Fire Effects on Stream Nutrients

Fire has the potential to impact water quality by increasing stream nutrients. Dissolved nutrients in streamflow are derived primarily from weathering, decomposition of plant material, and anthropogenic sources. Vegetative communities accrue and cycle large quantities of nutrients (Tiedemann et al. 1979). Fire can disrupt this cycle and cause nutrient leaching, volatilization, and transformation. When vegetation is consumed by fire, some of the litter-incorporated N, P, K, Ca, Mg, Cu, Fe, Mn, and Zn are volatilized and lost from the system, while metallic nutrients such as Ca, Mg, and K are converted into oxides and accumulated in ash (DeBano et al. 1998).

Once a burn has occurred, nutrient mobility determines whether or not a water source will be contaminated. Nitrate is a highly mobile ion and is at risk of being leached from burned areas and transported to either surface or ground water. On the other hand, phosphorus binds readily to sediment and other organic materials. Thus, phosphorus is predominately transported to a stream via soil erosion. Rates of soil erosion and phosphorus contamination of receiving waters will likely be dependent on the topographic relief of the site.

Despite the potentially important impact of fire on water quality, few studies have been conducted in the southeastern United States on the effects of fire on stream nutrients. Of the few studies available (Table: Fire effects on physical and chemical water quality in the southeast), results have shown either no effect or small increases in stream nutrients following fires. This contrasts with regions in the western United States where fires have a notably larger effect on water quality. For example, following a wildfire in the eastern Cascade Mountains of Washington, total N loads increased from 0.004 to 0.16 kg ha^-1yr^-1 and available phosphorus increased from 0.001 to 0.014 kg ha^-1yr^-1 in streams draining Douglas fir and ponderosa pine forests. Further, cations (specifically Ca, Mg, K, and Na) increased from an average of 1.98 to 54.3 kg ha^-1y^-1(Helvey et al. 1985). Whenever such high losses occur, not only can water quality potentially suffer, but site productivity may be reduced.

The long-term effects of fire on the soil nutrient status can be significant (Williard 1999). Repeated low intensity fires (prescribed burning) or single large fires can reduce soil nitrogen pools significantly (Gagnon 1965, Carreira et al 1994). Gagnon (1965) documented that a historical fire (20 years earlier) exhibited long-term effects on soil nitrogen and tree nutrition. Vegetation after the 500 km² burn showed deficiencies of both nitrogen and phosphorus in foliar analyses.

For management guidelines on reducing the impact of fire on water quality, see Management Guidelines for Reducing Fire Effects on Soil and Water.