Effects of substrate supply, pH, and char on net nitrogen mineralization and nitrification along a wildfire-structured age gradient in chaparral

•Examined controls on N cycling along a wildfire-structured age gradient in chaparral.•Nitrification was most powerfully constrained by substrate supply and pH.•Char did not influence N cycling, but did sustain heterotrophic microbial populations.

Mediterranean-type ecosystems are structured by fire. In California chaparral, fires uncouple inorganic nitrogen (N) production and consumption by enhancing nitrification and reducing plant uptake. Nitrate (NO3−) that accumulates after fire is vulnerable to leaching. However, the controls over N metabolism can change as chaparral ecosystems recover from fire, and these mechanisms are not well understood. To evaluate how ammonium (NH4+) availability, pH, and char influence N cycling, we measured inorganic N concentration and microbial biomass in chaparral soils that burned 1.5, 4, 23, and 49 years prior to sampling. We then experimentally adjusted NH4+ concentration, pH, and char content for all soils in a factorial design, and incubated them for 8 weeks. Each week, we measured respiration, exchangeable NH4+ and NO3− content, nitrification potential, microbial biomass, and pH. Within each watershed, NO3− was higher in soils collected from recently burned sites, than in soils from neighboring mature sites. Also, NO3− concentrations increased over the course of incubation in soils from all sites, especially with the addition of NH4+. When NH4+ was sufficiently high, pH determined the relative proportion of inorganic N that was nitrified. Char did not have a strong impact on N cycling, but in soils from 23- and 49-year-old sites, char treatments reduced the decline of microbial biomass C and N relative to control and fertilized treatments, perhaps by supplying C to soil microbes. These findings suggest that under some circumstances char may decelerate N cycling if it promotes immobilization and limits NH4+ supply to nitrifiers.