Wildfire-produced charcoal directly influences nitrogen cycling in ponderosa pine forests

Fire is the primary form of disturbance in temperate and boreal forest ecosystems. However, our knowledge of the biochemical mechanisms by which fire stimulates forest N cycling is incomplete. Charcoal is a major byproduct of forest fires and is ubiquitous in soils of most forest ecosystems, yet the biological function of charcoal in soils of forest ecosystems has been greatly overlooked. We conducted a suite of laboratory experiments on soils from ponderosa pine (Pinus ponderosa Laws) forests to determine the influence of charcoal on soil N dynamics and in particular, nitrification. The addition of NH4+ to forest soils had absolutely no effect on nitrification demonstrating that this process is not substrate limited. The amendment of these soils with NH4+ and field collected charcoal (1% w/w) significantly increased the nitrification potential, net nitrification, gross nitrification, and decreased the solution concentrations of plant secondary compounds (phenolics). Charcoal had no effect on nitrification in soils (from a grassland site) that had naturally high rates of nitrifier activity. The increase in gross nitrification in forest soils and lack of effect on grassland soils suggests that charcoal may alleviate factors that otherwise inhibit the activity of the nitrifying microbial community in forest soils. These results reveal the biological importance of charcoal and advance our mechanistic understanding of how fire drives nutrient cycling in temperate and boreal ecosystems.