Expanding our understanding of nitrogen dynamics after fire: how severe fire and aridity reduce ecosystem nitrogen retention

Fires release large pulses of nitrogen (N), which can be taken up by recovering plants and microbes or exported to streams where it can threaten water quality. The amount of N exported depends on the balance between N mineralisation and rates of N uptake after fire. Burn severity and soil moisture interact to drive these rates, but their effects can be difficult to predict. To understand how soil moisture and burn severity influence post-fire N cycling and retention in a dryland watershed, we quantified changes in plant biomass, plant N content, soil microbial biomass, inorganic N pools, and net N mineralisation for 2 years after fire. We compared sites that were unburned with those that burned at moderate or high severity, capturing variation in soil moisture within each severity category. Severe fire limited N uptake by plants. Dry conditions after fire limited both plant and microbial N uptake. When fire is severe or when soils are relatively dry after fire, recovering plants and microbes are less likely to take up post-fire N and therefore, N in these sites is more susceptible to export.