Mechanical fragmentation of leaf litter by fine root growth contributes greatly to the early decomposition of leaf litter

Understory plant root proliferation into the litter layer occurs in many forest ecosystems and dramatically accelerates litter decomposition and nutrient release in the early stages. Mechanical fragmentation of leaf litter by fine roots could be one main reason, but its mechanism is poorly understood. To quantitatively assess the influence of fine roots on the mechanical fragmentation of leaf litter in the early decomposition process, we examined the effects of fine root mass of Lolium multiflorum on morphological and physical characteristics of decomposing litters of Symplocos setchuensis, a dominant species in a mid-subtropical evergreen broad-leaved forest of China, via a litter bag simulation experiment. The results showed that there were various fine root growth forms in/on decomposing litters, including the penetration and tearing of leaf epidermis, and the “walking or inserting” between the mesophyll tissue and the leaf veins. Fine root mass had a significant positive effect on the mass loss during litter decomposition (F = 8.543, P < 0.01). In a duration of 170-day leaf litter incubation, the mass remaining ration was significantly higher in no roots treatments and fewer roots treatments than that in more roots treatments. In the case of more roots, the physical tensile strength of leaf litter fragments was 0.156 N/mm, which was significantly lower than that under the treatments with fewer roots and no roots (P < 0.05). More fine roots penetrating the litter layer could significantly promote the intensity of mechanical fragmentation by changing the morphology, structure, and physical properties of the leaf litter. The fine root biomass in the litter layer was positively correlated with the decomposition rate, and the specific root length and specific root tip density were two important indicators related to decomposition. In conclusion, fine roots growing into litter promoted the intensity of mechanical fragmentation by increasing root length and producing new root tips and by actively regulating litter traits, so as to improve the utilization efficiency of nutrient release in the early decomposition process.