Soil carbon and nitrogen eroded after severe wildfire and erosion mitigation treatments

Abstract

Erosion of soil carbon (C) and nitrogen (N) following severe wildfire may have deleterious effects on downstream resources and ecosystem recovery. Although C and N losses in combustion and runoff have been studied extensively, soil C and N transported by post-fire erosion has rarely been quantified in burned landscapes. To better understand the magnitude and temporal pattern of these losses, we analysed the C and N content of sediment collected in severely burned hillslopes and catchments across the western USA over the first 4 post-fire years. We also compared soil C and N losses from areas receiving common erosion-mitigation treatments and untreated, burned areas. The concentrations of C and N in the eroded material (0.23–0.98 g C kg−1 and 0.01–0.04 g N kg−1) were similar to those of mineral soils rather than organic soil horizons or combusted vegetation. Losses of eroded soil C and N were highly variable across sites, and were highest the first 2 years after fire. Cumulative erosional losses from untreated, burned areas ranged from 73 to 2253 kg C ha−1 and from 3.3 to 110 kg N ha−1 over 4 post-fire years. Post-fire erosion-mitigation treatments reduced C and N losses by up to 75% compared with untreated areas. Losses in post-fire erosion are estimated to be <10% of the total soil C and N combusted during severe wildfire and <10% of post-fire soil C and N stocks remaining in the upper 20 cm of mineral soil. Although loss of soil C and N in post-fire erosion is unlikely to impair the productivity of recovering vegetation, export of C and N may influence downstream water quality and aquatic ecosystems.