OOS 22-10
Anatomical changes and relative rates of mass loss of in-situ decomposing “woody” fine and coarse roots of American beech seedlings (Fagus grandifolia Ehrh.) growing under a mature pine canopy
After two decades of intensified research, belowground measurements of root decomposition dynamics and estimates of their contribution to the soil carbon and nitrogen cycles still remain highly variable. Current findings indicate that higher-order, older root segments with secondary development decompose at a faster rate than lower-order, younger fine roots, which are at a stage of transition from primary to secondary development or commonly being altered in their anatomy by mycorrhizal colonization. A reason for the high variability in decomposition rates might be the usage of traditional approaches, e.g. litter bags or intact soil cores, artificially altering the root litter or creating artificial access to decomposer organisms. In the current study, we investigate the relative rate of mass loss and track anatomical changes in roots with secondary (woody) development, utilizing 3D-laser ablation technology (3D-LAT), from in situ decapitated whole root systems of naturally established three to seven-year-old tree seedlings of American beech (Fagus grandifoliaEhrh.), growing under a 75 year-old pine canopy. Mass loss of five-millimeter long, dead, woody fine root segments (0.5 -1 mm diameter) were investigated in comparison to coarse woody root segments (2.5 - 3 mm diameter), as well as spatially and anatomically visualized (rendered).
Results/Conclusions
Initial and average woody fine and coarse root diameter ranged from 0.8 mm (+/- 0.14) and 2.7 mm (+/- 0.21), respectively. Similarly, the initial dry mass of the two root categories ranged from 2.1 mg (+/- 0.38) and 16.5 mg (+/- 0.2) with an average C/N ratio of 81 and 63, respectively. During the first six months, woody fine roots lost on average 18% of their initial relative mass, while woody coarse roots failed to show any change in relative mass, despite an obvious visual loss of a substantial portion of root bark (phellem) and cork cambium (phellogen) cell layers. The relative mass loss in woody fine roots could be linked to their comparative location in the root system, next to finest root orders with primary development, serving as main access points for decomposers. At the same time, the proximal location of woody coarse roots likely reduces the possibility of increased decomposer degradation. Our findings reveal that undisturbed, “in situ” root decomposition processes follow trends of carbon and nutrient cycling opposite to previously found decomposition dynamics with traditional approaches.