Chapter 19 Management Options for Mitigating Nitrogen (N) Losses from N-Saturated Mixed-Conifer Forests in California

Mixed-conifer forests of southern California are exposed to nitrogen (N) deposition levels that impair carbon (C) and N cycling, enhance forest flammability, increase the risk of fire occurrence and air pollution emissions in fire, and increase nitrate runoff and soil N emissions both pre- and postfire. N-deposition abatement policies and prescribed fire treatments have been proposed to mitigate the interactive effects of fire suppression, N deposition, and wildfire occurrence. To test the most effective management options for N-enriched forests, a simulation study was done using a parameterization of the DAYCENT model for a mixed-conifer forest site currently experiencing 70 kg N ha^-1 yr^-1. Five N deposition scenarios were defined, ranging from 5 to 70 kg N ha^-1 yr^-1. Five abatement strategies ranging from 0% to 100% reductions in N deposition were considered for each N-deposition scenario. The influence of prescribed fire was tested for the selected N deposition and abatement scenarios, considering 15-, 30-, and 60-year intervals (PF15, PF30, and PF60, respectively), or no prescribed fires. When the most extreme N-deposition scenario was compared to the lowest, fuel loads were increased by 121%, resulting in 70% increases in wildfire emissions of particulate matter (PM₁₀ and PM_2.5), methane (CH₄), carbon monoxide (CO), carbon dioxide (CO₂), and sulfur dioxide (SO₂). The estimated increase in wildfire nitrogen oxide (NO_x) emissions ranged from 56% to 210%. The larger values were derived when variations in fuel N content were taken into account. The combination of reduced N deposition and prescribed fire was most effective in reducing long-term N losses to the atmosphere and in runoff. The PF15 treatment combined with 50-75% reduced N deposition were the best options for reducing N losses before and after fire. However, even prescribed fire at longer intervals and in combination with 25-50% reduced N deposition still resulted in large reductions in ecosystem losses of N. Implementation of such treatments would be considered a major achievement towards mitigating the symptoms of N saturation, even though in sites chronically exposed to 70 kg N ha^-1 yr^-1 a 100% reduction in N deposition may require many years to return N losses to baseline levels.