Organic Mulching Increases Microbial Activity in Urban Forest Soil

Interactions between C accumulation, resulting from plant residue mulching, and the accelerated decomposition of litter and soil organic matter due to higher enzyme activities remain poorly understood, particularly in urban forests. Here, the activities of four enzymes associated with carbon (C) and nitrogen (N) cycling after organic mulching were investigated in the rhizosphere and bulk soil of a 15-year-old Ligustrum lucidum urban forest. The enzymatic activities in the rhizosphere were closely correlated to those in bulk soil, however, they were more strongly affected by organic mulching and exhibited faster C and nutrient cycling. Moreover, the activity of urease in the rhizosphere and peroxidase in the bulk soil in autumn, as well as invertase in the rhizosphere in spring, decreased. Microbial C and N were most responsible for the observed increase in invertase activity in spring, during which microorganisms exhibited rapid growth and the highest activities, whereas soil organic C regulated urease activity in winter. Additionally, the activity of dehydrogenase increased by up to 75%; the degree of increase was typically dependent on the mulching amount. Generally, invertase and dehydrogenase activity increased with the abundance of dissolved organic matter and microbial biomass, whereas peroxidase exhibited the opposite trend. Taken together, these findings indicate that organic mulching efficiently increases enzymatic activity, particularly within the rhizosphere, thus accelerating soil C and N cycling. Hence, mulching may represent an effective measure for urban forest management and soil conservation.