Soil carbon and nitrogen responses to snow removal and concrete frost in a northern coniferous forest

Abstract

Climate change in northeastern North America is resulting in warmer winters with reduced snow accumulation. Soils under a thin snowpack are more likely to experience freeze–thaw cycles, disrupting carbon (C) and nitrogen (N) transformations. We conducted a 2 year snow removal experiment in Maine to study the effects of soil freezing on soil C and N processes. O horizon soils were sampled during winter and spring of 2015 and 2016, and they were analyzed for labile inorganic N and water-extractable organic carbon (WEOC) concentrations, specific ultraviolet absorbance (SUVA254), and potential net N mineralization. The winter of 2015 was cold and snowy, whereas 2016 was warm with a shallow, short-term snowpack. Snow removal caused the soils to freeze, but winter rain-on-soil events in 2015 resulted in the formation of concrete frost, as opposed to granular frost in 2016. Concrete frost increased soil ammonium (NH4+-N) and WEOC concentrations and decreased SUVA254, which we attribute to microbial cell lysis. In contrast, granular frost did not alter soil nutrient concentrations, reflecting limited microbial distress. Our study demonstrates that moisture content influences the intensity of soil freezing, highlighting the importance of snowpack depth and winter rain events in regulating winter and spring biogeochemical processes and nutrient availability.

Northern forest winters have lost cold, snowy conditions that are important for ecosystems and human communities

“…, Patel et al. ). Our results indicate that the frequency of potential soil freezing days (i.e., Bare Ground Ice Days/Frozen Ground Days when T max < 0°C and snow depth = 0 mm) that might affect soil physical, biological, and biogeochemical processes and properties did not significantly change over our 100‐year time series, and in fact, were somewhat rare occurrences (Table ).…”

“…, Patel et al. ). While numerous studies have reported both earlier snowmelt (Dyer and Mote , Vincent et al.…”

“…), altering soil carbon and nitrogen availability (Patel et al. ), reducing groundwater permeability, and increasing surface water runoff (Shanley and Chalmers ). Mud Days may also become more frequent during the months of March and April as the ecosystem transitions between the growing and dormant seasons and the timing of snowmelt advances earlier in the year.…”

“…Indicators that featured both temperature and snowpack thresholds included Bare Ground Ice Days and Bare Ground Thaw Days. A Bare Ground Ice Day illustrates a situation where the combination of an absent snowpack plus cold air temperatures might result in soil freezing, microbial and fine root mortality, and nutrient leaching , Campbell et al 2005, Cleavitt et al 2008, Tatariw et al 2017, Patel et al 2018, Sanders-DeMott et al 2018a. By contrast, a Bare Ground Thaw Day exemplifies a scenario in which the absence of snow plus warm air temperatures might result in conditions that reduce access to winter forest harvest sites (Rittenhouse and Rissman 2015), expose hiking trails to erosion risk, and limit opportunities for skiing, snowmobiling, and other winter sports (Scott et al 2008).…”

“…When coupled with increasing trends of winter rainfall, such mid-winter Mud Days can result in saturated soils. If refrozen, concrete frost formation can occur (Fahey and Lang 1975, Proulx and Stein 1997, Tatariw et al 2017), altering soil carbon and nitrogen availability (Patel et al 2018), reducing groundwater permeability, and increasing surface water runoff (Shanley and Chalmers 1999). Mud Days may also become more frequent during the months of March and April as the ecosystem transitions between the growing and dormant seasons and the timing of snowmelt advances earlier in the year.…”