Prescribed Fire Alters the Impact of Wildfire on Soil Biochemical Properties in a Ponderosa Pine Forest

Abstract

Although studies have addressed the influence of fire on soil biochemical processes, there have been no reports on how prescribed fire followed by wildfire influences microbial activity and nutrient cycling. Over a 21-mo period we monitored changes in soil nitrogen (N) and carbon (C) of a ponderosa pine (Pinus ponderosa P.&C. Lawson) and Douglas-fir [Pseudotsuga menziesii var. glauca (Beissn.) Franco] forest (both O horizon and 0-10 cm of mineral soil) that had been exposed either to prescribed fire (PB), wildfire (WF), prescribed fire three months prior to wildfire (PBWF), or no fire as an unburned control. Total N, potentially mineralizable N (PMN), NH 4 + -N and NO 3 - -N concentrations in surface (0-10 cm) mineral soils were significantly increased immediately after WF. Soils exposed to prescribed fire prior to wildfire also had elevated concentrations of total N, PMN and NH 4 + -N, but were significantly lower than in WF alone. Potentially mineralizable N was significantly reduced on all fire-exposed sites from 9 mo to the end of the study period. Although mineral soil NO 3 - -N concentrations in fire-exposed soils were similar to the unburned control 12 mo after fire, resin sorbed NO 3 - -N was 88 μg capsule -1 in WF soils vs. 24 eg capsule -1 in PBWF soils, and 1.3 μg capsule -1 in the unburned control. Microbial biomass in the WF mineral soils was as low as 52 μg g -1 21 mo after fire while microbial biomass in PBWF soils remained above 100 μg g -1 throughout the study. It appears that prescribed fire prior to wildfire may attenuate the effects of wildfire on soil and may have predisposed the microbial community to the effects of heating.