Substrate, pH, and charcoal controls on nitrogen cycling along a wildfire-induced age gradient

Background/Question/Methods

Mediterranean-type ecosystems are structured by fire. In California chaparral, fires uncouple inorganic N production and consumption by enhancing nitrification and reducing plant uptake. NO₃^- that accumulates after fire is vulnerable to leaching. However, the extent to which fires decouple N fluxes can vary spatially and with timing of fire, and the specific mechanisms controlling N metabolism in recovering chaparral are not well understood. To evaluate how NH₄⁺ supply, pH, and charcoal influence N cycling, we measured inorganic N concentration and microbial biomass in chaparral soils that burned 1, 4, 20 and 40 years prior to sampling. We then experimentally adjusted NH₄⁺ concentration, pH, and charcoal 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. 

Results/Conclusions

NO₃^- production was highest in soils collected from the most recently burned sites. Overall, nitrification was most powerfully constrained by NH₄⁺. However, when NH₄⁺ was sufficiently high, pH determined the relative proportion of inorganic N that was nitrified. Increases in pH can directly stimulate nitrification by increasing the ratio of ammonia (NH₃) to NH₄⁺ in soils. In unburned chaparral, acidic soils mean that more mineralized N occurs as NH₄⁺, which is not available to ammonia oxidizing bacteria. Thus, the influence of pH might help explain nitrification differences between recently burned and mature ecosystems when NH₄⁺ is uniformly available. Char did not have a strong impact on inorganic N cycling, but in unburned soils, charcoal treatments enhanced microbial biomass C and N relative to control and fertilized treatments, perhaps by supplying C to soil microbes. These findings suggest that over longer timescales char may decelerate N cycling if it promotes N-immobilization and limits NH₄⁺ supply to nitrifiers.