EFFECT OF EXTRACELLULAR-ENZYME ACTIVITIES ON SOLUBILIZATION RATE OF SOIL ORGANIC NITROGEN

In a sandy soil containing (15)-labeled active (soluble and easily degradable) and non-labelled passive (recalcitrant) fractions of soil organic matter, the rate of net N mineralization (solubilization) was determined during a 55-day incubation at 25 degrees C, 63% water-holding capacity and different levels of soil extracellular-enzyme activities. The active fraction of soil N was labelled by preincubation (at 5 degrees C and 74% water-holding capacity for 6 months) of soil amended with N-15-labeled plant material. Increases in the activity of extracellular-enzymes in soil were induced by the addition of glucose and KH2PO4 at the beginning of the incubation. The results show that the contents of total soluble N (NO3-N+ NH4+-N+soluble organic N) were significantly higher in glucose-amended soil compared to the unamended soil. The increases in soluble N in soil amended with 1 and 2 mg glucose g(-1) dry soil corresponded to a mean rate of net solubilization of 7.9+/-1.4 and 18.8+/-0.7 nmol N g(-1) dry soil day(-1), respectively. The mean rate of net N solubilization (3.6+/-1.0 nmol N g(-1) dry soil day(-1)) in unamended soil was significantly lower than those of glucose amended soils. The content of N-15 in total soluble N in soil amended with 2 mg glucose, for example, was diluted from 3.11+/-0.08 atom % before the incubation to 2.77+/-0.03 atom % after 55 days. This indicates that 89% of soluble-N accumulated in soil by the end of the incubation originated from the active fraction of soil N and the rest, estimated at 11%, originated from the passive fraction. The activities of soluble and total proteases as well. as the rate of N solubilization in the soil increased with the application of glucose. The activity of these extracellular enzymes was highly correlated with the rates of net N solubilization. Thus, increases in extracellular-enzyme activities in glucose-amended soils had a priming effect on the solubilization of N-15-labeled active and non-labeled passive fractions of soil organic N. It seems that the activity of extracellular-enzymes expressed in terms of total and soluble protease activities could be a rate-limiting factor in the processes of soil organic N solubilization.