Manganese indicates root decomposition rates across soil layer, root order, and tree species: Evidence from a subtropical forest

Abstract

Different spatial locations provide different external conditions for root decomposition, while species and root order cause obvious differences in root substrate quality. Both differences in internal and external conditions may determine root decomposition rate. We explored the effects of decomposition position, root order, and species differences on root decomposition, and determined the role of manganese (Mn) in root decomposition. We conducted a fine root decomposition experiment using roots from Schima superba, Cleyera japonica, and Eurya loquaiana (separated into orders 1−3, 4−5, and 6) in the litter layer, and at 0−10 cm, 20−30 cm, and 40−50 cm soil depth in a subtropical forest. Root mass and concentration of C, N, P, Mn, condensed tannins and total phenolics were measured during two years of decomposition. Root decomposition rates decreased with increasing soil depth for all root samples. Compared to decomposition at 0−10 cm soil depth, remaining mass (% of initial) declined by 7%–34% in the litter layer and increased by 17%–37% at 40−50 cm soil depth. Lower-order roots had a higher decomposition rate than higher-order roots in the litter layer, but at 40−50 cm soil depth, we observed the opposite. Root litter Mn concentration and remaining mass were negatively correlated during the whole decomposition period including the early, middle, and late stages. Initial root Mn content was negatively correlated with remaining mass after two years of decomposition in the litter layer, and at 20−30 cm, 40−50 cm soil depth. However, no significant relationship between mass remaining and initial root N, P, condensed tannins and total phenolics were observed in the subsoil. Variance partitioning showed that the soil layer contributes significantly to the variance in remaining root mass, C, and Mn, whereas root order group contributed more than the soil layer to remaining root N and P. Our results indicate that soil depth is more important than root order for fine root decomposition rate at a site scale. Relative root decomposition rate between lower- and higher-order roots depends on the decomposition environment. Overall, this study strongly suggests that root Mn concentration (whether in initial or other decomposition stages) was much more consistently correlated with mass remaining than traditional predictors of N, P, condensed tannins, or total phenolics. Root Mn concentration greatly indicates root decomposition rates across soil layers, root orders, and species. Initial root Mn concentration and Mn supply capacity of the decomposition environment jointly control root decomposition rates throughout the decomposition process.