Shallow snowpack and early snowmelt reduce nitrogen availability in the northern hardwood forest

In seasonally snow-covered ecosystems such as northern hardwood forests of the northeastern U.S., spring snowmelt is a critical transition period for plant and microbial communities, as well as for the biogeochemical cycling of nitrogen (N). However, it remains unknown how shifting snowmelt dynamics influence soil and plant processing and uptake of N in these forests, which are experiencing reductions in N availability relative to demand, a process known as oligotrophication. We characterized the role of changing spring snowmelt timing on root production and N pools and fluxes by manipulating snowmelt timing along a climate elevation gradient at the Hubbard Brook Experimental Forest in New Hampshire. We manually halved or doubled snow water equivalent (SWE) in experimental plots in March of 2022 and 2023 to accelerate or delay by an average of one week, respectively, the onset of spring snowmelt. Earlier snowmelt led to reduced snowpack depth and duration, as well as deeper, more sustained soil frost during the snowmelt period in 2022, but soil freezing did not occur in 2023. Soil nitrate and net nitrification rates were significantly lower with shallower snowpack and earlier snowmelt compared to plots with deeper snow and later snowmelt. Shallower snowpack and early snowmelt were also associated with decreased foliar N concentrations and δ¹⁵N values, indications that earlier snowmelt contributes to lower N availability relative to plant N uptake and demand. Our study provides evidence that early snowmelt resulting from shallower snowpack contributes to N oligotrophication, primarily through impacts on soil nitrate supply and uptake of N by trees.