Stabilisation of pyrogenic carbon in tropical grasslands (SPECTRAL)

Every year, fires burn more than 500 million ha of the land surface on Earth. These fires leave residues of partially burned biomass which are deposited on the soil surface as "pyrogenic carbon" (PyC). It is estimated that as much as 114 to 379 Tg of PyC are added to soils each year, and global models predict that fire frequency and intensity will increase in many areas. Much of this PyC is deposited on to the soil surface and subsequently incorporated into deeper layers, so that it can account for as much as 50% of total carbon stored in soils in some fire prone ecosystems such as tropical savannahs.

Storage of pyrogenic carbon in soils matters a lot to the way in which natural and managed ecosystems interact with the global climate system. PyC has an inherently slow turnover once it is in the soil, meaning that it will persist much longer than organic matter deposited on or in soils that have not been exposed to fire. Remarkably, however, we largely ignore the fate and overall contribution of PyC to global carbon cycling. This is due to the fact that the scientific community so far only has a rudimentary understanding of what determines PyC distribution and turnover in soils.

There have been significant advances in understanding the role of charred biomass added to soil as an agricultural practice, and we have a reasonable understanding of these systems and how it interacts with soil organisms and physical factors. However, we require a much better understanding of processes underlying the changes of PyC particles, and stabilisation of carbon introduced to the soil after fire, in order to model current PyC dynamics, and be able to forecast these under future climates. Particularly biological processes, such as the role of invertebrate soil animals and different groups of soil microbial organisms have so far not received sufficient attention.

We propose a programme of research directed at creating a novel soil-PyC model that can be linked to regional and global carbon-climate models, which so far ignore PyC. We will achieve this through a number of experiments based in a tropical savannah system in Gabon. This provides an ideal experimental set-up, as these fire prone systems have a significant abundance of PyC in the soil profile, and we will be able to link to a long-term fire manipulation field experiments in the Lopé National Park.

Using targeted soil coring for contrasting fire return intervals, combined with the determination of the age of PyC (usingnatural 14C abundance) across the profile, we will be able to derive residence times of this stable soil carbon reservoir. By using isotopically labelled PyC (based on plants grown under enrichment with 13C, a non-harmful, stable isotope of carbon), we aim to gain a better mechanistic and quantitative understanding of soil PyC dynamics. This approach is linked to manipulations of soil animal presence, as these are hypothesised to have a significant influence over the distribution and turnover of PyC in the soil. Insights from the isotopic tracer study will be used to develop a novel soilorganic matter model to incorporate PyC dynamics for the first time, We will be able to parameterise this new model with our experimental data and validate simulated turnover against our independently assessed turnover time of PyC pools.

This is a 'Discovery Science' proposal, addressing a fundamental aspect of the terrestrial carbon cycle. This research has important implications for a number of beneficiaries in global change science, for ecosystem management and for broader society.

1. Intergovernmental Panel on Climate Change (IPCC): The outcome of this research is of direct relevance to policies to protect and enhance soil carbon (C) stores by incorporating climate feedbacks of the terrestrial biosphere into ecosystem models. We explicitly focus on the stabilisation and decomposition of pyrogenic carbon from vegetation fires, but these mechanisms are directly transferable to predicting the impact of geoengineering approaches such as biochar amendments of soil. The IPCC's 5th Assessment Report stresses the importance of understanding the residence time of charred biomass in soils in order to constrain uncertainty in ecosystem models (Working Group 1, Section 6.5.2.1 "Enhanced Carbon Sequestration by Land Ecosystems"). The proposed research will address the uncertainty posed by biotic feedback within terrestrial ecosystems and we will improve process models to predict future soil-atmosphere C exchange and soil C store developments by constraining uncertainties associated with biotic interactions. This project will contribute novel data to national data sets on carbon cycles and balances that are being used to inform Gabon's national policy for forest-based climate change mitigation and adaptation measures. Currently no data on the contribution of wildfire to carbon stock changes and dynamics in Gabon is available. Incorporating this to carbon accounting models will provide a valuable contribution to Gabon's climate commitments under the Paris Agreement.

2. Ecosystem modellers: The aim of current modelling efforts in the atmospheric and ecosystem communities is to understand future atmospheric CO2 burdens based on on-going and projected environmental conditions resulting from climatic change and human land use. The development of the MEMS-PyC model to incorporate abiotic and biotic interactions in the stabilisation of soil PyC as an outcome of this research is a significant step to enable ecosystem models and global coupled carbon-climate models to incorporate these dynamics.

3. Ecologists, ecosystem biogeochemists and plant- and soil scientists: The most important pathway for disseminating knowledge gained from this research is through peer-reviewed publications and presentation at conferences . We are confident that our findings will result in a number of high impact articles, published in key journals in the Global Change arena (such as Global Change Biology, New Phytologist, Global Biochemical Cycles, Nature Geoscience, Nature or Science). We think that this is the most effective way for us to communicate findings to ecosystem scientists and modellers as some of the main target groups.

4. Broader society: There is significant public interest in the way that climatic change affects society's ability to manage resources sustainably. Public understanding of the significance of climate change has probably never been greater than it is today, and there is genuine interest in understanding ways in which natural and managed ecosystems respond and interact with the Earth's climate system. Engagement of scientists and members of the public have improved considerably through dedicated events e.g. during festivals, school visits or local outreach (e.g. 'Pint of Science'). Our ability to shape policy towards a sustainable management of natural resources and the environment relies on a well-informed public that will drive societal and political opinions. We are committed to provide part of this education from our scientific research, by enthusing the public about the insights we gain about the way the biosphere functions and interacts with everyone's lives.