Black Carbon

In addition to microbial decomposition, combustion is a significant return route for carbon sequestered into vegetation via photosynthesis. On the millennial timescale fire has been an important feature in most landscapes, and in some natural ecosystems (such as tropical forests and savannah grasslands) the cycle of burning and regeneration is characteristic and defined. Fire has also been an important feature in agriculture, both in shifting "slash and burn" cultivation but also for disposal of crop wastes, eliminating disease or controlling nutrient status. Over time burning will not only have accelerated the general cycling of carbon between atmosphere and vegetation, but also diverted a small proportion into highly recalcitrant charcoal (biochar) fraction that resides in the soil (Figure 1).

Given the small proportion of carbon diverted from the biological cycle in this way, the levels of "black carbon" measured in soils may seem surprisingly high (Figure 2). However the period over which the additions have occurred, combined with extreme biological recalcitrance could explain this. In industrialised regions, a further factor to consider are particles of part-combusted fossil-fuel (e.g. soot) deposited from the atmosphere which, concentrated in time and space, may be an important constituent of the black carbon pool.

The presence of black carbon is clearly relevant to the modelling of the global soil carbon stock, since as well as constituting a variable and often large proportion of the total pool it is also thought to be quite inert. On the other hand, traditional measurements of soil organic carbon do not discriminate black carbon from those fractions produced through biological decomposition, and the actual turnover of the material has not previously been examined in detail. Further studies in Amazonia has uncovered evidence that ancient communities found great agronomic benefits to producing and adding charcoal to their soils, to the extent that it made continuous agriculture sustainable.

In collaboration with colleagues at Cornell University we are examining these soils (terra preta de Indio) as "models" for new strategies relevant to the sustainability and sequestration of carbon in modern-day farming systems. At Rothamsted we are contributing to this effort by applying the SOMA model and associated fractionation techniques as a unique tool to establish the impact of charcoal of contrasting age on the cycling of carbon and nitrogen in soils, the exchange carbon between different organic matter pools, and its eventual release through decomposition. To view the latest results from this work view our poster presented at the World Congress of Soil Science in July