Controls of bioavailability and biodegradability of dissolved organic matter in soils

Abstract In soils, dissolved organic matter (DOM) is probably the most bioavailable fraction of soil organic matter, since all microbial uptake mechanisms require a water environment. Bioavailability describes the potential of microorganisms to interact with DOM. It is a prerequisite for biodegradation and can be restricted, if DOM is present in small pores or within soil aggregates and therefore not accessible for microorganisms. DOM biodegradation is defined as the utilisation of organic compounds by soil microorganisms quantified by the disappearance of DOM or O2 or by the evolution of CO2. The controlling factors for DOM biodegradability can be divided into three groups, namely, intrinsic DOM quality parameters, soil and solution parameters and external factors. DOM characteristics that generally enhance its biodegradability are high contents of carbohydrates, organic acids and proteins for which the hydrophilic neutral fraction seems to be a good estimate. In contrast, aromatic and hydrophobic structures that can also be assessed by UV absorbance decrease DOM biodegradability, either due to their recalcitrance or due to inhibiting effects on enzyme activity. Effects of solution parameters such as Al, Fe, Ca and heavy metal concentrations on DOM biodegradability have been documented in various studies, however with different, sometimes conflicting results. Inhibitory effects of metals are generally attributed to toxicity of the organic complexes or the free metal ions. In contrast, the enhanced degradability observed in the presence of metal ions may be due to flocculation, as larger structures will provide better attachment for microbial colonies. As degradation is dependent on microbial activity, the composition and density of the microbial population used in the degradation studies also influence biodegradation. Site-specific factors, such as vegetation, land use and seasonality of meteorological parameters control DOM composition and soil and soil solution properties and therefore also affect its biodegradability. The major obstacle for a better understanding of the controls of DOM biodegradability is the lack of a standardised methodology or at least systematic comparisons between the large number of methods used to assess DOM biodegradability.