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The northern circumpolar permafrost region is experiencing considerable warming due to climate change, which is allowing agricultural production to expand into regions of discontinuous and continuous permafrost. The conversion of forests to arable land might further enhance permafrost thaw and affect soil organic carbon (SOC) that had previously been protected by frozen ground. The interactive effect of permafrost abundance and deforestation on SOC stocks has hardly been studied. In this study, soils were sampled on 18 farms across the Yukon on permafrost and non‐permafrost soils to quantify the impact of land‐use change from forest to cropland and grassland on SOC stocks. Furthermore, the soils were physically and chemically fractionated to assess the impact of land‐use change on different functional pools of SOC. On average, permafrost‐affected forest soils lost 15.6 ± 21.3% of SOC when converted to cropland and 23.0 ± 13.0% when converted to grassland. No permafrost was detected in the deforested soils, indicating that land‐use change strongly enhanced warming and subsequent thawing. In contrast, the change in SOC at sites without permafrost was not significant but had a slight tendency to be positive. SOC stocks were generally lower at sites without permafrost under forest. Furthermore, land‐use change increased mineral‐associated SOC, while the fate of particulate organic matter (POM) after land‐use change depended on permafrost occurrence. Permafrost soils showed significant POM losses after land‐use change, while grassland sites without permafrost gained POM in the topsoil. The results showed that the fate of SOC after land‐use change greatly depended on the abundance of permafrost in the pristine forest, which was driven by climatic conditions more than by soil properties. It can be concluded that in regions of discontinuous permafrost in particular, initial conditions in forest soils should be considered before deforestation to minimize its climate impact. [ABSTRACT FROM AUTHOR]

Many northern lake-rich regions are undergoing pronounced hydrological change, yet inadequate knowledge of the drivers of these landscape-scale responses hampers our ability to predict future conditions. We address this challenge in the thermokarst landscape of Old Crow Flats ( OCF) using a combination of remote sensing imagery and monitoring of stable isotope compositions of lake waters over three thaw seasons (2007-2009). Quantitative analysis confirmed that the hydrological behavior of lakes is strongly influenced by catchment vegetation and physiography. Catchments of snowmelt-dominated lakes, typically located in southern peripheral areas of OCF, encompass high proportions of woodland/forest and tall shrub vegetation (mean percent land cover = ca. 60%). These land cover types effectively capture snow and generate abundant snowmelt runoff that offsets lake water evaporation. Rainfall-dominated lakes that are not strongly influenced by evaporation are typically located in eastern and northern OCF where their catchments have higher proportions of dwarf shrub/herbaceous and sparse vegetation (ca. 45%), as well as surface water (ca. 20%). Evaporation-dominated lakes, are located in the OCF interior where their catchments are distinguished by substantially higher lake area to catchment area ratios ( LA/ CA = ca. 29%) compared to low evaporation-influenced rainfall-dominated (ca. 10%) and snowmelt-dominated (ca. 4%) lakes. Lakes whose catchments contain >75% combined dwarf shrub/herbaceous vegetation and surface water are most susceptible to evaporative lake-level drawdown, especially following periods of low precipitation. Findings indicate that multiple hydrological trajectories are probable in response to climate-driven changes in precipitation amount and seasonality, vegetation composition, and thermokarst processes. These will likely include a shift to greater snowmelt influence in catchments experiencing expansion of tall shrubs, greater influence from evaporation in catchments having higher proportions of surface water, and an increase in the rate of thermokarst lake expansion and probability of drainage. Local observations suggest that some of these changes are already underway. [ABSTRACT FROM AUTHOR]