Discovering The Molecular Basis For Carbon Storage In Soil For Food Security And Climate Change Mitigation

Recovering and enhancing carbon storage in soils is recognised globally as a key priority for protecting our environment, mitigating climate change and underpinning food security by improving agricultural sustainability. For over a century it has been the accepted wisdom that the persistence of organic compounds in soil was a function of the inherent 'decomposability' of their chemical structure, i.e. labile and readily hydrolysable/oxidised compounds are more readily mineralised compared to 'stable' less-easily degraded compounds. Current opinion places far more emphasis on physico-chemical protection, conferred by the soil matrix, as the major regulator of compound persistence. However, soil organic carbon (SOC) comprises a vast diversity of carbon forms ranging from low molecular weight compounds to large macromolecular structures, and from non-polar to highly polar molecules, each exhibiting a differing degree of protection in any soil. Small polar molecules (mainly microbially derived) may be protected by association with soil minerals whilst it has been suggested that macromolecules and non-polar lipids (mainly plant derived) are protected through occlusion within soil aggregates. If these are the dominant mechanisms controlling the long-term persistence of different organic compounds in soils, then this raises the potential that stabilisation capacities could saturate and may themselves need to be managed if carbon storage in agricultural soils is to be substantially enhanced.

Determination of compound-specific loading capacities, saturation levels and storage rates is now possible through the use of novel compound/compound class-specific radiocarbon measurements. The BRAMS facility at Bristol is a leading international centre for the extraction and purification of different compounds from soils for radiocarbon dating. We will apply these state-of-the-art techniques, together with established soil fractionation methods, to samples collected from the Long-Term Experiments (LTEs) at Rothamsted Research. By sampling contemporary and archived soils from the LTEs we can exploit the historical 14C-'bomb spike', arising from nuclear bomb testing in the 1950s and 1960s, to trace recent carbon into different compound classes in mineral-protected and aggregate pools. The contrasting organic and mineral fertiliser applications, agricultural managements (e.g. arable vs. permanent grass), and different soil types (e.g. more-'clayey' vs. more-'sandy') provide a unique opportunity for determining how soil carbon storage is controlled in the long term. Thus, for the first time it is possible to identify the relative roles of different stabilisation mechanisms in controlling the long-term storage of different compound classes, and the precise reasons why carbon stocks are increasing, decreasing or remaining constant in soils under different agricultural managements.

It is essential that advances in soil science are used to inform practical management strategies for enhancing soil carbon storage. Thus, to maximise the impact of our project, the mechanistic understanding produced will be used to develop, test and validate a new version of one of the most-used soil organic carbon models (RothC, which has ~3,500 users globally). RothC is conservative in terms of parameterisation making it highly attractive to end users, and, depending on which of our hypotheses are supported, we have identified structural changes that could be made to improve the model while maintaining its low input requirements. The updated model will be rigorously tested with independent data sets generated from other LTEs and observations of SOC variation across a range of environmental and land management scenarios. Once verified, the new RothC model will represent an important tool to inform how land management will affect SOC dynamics in different soil types and geographical locations.