The relationship of C and N stable isotopes to high-latitude moss-associated N.sub.2 fixation

Moss-associated N.sub.2 fixation by epiphytic microbes is a key biogeochemical process in nutrient-limited high-latitude ecosystems. Abiotic drivers, such as temperature and moisture, and the identity of host mosses are critical sources of variation in N.sub.2 fixation rates. An understanding of the potential interaction between these factors is essential for predicting N inputs as moss communities change with the climate. To further understand the drivers and results of N.sub.2 fixation rate variation, we obtained natural abundance values of C and N isotopes and an associated rate of N.sub.2 fixation with .sup.15N.sub.2 gas incubations in 34 moss species collected in three regions across Alaska, USA. We hypothesized that [delta].sup.15N values would increase toward 0â° with higher N.sub.2 fixation to reflect the increasing contribution of fixed N.sub.2 in moss biomass. Second, we hypothesized that [delta].sup.13C and N.sub.2 fixation would be positively related, as enriched [delta].sup.13C signatures reflect abiotic conditions favorable to N.sub.2 fixation. We expected that the magnitude of these relationships would vary among types of host mosses, reflecting differences in anatomy and habitat. We found little support for our first hypothesis, with only a modest positive relationship between N.sub.2 fixation rates and [delta].sup.15N in a structural equation model. We found a significant positive relationship between [delta].sup.13C and N.sub.2 fixation only in Hypnales, where the probability of N.sub.2 fixation activity reached 95% when [delta].sup.13C values exceeded - 30.4â°. We conclude that moisture and temperature interact strongly with host moss identity in determining the extent to which abiotic conditions impact associated N.sub.2 fixation rates.