Nitrogen cycling under elevated CO2: response of a key soil nutrient to global change

Increasing atmospheric CO2 concentrations in the atmosphere due to human activities, is projected to enhance photosynthesis and carbon storage of forest ecosystem. However, it is unclear how nutrient limitation will constrain the projected CO2 fertilization effect. Therefore, it is essential to evaluate how nutrient limitation will affect the response of forests to rising CO2 concentration and how it will feedback on nutrient availabilities and more especially nitrogen (N) which can become limiting with time.

The purpose of this research is to evaluate the response of N cycling processes to elevated CO2 fumigation in a N-limited northern deciduous temperate forest and a phosphorus (P)-limited Eucalyptus dominated forest. The research was conducted in two Free Air Carbon Dioxide Enrichment (FACE) facilities: BIFOR FACE located near Birmingham, UK and EucFACE in New South Wales, Australia. Furthermore, EucFACE (P-limited forest) received partial phosphorus fertilization to study its effect on N cycling.

A 15N pool dilution method will be used to quantify gross protein depolymerization and gross mineralization and estimate the microbial nitrogen use efficiency. N2O emissions from nitrification and denitrification will be assessed using a 15N Gas Flux method and free-living N fixation in soils will be assessed by the 15N assimilation method. Additionally, the isotopic measurements will be compared by a metagenomic approach quantifying the expression of N cycling genes.
We hypothesize that in a N limited forest, trees will allocate a greater proportion of carbon belowground to boost soil organic matter (SOM) decomposition for N supply to trees. This process will enhance N availability for trees but will also lead to nitrogen losses via leaching and denitrification. Conversely, in the phosphorus-limited forest, tree exudates will fail to effectively unlock phosphorus (P) availability, hindering the enhancement of photosynthesis and the feedback on N cycling in the soil. However, fertilizing these forests with phosphorus will shift the ecosystem from being P-limited to N-limited, triggering an enhancement of N supply processes.
In addition, EucFACE (P-limited forest) will be partially fertilized with phosphorus in order to explore how a sudden P limitation alleviation will affect N cycling.
Results from this research aim at improving our understanding of forest response to future climate by unveiling the role of nutrient limitation in future C uptake.

QUINTUS aims to clarify the role of nutrient availability on carbon sequestration by temperate forests in a changing atmosphere. This is science with profound societal impact; results contribute an essential evidence base informing the models used to predict future climate. The consortium has global reach in its partnership, and will have global reach because the terrestrial carbon cycle (and thus climate change) is a pressing concern to global communities of research users, policy-makers, climate-sensitive industrial sectors, and people.

BIFoR FACE is already a powerhouse of stakeholder engagement, having hosted an average of 3 events per fortnight throughout 2017 and 2018 at all levels from government to citizen.
Who might benefit? Potential socio-economic beneficiaries include: (i) the UK Met Office and other Earth-system modelling agencies; (ii) UK government departments of Business, Energy & Industrial Strategy (BEIS) and Environment, Food & Rural Affairs (Defra); (iii) the Intergovernmental Panel on Climate Change (IPCC) and other national and international policy-makers; (iv) forest authorities; (v) NGOs and concerned citizens. We have well-proven conduits to each of these stakeholders.

How might they benefit? We will interact with each impact constituency in the way best suited to each of them, to ensure reflexive development of change in practice.

1. UK Met Office and other Earth-system modelling agencies: The new process understanding and observational constraints in temperate forests under high CO2 will improve land surface models. We will work directly on the UK community land surface model, JULES. The improved JULES model, ready to be incorporated into the next generation UK Met Office Earth System Model, will contribute to IPCC assessments beyond the current cycle. Impact in this sphere is most effectively delivered through research co-production with the Met Office (Hemming, Wiltshire), cascading outwards through the Met Office's and Co-I Sitch's involvement in the annual Global Carbon Budget of the Global Carbon Project.

2. BEIS & Defra: we will provide the evidence to underpin future assessments of the natural capital of mature UK forests. By studying the throughput of macronutrients, water, and energy through the BIFoR FACE site, we will improve the evidence base for assessment of the regulating ecosystem services (including climate regulation) provided by UK deciduous woodlands. Impact in this sphere will be delivered by governmental advice channels including the West Midlands Forestry and Woodlands Advisory Group, of which the PI is a member.

3. IPCC and other national and international policy-makers: policy-makers require evidence quantifying the land carbon response to elevated CO2 in order to assess climate mitigation costs and pathways to sustainable living. Impact in this sphere will be delivered through expert involvement in international scientific bodies (Met Office partner provides several co-authors to the next IPCC AR6 WG1 report) and through government channels (item (2), above).

4. Forestry authorities: Forestry authorities will benefit from the new understandings on the nutrient factors in sustainable forest management. Impact in this sphere will be delivered through broader BIFoR knowledge exchange workshops, to which we will contribute during the project. All BIFoR projects come together each January to share outputs with stakeholders. See also our letter of support from one of the UK's largest and most innovative forest estates (Norbury LoS).

5. Citizens, particularly young people: the BIFoR-FACE facility is already being used as a flagship for STEM education on climate change issues through our strong links to the Royal Geographical Society and Royal Society of Biology. QUINTUS outreach will focus on the interaction of nutrient and carbon cycles, and be made available to stakeholder websites, e.g. www.schoolscience.co.uk.