Management options for mitigating Nitrogen (N) losses from N-saturated mixed-conifer forests in California
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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 Nha-1 yr-1. Five N deposition scenarios were defined, ranging from 5 to 70 kg Nha-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 PM2.5), methane (CH4), carbon monoxide (CO), carbon dioxide (CO2), and sulfur dioxide (SO2). The estimated increase in wildfire nitrogen oxide (NOx) 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 Nha-1 yr-1 a 100% reduction in N deposition may require many years to return N losses to baseline levels.