Benjamin W. Abbott

Climate change: High risk of permafrost thaw

Northern soils will release huge amounts of carbon in a warmer world, say Edward A. G. Schuur, Benjamin Abbott and the Permafrost Carbon Network.

Biomass offsets little or none of permafrost carbon release from soils, streams, and wildfire: an expert assessment

As the permafrost region warms, its large organic carbon pool will be increasingly vulnerable to decomposition, combustion, and hydrologic export. Models predict that some portion of this release w ...

Elevated dissolved organic carbon biodegradability from thawing and collapsing permafrost

As high latitudes warm, a portion of the large organic carbon pool stored in permafrost will become available for transport to aquatic ecosystems as dissolved organic carbon (DOC). If permafrost DOC is biodegradable, much will be mineralized to the atmosphere in freshwater systems before reaching the ocean, accelerating carbon transfer from permafrost to the atmosphere, whereas if recalcitrant, it will reach marine ecosystems where it may persist over long time periods. We measured biodegradable DOC (BDOC) in water flowing from collapsing permafrost (thermokarst) on the North Slope of Alaska and tested the role of DOC chemical composition and nutrient concentration in determining biodegradability. DOC from collapsing permafrost was some of the most biodegradable reported in natural systems. However, elevated BDOC only persisted during active permafrost degradation, with a return to predisturbance levels once thermokarst features stabilized. Biodegradability was correlated with background nutrient concentration, but nutrient addition did not increase overall BDOC, suggesting that chemical composition may be a more important control on DOC processing. Despite its high biodegradability, permafrost DOC showed evidence of substantial previous microbial processing, and we present four hypotheses explaining this incongruity. Because thermokarst features form preferentially on river banks and lake shores and can remain active for decades, thermokarst may be the dominant short-term mechanism delivering sediment, nutrients, and biodegradable organic matter to aquatic systems as the Arctic warms.

High risk of permafrost thaw

Northern soils will release huge amounts of carbon in a warmer world, say Edward A. G. Schuur, Benjamin Abbott and the Permafrost Carbon Network.

Elemental properties, hydrology, and biology interact to shape concentration‐discharge curves for carbon, nutrients, sediment, and major ions

To investigate the prevalence and cause of concentration-discharge (C-Q) relationships for carbon, nutrients, major ions, and particulates, we analyzed 40 years of water quality data from 293 monitoring stations in France. Catchments drained diverse landscapes and ranged from 50 to 110,000 km2, together covering nearly half of France. To test for differences during low and high flows, we calculated independent C-Q slopes above and below the median discharge. We found that 84% of all catchment-element combinations were chemodynamic for at least half of the hydrograph and 60% of combinations showed non-linear C-Q curves. Only two or three of the nine possible C-Q modalities were manifest for each parameter, and these modalities were stable through time, suggesting that intrinsic and extrinsic elemental properties (e.g. solubility, reactivity, and source dynamics) set basic C-Q templates for each parameter, which are secondarily influenced by biological activity during low flows, and the interaction between hydrology and catchment characteristics at high flows. Several patterns challenged current C-Q views, including low-flow chemostasis for TSS in 66% of catchments, low-flow biological mediation of NO3- in 71% of catchments, and positive C-Q for dissolved organic carbon independent of catchment size in 80% of catchments. Efforts to reduce nutrient loading decreased phosphorus concentration and altered C-Q curves, but NO3- continued to increase. While C-Q segmentation requires more data than a single analysis, the prevalence of non-linear C-Q slopes demonstrates the potential information loss associated with linear or monotonic analysis of C-Q relationships, and conversely, the value of long-term monitoring. This article is protected by copyright. All rights reserved.

Permafrost collapse alters soil carbon stocks, respiration, CH4 , and N2O in upland tundra.

