ALBERT

Description: We develop a multiscale zonation approach to characterize the spatial variability of Arctic polygonal ground geomorphology, and to assess the relative controls of these elements on land surface and subsurface properties, and carbon fluxes. Working within an ice-wedge polygonal region near Barrow AK, we consider two-scales of zonation: polygon features (troughs, centers, and rims of polygons) that are nested within different polygon types (high, flat, and low-centered). In this study, we first delineated polygons using a digital elevation map, and clustered the polygons into four types along two transects, using geophysical and kite-based landscape-imaging datasets. We extrapolated those data-defined polygon types to all the polygons over the study site, using the polygon statistics extracted from the digital elevation map. Based on the point measurements, we characterized the distribution of vegetation, hydrological, thermal and geochemical properties as well as carbon fluxes, all as a function of polygon types and polygon features. Results show that nested polygon geomorphic zonation – polygon types and polygon features – can be used to represent distinct distributions of carbon fluxes and associated properties, as well as co-variability among those properties. Importantly, the results indicate that polygon types have more power to explain the variations in those properties than polygon features. The approach is expected to be useful for improved system understanding, site characterization, and parameterization of numerical models aimed at predicting ecosystem feedbacks to the climate.

Description: Vegetation cover in dry regions is a key variable determining desertification. Soils exposed to rainfall by desertification can form physical crusts that reduce infiltration, exacerbating water stress on the remaining vegetation. Paradoxically, field studies show that crust removal is associated with plant mortality in desert systems, while artificial biological crusts can improve plant regeneration. Here, it is shown how physical crusts can act as either drivers of, or buffers against desertification depending on their environmental context. The behavior of crusts is first explored using a simplified theory for water movement on a uniform, partly vegetated slope subject to stationary hydrologic conditions. Numerical model runs supplemented with field data from a semiarid Long-Term Ecological Research (LTER) site are then applied to represent more realistic environmental conditions. When vegetation cover is significant, crusts can drive desertification, but this process is potentially self-limiting. For low vegetation cover, crusts mitigate against desertification by providing water subsidy to plant communities through a runoff-runon mechanism.

Description: Surface sediments from the Changjiang Estuary and adjacent shelf were analyzed using a variety of bulk and molecular techniques, including grain size composition, sediment surface area (SSA), elemental composition (C, N), stable carbon isotopic composition (δ 13 C), n -alkanes, lignin phenols, and glycerol dialkyl glycerol tetraether lipids (GDGTs), to obtain a more comprehensive understanding of the sources and fate of sedimentary organic carbon (SOC) in this dynamic region. Bulk N/C ratios of 0.09 to 0.15, δ 13 C of −24.4 ‰ to −21.1 ‰, branched/isoprenoid tetraether (BIT) index of 0 to 0.74, n -alkane content of 0.02 to 0.37 mg g −1 OC and lignin content (Λ 8 ) of 0.10 to 1.46 mg/100 mg OC and other related molecular indices in these samples indicate a mixed source of marine, soil and terrestrial plant derived OC in the study area. A three end-member mixing model using principal component analysis (PCA) factors as source markers and based on Monte-Carlo (MC) simulation was constructed to estimate the relative contributions of OC from different sources. Compared with traditional mixing models, commonly based on a few variables, this newly-developed PCA-MC model supported bulk and biomarker data and yielded a higher resolution OC inputs to different sub-regions of this system. In particular, the results showed that the average contributions of marine, soil and terrestrial OC in the study area were 35.3 %, 47.0 % and 17.6 %, and the highest contribution from each OC source was mainly observed in the shelf, inner estuary, and coastal region, respectively.

