An improved empirical model of soil carbon dynamics in temperate ecosystems

Lead Research Organisation: Cranfield University
Department Name: Cranfield University at Silsoe

Abstract

More than twice as much carbon is stored in soils as in vegetation or the atmosphere, and the global carbon budget and the concentration of carbon dioxide (CO2) in the atmosphere are sensitive to any changes in soil carbon. Previously it was thought that most soil carbon was safely locked away. But a recently-completed survey of the soils of England and Wales by members of the proposed project has shown losses of carbon over the past 25 years on an enormous scale: enough to cancel the UK's reductions in carbon emissions from fossil fuels. The survey measured carbon in soil samples from almost 6,000 sites across all types of land use / cropland, grassland, woodland, upland heath, bogs. It was first carried out in 1978-83 and then repeated at 40% of the sites between 1994 and 2003. Most sites showed losses, and the losses occurred across all types of land use. Whilst various changes in land management will have contributed to the losses / for example, the ploughing of grassland for crops in the 1950s / the fact that the losses occurred across all forms of land use suggests a link to climate change. The mechanisms for this are well known. At the moment, soils and vegetation are thought to absorb about a quarter of the CO2 emitted into the air from burning fossil fuels. The rate is increasing because plants are growing faster, fertilized by higher CO2 levels. But as temperatures rise, and the moisture content of the soil changes, the rate of turnover of dead vegetation and other organic matter in the soil and the corresponding release of CO2 are also increasing. At some point, the rate of release will overtake the rate of absorption, and the global land surface will switch from being a sink for CO2 to a source. Stabilising CO2 levels will then require much greater cuts in emissions. It was thought, based on computer modelling studies, this would take several decades. But the new results suggest it is happening already in England and Wales, implying an even greater urgency to cut emissions. Separating out the various possible causes of the losses across England and Wales is complicated. A huge variety of land management factors may be involved, and their effects interact with climate change. It is therefore impractical to make measurements of all the possible combinations of factors. In any case statistical relations derived from such measurements do not reveal cause-and-effect, and they are poor at revealing the sorts of non-linear relations and feedback processes that are operating. Some form of mathematical modelling is required, based on understanding of the underlying processes. But current models are inadequate for this. Here we propose to improve the best-existing soil carbon model for our purposes / the Century model / by improving its components dealing with soil carbon transformations, their dependences on soil moisture, temperature and other variables, and the calculations of soil moisture, temperature and other driving variables. We will bring together (1) the new, internationally-unique dataset discussed above on soil carbon changes over time across the large range of soils, climates and land uses in England and Wales; (2) the best existing models of soil carbon transformations and of the dynamics of associated soil conditions, and new approaches to modelling these; (3) new spatial statistical techniques for analyzing the datasets to be modelled and the soundness of the model's predictions; (4) four specialists in soil modelling with different perspectives on these topics, and two specialists in soil spatial statistics, all internationally-regarded for their expertise in these areas.