Release of greenhouse gases from thawing permafrost is potentially the largest terrestrial feedback to climate change and one of the most likely to occur; however, estimates of its strength vary by a factor of thirty. Some of this uncertainty stems from abrupt thaw processes known as thermokarst (permafrost collapse due to ground ice melt), which alter controls on carbon and nitrogen cycling and expose organic matter from meters below the surface. Thermokarst may affect 20-50% of tundra uplands by the end of the century; however, little is known about the effect of different thermokarst morphologies on carbon and nitrogen release. We measured soil organic matter displacement, ecosystem respiration, and soil gas concentrations at 26 upland thermokarst features on the North Slope of Alaska. Features included the three most common upland thermokarst morphologies: active-layer detachment slides, thermo-erosion gullies, and retrogressive thaw slumps. We found that thermokarst morphology interacted with landscape parameters to determine both the initial displacement of organic matter and subsequent carbon and nitrogen cycling. The large proportion of ecosystem carbon exported off-site by slumps and slides resulted in decreased ecosystem respiration postfailure, while gullies removed a smaller portion of ecosystem carbon but strongly increased respiration and N2 O concentration. Elevated N2 O in gully soils persisted through most of the growing season, indicating sustained nitrification and denitrification in disturbed soils, representing a potential noncarbon permafrost climate feedback. While upland thermokarst formation did not substantially alter redox conditions within features, it redistributed organic matter into both oxic and anoxic environments. Across morphologies, residual organic matter cover, and predisturbance respiration explained 83% of the variation in respiration response. Consistent differences between upland thermokarst types may contribute to the incorporation of this nonlinear process into projections of carbon and nitrogen release from degrading permafrost.

Thermo-erosion gullies increase nitrogen available for hydrologic export

Formation of thermokarst features, ground subsidence caused by thaw of ice-rich permafrost, can result in increased export of inorganic nitrogen (N) from arctic tundra to downstream ecosystems. We compared physical characteristics, N pools, and rates of N transformations in soils collected from thermo-erosion gullies, intact water tracks (the typical precursor landform to thermo-erosion gullies), and undisturbed tundra to test potential mechanisms contributing to export of inorganic N. Subsidence exposes mineral soils, which tend to contain higher abundance of inorganic ions relative to surface soils, and may bring inorganic N into contact with flowing water. Alternatively, physical mixing may increase aeration and drainage of soils, which could promote N mineralization and nitrification while suppressing denitrification. Finally, some soil types are more prone to formation of thermokarst, and if these soils are relatively N-rich, thermokarst features may export more N than surrounding tundra. Inorganic N pools in thermo-erosion gullies were similar to the mean for all tundra types in this region, as well as to water tracks when integrated across two sampled depths. Thus, soils prone to thermo-erosion are not intrinsically N-rich, and increased N availability in thermokarst features is apparent only at sub-regional spatial scales. However, vertical profiles of N pools and transformation rates were homogenized within thermo-erosion gullies compared to adjacent intact tundra, indicating that physical mixing brings inorganic N to the surface, where it may be subject to hydrologic export. Increased inorganic N availability caused by formation of thermo-erosion gullies may have acute, localized consequences for aquatic ecosystems downstream of positions within drainage networks that are susceptible to thermo-erosion.

Groundwater Isolation Governs Chemistry and Microbial Community Structure along Hydrologic Flowpaths

This study deals with the effects of hydrodynamic functioning of hard-rock aquifers on microbial communities. In hard-rock aquifers, the heterogeneous hydrologic circulation strongly constrains groundwater residence time, hydrochemistry, and nutrient supply. Here, residence time and a wide range of environmental factors were used to test the influence of groundwater circulation on active microbial community composition, assessed by high throughput sequencing of 16S rRNA. Groundwater of different ages was sampled along hydrogeologic paths or loops, in three contrasting hard-rock aquifers in Brittany (France). Microbial community composition was driven by groundwater residence time and hydrogeologic loop position. In recent groundwater, in the upper section of the aquifers or in their recharge zone, surface water inputs caused high nitrate concentration and the predominance of putative denitrifiers. Although denitrification does not seem to fully decrease nitrate concentrations due to low dissolved organic carbon concentrations, nitrate input has a major effect on microbial communities. The occurrence of taxa possibly associated with the application of organic fertilizers was also noticed. In ancient isolated groundwater, an ecosystem based on Fe(II)/Fe(III) and S/SO4 redox cycling was observed down to several 100 of meters below the surface. In this depth section, microbial communities were dominated by iron oxidizing bacteria belonging to Gallionellaceae. The latter were associated to old groundwater with high Fe concentrations mixed to a small but not null percentage of recent groundwater inducing oxygen concentrations below 2.5 mg/L. These two types of microbial community were observed in the three sites, independently of site geology and aquifer geometry, indicating hydrogeologic circulation exercises a major control on microbial communities.

