An Inter-Disciplinary Approach to Assess Adaptation Processes Resulting From a Long-Term Drainage Disturbance of a Permafrost Ecosystem

Abstract:

Hydrology plays a pivotal role for the sustainability of the permafrost carbon reservoir in the Northern high latitudes under climate change. Changes in moisture conditions may significantly carbon pools and fluxes, and in addition trigger secondary shifts in biogeochemical and biogeophysical ecosystem properties, with feedbacks to the carbon cycle. The net impact of hydrologic disturbance, e.g. as a result of changes in drainage conditions, on the links between carbon processes and climate is therefore highly uncertain.

This study presents findings from an interdisciplinary experiment established on the floodplain of the Kolyma River near Chersky, Northeast Siberia. Parts of our study site have been artificially drained by ditches since 2004, simulating the formation of a channel system triggered by the thawing of ice wedges in ice-rich permafrost. Observations from this area are directly compared to data from a nearby undisturbed reference site. CO2 and CH4 flux measurements are available from 2 eddy-covariance towers operated year-round, and a distributed array of soil chambers. Additional observations target e.g. microbial and vegetation community structures, radiocarbon signals, nutrient availability and seasonal dynamics in phenology.

Drainage has lowered average soil water levels by about 20cm, resulting in systematic shifts in e.g. soil temperature regime and snow cover. These dryer and warmer conditions have shifted the vegetation structure away from the formerly dominating cotton grasses towards tussock-forming sedges and shrubs. Microbial communities were found to have adapted to the new environment, e.g. concerning the dominating orders of methanogenetic archaea. Radiocarbon signals indicate that the drainage has unlocked older carbon pools from deeper layers. The combined effects of these changes reduce the CO2 uptake and significantly lower the CH4 emissions in the drained area, resulting in a higher net carbon sink compared to the reference. Seasonal dynamics of these shifts were found to be complex, particularly concerning the role of the shoulder seasons. Based on the altered functional relationships between environmental drivers and carbon fluxes, we expect that drainage will systematically affect the trajectories of carbon pools and fluxes under future climate change.