Landscape heterogeneity in the soil nitrogen cycle of an alpine-subalpine ecosystem

The soil nitrogen (N) cycle is an important part of alpine-subalpine ecosystems for two reasons: (1) It affects plant species composition and production rates; and (2) anthropogenic N deposition is increasing the amount of available N. One of the major obstacles to understanding the soil N cycle at larger scales is landscape heterogeneity. The goal of this work was to examine landscape heterogeneity in soil N cycling pools and fluxes within a 0.89 km2 ecosystem at the alpine-subalpine ecotone in the Front Range of the Rocky Mountains. N cycling pools and fluxes spanning a range of timescales were measured. Physical, chemical, and biotic conditions across the study site were also quantified. Measurements of differently aged soil organic matter (SOM) constituents showed that older SOM is less heterogeneous than younger SOM across the study site, supporting the hypothesis that decomposition chemically homogenizes soil over time. A quantitative analysis of N cycling hot spots demonstrated that a large percentage of total inorganic N pool sizes and N cycling rates are attributable to a small percentage of the landscape area, showing that hot spots are an important feature of the N cycle at the study site. Statistical models of the pools and fluxes of the soil N cycle showed that while plant canopy structure is an important determinant of landscape-level N cycling rates, the identity and abundance of particular plant species are not effective as predictors beyond their covariance with abiotic conditions. Finally, a strong relationship was observed between SOM N concentrations and log-transformed IN levels within soil cores. Taken together, the results of these studies suggest that the size of the SOM pool is a key determinant of N cycling activity and that this pool is highly variable across the landscape and not easily linked to abiotic gradients.