Does increasing soil BC content lead to greater aggregation and soil carbon preservation?
Barry Rawlins, Vicky Moss-Hayes and Christopher Vane
email: [email protected]
In a recent study, Rawlins et al. (2008) reported a wide range of
black carbon (BC) concentrations in urban soils of Glasgow, UK. The
provenance of this BC is likely due to local dispersal of domestic and
industrially coal-ash between around 1945 and 1960. An investigation
of aggregate-occlusion of BC in agricultural soils showed that BC was
enriched in the microaggregate fraction (Brodowski et al., 2006). In
this study we wish to investigate whether differing soil BC contents
result in different amounts of aggregation which may account for
preservation of i) BC and ii) labile carbon in urban soil.
We are currently determining the BC content and TOC contents of
water-stable aggregates (<60μm, 60-250μm, 250μm -1mm and 1-2mm)
and particle density fractions (mineral, FPOM<1.6 g cm-3, OPOM<1.6 g
cm-3 and OPOM>1.6 g cm-3) of five samples across a wide-range of BC
contents (0.5 to ca. 6%). Each of the samples is geochemically and
mineralogically similar. The fractionation procedure adopted (shown
to the right) is similar to that of Brodowski et al. (2006). In addition
we are characterising the BC using solid-state 13C NMR and
determining surface area of the soils. We had hoped to have our
results to present in this poster, but due to unforseen circumstances
our analyses have been delayed.
A
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This SEM image shows a black
carbon particle from the one of the
Glasgow soil samples with a partial
coating of clay minerals. The EDX
atomic ratio (O/C) at position
marked A is ca. 0.45.
Density fractionation scheme using sodium
polytungstate (SPT)
Soil Sample with SPT (1.6g cm-3)
From Glasgow sites
Gently shake 5 times,
centrifuge for 2hrs @ 4000 rpm
Soil Particles
Shake 16h @60 rpm with glass beads & SPT (1.6g cm-3)
centrifuge 2hs @ 4000 rpm
Supernatant
fPOM (<1.6g cm-3)
Supernatant
oPOM (<1.6g cm-3)
Supernatant
oPOM (<1.6-2g cm-3)
Soil Particles
Shake 10min @ 100 rpm with SPT (2.0g cm-3)
centrifuge for 2hs @ 4000 rpm
Soil Particles
X 3
Rinse distilled water
centrifuge 20min @ 4000 rpm
Soil Particles
Mineral (>2.0g cm-3)
oPOM - Occluded particulate organic matter
fPOM – free particulate organic matter
The four size fractions
1000 - 2000
µm
250 – 1000
µm
60 - 250 µm
<60 µm
References:
Brodowski, S., John, B., Flessa, H & Amelung, W. 2006. Aggregate-occluded black carbon in soil.
European Journal of Soil Science, 57, 539-546.
Rawlins, B.G., Vane, C.H., Kim, A.W., Tye, A.M., Kemp, S., & Bellamy, P.H., 2008. Methods for
estimating types of soil organic carbon and their application to surveys of UK urban areas. Soil Use
and Management, 24, 47-59.