U. Choromanska and T. H. DeLuca*

Abstract

Although studies have addressed the influence of fire on soil bio chemical processes, there have been no reports on how prescribedfire 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-Nconcentrations in surface (0–10 cm)mineral soils were significantly increased immediately after WF. Soils exposed to prescribed fireprior to wildfire also had elevated concentrations of total N, PMN andNH1 4 –N, but were significantly lower than inWFalone. 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 g capsule^-1 in WF soils vs. 24 g capsule^-1 in PBWF soils, and 1.3 g capsule^-1 in the unburned control. Microbial biomass in the WF mineral soils was as low as 52 g g^-1 21 mo after fire while microbial biomass in PBWF soils remained above 100 g 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.

Keywords: Microbial biomass, Soil Biochemical, Pseudotsuga menziesii var. glauca, Pinus ponderosa, nutrient cycling,Pseudotsuga menziesii, Organig farming.