6:15 PM - 7:30 PM
[BGM22-P04] Biodegradation activity of organic matter in the buried humic horizons in volcanic ash soils
Soil samples (volcanic ash soil) were collected from the soil profiles in three forest sites and one pasture site in Hokkaido, Japan. These “fresh” field moist, un-sieved soils were used for mineralization kinetic studies. A 14C-glucose solution (100 μL; 10 - 300000 μM) was added to 1 g of field-moist soil and incubated for 24 h at 20℃. The 14CO2 production was trapped in the scintillation vial containing NaOH and determined by liquid scintillation counting. The experiments were performed in triplicate. The data of mineralization kinetics were fitted to a single Michaelis-Menten equation: V = VMAXC/(KM+C), where V is the mineralization rate (nmol g-1 h-1), C is the substrate concentration (μM) in soil solution, VMAX is the maximum mineralization rate (nmol g-1 h-1), and KM is the Michaelis constant (μM) representing the concentration at which 1/2 VMAX is achieved. We also measured soil microbial biomass, fungal/bacterial (F/B) ratio, and fine root biomass.
Fine root biomass in the soil profiles decreased with depth, suggestion the low rates of organic substrate supply in the buried humic horizon. Their Michaelis-Menten kinetic parameters (VMAX and KM) varied widely from 303 to 18598 nmol g-1 h-1 and 198 to 1294 μM. The parameters decreased with soil depth. The kinetic parameters of the surface soil horizon exhibited high mineralization capacity, while parameters of the buried humic horizons were similar to those of the other soil types (11 to 2406 nmol g-1 h-1 for VMAX and 198 to 30786 μM for KM). The both of VMAX and KM parameters were correlated positively with microbial biomass-C and -N, respectively. This indicates microbial biomass is a primary factor regulating the potential degradation activity in the buried humic horizons. Microbial biomass decreased with soil depth, consistent with the low input of fresh organic substrates in the deeper soil horizons. The higher F/B ratios were observed in the buried humic horizon, compared to the surface horizon. Since F/B ratios has influence on the mineralization kinetic parameters due to differences of substrate use efficiency and growth speed between fungi and bacteria, the high potential degradation activity relative to the other soil types may be due to high F/B ratios in the buried humic horizons. The high potential degradation activities of soil microorganisms in the buried humic horizons suggest that OM decomposition can be accelerated by addition of easily-biodegradable OM which stimulates soil microbial activity.