Effects of snow depth and soil freezing on nitrogen cycling in deciduous vs. coniferous temperate forests

Background/Question/Methods

In the northern hardwood forest zone, climate change is projected to result in both decreasing snow cover and replacement of evergreen conifers by deciduous angiosperm tree species. Previous studies have identified increased soil freezing due to reduced snow cover as a significant disruptor of nitrogen (N) cycling in deciduous forests of the northeastern USA. We examined snow cover effects on nitrogen cycling in forests dominated by sugar maple (Acer saccharum) and red pine (Pinus resinosa) growing on sandy soils in the eastern Upper Peninsula of Michigan, USA. We hypothesized that the sugar-maple-dominated ecosystem would be more susceptible to N cycle disruptions, largely due to the more open N cycle, bare winter canopy and thinner forest floor layer. To test this hypothesis, we manipulated snow cover in experimental plots (control, augmented, removed) at three replicate sites per ecosystem. At each, we monitored soil temperatures at depths of 5, 20 and 45 cms. We used zero-tension lysimeters to capture water moving out of the forest floor, as well as from the E and B horizons. Soil nitrous oxide (N₂O) fluxes were estimated using both gas wells deployed from 0 to 100 cm depth, as well as from static chambers at the soil surface.

Results/Conclusions

Snow removal produced greater soil freezing (colder temperatures and greater depth of frost) in sugar maple stands. In red pine stands, dissolved organic N dominated soluble N pools, with no change in concentrations across treatments. In contrast, in the sugar maple stands, snow removal caused large increases in soluble N concentrations, as well as significant increases in the proportion of soluble N in inorganic forms. Despite the strong increase in solution N concentrations in the snow removal plots, the hydrological effects of snow manipulation exerted the greatest impact on N leaching losses; total N losses increased from removal to control to augmentation treatments. Nitrate moving to deeper soil horizons in snow-removal plots under sugar maple provided substrate for denitrification, resulting in elevated soil N₂O concentrations at depth as well as greater rates of surface N₂O efflux. In contrast, N₂O fluxes in red pine stands were unaffected by snow manipulation. These results demonstrate that the impacts of changing snow cover are ecosystem specific: N cycling in conifer ecosystems appears to be more strongly buffered against declines in snow cover due to the relatively closed N cycle and the insulating properties of the evergreen canopy and thick forest floor.