Factors regulating nitrogen retention during the early stages of recovery from fire in coastal chaparral ecosystems

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Climate change models predict that interannual rainfall variability will increase in California over the next several decades, and these changes are expected to alter fuel characteristics and fire regimes in chaparral. Fires uncouple N mobilization and uptake by increasing nitrification and destroying plant biomass. Following fire, heavy winter rains can leach N into streams, particularly from slopes that have been denuded. The extent to which N is transported from burned slopes to streams depends on how rapidly soil microbes metabolize it into mobile forms such as nitrate, and how rapidly recovering plants take up mineral N. However, the long-term impacts of a changing climate and fire regime on N dynamics remain unknown. We combined empirical measurements with the ecohydrologic model RHESSys to better represent the effects of fire on soil N dynamics and then to project the effects of changing climate and fire timing on N cycling and retention in chaparral. To evaluate how NH₄⁺ supply and pH 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 and pH in a factorial design, and incubated the treated soils for 8 weeks. Nitrification was highest in soils collected from the most recently burned sites, and was most powerfully constrained by NH₄⁺. However, when NH₄⁺ was sufficiently high, pH determined the relative proportion of inorganic N that was nitrified.

After incorporating these relationships into the RHESSys framework, we modeled mineralization, nitrification, N leaching, net-primary production (NPP), and plant N-uptake under a range of climate and fire timing scenarios.  We considered a range of scenarios where fires were imposed either at the beginning or end of the growing season followed by 15 years of recovery. We considered 15 possible climate trajectories. Modeling results suggest that chaparral systems are vulnerable to rapid nitrification and leaching immediately after fire, however recovering plants rapidly immobilize soluble N under most climate scenarios. The strongest variability in NPP and plant N-uptake between different climate trajectories occurred during the first few years of recovery. Notably, following drier years, NPP and plant N-uptake recovered more slowly. Because N is rapidly mineralized and nitrified after fire, slow recovery makes N more vulnerable to leaching. In cases where drought and fire were followed by heavy winter rain, nitrate losses were more severe, which can further slow plant recovery and promote a positive feedback on nutrient export.