Description: Natural and anthropogenic disturbances influence ecological succession and impact the carbon cycle. Understanding disturbance effects and ecosystem recovery is essential to carbon modeling. We hypothesized that: (1) species-specific disturbances impact the carbon cycle differently from non-specific disturbances. In particular, disturbances that target early-successional species will lead to higher carbon uptake by the post-recovery, mid- and late-successional community; and (2) disturbances that affect the mid-successional deciduous species have more intense and long-lasting impacts on carbon uptake than disturbances of similar intensity that only affect the early-successional species. To test these hypotheses, we employed a series of simulations conducted with the Ecosystem Demography model version 2 to evaluate the sensitivity of a temperate mixed-deciduous forest to disturbance intensity and type. Our simulation scenarios included a control (undisturbed) case, a uniform disturbance case where we removed 30% of all trees regardless of their successional status, five cases where only early-successional deciduous trees were removed with increasing disturbance intensity (30%, 70%, 85%, and 100%), and four cases of mid-successional disturbances with increasing intensity (70%, 85%, and 100%). Our results indicate that disturbances affecting the mid-successional deciduous trees led to larger decreases in carbon uptake as well as longer recovery times when compared to disturbances that exclusively targeted the early-successional deciduous trees at comparable intensities. Moreover, disturbances affecting 30% to 100% of early-successional deciduous trees resulted in an increased carbon uptake, beginning 6 years after the disturbance and sustained through the end of the 100-year simulation.

Description: We assessed spatial and temporal patterns of dissolved organic carbon (DOC) lability and composition throughout the alluvial aquifer of the 16 km 2 Nyack Floodplain in northwest Montana, USA. Water influx to the aquifer derives almost exclusively from the Middle Fork of the Flathead River, and water residence times within the aquifer range from days to months. Across seasons and channel discharge conditions, we measured DOC concentration, lability, and optical properties of aquifer water sampled from 12 wells, both near and ~3 m below the water table. Concentrations of DOC were typically low (542 ± 22.7 µg L -1 ; mean ± se) and the percentage of labile DOC averaged 18 ± 12% during 3-day laboratory assays. Parallel factor analysis of fluorescence excitation-emission matrices revealed two humic-like and two amino acid-like fluorescence groups. Total DOC, humic-like components, and specific UV absorbance decreased with water residence time, consistent with sorption to aquifer sediments. However, labile DOC (both concentration and fraction) increased with water residence time, suggesting a concurrent influx or production of labile DOC. Thus, although the carbon-poor, oxygen-rich aquifer is a net sink for DOC, recalcitrant DOC appears to be replaced with more labile DOC along aquifer flow paths. Our observation of DOC production in long flow paths contrasts with studies of hyporheic DOC consumption along short (cm to m) flow paths, and highlights the importance of understanding the role of labile organic matter production and/or influx in alluvial aquifer carbon cycling.

Description: We report methane (CH 4 ) concentration and methane oxidation (MO x ) rate measurements from the eastern tropical north Pacific (ETNP) water column. This region comprises low-CH 4 waters and a depth interval (~200-760 m) of CH 4 supersaturation that is located within a regional oxygen minimum zone (OMZ). MO x rate measurements were made in parallel using tracer-based methods with low-level 14 C-CH 4 (LL 14 C) and 3 H-CH 4 ( 3 H). The two tracers showed similar trends in MO x rate with water depth, but consistent with previous work, the LL 14 C rates (range: 0.034-15 x 10 -3 nmol CH 4 L -1 d -1 ) were systematically slower than the parallel 3 H rates (range: 0.098-4000 x 10 -3 nmol CH 4 L -1 d -1 ). Priming and background effects associated with the 3 H-CH 4 tracer and LL 14 C filtering effects are implicated as the cause of the systematic difference. The MO x rates reported here include some of the slowest rates measured in the ocean to date, are the first rates for the ETNP region and show zones of slow CH 4 turnover within the OMZ that may permit CH 4 derived from coastal sediments to travel great lateral distances. The MO x rate constants correlate with both CH 4 and oxygen concentrations, suggesting their combined availability regulates MO x rates in the region. Depth-integrated MO x rates provide an upper limit on the magnitude of regional CH 4 sources and demonstrate the importance of water column MO x , even at slow rates, as a sink for CH 4 that limits the ocean-atmosphere CH 4 flux in the ETNP region.

Description: The Atacama Desert is the driest and one of the most life-limiting places on Earth. Despite the extreme conditions, microbial endolithic communities have been found inside halite rocks. The presence of these microbial communities is possible due to the hygroscopic properties of evaporitic rocks composed of sodium chloride. It is important to elucidate every possible water source in such a hyper-arid environment. Therefore, in the present study, an artificial neural network (ANN) based model has been designed to predict the presence of liquid water on the surface of halite pinnacles. The model predicts the moisture formation using two basic meteorological variables, air temperature and air relative humidity. ANNs have been successfully employed for the first time as a tool for predicting the appearance of liquid water, a key factor for the endolithic microbial communities living in the driest part of the Atacama Desert. The model developed is able to correctly predict the formation of water on the surface of the halite pinnacles 83% of the cases. We anticipate the future application of this model as an important tool for the prediction of the water availability, and, therefore, potential habitability of lithic substrates in extreme environments on Earth and perhaps elsewhere.

