Assessing the effects of woody mulch on post-fire soil and stream nitrogen at experimental and operational scales

Severe wildfires often increase nitrogen (N) losses from burned forests which can impact downstream water quality, water treatability, and aquatic habitat. Woody mulch is commonly applied to mitigate post-fire soil erosion and enhance revegetation, but it is also a source of labile carbon (C) that can stimulate microbial immobilization and limit N release to soil and streams. The objective of our study was to evaluate whether woody mulch application could reduce N losses from soils and export from watersheds affected by the 2020 Cameron Peak fire in Northern Colorado, USA. Our assessment evaluated the potential for mulch to influence N cycling at various spatial scales including laboratory trials using mulched soil columns, field measurements on aerially mulched hillslopes, and replicate mulched and unmulched catchments. As expected after severe wildfire, we found elevated levels of various forms of N in burned hillslope soils and stream water in extensively burned catchments. Woody mulch released high concentrations of soluble C into soils and decreased nitrate leaching both from soil columns and burned hillslopes. However, there was no detectable effect of mulch on stream water N or C, likely due to low mulch coverage and high landscape complexity at the catchment scale. Though we observed that mulch has the potential to reduce post-fire N losses, soil N remained elevated compared to unburned soils. Logistical constraints on aerial mulching limited mulch application rates in our study. Uncertainties about where mulch applications are most effective constrain the utility of this treatment to alter post-fire water quality at magnitudes relevant to downstream users. Land managers would benefit from a better understanding of how to match the positive outcomes of post-fire mulching to the most biogeochemically sensitive locations within burned watersheds.