W.W. Miller

Watershed disturbance has been suggested as a possible mechanism for accelerated nutrient input into Lake Tahoe, California/Nevada. However, little is known regarding how nutrient discharge is coupled to physicochemical watershed processes. Recent investigations in the Lake Tahoe Basin have suggested that suspended/dissolved‐organic nutrient transport may play an important role in lake and tributary water quality. The mobility of inorganic and suspended/dissolved‐organic N in soils of a Lake Tahoe watershed was assessed using constant head permeameter leaching experiments with intact soil cores. We evaluated the interaction of plot condition (riparian, nonforested, and forested) and soil depth (0–15 and 15–30 cm) on magnitude and form of N discharge. Incremental leachate discharge was analyzed for concentrations of inorganic (NH₄‐N and NO₃‐N) and suspended/dissolved‐organic N. Leachate from the riparian soil cores had significantly higher (P < 0.05) concentrations and total discharge NO₃‐N than that from the nonforested or forested areas. Loading of NH₄‐N was more consistent among vegetative cover types, but the riparian leachate again contributed a significantly (P < 0.05) greater amount. Suspended/dissolved‐organic N was mobile and the most dominant form of N for nonforested and forested soil cores with discharge loading ratios (suspended/dissolved‐organic:inorganic) of 17:1 and 7:1, respectively. Although the loading ratio was approximately 1:1 for the riparian soil cores, the amount of suspended/dissolved‐organic N discharged was greatest. The mobility and presence of significant amounts of suspended/dissolved‐organic N indicate that this once unrecognized nutrient form is an important component in at least one Sierra Nevada watershed, and should be more fully investigated elsewhere.