Description: The underlying mechanisms driving the coupled interactions between inorganic nitrogen uptake and dissolved organic matter are not well understood, particularly in surface waters. To determine the relationship between dissolved organic carbon (DOC) quantity and nitrate (NO 3 - ) uptake kinetics in streams, we performed a series of NO 3 - Tracer Additions for Spiraling Curve Characterization (TASCC) experiments in four streams within the Lamprey River Watershed, New Hampshire, across a range in background DOC concentrations (1 – 8 mg C/L). Experiments were performed throughout the 2013 and 2014 growing seasons. Across streams and experimental dates, ambient uptake velocity (V f ) correlated positively with increasing DOC concentrations and DOC:NO 3 - ratios but was only weakly negatively associated with NO 3 - concentrations. Ambient NO 3 - V f was unrelated to pH, light, temperature, dissolved oxygen and SUVA 254 . Although there were general tendencies across the entire Lamprey River Watershed, individual sites behaved differently in their uptake kinetics. NO 3 - uptake dynamics in the Lamprey River Watershed are most strongly influenced by DOC concentrations rather than NO 3 - concentrations or physico-chemical parameters, which have been identified as regional- to continental-scale drivers in previous research. Understanding the fundamental relationships between dissolved organic matter and inorganic nutrients will be important as global and climatic changes influence the delivery and production of DOC and NO 3 - in aquatic ecosystems.

Description: Permafrost dynamics play an important role in high-latitude peatland carbon balance and are key to understanding the future response of soil carbon stocks. Permafrost aggradation can control the magnitude of the carbon feedback in peatlands through effects on peat properties. We compiled peatland plant macrofossil records for the northern permafrost zone (515 cores from 280 sites) and classified samples by vegetation type and environmental class (fen, bog, tundra and boreal permafrost, thawed permafrost). We examined differences in peat properties (bulk density, carbon (C), nitrogen (N) and organic matter content, C/N ratio) and C accumulation rates among vegetation types and environmental classes. Consequences of permafrost aggradation differed between boreal and tundra biomes, including differences in vegetation composition, C/N ratios, and N content. The vegetation composition of tundra permafrost peatlands was similar to permafrost-free fens, while boreal permafrost peatlands more closely resembled permafrost-free bogs. Nitrogen content in boreal permafrost and thawed permafrost peatlands was significantly lower than in permafrost-free bogs despite similar vegetation types (0.9% versus 1.5% N). Median long-term C accumulation rates were higher in fens (23 g C m -2 y -1 ) than in permafrost-free bogs (18 g C m -2 y -1 ), and were lowest in boreal permafrost peatlands (14 g C m -2 y -1 ). The plant macrofossil record demonstrated transitions from fens to bogs to permafrost peatlands, bogs to fens, permafrost aggradation within fens, and permafrost thaw and re-aggradation. Using data synthesis, we've identified predominant peatland successional pathways, changes in vegetation type, peat properties, and C accumulation rates associated with permafrost aggradation.

Description: It is recognized that anthropogenic factors have had a major impact on carbon fluxes from land to ocean during the past two centuries. However, little is known about how future changes in climate, atmospheric CO 2 and land use may affect riverine carbon fluxes over the 21 st century. Using a coupled hydrological-biogeochemical model, the Dynamic Land Ecosystem Model, this study examines potential changes in dissolved inorganic carbon (DIC) export from the Mississippi River basin to the Gulf of Mexico during 2010–2099 attributable to climate-related conditions (temperature, precipitation), atmospheric CO 2 and land use changes. Rates of annual DIC export are projected to increase by 65% under the high emissions scenario (A2) and 35% under the low emissions scenario (B1) between the 2000s and the 2090s. Climate-related changes along with rising atmospheric CO 2 together would account for over 90% of the total increase in DIC export throughout the 21 st century. We are aware that climate change would largely increase DIC export from the Mississippi River basin and hence alter the chemistry of coastal ocean if appropriate climate mitigation wouldn't be taken into action in the future.