Soil organic matter quality as a link between microbial community structure and vegetation composition along a successional gradient in a boreal forest

In this study we hypothesized that a change in the composition of vegetation communities is closely linked to a concurrent change in the structure of microbial communities through the decisive role of plant residues in regulating the quality of carbon sources for soil microbes. The study was carried out in a boreal forest along an 80-m-long successional transect (willow, alder, birch, spruce) located on the land-uplift coast of the Gulf of Bothnia, western Finland. The field and ground layer vegetation were recorded by species percentage cover on 2 m2 sample plots located at 10 m intervals. Organic layer samples were taken from around each vegetation sample plot for the determination of phospholipid fatty acid (PLFA) profiles and chemical composition of the organic layer, as characterized by the concentrations of neutral and acidic sugar units in non-cellulosic polysaccharides, cellulose, the acid-soluble lignin (ASL), and acid-insoluble residue (AIR). The organic layer along the transect showed a decrease in pH (from 5.2 to 4.4), moisture content and concentrations of ASL and the hemicellulosic sugars arabinose, rhamnose, xylose, galactose, and glucuronic acid, while the AIR concentration increased. These organic matter fractions and pH also showed a strong positive or negative correlation with the vegetation and the PLFA ordination. The percentage cover of grasses significantly correlated with the PLFA data, as well as with that of herbs and field layer species. However, PLFA ordination showed a higher correlation with the organic layer properties than with the coverage of plant species groups. In conclusion, our data showed a strong directional relationship between the organic matter characteristics and structure of the vegetation and microbial communities along the study transect. This study is, to our knowledge, the first investigation to apply acid methanolysis in the characterization of non-cellulosic polysaccharides in the organic layer.

Research highlights▶ Structure of vegetation and of soil microbial communities showed a strong relationship. ▶ Organic layer quality followed the change in the vegetation and microbial communities. ▶ Acid methanolysis provided valuable new knowledge on the organic matter quality.