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ABSTRACT The intensity, duration, and spatial distribution of frozen soil influences hydrologic flow paths, soil biogeochemistry, and slope geomorphology. In cold regions, where the ground thermal regime is controlled by the seasonal snowpack, representation of the snowpack in models simulating seasonally frozen ground is required and leads to significant improvements in soil temperature estimates. With long‐term climate and ground temperature observations, Niwot Ridge, a seasonally snow‐covered alpine catchment in the headwaters of the Boulder Creek watershed, serves as an ideal location for analyzing frozen ground under a changing climate. In this study, we use a coupled thermo‐hydrologic model to provide novel perspectives on cryosphere research at Niwot Ridge. We project how end‐of‐21st‐century changes in Front Range air temperature, snowfall, and snowpack cold content will influence the ground thermal regime, including seasonally frozen ground and permafrost, in comparison to the 1952–1970 period. In projections of seasonally frozen ground, the model predicts two additional months of unfrozen soils by the end of the 21st century compared with the 1952–1970 time period, which is expected to lead to an increase in the number of days favorable for microbial respiration. Our permafrost analysis supports the occurrence of permafrost above 3800 m with active layer thickness 1.8 m (1952–1970), 2.2 m (2001–2013), and 38 m by end of 21st century. The simulated deep soil thaw over the last several decades (1970–2020) is small compared with projected deep soil thaw through the current century, which is expected to lead to reductions in frost cracking. 

Abstract Microbial processing of atmospheric nitrogen (N) deposition regulates the retention and mobilization of N in soils, with important implications for water quality. Understanding the links between N deposition, microbial communities, N transformations, and water quality is critical as N deposition shifts toward reduced N and remains persistently high in many regions. Here, we investigated these connections along an elevation transect in the Colorado Front Range. Although rates of N deposition and pools of extractable N increased down the elevation transect, soil microbial communities and N transformation rates did not follow clear elevational patterns. The subalpine microbial community was distinct, corresponding to a high C:N ratio and low pH, while the microbial communities at the lower elevation sites were all very similar. Net nitrification, mineralization, and nitrification potential rates were highest at the Plains (1,700 m) and Montane (2,527 m) sites, suggesting that these ecosystems mobilize N. In contrast, the net immobilization of N observed at the Foothills (1,978 m) and Subalpine (3,015 m) sites suggests that these ecosystems retain N deposition. The contrast in N transformation rates between the plains and foothills, both of which receive elevated N deposition, may be due to spatial heterogeneity not captured in this study and warrants further investigation. Stream N concentrations from the subalpine to the foothills were consistently low, indicating that these soils are currently able to process and retain N deposition, but this may be disrupted if drought, wildfire, or land‐use change alter the ability of the soils to retain N. 

Abstract Considerable debate revolves around the relative importance of rock type, tectonics, and climate in creating the architecture of the critical zone. We demonstrate the importance of climate and in particular the rate of water recharge to the subsurface, using numerical models that incorporate hydrologic flowpaths, chemical weathering, and geomorphic rules for soil production and transport. We track alterations in both solid phase (plagioclase to clay) and water chemistry along hydrologic flowpaths that include lateral flow beneath the water table. To isolate the role of recharge, we simulate dry and wet cases and prescribe identical landscape evolution rules. The weathering patterns that develop differ dramatically beneath the resulting parabolic interfluves. In the dry case, incomplete weathering is shallow and surface parallel, whereas in the wet case, intense weathering occurs to depths approximating the base of the bounding channels, well below the water table. Exploration of intermediate cases reveals that the weathering state of the subsurface is strongly governed by the ratio of the rate of advance of the weathering front itself controlled by the water input rate, and the rate of erosion of the landscape. The system transitions between these end‐member behaviours rather abruptly at a weathering front speed ‐ erosion rate ratio of approximately 1. Although there are undoubtedly direct roles for tectonics and rock type in critical zone architecture, and yet more likely feedbacks between these and climate, we show here that differences in hillslope‐scale weathering patterns can be strongly controlled by climate.