Quantifying the loss of soil C from soil organic matter, as a consequence of climate or land-use change

Figure 1. Equipment used to collect the soil surface CO2 efflux in order to measure its isotopic composition.

Soils are the largest pool of C in terrestrial ecosystems, and some of the C in soil organic matter (SOM) is resistant to decomposition, taking from decades to millennia to decompose (hence its description as ‘historical’ C). Yet it is the carbon dioxide (CO₂) from ‘historical’ C that, with photosynthesis, determines an ecosystem’s source-sink balance of C in and C out.

To measure this flux, it must be distinguished from the CO₂ arising from the respiration of living roots, fungal hyphae and microbes, as all contribute to the soil flux. Conventional methods involve killing roots and hyphae, removing litter from the soil surface, or introducing SOM whose C is distinct from that of native soil C. All these invasive procedures can, worryingly, produce estimates of CO₂ fluxes that do not necessarily reflect those of undisturbed systems. Knowing how the ‘historical’ CO₂ flux varies between different ecosystems and in relation to temperature and moisture would be a major step towards improving climate models, most of which use information about these CO₂ fluxes.

Our solution to this long-standing problem has been to develop a method that measures ‘historical’ CO₂ fluxes directly, but with minimal disturbance. We do this by accurately measuring the natural isotopic (13C) composition of CO₂ derived from the decomposition of SOM and from the respiration of living roots. These sources of CO₂ usually differ in their 13C content by a small, but measurable, amount. By measuring the CO₂ flux at the soil surface and its 13C content (Figure 1), we can estimate directly how much of it derives from ‘historical’ C. We have tested this approach at a range of field sites and found we can measure the loss of this ‘historical’ C without any disturbance of the system. We are now assessing the impact of soil temperature on SOM turnover in order to improve the description of temperature sensitivity of SOM decomposition in existing models that predict interactions between climate and the C cycle and that forecast resulting changes in the C stocks of ecosystems.

One finding from this work is that when soil is disturbed, the 13C signature of the CO₂ arising from respiration rapidly changes (Figure 2). Systematic loss of soil C following soil disturbance is attributed to loss of physically protected SOM. Such protection occurs due to a range of factors, including reduced oxygen diffusion into soil aggregates and physical separation from soil microbes. Any soil disturbance through land-use change or cultivation that leads to a break up of soil aggregates releases labile C, which is then accessible to decomposition. Our new isotope approach offers the possibility of quantifying labile soil C pools that are vulnerable to loss through disturbance.

This research is being conducted collaboratively with Andy Midwood of the Macaulay Institute, Aberdeen, Scotland.

Peter Millard, John Hunt, Graeme Rogers & David Whitehead

Figure 2. The effect of length of incubation on the 13C isotopic signature of CO₂ from soil respiration, following a disturbance to the soil. The change in signature is due to the start of a rapid turnover of labile soil C. The blue lines show 95% confidence intervals.