Understanding the carbon-water coupling over permafrost regions is essential to projecting global ecosystem carbon sequestration and water dynamics. Ecosystem water use efficiency (EWUE), defined as the ratio of gross primary productivity (GPP) and evapotranspiration (ET), reflects plant acclimation strategies with varying ecosystem functioning against environmental stress. Yet EWUE change and its potential drivers across the northern permafrost regions remain poorly quantified, hampering our understanding of permafrost carbon-climatefeedback. Here, we compared and analyzed the difference using satellite observations and process based models to estimate the spatio-temporal variations of EWUE in 1982-2018 over northern permafrost regions. Using flux measurements as truth data, satellite-derived EWUE was more reliable than model-based EWUE. Satellite-derived EWUE showed biome-dependent spatial patterns, with a steady temporal trend (0.01 g C mm-1 decade-1, P > 0.05) for spatially averaged EWUE over northern permafrost regions. Carbon dioxide (CO2) concentration and nitrogen deposition positively affected interannual variations of EWUE, while vapor pressure deficit and other climatic factors (i.e., temperature, precipitation, and radiation) negatively controlled EWUE. Compared to satellite-derived EWUE, we found that EWUEs derived from an ensemble of process-based carbon cycle models are overestimated in seven out of ten models, with an increasing trend of 0.11 g C mm-1 decade 1 (P < 0.001) for spatially averaged EWUE of the ensemble mean. The relationships between climatic factors and EWUE are partially misinterpreted in model estimates, especially with overstated CO2 sensitivity and the opposite temperature effect. The fluctuating sensitivities to climate over time and the diminishing effect of CO2 fertilization on gross primary productivity (GPP) may partially explain the discrepancy observed between satellite-derived and model-based estimates of EWUE. Thus, this study calls for caution concerning model-based EWUE and aids in understanding permafrost-climate feedbacks and projections of carbon and water cycles.

It is of prime importance to understand feedbacks due to the release of carbon (C) stored in permafrost soils (permafrost-climate feedback) and direct impacts of climatic variations on permafrost dynamics therefore received considerable attention. However, indirect effects of global change, such as the variation in soil nutrient availability and grazing pressure, can alter soil and surface properties of the Arctic tundra, with the potential to modify soil heat transfers toward the permafrost and impact resilience of Arctic ecosystems. We determined the potential of nutrient availability and grazing to alter soil energy balance using a 16-year split-plot experiment crossing fertilization at different doses of nitrogen (N) and phosphorus (P) with protection from goose grazing. Moss biomass and some determinants of the surface energy budget (leaf area index (LAI), dead vascular plant biomass and albedo) were quantified and active layer thaw depth repeatedly measured during three growing seasons. We measured soil physical properties and thermal conductivity and used a physical model to link topsoil organic accumulation processes to heat transfer. Fertilization increased LAI and albedo, whereas grazing decreased dead vascular plant biomass and albedo. Fertilization increased organic accumulation at the top of the soil leading to drier and more porous topsoil, whereas grazing increased water content of topsoil. As a result, topsoil thermal conductivity was higher in grazed plots than in ungrazed ones. Including these properties into a simulation model, we showed that, after 16 years, nutrient addition tended to shallow the active layer whereas grazing deepened mean July active layer by 3.3 cm relative to ungrazed subplots. As a result of OM accumulation at the surface, fertilization increased permafrost vertical aggradation rate by almost an order of magnitude (up to 5 mm year(-1) instead of 0.7 mm year(-1)), whereas grazing slowed down permafrost aggradation by reducing surface uprising and deepening thaw depth. Synthesis. We demonstrated that long-term grazing and N and P addition, through their impact on vegetation and soil properties have the potential to impact permafrost dynamics to the same extent as contemporary temperature increase in High Arctic polygonal wetlands.