Trends and seasonality of river nutrients in agricultural catchments: 18 years of weekly citizen science in France

Agriculture and urbanization have disturbed three-quarters of global ice-free land surface, delivering huge amounts of nitrogen and phosphorus to freshwater ecosystems. These excess nutrients degrade habitat and threaten human food and water security at a global scale. Because most catchments are either currently subjected to, or recovering from anthropogenic nutrient loading, understanding the short- and long-term responses of river nutrients to changes in land use is essential for effective management. We analyzed a never-published, 18-year time series of anthropogenic (NO3- and PO43-) and naturally derived (dissolved silica) riverine nutrients in 13 catchments recovering from agricultural pollution in western France. In a citizen science initiative, high-school students sampled catchments weekly, which ranged from 26 to 1489km2. Nutrient concentrations decreased substantially over the period of record (19 to 50% for NO3- and 14 to 80% for PO43-), attributable to regional, national, and international investment and regulation, which started immediately prior to monitoring. For the majority of catchments, water quality during the summer low-flow period improved faster than during winter high-flow conditions, and annual minimum concentrations improved relatively faster than annual maximum concentrations. These patterns suggest that water-quality improvements were primarily due to elimination of discrete nutrient sources with seasonally-constant discharge (e.g. human and livestock wastewater), agreeing with available land-use and municipal records. Surprisingly, long-term nutrient decreases were not accompanied by changes in nutrient seasonality in most catchments, attributable to persistent, diffuse nutrient stocks. Despite decreases, nutrient concentrations in almost all catchments remained well above eutrophication thresholds, and because additional improvements will depend on decreasing diffuse nutrient sources, future gains may be much slower than initial rate of recovery. These findings demonstrate the value of citizen science initiatives in quantifying long-term and seasonal consequences of changes in land management, which are necessary to identify sustainable limits and predict recovery timeframes.

Dating groundwater with dissolved silica and CFC concentrations in crystalline aquifers

Estimating intermediate water residence times (a few years to a century) in shallow aquifers is critical to quantifying groundwater vulnerability to nutrient loading and estimating realistic recovery timelines. While intermediate groundwater residence times are currently determined with atmospheric tracers such as chlorofluorocarbons (CFCs), these analyses are costly and would benefit from other tracer approaches to compensate for the decreasing resolution of CFC methods in the 5-20 years range. In this context, we developed a framework to assess the capacity of dissolved silica (DSi) to inform residence times in shallow aquifers. We calibrated silicate weathering rates with CFCs from multiple wells in five crystalline aquifers in Brittany and in the Vosges Mountains (France). DSi and CFCs were complementary in determining apparent weathering reactions and residence time distributions (RTDs) in shallow aquifers. Silicate weathering rates were surprisingly similar among Brittany aquifers, varying from 0.20 to 0.23 mg L-1 yr-1 with a coefficient of variation of 7%, except for the aquifer where significant groundwater abstraction occurred, where we observed a weathering rate of 0.31 mg L-1 yr-1. The silicate weathering rate was lower for the aquifer in the Vosges Mountains (0.12 mg L-1 yr-1), potentially due to differences in climate and anthropogenic solute loading. Overall, these optimized silicate weathering rates are consistent with previously published studies with similar apparent ages range. The consistency in silicate weathering rates suggests that DSi could be a robust and cheap proxy of mean residence times for recent groundwater (5-100 years) at the regional scale. This methodology could allow quantification of seasonal groundwater contributions to streams, estimation of residence times in the unsaturated zone and improve assessment of aquifer vulnerability to anthropogenic pollution.