Fjölrit RALA - 15.06.2004, Page 50

Size and activity of the soil microbial community from a range of European
volcanic soils
D.W. Hopkins1 and F. Bartoli2
1School of Biological and Environmental Sciences, University ofStirling, Scotland UK,
2Laboratoire Sols et Environnement INPL(ENSAIA)-INRA, Vandoeuvre-lés-Nancy France,
Despite previous work, biological characterisation of Andosols has not been fully explored. In
this paper, we report the results of two investigations of the biological properties of volcanic
soils that builds on the work of many collaborators. First, we present the results of a survey of
the soil microbial biomass, respiratory activity of ten topsoils from the COST 622 reference
volcanic soils of Europe (Soil Resources of European Volcanic Systems). The soils used were
sampled from reference profiles in five countries (Italy, Portugal [Azores], Iceland, Spain
[Tenerife] and France) with two profiles in each country. They were all Andosols (six Silic,
two Aluandic and one Mollic Andosols according to the WRB classification), except one
from Tenerife which was a Pachic Andic Umbrisol. Second, we report the results of detailed
investigations on the mineralization of carbon in soils collected from Mt Etna, Sicily. Seven
of the eight soils from Etna were also Andosols, and included soils at different stages of
development in dated lava flows which we used to test two hypotheses of community
development on volcanic chronosequences.
For the COST 622 volcanic topsoils, the hypothesis was that availability of soil organic
matter to microorganisms decreases with increasing of Al-humus content, following the
observation by Boudot (1992) that biodegradation of citric acid was low when adsorbed on
poorly-ordered Al-hydroxide or Al-organic complexes, but not on allophane or imogolite.
This hypothesis was not validated for the Andosols studied. First, from the soil survey the
soil microbial biomass and respiration rates fell within the typical range for soils and both
microbial biomass C concentration and respiration rate were positively correlated with total
soil organic C (Fig. 1).
Figure 1
Relationships of
biomass C (a) or
respiration rate
(b) with soil
organic C. The
outliers (diamond
symbols) were
thosefrom
FaialfAzores] (a)
and thosefrom
Tenerife (b).
Second, excluding the Faial [Azores] soil, the mean biomass C-to-soil C ratio was 1.3 %
and with the Faial soil it is 1.66 %. These ratios are much larger than the extremely small
biomass C-to-soil C ratios in Andosols from Japan (mean 0.2 %) reported by Murata et al.
(1998) or from Costa Rica (mean 0.3 %) by Mazzarino et al. (1993). Furthermore, both
biomass C-to-soil C ratio and respiration rate-to-soil C ratio were not related to Al-humus
content, but were positively related to total porosity (Fig. 2), which itself was positively
related to soil organic C (aggregation) and to capillary porosity (90 to 98 % of total porosity).
y = 21,28 x-713,56 r = 0,851; p < 0,01
Soil organic C
(g.kg 1 105°C dried soil)
y = 0,339 x- 1,374 r = 0,852; p < 0,01
Soil organic C
(g.kg1 105°C dried soil)
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