High-latitude climate change has impacted vegetation productivity, composition, and distribution across tundra ecosystems. Over the past few decades in northern Alaska, emergent macrophytes have increased in cover and density, coincident with increased air and water temperature, active layer depth, and nutrient availability. Unraveling the covarying climate and environmental controls influencing long-term change trajectories is paramount for advancing our predictive understanding of the causes and consequences of warming in permafrost ecosystems. Within a climate-controlled carbon flux monitoring system, we evaluate the impact of elevated nutrient availability associated with degraded permafrost (high-treatment) and maximum field observations (low-treatment), on aquatic macrophyte growth and methane (CH4) emissions. Nine aquatic Arctophila fulva-dominated tundra monoliths were extracted from tundra ponds near Utqiagvik, Alaska, and placed in growth chambers that controlled ambient conditions (i.e., light, temperature, and water table), while measuring plant growth (periodically) and CH4 fluxes (continuously) for 12 weeks. Results indicate that high nutrient treatments similar to that released from permafrost thaw can increase macrophyte biomass and total CH4 emission by 54 and 64%, respectively. However, low treatments did not respond to fertilization. We estimate that permafrost thaw in tundra wetlands near Utqiagvik have the potential to enhance regional CH4 efflux by 30%. This study demonstrates the sensitivity of arctic tundra wetland biogeochemistry to nutrient release from permafrost thaw and suggests the decadal-scale expansion of A. fulva-dominant aquatic plant communities, and increased CH4 emissions in the region were likely in response to thawing permafrost, potentially representing a novel case study of the permafrost carbon feedback to warming. Plain Language Summary Over the past half century near the town of Utqiagvik (formerly Barrow) Alaska, plants growing in wetlands have expanded, over the same time period as increases in air/pond temperatures, permafrost thaw, and nutrient availability. Although circumstantial evidence suggests nutrients released from permafrost thaw may have influenced past vegetation expansion and land-atmosphere carbon exchange, direct evidence is lacking. We built a climate and environmentally controlled carbon flux monitoring system to evaluate the impact of nutrient availability on plant growth and CH4 emissions, associated with (1) permafrost thaw and (2) the maximum field-based observations. We found nutrients released from permafrost thaw/degradation to increase emergent plant biomass and CH4 emissions by 54 and 64%, respectively. While, nutrient concentrations similar to maximum field concentrations had no effect. Assuming permafrost thaw only occurs in aquatic tundra (similar to 9% of the land surface area), our estimates suggest that regional CH4 emissions may be enhanced by 30%. We conclude that long-term patterns of emergent vegetation expansion and increased CH4 emissions in this region were likely due to thawing permafrost, which may represent a novel well-documented case study of the permafrost carbon feedback to warming.

World soils and terrestrial ecosystems have been a source of atmospheric abundance of CO2 ever since settled agriculture began about 10-13 millennia ago. The amount of CO2-C emitted into the atmosphere is estimated at 136 +/- 55 Pg from terrestrial ecosystems, of which emission from world soils is estimated at 78 +/- 12 Pg. Conversion of natural to agricultural ecosystems decreases soil organic carbon (SOC) pool by 30-50% over 50-100 years in temperate regions, and 50-75% over 20-50 years in tropical climates. The projected global warming, with estimated increase in mean annual temperature of 4-6 degrees C by 2100, may have a profound impact on the total soil C pool and its dynamics. The SOC pool may increase due to increase in biomass production and accretion into the soil due to the so-called CO2 fertilization effect, which may also enhance production of the root biomass. Increase in weathering of silicates due to increase in temperature, and that of the formation of secondary carbonates due to increase in partial pressure of CO2 in soil air may also increase the total C pool. In contrast, however, SOC pool may decrease because of: (i) increase in rate of respiration and mineralization, (ii) increase in losses by soil erosion, and (iii) decrease in protective effects of stable aggregates which encapsulate organic matter. Furthermore, the relative increase in temperature projected to be more in arctic and boreal regions, will render Cryosols under permafrost from a net sink to a net source of CO2 if and when permafrost thaws. Thus, SOC pool of world soils may decrease with increase in mean global temperature. In contrast, the biotic pool may increase primarily because of the CO2 fertilization effect. The magnitude of CO2 fertilization effect may be constrained by lack of essential nutrients (e.g., N, P) and water. The potential of SOC sequestration in agricultural soils of Europe is 70-190 Tg C yr(-1). This potential is realizable through adoption of recommended land use and management, and restoration of degraded soils and ecosystems including wetlands.