LOCKED UP: The role of biotic and abiotic interactions in the stabilisation and persistence of soil organic carbon

Loss of soil organic carbon (SOC) through human land use is one of the most pressing environmental challenges of the 21st century. SOC loss contributes to climate change, makes soils less suitable for crops, reduces soil fertility through associated loss of nitrogen (N) and phosphorous (P) as plant nutrients, and reduces water holding capacity and drainage to aquifers - adversely impacting drought and flood resistance, water quality and water availability. The international initiative "4 per mille" addresses the threat of SOC loss to food security, climate regulation and water resources and aims to reverse global SOC losses through sustained, incremental (e.g. 0.4 % per year) increases.

Our research project aims to transform fundamental knowledge of the processes and mechanisms of SOC production and persistence in soil to inform land management innovation, and quantify the capacity and time scale to increase persistent - i.e. "LOCKED UP" - SOC stocks. Our hypothesis is that persistent SOC is produced by a series of complex but testable interactions between soil microbes and soil minerals: 1) relatively rapid microbial transformation of plant biomass input to soil, which produces; 2) specific classes of SOC compounds including extracellular products and components of dead cells that are essential precursors to persistent forms, which are then 3) stabilised against microbial degradation through chemical sorption to soil minerals, which can remove SOC from the microbially accessible C pool; and 4) physically protected against microbial degradation through aggregation of soil particles and soil organic matter, where SOC is protected from microbial degradation in inter and intraparticle pore spaces.

Our approach is to undertake linked laboratory studies, field sampling and modelling to obtain fundamental knowledge of key functional groups of soil microbes, the microbial operations and their rates which transform SOC to forms which then persist with minerals and within mineral aggregates; and to quantify how these transformations and persistent forms respond to changing environmental factors - plant input C:N ratios, water stress, indigenous microbial community composition, redox status, ionic composition and nutrient status of pore waters, temperature, and physical disturbance. The complex and interactive stages of forming persistent SOC will be quantified in stages, in model systems of microbial cultures, aqueous media and selected minerals in built and real soil matrices, as an idealised and experimentally tractable representation of the soil environment. In multi-factorial experiments that account for the range of environmental conditions, we will quantify rate laws and constants for SOC transformations based on first principles of mass balance, biological growth, chemical mass action and physical-chemical colloid interactions. The results will be implemented into an existing soil process model. This advance in mechanistic knowledge will allow us to build model simulations from a strong first principles understanding of the SOC transformation dynamics and resulting changes in soil structure and bulk properties. We will test these advances against independent data from manipulation experiments on whole soil cores from agricultural sites. Manipulation of additional soil cores - obtained from selected soil types and biomes to reflect specific regions and land uses around the world - will be carried out with application of the mechanistic soil process model. The experimental and model results will be used to assess - for key soil types, climate regions and land uses - the potential maximum, time scale and persistence of SOC that can be obtained from hypothesised land-use practices to increase stocks of persistent SOC - e.g. by changing tillage practices, vegetation cover and water management.

The 'Locked up' consortium brings together a multidisciplinary team with expertise, infrastructure and credibility from three leading UK soil carbon research organisations to deliver new discovery and impact for the NERC Highlights 4 per mille initiative.

Our research impact activity will:
Deliver novel research to advance fundamental scientific understanding of microbial and physico-chemical mechanisms of SOC persistence that will:
1. Contribute evidence to assess the feasibility of achieving and maintaining 4 per mille increases in SOC sequestration across UK and global soils under different land use and climate change scenarios,
2. Define soils that can most effectively contribute to achieving 4 per mille whilst considering wider trade-offs in land demand and socio-economic needs,
3. Provide evidence to support the UK and international ambition (Defra 25 year Environment Plan and BEIS Clean Growth Strategy, COP21) that 'soils should be managed sustainably by 2030, supporting profitable and productive farming, and underpinning targets for clean water and air and the mitigation of / adaptation to climate change'.

Who will benefit and how?
Research - The research is expected to advance scientific knowledge for the wider global research community challenged to address the potential for climate mitigation scale enhancement of soil carbon sequestration.
Industry - Opportunity for engagement with UK and globally relevant business (e.g. Shell International, UK farming sector) to share and evolve understanding and co-design plans for development of sustainable actions.
Policy - To provide scientific evidence to support the development of UK and European policy on soil carbon sequestration and health to support food security, water quality and climate change mitigation goals.
Public - To raise awareness of the importance of soils and climate action in the general public including schools.

Our project partners (see LoS) will be important in achieving impact from this project through collaborative working and their guiding role on the impact advisory board.
CIRCASA (Coordination of International Research Cooperation on soil CArbon Sequestration in Agriculture (CIRCASA). The team have strong links with INRA and Max-Planck both partners in CIRCASA. Through joint working we aim to contribute to the CIRCASA objective of co-designing a strategic research agenda with stakeholders on soil carbon sequestration in agriculture.
Shell (see LoS). CEH have a long-standing collaboration with Dr Christian Davies (Shell) who is a principal soil scientist and leads Shells Nature Based Solutions project. The company are interested in soil carbon sequestration and carbon markets as tools to achieve a significant reduction in their global carbon footprint. Dr Davies has committed to support and enable industrial linkages with the consortium to facilitate co-design of engagement activities and research which addresses stakeholder needs.

We will exploit consortium networks and impact mechanisms to engage a wide range of relevant stakeholder and potential beneficiaries including:
- Policy stakeholders: UK government departments, agencies and committees (DEFRA, BEIS, CCC, Natural England, Environment Agency) and devolved administrations. International policymakers: FAO, UNFCCC. The consortium have strong working relationships with many of these organisations through existing impact activities and research projects.
- Industry and representative associations e.g. Shell, water companies, NFU.
- Stakeholder networks: Sustainable Soils Alliance, ADHB Soil Biology and Soil Health Partnership, LEAF Linking Environment and Farming, 4 per mille initiative, Global Alliance for Climate-Smart Agriculture. Across the consortium we have active links with these organisations who are key in developing soil-relevant policy and guiding industry action.
- Third sector organisations: Campaign for the Farmed Environment, RSPB, LEAF.