Forests in a changing environment: A study of nitrogen cycle feedbacks

Project Highlights:

• Novel research to describe forest nitrogen cycle dynamics in response to global change.
• Access to one of the most extensively characterised ancient woodlands in the UK, facilitating opportunities for wider collaboration in forest ecology research.
• A blend of laboratory and field experience, accessing skills ranging from molecular techniques to field measurements of trace gases.  

Overview:

Temperate forests are currently experiencing rapid alteration owing to global change pressure such as elevated CO₂, unpredictable weather patterns, and atmospheric deposition (Fei et al. 2017; Jo et al. 2019). While it is well-known that such changes affect the organisms that inhabit forests and woodlands, the degree to which such changes affect forest functioning is largely unknown (Clark et al. 2016). Forests perform numerous functions, perhaps none more critical than retaining nutrients and exchanging climate-active gases with the atmosphere. As such, changing forest composition has the potential to affect both air and water quality, as well as feedbacks to the climate system. However, we do not yet know whether those changes will be positive (amplifying) or negative (mitigating).

The overarching objectives of this project are to (i) define how dominant tree species, and the effect of global change processes on those species, influence nitrogen cycling in forest soils and (ii) explore the applicability of trait-based framework(s), such as the Mycorrhizal Associated Nutrient Economy hypothesis (Phillips et al 2013; Mushinski et al. 2019, 2021), to predict the consequences of forest community change. The project will utilise a combination of field studies, environmental chamber experiments, laboratory mesocosms, molecular microbiology, and ecosystem models to accomplish these objectives. We will leverage an extensively characterised ancient woodland (Wytham Woods) to facilitate the collection of high-resolution data in situ and from laboratory mesocosms.

The student and PI will collaboratively define the specific research direction of the project. Potential lines of inquiry include:

(1) Linking aboveground community composition to nitrogen gas emissions from plots differing in the relative abundance of trees that associate with various groups of root symbionts and underlying soil characteristics;

(2) Determining forest soil nitrogen cycle sensitivity to elevated CO₂, drought, water logging, and nitrogen deposition;

(3) Developing statistical models of species-soil-climate relationships to produce maps of future forest composition that can be coupled to process-based biogeochemical models – allowing us to predict the consequences of shifts in forest community composition in the wake of global change.

Methodology:

Please note that exact protocols will be defined at the onset of the project. We will make seasonal nitrogen flux measurements as well as evaluate the soil microbial community in forest plots at Wytham Woods. To assess the impact of tree species and global change drivers on nitrogen fluxes, we will grow saplings of tree species known to associate with differing microbial symbionts in laboratory mesocosms. Flux values gathered from in situ and ex situ experiments will be used to parameterise ecosystem models, facilitating predictions of forest community change.

Training and skills:

Students will be awarded CENTA2 Training Credits (CTCs) for participation in CENTA2-provided and ‘free choice’ external training. One CTC equates to 1⁄2 day session and students must accrue 100 CTCs across the three years of their PhD.

Training during this fellowship includes a wide range of molecular techniques and analyses (microbial culturing, DNA extraction from soil, PCR, sequencing, and bioinformatics) as well as analytical chemistry (nitrogen oxide quantification, reactive oxygen extraction from soil and subsequent quantification, and building sampling mesocosms). Field-based sampling and measurements from forest ecosystems will also be emphasized.

Partners and collaboration (including CASE):

Dr. Mushinski leads the Environmental Processes Laboratory (SLS-Warwick). His research group studies nitrogen cycle biogeochemistry and soil-microbe interactions in a range of natural and manged ecosystems. Professor Bending leads the Microbial Ecology Laboratory (SLS-Warwick), where his group studies the structure, diversity and function of microbial communities inhabiting plants, soil, and water - often integrating a variety of 'omics approaches. Professor Malhi leads the Ecosystem Dynamics Laboratory at Oxford and is Principal Scientist for the Wytham Woods ForestGEO plot. Mushinski and Bending will be the primary advisors for the project, while Malhi will facilitate access to Wytham Woods.    

Possible timeline:

Year 1: Define experimental design. Conduct field sampling – measuring nitrogen fluxes from forest soils at Wytham Woods.

Year 2: Assess how individual tree species influence the production or suppression of nitrogen fluxes in response to global change. Define how plant, soil, and microbial characteristics influences fluxes.      

Year 3: Utilise ecosystem models and Earth observation data to define how forest community change may influence nitrogen cycle feedbacks.