Microbial activity and nitrogen mineralization in forest mineral soils following heating: evaluation of post-fire effects

Heat generated during tire induces chemical oxidation of soil organic matter thereby altering carbon (C) and nitrogen (N) transformations. Prior soil fire history and soil moisture content at the time of heating can be confounding factors in the interpretation of the influence of heat on soil processes. In this study we evaluated how soil heating (160 and 380degreesC) under three moisture regimes (-0.03, -1.0, and -1.5 MPa) influences microbial activity and N mineralization in two soils: (1) not exposed to tire for the past 80 years, (2) recently exposed to wildfire. Initially, the fire exposed soil had lower basal respiration rates and lower concentrations of microbial biomass C, potentially mineralizable nitrogen (PMN), soluble hexose sugars, and NH4+-N, but higher NO3--N concentrations than the soil not exposed to fire. Both soils responded similarly to elevated temperatures. Higher temperatures resulted in greater microbial mortality and a greater release of soluble sugars and NH4+-N. PMN concentrations increased at 160degreesC, but decreased at 380degreesC in both soils. The highest NH4+-N concentrations were observed in soils not previously exposed to fire that were incubated at -0.03 MPa after heating. Soils previously exposed to fire had low NH4+-N concentrations and high NO3--N concentrations. Heating at low soil water potentials resulted in elevated concentrations of microbial biomass C and soluble sugars, and lower NH4+-N and NO3--N concentrations. Initial C availability appeared to be an important factor in the recovery of microbial biomass during 14-d post-heating incubation. which was greatest after heating at 380degreesC and -1.5 MPa. Both soils demonstrated slow rates of recovery of nitrifying organisms despite high rates of net NH4+-N accumulation. It appears that low soil water potential at the time of heat exposure reduces losses of mineralizable N. (C) 2002 Elsevier Science Ltd. All rights reserved.