Integrating pH, substrate, and plant regrowth effects on soil nitrogen cycling after fire

Abstract:

Mediterranean-type ecosystems are structured by fire. In California chaparral, fires uncouple 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. We combined empirical analysis and modeling in two chaparral watersheds to better understand how these systems recover from fire, and to explore their sensitivity to changing climate and fire regimes. To evaluate how pH, charcoal, and NH₄⁺ supply influence N cycling, we measured mineralization and nitrification rates in chaparral soils that burned 1, 4, 20 and 40 years prior to sampling. We then experimentally adjusted pH, charcoal, and NH₄⁺ concentrations 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. Then to project the effects of altered precipitation patterns and fire timing on nitrogen dynamics and recovery, we used the hydro-biogeochemical model RHESSys. Fires were imposed at the beginning and end of the growing season under various climates.

NO₃^- production was highest in soils collected from the most recently burned sites. Also, NO₃^- concentrations increased over the course of incubation in soils from all sites, especially at high pH, and with NH₄⁺ addition. Charcoal slightly augmented the effects of elevated pH and NH₄⁺ on NO₃^- production iduring the early stages of incubation in 1 and 4-year old sites, while it slightly dampened their effects by week 8. However, in 20 and 40-year old sites, charcoal had no effect. Overall, nitrification was most powerfully constrained by NH₄⁺ supply. However, increases in pH that occur after fire may enhance nitrification rates when substrate is available. Also, charcoal might enhance N cycling immediately after fire, perhaps by supplying C to microbes, but impacts are short-lived. Modeling results suggest that soil acidity and rapid plant recovery reduce leaching. However, during drought and temperature extremes nutrients pools recover more slowly when fires occur prior to the hot, dry summer.