Evidence for late winter biogeochemical connectivity in permafrost soils

Abstract

The permafrost active layer is a key supplier of soil organic carbon and mineral nutrients to Arctic rivers. In the active layer, sites of soil-water exchange are locations for organic carbon and nutrient mobilization. Previously these sites were considered as connected during summer months and isolated during winter months. Whether soil pore waters in active layer soils are connected during shoulder seasons is poorly understood. In this study, exceptionally heavy silicon isotope compositions in soil pore waters show that during late winter, there is no connection between isolated pockets of soil pore water in soils with a shallow active layer. However, lighter silicon isotope compositions in soil pore waters reveal that soils are biogeochemically connected for longer than previously considered in soils with a deeper active layer. We show that an additional 21% of the 0–1 m soil organic carbon stock is exposed to soil - water exchange. This marks a hot moment during a dormant season, and an engine for organic carbon transport from active layer soils. Our findings mark the starting point to locate earlier pathways for biogeochemical connectivity, which need to be urgently monitored to quantify the seasonal flux of organic carbon released from permafrost soils.

Permafrost Carbon: Progress on Understanding Stocks and Fluxes Across Northern Terrestrial Ecosystems

“…Shoulder seasons, the transition periods close to the growing season (i.e., spring and fall), may be particularly important. For example, in fall and early winter, deeper soils are often thawed despite soils at the surface being frozen, boosting decomposition of deeper (and potentially older) soil organic matter while plant activity remains limited (Euskirchen et al, 2017;Pedron et al, 2022;Schuur et al, 2009); increased connectivity with groundwater pathways may enhance export (Hirst et al, 2023). As the soils freeze and thaw during the "zero-curtain" window (Outcalt et al, 1990), microbial activity can persist at low rates even when average soil temperatures are at or below zero (Clein & Schimel, 1995;Öquist et al, 2009).…”