ALBERT

Description: As Arctic regions warm, the large organic carbon pool stored in permafrost becomes increasingly vulnerable to thaw and decomposition. The transfer of newly mobilized carbon to the atmosphere and its potential influence upon climate change will largely depend on the reactivity and subsequent fate of carbon delivered to aquatic ecosystems. Dissolved organic carbon (DOC) is a key regulator of aquatic metabolism and its biodegradability will determine the extent and rate of carbon release from aquatic ecosystems to the atmosphere. Knowledge of the mechanistic controls on DOC biodegradability is however currently poor due to a scarcity of long-term data sets, limited spatial coverage of available data, and methodological diversity. Here, we performed parallel biodegradable DOC (BDOC) experiments at six Arctic sites (16 experiments) using a standardized incubation protocol to examine the effect of methodological differences used as common practice in the literature. We further synthesized results from 14 aquatic and soil leachate BDOC studies from across the circum–arctic permafrost region to examine pan-Arctic trends in BDOC. An increasing extent of permafrost across the landscape resulted in higher BDOC losses in both soil and aquatic systems. We hypothesize that the unique composition of permafrost-derived DOC combined with limited prior microbial processing due to low soil temperature and relatively shorter flow path lengths and transport times, resulted in higher overall terrestrial and freshwater BDOC loss. Additionally, we found that the fraction of BDOC decreased moving down the fluvial network in continuous permafrost regions, i.e. from streams to large rivers, suggesting that highly biodegradable DOC is lost in headwater streams. We also observed a seasonal (January–December) decrease in BDOC losses in large streams and rivers, but no apparent change in smaller streams and soil leachates. We attribute this seasonal change to a combination of factors including shifts in carbon source, changing DOC residence time related to increasing thaw-depth, increasing water temperatures later in the summer, as well as decreasing hydrologic connectivity between soils and surface water as the seasons progress. Our results suggest that future, climate warming-induced shifts of continuous permafrost into discontinuous permafrost regions could affect the degradation potential of thaw-released DOC as well as its variability throughout the Arctic summer. We lastly present a recommended standardized BDOC protocol to facilitate the comparison of future work and improve our knowledge of processing and transport of DOC in a changing Arctic.