The N goes up, the N goes down: The effect of an arctic sedge on transport of soil nitrogen

Background/Question/Methods

Moist acidic tussock tundra is a world of highly-organic soils underlain by permafrost and dominated by the tussock-forming sedge Eriophorum vaginatum. Here, available soil nitrogen (N) is a precious resource for plants and microbes. In contrast to many other plants in nutrient-limiting environments, E. vaginatum is highly successful without maximizing its root surface area to volume ratio. Instead, it has distinctive, long, fast-growing, non-branching, non-mycorrhizal roots. This makes the success of E. vaginatum somewhat mysterious, but also provides a perfect system for the study of nutrient translocation in the soil profile. To better understand the role of E. vaginatum in vertical N movement, we used tussock microcosms (10 cm x 10 cm x 20 cm). The microcosms were maintained under identical conditions save for one factor: presence/absence of light. They were then divided into three layers based on soil characteristics. Labeled ammonium (¹⁵NH₄Cl) was injected into the lowest layer. Carbon (C) and N were then measured in plants, microbial biomass, and soil.

Results/Conclusions

The microcosms kept in the light had more roots per cm³ soil at the surface than those moved to the dark, while those in the dark had more roots at depth than those in the light, though variation was high. Differences between the layer of ¹⁵NH₄ injection (“Low”) and the surface horizon (“High”) were compared. The extractable soil N gradient (Low/High) is greater in microcosms that were grown in the light; microcosms grown in the dark showed a greater similarity between the Low layer and the surface horizon. N was higher at depth in both treatments. Microbial biomass C also showed a greater difference between the Low and High horizons in microcosms grown in the light, being higher at the surface. Differences in microbial N were insignificant.  Vertical N fluxes in tussock soil vary depending on E. vaginatum activity. This study will be used to understand seasonal changes in N cycling in tussock soil.