Fine Root Biomass Stimulates Microbial Activity Of Glucose Mineralization In Buried Humic Horizon Of Volcanic Ash Soils

Burial of the surface soil by volcanic ash deposition plays roles in organic matter accumulation. Assuming that decomposition of the buried humus is limited solely by labile substrate inputs, glucose inputs are hypothesized to stimulate microbial activity in buried humic horizons. We describe patterns of microbial mineralization of C-14-labeled glucose in volcanic soil profiles to test whether glucose inputs increase microbial activity similarly in the surface soil and buried humic horizons. We found that microbial biomass was dependent on fine root biomass rather than soil carbon concentration in the volcanic soil profiles. Both bacterial and fungal biomass correlated with fine root biomass. The microbial capacities of glucose mineralization in the buried humic horizons were lower than in the surface horizons. The microbial activity of glucose mineralization in the buried humic horizons is limited by (1) the smaller microbial biomass due to smaller fine root biomass and (2) the dominance of microbial community adapted to the low concentrations of substrates with low mineralization capacity. Stimulation of microbial activities in the buried humic horizons requires both fine root biomass and inputs of labile substrates.