| Soil organic matter (SOM) is the largest carbon pool in the terrestrial ecosystem, and plays crucial role in controlling the global biogeochemical cycle of carbon. The variation of the stability of SOM could significantly alter the CO2 concentration in the atmosphere, and hence affect the global climate change. Soil fauna, due to their large biomass and abundant biodiversity, play an important role in regulating stability of SOM through their feeding activity in soil, improve the properties of soil habitat and intensively affect the microbial activity. Earthwoms are the predominant biomass in most temperate terrestrial ecosystems. As the "soil ecological engineer", earthworms, particularly the geophagous earthworms, play crucial role in nutrient turnover, organic matter decomposition and carbon cycling in soil. Also the fate of the organic pollutants in soil can be intensively affected by the activities of earthworms. However, the degradation of SOM by the geophagous earthworms is poorly understood and it is still unclear which components of SOM the earthworms feed on and whether they can degrade the refractory humic substances. The degradation and assimilation of microbial biomass and their cell components by the geophagous earthworms is also obscure. Much attention has been paid to the degradation and transformation of traditional organic pollutants (such as pesticide, oil hydrocarbons, PAHs, and PCBs) by earthworms, but the bioaccumulation and transformation of the emerging pollutants deriving from the land application of biosolids by the earthworms has not yet been investigated. In addition, it is yet unclear how the earthworms affect the degradation and transformation process of the emerging organic pollutants in soils. In the present study, using 14C tracer technique, degradation and transformation of the natural organic matters (including humic model compounds, microbial biomass, and their cell components) by the geophagous earthworms, bioaccumualtion and transformation of nonylphenol in the geophagous earthworms, and influence of the geophagous earthworms on the bioavailability and fate of phenolic organic pollutants (including chlorophenols, nonylphenols, and bisphenol A) were investigated. The major results are summarized as the followings:1. Selective digestion of the proteinaceous components of humic substances by the geophagous earthworms Metaphire guillelmi (anecic) and Amynthas corrugatus (endogeic) were investigated. The results demonstrated that M. guillelmi and A. corrugatus were capable to digest the proteinaceous components of humic substances, indicating that the geophagous earthworms are available to a wide reservoir of high quality of nitrogen in humic substances although the assimilation of the humic substances by the earthworms was low (less than 3%). The degradation of the proteinaceous component of soil humic substances by geophagous earthworms resulted in a release of inorganic nitrogen into the soil, which could potentially affect the nitrogen cycling in soil.2. Digestion of microbial biomass, structural polysaccharides, and protein by the geophagous earthworm M. guillelmi was studied. The results clearly showed that M. guillelmi can not only enhance the mineralization of bacterial (Escherichia coli and Bacillus megaterium), fungal biomass (Penicillium. chrysogenum) and their cell components (i.e. peptidoglycan, protein, and chitin), but also utilize them as its nutrient and energy sources. High efficiency of fungi and chitin utilization as compared to bacterial biomass and other cell components by M. guillelmi implied that fungi may be a more important food source for the geophagous earthworms. The feeding activity of M. guillelmi also affected the fate of the residues of microbial biomass and their cell components in soil.3. The sorption and desorption of 2,4-dichlorophenol,2,4,6-trichlorophenol, and pentachlorophenol on geophagous earthworm (M. guillelmi) casts of various aging times and on the parent soil were studied. The sorption of the chlorophenols on the soil and casts were well fitted to linear isotherms, with sorption capacity in the order of pentachlorophenol>2,4-dichlorophenol>2,4,6-trichlorophenol. The sorption on the casts with different aging time was quite similar and was higher than on the parent soil. The sorption on the soil did not change between pH 7.07 of the soil and pH 6.76 of the casts. The higher sorption capacity of the casts was not owing to the lowered pH of the casts, but mainly to the higher fine particles in the casts and the possible changes of nature of the soil organic matter through the earthworm gut passage.4. Using 14C- and 13C-ring-labelling, degradation of five para-nonylphenol (4-NP) isomers including four branched (14C-4-NP111,4-NP112,4-NP65, and 13C-4-NP38) and one linear (4-NP1) isomers in a rice paddy soil was studied under oxic conditions. The degradation of 4-NP isomers followed an availability-adjusted first order kinetics, with the decreasing order of half-life:4-NP111 (10.3 d)>4-NP112 (8.4 d)>4-NP65 (5.8 d)>4-NP38 (2.1 d)>4-NP1 (1.4 d), which is in agreement with the order of their reported estrogenicities. One metabolite of 4-NP111 with less polarity than the parent compound occurred rapidly and remained stable in the soil. At the end of incubation, bound residues of 4-NP111 amounted to 54% of the initially applied radioactivity and resided almost exclusively in the humin fraction of SOM.5. The bioaccumulation, elimination, and biotransformation of 4C-4-NP111 in the geophagous earthworm M. guillelmi in a rice paddy soil were studied. Earthworms rapidly bioaccumulated 14C-4-NP111 following a two-compartment first-order kinetics model. At steady state (after 20 d exposure),77% of the accumulated radioactivity were present as non-extractable bound residues. The total radioactivity was eliminated from the earthworm following an availability-adjusted decay model and controlled by the elimination rate of the bound residues (half-life=22.6 d). The organic extractable residues of earthworms consisted mainly of one less-polar metabolite (37%) and polar compounds (50%), including glucuronide conjugates of 4-NP111 and the metabolite; and free 4-NP111 accounted for only 9% of the total extractable residues.6. Fate of bisphenol A (BPA) and a branched 4-nonylphenol isomer (4-NP111) in soil as affected by the geophagous earthworm Aporrectodea longa was investigated. Mineralization and degradation of BPA were faster than 4-NP111 in the soil. The distribution of BPA residues within humic substances was relatively homogenous, while residues of 4-NP111 within humic substances was mainly associated with the humic substances of high molecular weights. Large amounts of bound residues were formed during the degradation of BPA and 4-NP111 in the soil. The earthworms changed only the distribution of BPA residues within humic substances, while the earthworms inhibited the mineralization and bound residue formation of 4-NP111 and reduced the amounts of chemcially humin-bound residues of 4-NP111 in soil.Results of this study provide the first direct evidence that geophagous earthworms can degrade and utilize the refractory humic substances. Both microbial biomass and their cell components can be assimilated by the geophagous earthworms, in which fungi may be a more important food source for the earthworms. Gut passage of the geophagous earthworms, combined with the selective digestion of humic substances by the earthworms, altered sorption behaviour of chlorophenols in soil which may potentilly affect the bioavaibility of chlorophenols. Degradation of nonylphenols in an oxic soil in this study provides the first direct evidence for isomer-specific fate of nonylphenols in the environment. Nonylphenols accumulated in the geophagous earthworms as free form, conjugates, and bound residues, among which the bound residues were most abundant. Effects of the geophagous earthworms on the fate of estrogenic organic pollutants (i.e. BPA and nonylphenols) in soil were compound-specific, and were more intensive with nonylphenol.
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