PNNL

Abstract

Topography and canopy cover influence ground temperature in warming permafrost landscapes, yet the small-scale heterogeneities of soil temperatures introduced at the meter-scale and subsequent impacts to carbon dynamics are understudied. Buffering of permafrost-affected soils against warming air temperatures in boreal forests reflects the degree and type of canopy cover where open cover results in increased ground warming compared with closed cover. We sampled a hillslope catena transect in a discontinuous permafrost zone near Fairbanks, Alaska to test the small-scale (1 to 3 meter) impacts of slope position and cover type on soil organic matter composition. Mineral active layer samples were collected from 19 to 60 cm depths in backslope, low backslope, and footslope positions. We examined soil mineralogical compositions, soil moisture, total carbon and nitrogen content, and organic mat thickness in conjunction with an assessment of soil organic matter (SOM) composition using Fourier-transform ion Cyclotron Resonance Mass Spectrometry (FT-ICR-MS). Soils at the footslope position had a higher relative contribution of lignin-like compounds while backslope soils had more aliphatic and condensed aromatic compounds. The effect of open vs closed tree canopy cover varied with slope position. On the backslope, we found higher oxidation of molecules under open cover compared with closed cover, indicating an effect of warmer soil temperature on decomposition. Little to no effect of canopy was observed for soils at the footslope position attributed, in part, to the strong impact of soil moisture content in soil organic carbon (SOC) dynamics in the water-gathering footslope position. The thin organic mat under open cover on the backslope position may have contributed to differences in soil temperature and thus carbon oxidation under open vs closed canopy. Here, the thinner organic mat did not appear to buffer the underlying soil against warm season air temperatures and thus increased carbon decomposition as indicated by higher oxidation of carbon molecules and a lower contribution of simple molecules under open cover compared with closed. Our findings suggest that role of canopy cover in carbon dynamics varies as a function of landscape position and soil properties. Condition-specific heterogeneity of SOM composition under open and closed canopy cover highlights the protective effect of canopy for soils on backslope positions.