Soil temperature increase effects on maritime Antarctic soil microbial community and humic acid degradation

Abstract

Soil humic substance (HS) is the largest constituent of soil organic matter. A major extractable component, humic acid (HA), of HS is dark brown to black high molecular weight organic polymer. To assess the effects of warming both on HA degradation and microbial community, microcosm beakers with HA-rich soils from King George Island in the maritime Antarctic, were incubated at elevated temperature of 5C and 8C, compared to the soil temperature (below 2.0C) during thawing period. Under the microcosm systems, HA content steadily decreased to approximately 63% and 55%, compared to untreated control (100%, 287.0±2.8 mg/g soil), until 90 days-incubation at 5C and 8C, respectively, presumably by microbial degradation process. Culture-independent community analysis of 16S rRNA genes showed that, during the microcosm experiments, the relative abundances of bacterial phyla Proteobacteria (copiotrophic) and Actinobacteria (polymer-degrading) slightly increased and decreased, respectively, in parallel with the incubation temperature rising to 5C and 8C. In contrast, archaeal community, dominated by phylum Thaumarchaeota, hardly responded to the soil temperature increase, indicating that bacterial community was much more affected than archaea by warming, although fungal response was not consistent. Culture-dependent community analyses were subsequently performed for the indigenous bacteria at 5C and 8C which were enriched in an artificial mineral medium containing HA. Consequentially, addition of HA resulted in a rapid increase of Proteobacteria dominance at both 5C and 8C, with the relative abundance of class Alphaproteobacteria-related bacteria being highly increased to over 72.7% among Proteobacteria (100%) under HA-degradation process. The overall results of this study indicate that HA degradation is in progress by bacteria in maritime Antarctic soil, and soil temperature rise by global climate change can change the bacterial community structure and HA degradation rate