Soil Carbon

Across the planet, soils store about twice as much carbon as the atmosphere, making soils a large and important reservoir of carbon. However, carbon cycling in soils (the routes by which carbon enters the soil, is modified by physical, chemical and biological processes, and ultimately leaves the soil through microbial respiration or leaching) is not well understood.

There is particular interest in understanding the soil carbon cycle, since we both need to be able to predict what will happen to soil carbon as climate changes due to global warming; and we want to preserve carbon stocks in agricultural soils that aid structure, fertility and water-holding capacity.

CEH undertakes the Countryside Survey every eight to ten years, measuring ecosystem parameters at the national scale, including soil carbon concentration across Great Britain. The survey, which was initiated in the 1970s, is beginning to show us how carbon concentration in soils is altering in different habitats. This is of national importance, given the UK’s commitment to tackling and adapting to climate change.

Carbon cycling in soils is biologically driven by bacteria, fungi and invertebrates, as they act upon plant litter-derived carbon (above ground), root deposition and soil organic matter (below ground).

The impact of these organisms on carbon cycling depends upon biological community structure and activity. We determine invertebrate and microbial community structure by extracting the organisms, or biochemical markers of the organisms, from the soil. This is done using techniques such as dry and wet extraction for invertebrates, traditional microbiological methods such as plate counts and respiration measurement, and chemical and molecular methods such as phospholipid fatty acid (PLFA) analysis and DNA/RNA based analysis.

Amongst other methods, we measure biological activities by determining microbial respiration rates, especially rates of release or consumption of the trace gases carbon dioxide, methane and nitrous oxide. We have a mobile laboratory that can measure emitted soil gases directly and can also store gases for more detailed laboratory analysis, e.g. stable isotopic composition. We are also developing methods for in situ monitoring of gases in both water and soils.

Much of our research is field based and we run a number of sites across Great Britain studying carbon dynamics, including Carbon Catchments at Moor House, Conwy, Auchencorth Moss and Forsinard.

One of our research tools is the use of stable isotopic methods, in which the 13C content of soils, trace gases, individual soil-derived compounds and dissolved organic carbon (DOC) are measured. The 13C content of a material gives us information on its source and the underlying soil processes. We work closely with the NERC Stable Isotope Facility (Lancaster Node) to perform these analyses.

Most of the carbon flow through soil, and much of the cycling of associated nutrients such as nitrogen and phosphorus, involves the decomposition of recent litter operating over a few years.  But the great majority of the soil carbon – indeed the single largest pool of organic matter on the planet – resides in relatively stable pools that return their carbon to the atmosphere as CO2 on timescales of decades to millennia.  This soil organic matter is important for functions such as holding water and nutrients, filtering pollutants, and structuring the soil.   

A powerful tool to investigate the longer-lived material is radiocarbon (14C).  Loss of 14C by radioactive decay (“carbon dating”) provides information about the most stable forms, while the pulse of 14C added to the atmosphere by weapons testing in the mid-20th century (“bomb carbon”) is useful to estimate cycling over decades.  Radiocarbon measurements require sophisticated equipment, and we collaborate extensively with staff at the dedicated NERC Radiocarbon Facility at East Kilbride in Scotland.  CEH scientists have developed models at different levels of complexity to interpret the radiocarbon data, working on sites in the UK, Scandinavia, mainland Europe and North America.