Snowfall amounts, duration of snow on the ground, and snowpack thickness are all components of climate that are projected to change in the next few decades, as climate warms and snowfall declines. Previous work by soil geographers had demonstrated much stronger soil development in areas of naturally high snow fall associated with lake-effect snow belts in the Great Lakes Region.  This led us to hypothesize that changes in winter snow cover could affect soil processes in ways that could be important in terms of interactions of soils with the global climate system.  To test this hypothesis we installed instrumentation (see Figure 1) at six sites in the Upper Peninsula of Michigan in order to measure 1) fluxes of carbon (C) and nitrogen (N) in water percolating through soil, and 2) fluxes of greenhouse gases from the soil to the atmosphere.  Three of the sites were under deciduous forest vegetation, which is projected to increase across the region with climatic warming; three of the sites were under coniferous vegetation, which is forecast to decline across the region with climatic warming.  Within each site we set up experimental plots with three treatments: 1) control plots which received natural ambient snowfall (Figure 2), 2) augmentation plots which received double the natural snowfall (Figure 3), and 3) removal plots which received no snow during the winter (Figure 4).  We maintained these treatments through three winters (2012-2013, 2013-2014 and 2014-2015) and measured the impacts of the treatments on movement of C and N in soil solutions, as well as emissions of greenhouse gases from soils to the atmosphere.

                Overall, we found that snow manipulations had marked effects on the cycling of C and N and on soil greenhouse gas emissions.  We found that over the course of the experiment transport of C from surface plant litter (dead leaves, twigs etc) to deep soil horizons was about 20% higher than the controls in augmented plots, and about 20% lower in removal plots.  This finding highlights the importance of the brief (2-3 week) period of snowmelt in the spring to soil C cycling.  We also found marked effects of snow manipulation on soil N cycling in the deciduous forest ecosystem, and very little impact in the coniferous forest ecosystem.  In the deciduous forest ecosystem, even mild soil freezing in the winter elicited a sharp increase in the production and movement of nitrate (NO₃^-) through soil water in the spring.  When this NO₃^- reached deeper soil layers (around 1 meter deep) microroganisms in the soil converted this NO₃^- to nitrous oxide (N₂O) which diffused back up through the soil profile and was released to the atmosphere.  This is important because N₂O is a potent greenhouse gas with about 300 times the heat-trapping capacity of carbon dioxide (CO₂).  After the severely cold winter of 2013-2014 we observed an approximately 6-fold increase in the amount of N₂O released from removal plots compared to the control plots in the deciduous forest.  Interestingly, we found that soil freezing in the coniferous forest sites had little to no impact on soil N cycling. This is new knowledge in the field which has important implications for ecosystem change with a changing climate.  As the climate warms, soil freezing is projected to increase, and conifers are projected to decline and be replaced by deciduous species.  This suggests that N cycle impacts of soil frost may be amplified by forest cover change.

Last Modified: 12/01/2015
Modified by: David E Rothstein