Scientists map soil carbon from the air
8 January 2009, by Tom Marshall
Geologists have found a way to improve estimates of the concentration of organic carbon in the soil.
The researchers have created an accurate map of the carbon content of Northern Ireland's soils by using information from airborne surveys of ground radiation to supplement data from soil samples taken across the country.
Sampling soil on the ground is time-consuming and expensive; airborne geophysical measurements are far easier and cheaper to make across large areas.
The problem until now was that airborne sensors only looked at the Earth's surface, not at what was hidden beneath. Especially in areas of heavy vegetation, they are of limited use for looking at the carbon underground. Instruments to measure soil radiation, by contrast, look deeper into the earth.
'The beauty of using airborne radiometric sensors is that it lets us look beneath the surface of the soil,' says Dr Barry Rawlins, a soil scientist at the British Geological Survey (BGS) who worked on the research along with co-author Dr Ben Marchant at Rothamsted Research.
By using these airborne readings of soil radiation as a proxy for carbon content and taking them into account in models, the scientists found that soil carbon could be accurately estimated across a wide area based on only a few direct carbon measurements.
Indeed, they say that instead of taking more than 6500 soil samples in a recent survey, they could have arrived at a comparably accurate soil carbon map with just 300 core samples if these were supplemented by airborne sensing.
Taking stock of carbon
Accurately assessing stocks of carbon in the soil is important because in many areas the soil is expected to emit carbon into the atmosphere as temperatures rise and microbial activity increases.
The United Nations Framework Convention on Climate Change obliges countries to understand the carbon-related emissions from their soils. Before we can predict how much carbon is being given off, and how this may change in future, we need to understand how much is there already.
Scientists from BGS and Rothamsted Research were joined on the project by collaborators from Belfast's Agri-Food and Biosciences Institute and the Geological Survey of Northern Ireland. Their findings appear in the European Journal of Soil Science.
Most of the data behind the study comes from the Tellus programme, under which scientists surveyed the geology of Northern Ireland during the summers of 2005 and 2006, covering some 13,550 square kilometres using low-flying aircraft. The airborne survey and associated soil sampling, both managed by the Geological Survey of Northern Ireland, were financed by the Northern Ireland government.
Among the variables they measured was radioactivity coming from the decay of elements like potassium, thorium and uranium in topsoil. Rawlins and colleagues focused on the presence of potassium, since this signal is clearer when measured from the air.
This radiation is closely connected to the amount of carbon in the soil for two main reasons. First, in environments such as that of Northern Ireland wetter soils tend to contain more organic matter, which decomposes more slowly in these conditions. Soil water also absorbs radiation produced by the decay of potassium, so a wetter soil emits less radiation to the sensor.
And secondly, soils containing a higher proportion of organic carbon must by definition contain less minerals and crushed rock, which are the source of radiation.
The scientists combined the airborne survey data with direct measurements of soil carbon from another part of the Tellus survey that collected 6862 samples of earth from sites across Northern Ireland. Half of these samples were drawn on to make the predictions; the other half were held back and later used to test their accuracy.
Also included in the models was information on elevation above sea level. Including the radiation and topographic data let the researchers accurately map levels of carbon in places that weren't directly sampled in the earlier study.
The technique could be used in other areas, though not everywhere - Rawlins believes it is probably most effective in wet, carbon-rich soils like the 'gley' found over much of Northern Ireland. 'I wouldn't expect the same pattern where soils have a much smaller carbon content, or in soils that aren't wet for much of the year,' he explains.
This still leaves large areas of north-western Europe as possible targets, though. 'People tend to want to measure soil carbon in areas where there's a lot of organic carbon,' Rawlins says; ascertaining the carbon contents of more arid southern European soils is unlikely to be high on the agenda.
Likewise, airborne radiometric sensing may be of limited use in permafrost soils, where research suggests carbon tends to migrate lower into the soil and hence may mostly be stored in soil layers that are too deep for the sensors to reach. Gamma radiation is almost all absorbed by 30 cm of soil or rock, which is the effective maximum depth of observation.
One other caveat is that scientists need to take care not to take the measurements in unusually wet or dry periods, which could distort results.
Rawlins says that more work is needed to confirm the technique's capabilities and limitations; he suggests this could be done quickly and cheaply, without a lengthy and intensive survey like Tellus, by sampling carbon and then taking airborne radiometric readings along transects cutting across the countryside in a suitable area.
Repeating these experiments at intervals could provide insight into how soils' carbon contents are changing over time as climate change takes effect.
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