'Bound' residues from biomass and CO 2 in soils : formation, fate and stability during biotic incubation

Autor: Nowak, Karolina Malgorzata
Přispěvatelé: Schäffer, Andreas
Jazyk: angličtina
Rok vydání: 2011
Předmět:
Zdroj: Aachen : Publikationsserver der RWTH Aachen University, Dissertation / Helmholtz-Zentrum für Umweltforschung, UFZ 2011,10 XVI, 118 S. : Ill., graph. Darst. (2011). = Aachen, Techn. Hochsch., Diss., 2011
Popis: Biodegradation of organic pollutants in soil generally results in the formation of metabolites, microbial biomass, mineralisation products and bound or non-extractable residues (NER). It is speculated that NER can pose a risk for humans due to the remobilisation and further distribution of active parent compounds or their metabolites over the food web (BARRACLOUGH ET AL., 2005). Although there are many studies on NER formation available, their chemical structures are still unknown. However, without this knowledge, a proper risk assessment for the pollutant and the related NER during its transformation in soil is impossible. Part of the NER may be biogenic, since the pollutant-derived C and CO2 released from its mineralisation are assimilated by microorganisms into their cellular components [e.g. fatty acids (FA) and amino acids (AA)], which are subsequently incorporated into soil organic matter (SOM) after cell death. In order to study the microbial biomass contribution to NER formation, soil was incubated with either 13C-labelled 2,4-Dichlorophenoxy acetic acid (2,4-D) or ibuprofen (Ibu) for 64 and 90 days, respectively. At different sampling dates, the soil was analysed for the presence of the 13C label in FA and AA. The 13C label assimilated in biomolecules was determined in both the living and the non-living SOM fractions. Moreover, for investigating the relevance of heterotrophic CO2 fixation for the incorporation of 13C into biogenic NER in soil and to distinguish between formations of NER directly from 2,4-D and via CO2, an experiment with unlabelled 2,4-D and labelled CO2 was conducted. In addition, to understand both the processes of the biogenic NER formation and the incorporation of 13C label into FA and AA from either 13C6-2,4-D or 13CO2 in the complex soil environment, a simple biological model system with the known 2,4-D degrader (Cupriavidus necator JMP 134), was studied. After 64 days of 13C6-2,4-D incubation in soil, the total contents of 13C detected in AA in SOM indicated that 44% of the initially applied 13C6-2,4-D equivalents had been converted to microbial biomass and finally to biogenic residues. The intermediate maximum of 13C-FA in SOM indicated a 20% conversion of 13C6-2,4-D to biomass. However, 13C-FA in the non-living SOM fraction decreased to 50% indicating their metabolisation and their further distribution within the food web. Contrary to the 13C-FA, 13C-AA in non-living SOM pool were surprisingly stable. The soil experiment with 13CO2 showed that after 16 days of incubation, the heterotrophic CO2 fixation was relevant in the assimilation of 13C label into biomass components, in particular FA. In addition, the liquid culture experiment with C. necator JMP 134 demonstrated the high importance of CO2 fixation in the incorporation of 13C label into the biomass components in the later phase of incubation. In this study about 4% of 13C derived from 13C6-2,4-D was assimilated in the biomass of this bacterial strain via CO2 fixation. A total content of biogenic NER of 54% of 13C6-ibu equivalents at the end of the 13C6-ibu soil experiment was indicated by the amount of 13C found in AA in SOM. From the maximum content of 13C-FA detected in this experiment, at least 24% (of 13C6-ibu equivalents) biomass must have been formed. Contrary to the 13C6-2,4-D soil incubation experiment, the 13C-FA remained stable until the end. However, the formation of NER in the soil incubated with 13C6-ibu started much later than that in the 13C6-2,4-D study, due to the longer lag phase and the interruption of the incubation before the destabilisation of biogenic NER. The 13C-AA in SOM remained also stable as found in 13C6-2,4-D experiment. The results from these soil biodegradation experiments provide the first evidence that nearly all NER both from 2,4-D and Ibu are biogenic, containing only natural microbial biomass components stabilised in SOM. However, biogenic residues are formed if the respective organic contaminant is readily degraded by microorganisms under significant formation of CO2. Depending on the yield coefficients of C conversion into biomass, we expect ratios of biomass plus biogenic residues to CO2 of about 0.2 to 1. In the 13C6-2,4-D study, the ratio was ~ 0.8 and in the 13C6-ibu was 1.2. Biogenic residues are clearly excluded from the definition of NER according to IUPAC. However, they are usually not analysed separately. The potential risk of NER from readily metabolised organic contaminants in soils is thus highly overestimated in many cases due to biogenic residue formation. Therefore, the formation of biogenic residues must be taken into consideration, when determining the mass balances of contaminants during their microbial degradation in soil.
Databáze: OpenAIRE