• Energy Research

Abstract Permafrost is vulnerable to rapid changes in climate, and increasing air temperatures have recently resulted in the increase of active layer thickness, thaw subsidence and warming of the underlying permafrost. Such changes have important implications for geotechnical properties and the stability of infrastructures in permafrost-affected areas. Many studies focus on the sensitivity of the active layer with respect to changes in climate conditions, but few assess the sensitivity of active layer thermal properties in relation to sediment types and soil water contents, and the importance of direct measurements of thermal property sensitivity with respect to soil water content compared to default physical relationships incorporated in process-based models. In this study, we use on-site data and samples to measure thermal conductivity (TC) at different gravimetric water/ice contents (GWC) in frozen and thawed permafrost. The samples, obtained from an emerged delta and an alluvial fan in the Zackenberg Valley, NE Greenland, are characterized by contrasting grain-size distribution and mineralogy. We calibrated a coupled heat and water transfer model, the “CoupModel”, to simulate permafrost temperatures at two sites on the delta. The sites have different snow depth characteristics and were simulated using both observed and default values of TC, and observed liquid soil water content. The results show that depth- and sediment type-specific TC values are crucial for a successful model simulation, and that transfer function derived values of TC are useful for modeling permafrost temperatures as long as site- and depth-specific grain size distribution and ice contents are defined. A thicker snow pack increased ground surface temperatures and resulted in a 1 °C higher annual mean ground temperature at the depth of zero annual amplitude. Permafrost temperatures increased by 1.5 °C and 3.5 °C at the depth of 18 m with 3 °C and 6 °C ground surface warming, but warming combined with increased soil water content had no important additional effect on the thermal regime when ground surface temperatures were prescribed as upper boundary conditions. Precipitation in the form of snow, however, may have a larger effect on ground temperatures directly, due to the surface temperature changes, than will the subsequent changes in thermal properties following increase in soil water content.

Abstract Future Arctic tundra primary productivity and vegetation community composition will partly be determined by nitrogen (N) availability in a warmer climate. N mineralization rates are predicted to increase in winter and summer, but because N demand and –mobility varies across seasons, the fate of mineralized N remains uncertain. N mineralized in winter is released in a “pulse” upon snowmelt and soil thaw, with the potential for lateral redistribution in the landscape. In summer, the release is into an active rhizosphere with high local biological N demand. In this study, we investigated the ecosystem sensitivity to increased lateral N input and near-surface warming, respectively and in combination, with a numerical ecosystem model (CoupModel) parameterized to simulate ecosystem biogeochemistry for a tundra heath ecosystem in West Greenland. Both model and measurements indicated that plants were poor utilizers of increased early-season lateral N input, indicating that higher winter N mineralization rates may have limited influence on plant growth and carbon (C) sequestration for a hillslope ecosystem. The model further suggested that, although deciduous shrubs were the plant type with overall most lateral N gain, evergreen shrubs had a comparative advantage utilizing early-season N. In contrast, near-surface summer warming increased plant biomass and N uptake, moving N from soil to plant N pools, and offered an advantage to deciduous plants. Neither simulated high lateral N fluxes nor near-surface soil warming suggests that mesic tundra heaths will be important sources of N2O under warmer conditions. Our work highlights how winter and summer warming may play different roles in tundra ecosystem N and C budgets depending on plant community composition.

Understanding N budgets of tundra ecosystems is crucial for projecting future changes in plant community composition, greenhouse gas balances and soil N stocks. Winter warming can lead to higher tundra winter nitrogen (N) mineralization rates, while summer warming may increase both growing season N mineralization and plant N demand. The undulating tundra landscape is inter-connected through water and solute movement on top of and within near-surface soil, but the importance of lateral N fluxes for tundra N budgets is not well known. We studied the size of lateral N fluxes and the fate of lateral N input in the snowmelt period with a shallow thaw layer, and in the late growing season with a deeper thaw layer. We used 15N to trace inorganic lateral N movement in a Low-arctic mesic tundra heath slope in West Greenland and to quantify the fate of N in the receiving area. We found that half of the early-season lateral N input was retained by the receiving ecosystem, whereas half was transported downslope. Plants appear as poor utilizers of early-season N, indicating that higher winter N mineralization may influence plant growth and carbon (C) sequestration less than expected. Still, evergreen plants were better at utilizing early-season N, highlighting how changes in N availability may impact plant community composition. In contrast, later growing season lateral N input was deeper and offered an advantage to deeper-rooted deciduous plants. The measurements suggest that N input driven by future warming at the study site will have no significant impact on the overall N2O emissions. Our work underlines how tundra ecosystem N allocation, C budgets and plant community composition vary in their response to lateral N inputs, which may help us understand future responses in a warmer Arctic. (Less)