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FACTORS AFFECTING MINERAL NITROGEN TRANSFORMATIONS BY SOIL HEATING: A LABORATORY-SIMULATED FIRE STUDY

Two forest soils from the Sierra Nevada Mountains of California were brought into the laboratory and subjected to simulated burning in a muffle furnace at several durations, oven temperatures, and water contents. Soils were analyzed for NO3−, NH4+, total N, total C, and C:N responses to the treatments. Ammonium, which was the dominant form of inorganic N, generally increased with greater burn temperatures and durations (interpreted as higher fire severity), but decreased at the highest severity in dry soils. Increasing initial water content (IWC) diminished these responses in one soil but not in the other. Nitrate responses varied between soils, but generally increased with increasing IWC. Little or no change was seen in total N, total C, and C:N ratio except at highest severity, where C and N decreased regardless of initial moisture content. Soil temperatures measured immediately after burning increased with greater applied furnace temperatures and durations to near 100 °C, leveled off for periods depending on IWC, and then increased to 300 °C to 400 °C at higher temperatures and durations. This experiment shows that low-severity fire can cause very large and highly variable changes in NO3− and NH4+, with little or no change in total C or N. High-severity fire has varying effects on NO3− and NH4+, depending on IWC and soil type, but generally causes losses of total C and N.