Soil Organic Matter Management: The Roles of Residue Quality in C Sequestration and N Supply - Sustainable Management of Soil Organic Matter - page 146

Fig. 3.6.3. Relative distribution of [ 14 C]pyrene-radioactivity amongst different extracts of compost
material during various stages of composting. Er = extractable radioactivity; NER = non-extractable
residues.
14 C-radioactivity was bound to the compost matrix and could not
be extracted with the described methods. During this period, the mineral-
ization rate was negligible. With an increasing mineralization rate of pyrene
after the thermophilic phase, the fraction of NERs grew rapidly, finally
reaching 25% of the initial 14 C-radioactivity. The results indicate that there
was a correlation between mineralization and formation of NERs. This
corresponds with the results of Eschenbach (1995), who found no further
formation of NERs in sterile soil samples. It can therefore be concluded
that the formation of NERs, at least to some extent, can be attributed to
microbial-mediated processes.
Moreover, the ratio of acetonitrile-extractable to toluene-extractable
pyrene showed a decrease with time (Fig. 3.6.3). This indicates that apart
from microbial processes there was a sorption of pyrene due to adsorption
and ageing processes. Accordingly, the formation of NERs cannot be
explained by one mechanism alone.
Release of NERs is very unlikely, which is demonstrated by the strong
decrease in the Er : NER ratio and the small deviations of the data. This
corresponds with investigations of Eschenbach (1995), who could not find
any remobilization of PAHs from residual waste soil.