Nitrogen Saturation

Nitrogen saturation occurs when supplies of ammonium and nitrate are in excess of the total combined plant and microbial demand. The termis applied toecosystems where the biota are unable to utilize all of the N that is added to the system, either through N fixation, atmospheric N inputs or other sources (Aber and others 1989, Stoddard 1994). Often nitrogen saturationleads tochronic acidification of streams (Adams 1999) .

Nitrogen leakage into streams may be caused by several factors including the maturation of forests, effects of insect infestation, and excess nitrogen supply in deposition. Fixed nitrogen is an important nutrient for plant growth, but as forests mature, a balance is reached between plant use and recycling back into the system by decaying plant materials. Insect defoliation, for example, by gypsy moths or balsam wooly adelgids, also causes rapid recycling of nitrogen (Webb and others 1995, Nodvin and others 1995, SAMAB 1996).

A nitrogen saturated system may show one or more of the following symptoms (Aber and others 1989, Stoddard 1994):

• elevated concentrations of nitrate, aluminum and hydrogen in streams,
• frost damage or other disruptions of physiological function,
• increased emissions of trace gases such as nitrous oxide, and
• increased cation leaching from soils.

High-elevation spruce-fir forests of the Northeast (McNulty 1993) and hardwood forests of the central Appalachian region (Adams and others 1997) are believed to be N saturated as a result of high N deposition (Adams 1999).

Nitrogen saturation can result in significant decreases in soil fertility and nutrient deficiencies. As nitrate and sulfate anions move through the soil, cations (most often Ca and Mg) are removed from exchange sites to maintain charge neutrality of the soil solution. Adams and others (1997) reported that at the Fernow Experimental Forest, 5 years of experimental additions of ammonium sulfate at twice the ambient input levels (additions of 40 kg S ha ^-1 year ^-1 and 35 kg N ha ^-1 year ^-1 ) resulted in increased leaching of nitrate from a 25-year-old mixed hardwood forest, along with significant increases in leaching of Ca and Mg. By the fifth year of treatments, mean annual export of Ca was 2-3 times greater than inputs. Results from other watershed manipulation studies suggest that bed weathering did not increase (Norton and others 1997). Ca deficiencies have been identified in high-elevation red spruce (McLaughlin and others 1991) and may also be linked to sugar maple decline (Long and others 1997) on nonglaciated soils. As acidic loading continues, base saturation of the soil will decrease until base cations no longer are being desorbed from the soil, and aluminum concentrations in soil solution and stream water will increase. This effect is of concern because elevated soil solution aluminum concentrations have been found to interfere with root growth in red oak seedlings (Joslin and Wolfe 1989), and with Ca uptake in spruce (Shortle and Smith 1988). The acidification associated with N saturation also affects the cycling of nutrients by changing rates of litter decomposition (Adams and Angradi 1996), nutrient uptake and concentration in aboveground biomass (Adams and others 1995), soil base saturation, and N mineralization and nitrification (Aber and others 1989, Adams 1999).

In Great Smoky Mountains National Park, many streams have higher concentrations of baseflow nitrate than sulfate. In fact, streams in Great Smoky Mountains National Park have the highest recorded nitrate concentrations of any streams draining undisturbed watersheds in the United States. Silsbee and Larson (1982) report nitrate concentrations in Great Smoky Mountains National Park streams ranging from 0.2 to 90 µeq/l, often higher concentrations than are found in deposition. This finding suggests that watersheds in this region are net sources of nitrogen to streams. Old-growth forests, such as those in Great Smoky Mountains National Park, may no longer be acting as nitrate sinks, and nitrate may be leaching out of these old-growth watersheds (SAMAB 1996).