Combining results on organic pollutant degradation and on soil organic matter turnover gives indications for the control of key microbial transformation processes in soils under global change

Microbial transformation of organic compounds is a key process in soil. Microorganisms are responsible for important soil functions such as degradation of organic pollutants, but also for the formation and transformation of soil organic matter (SOM). Research on both processes suffers from missing knowledge on the process details, but also on the controlling factors and transformation products formed. In case of organic pollutants, most studies relied on the use of radiolabelled compounds, as such studies are required for market approval of chemicals, e.g. for pesticides. Such studies regularly show formation of non-extractable residues (NER), even under optimal conditions for microbial activity. The characterization of NER is difficult, preventing adequate risk assessment. In contrast, SOM studies usually comprise general C balances, without focus on detailed microbial processes. We are now using considerations on general principles of microbial metabolism and a combination of knowledge on the degradation of natural organic compounds as well as organic pollutants to get more detailed insight into both processes. This includes better characterization of NER for improved environmental risk assessment, an improved framework describing SOM transformation and stabilization as well as the identification of controlling factors for both types of metabolic processes.

For heterotrophic organisms, each growth substrate is both a C- and an energy source. The allocation depends on the actual requirements of the organisms as well as on the chemical structure of the substrate. Therefore, each substrate will only partially be mineralized (catabolism), the other part will be used as C source for biomass synthesis (anabolism). As a consequence, microbial biomass will be formed, and its residues will contribute to SOM after cell death. We already showed in earlier studies on pesticide degradation in soils that biomass residues of the degraders (biogenic NER = bioNER) account for a substantial part of the NER found by isotope mass balances. The close link between mineralization and biomass formation suggests a relation between mineralization and formation of biomass and bioNER, which represents the carbon use efficiency (CUE). CUE may vary and controls the allocation of natural organic substrates to respiration vs. microbial biomass formation, which is a key prerequisite for necromass accumulation. However, CUE is strongly affected by microbial metabolism, which adapts to environmental conditions.

Mass balances for a number of pesticides including bioNER formation were performed under different conditions, by manipulating pH and TOC concentrations of a study soil and incubating at different temperatures or in different soils. In particular, incubation temperature had a strong effect on the total degradation efficiency, but also on CUE during pollutant degradation and therefore on the contribution of bioNER to total NER. We can expect similar effects for the turnover of natural organic compounds and thus SOM formation. If supported by modelling of microbial biomass formation, this information can significantly improve the risk assessment in the framework of market approval of chemicals and advance our knowledge on factors controlling organic compound turnover and SOM formation under climate change.