Fine root extension in urban forest soil depends on organic mulching

As an effective measure of urban forest management, organic mulching impacts soil properties, the environment, and tree growth. Fine roots are vital for resource acquisition and nutrient cycling in forest ecosystems, but the effects of organic mulching on root morphology and biomass are unknown. We conducted a field experiment in a 15-year-old Ligustrum lucidum urban forest with and without mulch addition (0 cm, 5 cm, 10 cm and 20 cm of mulch). Five cm of organic mulch decreased the specific root length (SRL) and specific surface area (SSA) but increased root tissue density and fine root biomass (FRB) with decreasing bulk density. The SRL and SSA of low-order roots (1st- and 2nd-order roots) and FRB of high-order roots (3rd- to 5th-order roots) were more sensitive to mulching as more nutrients were transported between plants and soil. The SRL and SSA decreased in the topsoil (0–20 cm) and increased gradually in the subsoil (20–40 cm) with mulch, while the root tissue density decreased. Consequently, fine roots extend into the deeper soil after mulching with nutrient loss in the subsoil. The FRB increased during the growing season but decreased in the non-growing season after mulching, indicating that mulching stimulated the growth and turnover of fine roots. Bulk density was the key factor that affected the root morphology and biomass, and root order and soil depth were considerable factors for the fine roots. Organic mulching is an effective measure for urban forest management that benefits root and plant growth.