Evolution of selenium behaviour in soils resulting from soil organic matter degradation
Congress title :Consoil
Congress town :Bordeaux
Congress date :03/10/2005
In soils, the behaviour of radionuclides and heavy metals highly depends on soil bio-physico-chemical properties. Both mineral and organic matter interact with most of trace elements by sorption and/or complexation processes and some of them strongly interacts with the microbiological compartment (fungi and bacteria). Although several works have been devoted specifically to the study of pollutants interaction with one or several soil component(s) (mineral, organic matter, microbiological), only few take into account the evolution in time of the whole soil system as a result of soil organic matter (SOM)
biological degradation. In this work, we studied how the SOM ageing could modify the soil physical, chemical and biological properties and how it could affect, or not, the mobility of an element. Selenium was chosen as it interacts strongly with micro-organisms and with many other soil components.
Three different soils from the Rothamsted Institute (U.K.) with a very similar mineralogical composition, but with contrasted organic matter qualities and contents (1.0, 4.1 and 4.9 % of organic carbon for the 3 soils, called respectively Roth1, Roth2 and Roth3) were initially contaminated with
selenium (spiked with 75Se) and then incubated at a constant temperature and moisture. The
incubation parameters were adjusted to increase the carbon turnover without disturbing the soil microorganisms.
The effect of time was studied according to a dual approach combining the characterisation of selenium mobility (batch, column experiments and Se solid partition ) with an
exhaustive characterisation of the physical (granular size, aggregates stability, etc.), chemical (DOC,major organic and inorganic ions, etc.) and microbiological (biomass, community structure) properties of the soil, the SOM and the soil solution. Firsts results revealed the importance of the interaction
between organic matter and selenium. In one hand, when a particulate organic matter fraction (POM, which correspond to the organic matter bigger than 50 μm) was isolated from the soil sample (Roth2 and Roth3) more than 10% of the total selenium was bound to that fraction. According to the relatively fast degradation kinetic of the POM (few years), it has to be expected that the decomposition of this fraction may have an observable impact on the redistribution of the POM-selenium fraction within the soil components. In the other hand, sequential extraction results showed that about 60% of the selenium was associated to the humic substances extracted with NaOH (fulvic and humic acids).
Despite the different amounts of the SOM, no significant difference was observed between the three soils indicating that humic substances quality seemed more important than its quantity.
Finally, the estimation of selenium mobility (desorption from batch and column experiments) did not reveal any direct correlation between selenium releases and the SOM quantity either, but confirmed the dependence between selenium behaviour and the SOM quality.
The qualitative and quantitative evolution of the humic substances, caused by the (microbial)
humification process will be presented in relation with the selenium solid partition and its mobility in the different soils as a function of time. The experimental results concerning SOM evolution will be compared to modelled SOM turnover using the model ROTH C (Jenkinson, 1977).