Climate Change and Fire impacts on Ecosystem Critical Nitrogen Load

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The federal agencies of the United States (US) are currently developing guidelines for critical nitrogen loads for US forest ecosystems. These guidelines will be used to establish regulations designed to maintain nitrogen inputs below the level shown to damage forests and streams. Traditionally, an ecosystem is considered to be at risk for health impairment when the critical nitrogen load exceeds a level known to impair forest health. The excess over the critical nitrogen load is termed the exceedance, and the larger the exceedance, the greater the risk of ecosystem damage. This definition of critical nitrogen load applies to a single, long-term pollutant exposure. Unfortunately, a single critical nitrogen load level may not accurately reflect ecosystem health risk when an ecosystem is subjected to multiple environmental stresses. In other US regions, fire is a major component of the forest ecosystem. Fire volatilizes organic nitrogen, reduces plant nitrogen uptake, increases nitrogen mineralization and nitrification, and can change the pH level of surface horizons. If multiple stress impacts (i.e., drought and fire) are included in critical nitrogen load assessments, critical nitrogen load may need to be lowered in many areas. This paper explores how fire and climate change and variability influence ecosystem critical nitrogen loads.