PS 47-30
Fine roots are the dominant source of recalcitrant plant litter in northern hardwood forests
Plant litter is a major source of the soil organic matter. Leaf litter traits are often used to represent the whole-plant traits controlling the plant-originated organic matter cycling. Growing evidence, nevertheless, indicates that fine roots contribute a substantial fraction of annual plant litter input, and that fine roots have significantly different litter chemistry than leaf litter. Hence, fine roots and leaf litter could play highly different roles in soil organic matter dynamics and terrestrial biogeochemical cycles. Here, we compare the biochemical composition and decomposition of sugar maple (Acer saccharum) leaf litter and fine roots at four sites in northern lower and western upper Michigan that are spread across a 500 km climate and air pollution gradient. We quantified nine major biochemical classes for each litter type. Then we used litter bags to track the mass loss and the dynamics of these biochemical classes in different stages of decomposition, in order to investigate which biochemical classes are responsible for the difference of decomposition rates, if exists, between leaf litter and fine roots.
Results/Conclusions
Fine roots showed higher concentrations of recalcitrant biochemical classes such as lignin and condensed tannins than leaf litter (P< 0.001). Lignin concentrations were approximately three-fold higher in fine roots than in leaf litter (45.4±0.3% and 14.6±0.2%, respectively). Condensed tannins, which can both form recalcitrant complexes with proteins and inhibit extracellular enzyme activities, were also more abundant in fine roots than in leaf litter (129.9±3.8mg g-1 and 49.9±4.5mg g-1, respectively). In contrast, fine roots showed lower concentrations of nonstructural carbohydrates, soluble phenolics, and cellulose than that of leaf litter (P< 0.001). Taking into account of the litter production, fine roots contributed >70% of lignin and >80% of the condensed tannins produced annually in these ecosystems, indicating that fine roots are the dominant source of recalcitrant plant litter. Leaf litter exhibited an approximately two-fold greater mass loss than fine roots in the early stage of decomposition (24.3±5.8% and 12.5±2.7% respectively, in 30 days). The greater loss of soluble phenolics and holocelllulose in leaf litter explained 72% of this difference of mass loss between leaf litter and fine roots. Non-structural carbohydrates were the most degraded biochemical class in both litter types, yet lignin showed no sign of degradation in this stage of decomposition.