Lignin and cellulose concentrations in roots of Douglas fir and European beech of different diameter classes and soil depths

Abstract

Comparing two tree species, we tested the effects of root diameter (up to 30 mm) and soil depth (down to 1.2 m) on the concentrations of lignin, cellulose and nitrogen (N) in roots of approximately 50-year-old Douglas fir and European beech growing in a temperate forest in South-western Germany. Fine roots (diameter 0.5–2 mm) exhibited significantly higher lignin concentrations, but lower cellulose concentrations than medium or coarse roots (diameter >5 mm). The cellulose and lignin concentrations of the roots as well as their lignin:cellulose ratios did not differ significantly among soil depths. In the Douglas fir, there was a tendency of decreasing N concentrations and increasing lignin:N ratios with increasing soil depth. This trend was absent or less pronounced in the beech. Beech roots displayed significantly higher cellulose and N concentrations and lower lignin:cellulose and lignin:N ratios than roots of the Douglas fir. Generally, the lignin concentrations of the roots did not differ between the tree species. Cellulose and lignin concentrations exhibited a significantly negative correlation. As several studies have demonstrated that plant litter decomposition is governed by the lignin:cellulose and lignin:N ratios more than by the lignin concentration of the detritus, the fraction of individual tree species in the stand composition might affect the decomposability of roots in beech–Douglas fir forests, and might also have an influence on soil carbon sequestration.