Soil C:N ratio is the main driver of soil δ15N in cold and N-limited eastern Canadian forests

Significant relationships have been observed between soil N isotopic natural abundance (delta N-15) and both climate and soil characteristics across a large range of ecosystems over the globe, suggesting strong and consistent effects of these variables on N cycling. However, whether the strength and the nature of these relationships vary at regional scales and with soil depth is less documented, especially in northern cold and N-limited forest ecosystems. In this study, we analyzed delta N-15 in soil horizons at 21 forest sites in eastern Quebec along a gradient of concomitant decreasing N deposition and temperature (MAAT) and increasing precipitation (MAP). We hypothesized that both soil delta N-15 and the magnitude of increase in soil delta N-15 would decrease along this gradient, in accordance with relationships reported at a global scale. The data show an increase in delta N-15 with soil depth, although it remained constant or sharply decreased between the B- and the C-horizon at most sites. The natural abundance of N-15 in the forest floor (FF), in the B-horizon and in the C-horizon averaged 2.2 +/- 0.9 parts per thousand, 6.2 +/- 1.2 parts per thousand and 4.9 +/- 2.0 parts per thousand, respectively while total soil profile delta N-15 ranged from 3.8 parts per thousand to 7.4 parts per thousand. Contrary to our hypothesis, soil delta N-15 was poorly correlated with climate, vegetation and most soil metrics. As a consequence, there was no spatial gradient in soil delta N-15 values and in the magnitude of increase in delta N-15 with soil depth across the study area. Soil C:N ratio was the only variable significantly correlated with soil delta N-15. Multivariate models including the C:N ratio explained 47%, 60% and 36% of the inter-sites delta N-15 variation in B-horizon, C-horizon and total soil, respectively. In contrast with global scale studies, which have reported higher soil delta N-15 at sites with low soil C:N ratio, the relationship between these two variables was positive across the study area. The possible influence of ecto-myccorhizal association on this pattern is discussed. Overall, our data show that soil delta N-15 is controlled by complex mechanisms influenced by several variables with potential antagonist effects. Climate and most soil metrics appear to have no direct influence in the cold and N-limited forest ecosystems studied here and soil C:N ratio can affect soil delta N-15 in an opposite manner to what has been commonly observed.