Winter climate change and the legacy effect of soil frost on carbon and nitrogen fluxes from northern forest soils: A microcosm study

Background/Question/Methods

In northern forests, soil frost is a natural event that occurs when there is insufficient snowpack accumulation to insulate the soil. However, climate models predict a reduction in snowpack depth and duration by the end of the 21^st century in the northeastern U.S., which could alter the severity and frequency of soil frost. We conducted a microcosm experiment to determine if there is a legacy effect of winter soil frost on carbon (C) and nitrogen (N) fluxes during snowmelt, a biologically important period. Organic soils were collected from an Acer saccharum (sugar maple) forest and a high elevation Picea rubens - Abies balsamea (red spruce - balsam fir) forest at the Hubbard Brook Experimental Forest, NH. After incubating in situ for 3 months, the cores were subjected to one of three different temperature treatments (5, -2, and -15º C) in the lab for one week. The cores were then transferred to a 5º C refrigerator to simulate spring snowmelt. Soil leachate was collected and analyzed for nitrate (NO₃^-), ammonium (NH₄⁺), total dissolved N (TDN), dissolved organic nitrogen (DON), and dissolved organic carbon (DOC). Trace gas fluxes (CO₂, CH₄, and N₂O) were also measured using the closed chamber method.

Results/Conclusions

Leaching of NO₃^-, NH₄⁺, TDN, and DON were greater from sugar maple soils than spruce – fir soils. Relative to the unfrozen soils (5º C), mild soil frost (-2º C) increased N leaching from sugar maple soils, while it reduced leaching from the spruce – fir soils. However, severe soil frost (-15º C) reduced N leaching from both forest soil types by one to two orders of magnitude. DOC leaching was not influenced by forest type. The only treatment response for DOC leaching was the severe soil frost where leaching was reduced by an order of magnitude relative to the other treatments. All treatments resulted in a net loss of DOC from soils. A budget calculated from known inputs of N in snowmelt and leaching losses suggests net losses of NO₃^-, NH₄⁺, TDN, and DON in response to the 5 and -2º C treatments and net retention in response to the -15ºC treatment. There was an interaction between the effects of frost severity and forest type on losses of N during snowmelt. These results highlight the complex relationship between climate change and belowground N and C cycling in northern forests.