Seasonal variations in enzyme activity and organic carbon in soil of a burned and unburned hardwood forest

This study examined variations in soil organic C content and the activity of acid phosphatase, alpha-glucosidase, phenol oxidase, chitinase, and L-glutaminase in ultisols of burned and unburned areas in Quercus-dominated forests in Ohio, USA. The low intensity, prescribed fires were conducted in April 2001, with temperature 10 cm above the forest floor averaging 160-240 degrees C. Sampling was conducted throughout the six month growing season (May-October) of 2003, two years after the fire. Organic C content in these ultisols varied between 20 and 30 g C/kg soil, and varied little through the growing season, except for a late season increase to approximately 32 g C/kg soil in the burned areas. When enzyme activity was expressed per unit soil organic C, there was no statistically significant variation among sample dates in soil enzyme activity except L-glutaminase, which demonstrated a distinct maximum in activity in spring. Non-metric multidimensional scaling (NMS) ordination resulted in no clear separation of burned and unburned sample areas based on soil organic C and enzyme activity. When the growing season was divided into three segments (early spring, late spring/early summer, and late summer/early autumn), there was again a lack of separation between burned and unburned areas in the earlier two segments, whereas in the late summer/early autumn segment the burned and unburned areas were clearly separated on the basis of differences in soil organic C and L-glutaminase activity. As environmental factors (e.g. soil temperature, moisture) and substrate availability do not vary in parallel through the growing season in this region, seasonal patterns often differ among enzyme systems based on their predominant control mechanism. Sampling time during the growing season appears to have little effect on holistic judgments of fire effects based on soil enzymes, except under restrictive conditions.