Carbon release from a warmer, wet, organic-rich active layer across a gradient of polygonal tundra in the Arctic may provide an important feedback to the atmosphere

Background/Question/Methods

Atmospheric warming, which is expected to occur more rapidly in the Arctic compared with lower latitudes, may lead to permafrost degradation, potentially increasing feedbacks of carbon (C) to the atmosphere. While C stored in permafrost has been the focus of much research, we were interested in feedbacks from warming of the active layer that thaws each year. Microbial processing of organic matter in the active layer is controlled by the balance among soil organic matter content, moisture, and temperature, which are influenced by landscape position and plant community composition. We collected active layer soil across a gradient of microtopographic positions in polygonal tundra in Barrow, Alaska. Soils from three depth horizons (organic, mineral, and organic-rich mineral soil at the permafrost boundary) from low- to high-centered polygons, and center to trough microtopography, were incubated at 4, 8, and 12°C to quantify the effects of warming on aerobic C mineralization. We hypothesized that C mineralization would be greatest in the warmest soils with the greatest soil organic matter content.

Results/Conclusions

Total C mineralization and C mineralization rates increased with increasing incubation temperature in all soils. C mineralization rates were generally highest in highly-organic soils, which included the low-lying troughs across all polygons types and the centers of low-centered polygons. C mineralization rates were 6.5 times greater on a soil mass basis and 50% greater on a volume basis in the organic soil horizon than the deeper mineral horizons; the organic layer accounted for roughly half of total active layer soil column C mineralization after adjusting for soil horizon thickness. Highly-organic soils also tended to be most sensitive to warming, with Q10 values nearing 4. This sensitivity could be due to increased moisture in these soils, given that soil C mineralization rates were positively correlated with soil water content (R2 = 0.6). This was surprising given that gravimetric moisture was high in all soils (averaging 200%), and we hypothesize that water content might be correlated with more labile, dissolved organic carbon, which was not measured here. The results highlight the importance of considering the response of the active layer to projected warming in the Arctic, especially organic-rich soil horizons formed by complex interactions between topography, soil moisture, and vegetation community composition.