Ecosystem type differences in nitrogen process rates and controls in the riparian zone of a montane landscape

Nitrogen (N) inputs to the world's aquatic ecosystems have increased over the past 100 years. Riparian zones and other wetlands are known to remove N from ground and surface waters before they enter lakes and streams. However, the effect of riparian wetlands on water quality has been shown to differ dramatically among sites. Sources of landscape variation in riparian N dynamics have been poorly documented and rarely quantified. Within the Lake Tahoe Basin (LTB), we hypothesized that differences in riparian N dynamics correlate to differences in ecosystem type. To test this hypothesis, we randomly selected 20 plots, four each of five ecosystem types, along a tributary to Lake Tahoe. Throughout the snow-free season, we measured soil N transformations and groundwater chemistry in each plot. We found significant differences in denitrification potential, net mineralization, net nitrification and groundwater nutrient flux among different ecosystem types. These results suggest that classification of riparian zones into ecosystem types might be useful in predicting landscape differences in riparian water quality effects.We also tested for factors expected to directly and indirectly control riparian N process rates using data collected at each study site. Using correlation and multiple regression analysis, we found that N process rates were most highly correlated with different control factors according to ecosystem type. Differences in direct and indirect control factors among ecosystem types indicate that construction of mechanistic, landscape models of riparian processes might be most accurately done by stratifying the riparian zone into ecosystem types rather than by viewing it as a single, homogeneous part of the watershed.