As western wildfires increase in frequency and intensity, it is urgent to elucidate the mechanisms that drive post-fire ecosystem recovery. During fire in California chaparral shrublands, large net losses of nitrogen occur through volatilization from biomass, litter, and soils. However, the ash layer deposited on chaparral soils is rich in ammonium (NH4+), which can be converted to nitrate (NO3-) and leached from soils, polluting nearby bodies of water and slowing shrub recovery. In the first two years after chaparral fire, fast-growing herbaceous species dominate the landscape, potentially immobilizing and retaining ash-derived mineral N. To test the impact of herb functional diversity on N-cycling, I introduced a large-scale herb-manipulation experiment in immediate post-fire chaparral (Mendocino County, CA, 2016). I weeded 2mx3m plots so that they contained: (1) all naturally-occurring herbs, (2) non-N-fixers only, (3) N-fixers only, or (4) no herbs. In tandem, I established fenced plots to exclude mammalian herbivores, which are predicted to increase N retention in N-rich post-fire chaparral. During the first two post-fire rainy seasons (2017-2018), I measured N mineralization monthly and peak biomass in June.
Results/Conclusions
First-year results: In the herb-removal experiment, total biomass in plots was approximately equal between all plots where herbs were present, regardless of the functional group allowed to grow. Where N-fixers grew without non-N-fixer competitors, their biomass increased significantly, while non-N-fixer biomass was not impacted by the removal of N-fixers. Biomass of recovering shrubs was not impacted by any herb-removal treatments. Preliminary N mineralization data showed no significant differences between herb-removal reatments, but suggested slightly elevated NH4+ accumulation in N-fixer plots. Excluding herbivores significantly increased the total biomass of non-N-fixers, but did not impact total shrub or N-fixers biomass. Although overall rates of N mineralization were not-significantly different between grazed and non-grazed plots, nitrification and soil NO3- accumulation were slightly elevated in non-grazed plots. Overall, in the high-N conditions of the first year post-fire, non-N-fixer biomass is limited by herbivory and N-fixer biomass is reduced by competition with non-N-fixers. Although there were minimal impacts of herb removal and herbivore exclusion on first-year N cycling, the decomposition of annual plant litter may drive differences in year two. Herbivory and plant competition impact herb functional group biomass, but effects on first-year N cycling are minimal.