Pulse-labeling studies of carbon cycling in arctic tundra ecosystems: Contribution of photosynthates to soil organic matter

[1] To increase our understanding of carbon (C) cycling and storage in soils, we used ¹⁴C to trace C from roots into four soil organic matter (SOM) fractions and the movement of soil microbes in arctic wet sedge and tussock tundra. For both tundra types, the proportion of ¹⁴C activity in the soil was 6% of the total ¹⁴C-CO₂ taken up by plants at each of the four harvests conducted 1, 7, 21, and 68 days after labeling. In tussock tundra, we observed rapid microbial transformation of labile C from root exudates into more stable SOM. In wet sedge tundra, there appears to be delayed or indirect microbial use of root exudates. The net amount of ¹⁴C label transferred to SOM by the end of the season in both tundra types was approximately equal to the amount transferred to soils 1 day after labeling, suggesting that transfer of ¹⁴C tracer from roots to soils continued through the growing season. Overall, C inputs from living roots contributes 24 g C m⁻² yr⁻¹ in tussock tundra and 8.8 g C m⁻² yr⁻¹ in wet sedge tundra. These results suggest rapid belowground allocation of C by plants and subsequent incorporation of much of this C into storage in the SOM.