How much carbon can realistically be sequestered in arable soils and can we measure it?

Soil organic matter (SOM), which contains about 50% carbon (C), influences virtually all soil properties. The management practices leading to increased soil organic carbon (SOC) content are generally well known and include the addition of manures or crop residues, organic inputs through cover crops where soil would otherwise be bare, and inclusion of periods of pasture within arable cropping systems. Changes in SOC in response to alterations in management practice occur slowly, so long-term experiments are valuable in detecting and quantifying them.

New research approaches include the use of non-destructive imaging techniques that provide information on pore space within the soil matrix and the connectivity of pores. Inputs of organic matter are processed by soil microbes and the metabolites form associations with mineral particles. These processes at the small-scale lead to structural reorganisation at larger scales which, in turn facilitate changes in oxygen diffusion into soil and the microbial processing of organic C and nutrients. Imaging of pore networks clearly shows how a clay loam soil adapts to differences in organic inputs, but a sandy loam does not adapt in a similar manner. Thus the physical structure and resulting changes in biological activity, are less influenced by organic inputs in sandy textured soils. This is consistent with the observation that sandy soils have much less capacity to sequester carbon than soils of finer texture. The results also indicate that, in responding to organic inputs, soil can be regarded as a self-organising system.

There is much interest in sequestering carbon in soil as a means of mitigating climate change. But, from the viewpoint of soil functioning, we argue that it is the flow of carbon through soil that is key, rather than actual stock at a given time. We thus propose a dynamic rather than static view of soil.

There is currently considerable interest in making payments to farmers in return for carbon sequestered. We consider that there are formidable challenges to doing this in an equitable manner in practical farming situations. This is in large part because of the difficulty of measuring the likely small changes in SOC within a short time period; but there are also issues relating to the potential for C sequestration being strongly influenced by soil type and the starting SOC content, which will be a legacy of past practices. There are various measurements, including soil microbial biomass and a range of technically defined so-called ‘active’ fractions of SOM that are useful in showing whether SOC is increasing or decreasing in response to a change of management. However, these measurements provide neither an estimate of SOC stocks nor a prediction of absolute changes in SOC. If a system of policy requirements or financial incentives for increasing SOC is to be instituted, we propose an alternative approach using SOC models to predict probable SOC changes taking account of soil type, climate, cropping practices and initial SOC content. This could be combined with closer monitoring at a network of benchmark sites. Interactions between findings at these sites, observations within farmers’ fields, and modelled SOC changes would provide a sound basis for improved understanding of the underlying processes and modification of management practices in a range of situations.