Assessing Nutrient Status Along a Boreal Forest Climate Transect.

In the boreal forest, temperature and precipitation increases are projected to be significant in the next century.  Direct temperature effects have been observed for carbon fluxes associated with boreal forest soil. However, indirect effects on carbon (C) cycling associated with longer term climate change are likely driven by the impact of increased temperature and precipitation on nutrient cycling and weathering processes.  By investigating replicated forest sites along a climate transect, where soil carbon fluxes increase with decreasing latitude, we assessed whether nitrogen (N) and phosphorus (P) availability increase or decrease with climate warming. The study sites, located in eastern Canada, span a mean annual temperature range of 0 to 5.2˚C with sites monitored for soil C, N, and P since 2011.  Total soil N stocks soils were found to increase approximately 20% from the cold to warm region.  With increasing latitude, total litterfall increased with a 3-fold increase in annual litterfall N.  Despite the differences in soil N stocks, the C:N of green needle litterfall was similar across regions on an annual basis.  However, C:N of brown needle litterfall was greater in the cold region compared to the two warmer regions indicating greater N resorption by trees in the cold region and suggesting N limitation compared to the warmer regions.  Greater soil C:P ratios, however, suggest that the warm region may be more P-limited relative to the colder regions. Further analyses of P stocks and fluxes will enable us to confirm whether increased N is coupled with decreased P availability with warming along this climate transect. Results of this study indicate that climate warming induced increases in soil C fluxes are associated with increased N availability and further suggest potential for such increases to be limited by P in a future warmer and wetter climate.