglucosamine (GlcNac). The aboveground lifespan of cicadas is just ~6 weeks, after which the large deposition of low C:N content cicada carcasses is a significant N-rich input into the environment, particularly to forest soils.
Decomposition of insect carcasses occurs more readily than decomposition of other organic material, making insects an important and currently understudied input of N to soils that can contribute to emissions of N species from soils.It has been found that emissions of N2O are enhanced when cicada carcasses are added to soil chambers. We have examined the soil microbiome, and the cicada necrobiome using quantitative PCR (qPCR) and 16S rRNA amplicon sequencing to elucidate the potential microbial source of these enhanced emissions. We suggest a currently unknown microbial pathway for the decomposition of cicada chitin and the resulting enhanced N cycle processes that lead to “hotspots” of N2O emissions from forest soils following cicada emergence. Future work will involve time-series experiments to ascertain the links between decomposition state, microbial community of the soil and carcasses, and N2O emissions.
CENTA Conference 2023
University of Warwick - September 2023
Organising Committee
Short Talk - 2nd Place
PGR Symposium 2023
University of Warwick - March 2023
School of Life Sciences Best Research Poster Award for presenting one of the five best posters at the SLS Postgraduate Research Symposium
Abstract
Volatile reactive nitrogen oxides (NOy) are significant atmospheric pollutants, including NO + NO2 (NOx) and HONO + HNO3 + NO3(NOz). Biogenic sources, including soil, account for over 50% of natural NOy emissions to the atmosphere. Despite their importance, NOy emissions from soils are generally not included in atmospheric models due to a lack of mechanistic data.
Spatial heterogeneity amongst landscapes and across population gradients likely influences NOy fluxes due to differences in atmospheric deposition rates and anthropogenic soil modifications – likely altering microbial and abiotic cycling of NOy; however, this has not been explored in mechanistic detail. Here, we link nitrifying and denitrifying soil microbes, across a gradient of urbanisation and land-use, to NOy gas fluxes from soil. To resolve temporal discrepancies, soil sampling (0-10 cm) occurred seasonally.
Results show significant changes in relative abundances of microbial orders associated with nitrification, denitrification and nitrogen fixation across a human population gradient, and between land-use types, suggesting anthropogenic impact on soil microbiology. From structural equation modelling (SEM), we see NOy and NO fluxes significantly affected by land-use type. Soil physicochemistry is a major influence on other measured variables and is therefore likely to be an important predictor of soil NOy fluxes.
Keywords: Soil, Biogeochemical cycling, Anthropogenic impacts
Net Zero Doctoral Summer Showcase 2022
Loughborough University, UK
Poster - 1st Place
‘Science of Future Food' Event 2022
University of Warwick, Stratford Innovation Campus
Poster