Rapid climate warming may already be releasing the vast stocks of carbon that have been locked away in cold Arctic tundra soils for thousands of years. When these soils warm, microbes speed up the process of decomposition which breaks down organic plant material and releases the greenhouse gas, carbon dioxide (CO2), through respiration. When CO2 reaches the atmosphere, it causes further climate warming. However, this same microbial breakdown of organic matter also releases nitrogen which acts as a fertilizer to increase plant growth rates. Increased plant growth and photosynthesis draws CO2 out of the air, potentially counteracting soil carbon losses. In turn, increased plant growth shuttles more carbon into soils, which may stimulate microbes to attack soil organic matter to acquire nitrogen, resulting in net carbon losses through a process called priming. Since nitrogen uptake rates differ among plant species, they may have different potentials to drawdown CO2 from the air and release CO2 through priming.

We conducted field and laboratory experiments to quantify the interactions of biological, chemical, and physical controls on soil carbon stability. Using isotope-tracing techniques, we found that the amount of priming depends on vegetation type and soil nitrogen concentrations. Under non-limiting nitrogen conditions, increased carbon inputs from plant growth primed soil organic matter decomposition in tussock soils, but did not prime shrub soils. If warming enhances decomposition and nitrogen availability, increasing shrub abundance may dampen soil carbon losses. This hypothesis was supported by results from a field tracer experiment, where labile carbon inputs reduced loss of soil organic matter stocks by increasing the efficiency with which microbes transformed soil carbon into biomass. The quality and mobility of carbon remaining in the soil solution will determine the overall strength of the soil carbon sink.

Permafrost thaw changes the flowpaths through which surface and subsurface water flows, influencing the rate at which dissolved organic carbon moves through Arctic watersheds. Increases in flow between soil horizons (organic-mineral) and landscape positions (hillslope-riparian) could give microbes less time to process this carbon, and change its chemical composition. To quantify soluble carbon transport rates, we used a chemical tracer, and assessed dissolved organic matter chemistry along a hillslope.  While pore waters collected from the organic horizon were associated with plant-derived molecules, those collected from permafrost-influenced mineral horizons had a microbial origin. We found greater chemical diversity near streams compared to the hillslope, implying that a more complex mix of carbon forms could reach streams as permafrost thaws. Changing carbon chemistry could have important effects on the food webs within these streams.

In combination, our findings revealed how changing plant communities, nitrogen availability, and dissolved organic matter chemistry regulate the fate of soil carbon, and highlight the complex interactions required to improve predictions of the magnitude and direction of the Arctic carbon-climate feedback.

Last Modified: 09/28/2019
Modified by: Matthew D Wallenstein