Papers by Asmeret Asefaw Berhe
Geoderma, 2018
The chemical characteristics of dissolved organic matter (DOM) in soils that experience erosion a... more The chemical characteristics of dissolved organic matter (DOM) in soils that experience erosion and deposition are key to the biogeochemical cycle of carbon on the earth's surface. However, data related to the transport and fate of DOM from soils that experience erosion and different management practices are scarce, particularly at catchment scales. In this study, soil samples (uppermost 10 cm) were collected from uplands representing four land use types (cropland, fallow, grassland, and forests) as well as gullies, and sediment samples (100 cm sampled at 10 depths) were collected from sediments retained by a check dam. Chemical characteristics of DOM in soils and sediments, as well as subsequent source identification, were inferred from UV-Visible absorption and fluorescence excitation emission matrix (EEM)-parallel factor analysis (PARAFAC) as well as principal component analysis (PCA). The results indicated higher aromaticity, hydrophobic fraction, and molecular size in DOM from forest soils than those from other land use types and gullies. These factors were also higher in soils at the eroding sites than in sediments. EEM-PARAFAC analysis demonstrated that more protein-like components (tyrosine-like and tryptophan-like combined, accounting for > 42.77%) were present in sediments compared to soils with terrestrial humic-like substances. PCA results revealed that approximately 72% of the variance in the DOM characteristics was explained by the first two principal components and that the DOM in upland and gully soils had a negligible contribution to DOM in sediments. Combined our results indicate that, despite the large amount of sediment-associated carbon that is transported by erosion and trapped in check dams, DOM is likely mineralized during soil transport. Furthermore, biological production of new organic compounds (auto-chthonous sources) are likely the major source of sediment DOM in depositional settings.
Biogeosciences, 2019
Soil water status is one of the most important environmental factors that control microbial activ... more Soil water status is one of the most important environmental factors that control microbial activity and rate of soil organic matter (SOM) decomposition. Its effect can be partitioned into effect of water energy status (water potential) on cellular activity, effect of water volume on cellular motility, and aqueous diffusion of substrate and nutrients , as well as the effect of air content and gas-diffusion pathways on concentration of dissolved oxygen. However, moisture functions widely used in SOM decomposition models are often based on empirical functions rather than robust physical foundations that account for these disparate impacts of soil water. The contributions of soil water content and water potential vary from soil to soil according to the soil water characteristic (SWC), which in turn is strongly dependent on soil texture and structure. The overall goal of this study is to introduce a physically based modeling framework of aerobic microbial respiration that incorporates the role of SWC under arbitrary soil moisture status. The model was tested by comparing it with published datasets of SOM decomposition under laboratory conditions.
Pyrogenic carbon (PyC) is an incomplete combustion by-product with longer soil residence times co... more Pyrogenic carbon (PyC) is an incomplete combustion by-product with longer soil residence times compared with nonpyrogenic components of the soil carbon (C) pool and can be preferentially eroded in fire-affected landscapes. To investigate geomorphic and fire-related controls on PyC erosion, sediment fences were established in three combinations of slope (high 13.9-37.3%; moderate 0-6.7%) and burn severity (high; moderate) plots within the perimeter of the Rim Fire in 2013, Yosemite National Park, California, USA. After each major precipitation event following the fire, we determined transport rates of total sediment, fine and coarse sediment fractions, and C and nitrogen (N). We measured stable isotope (δ 13 C and δ 15 N) compositions and 13 C-nuclear magnetic resonance spectra of soils and eroded sediments. The highest total and fine (<2 mm) sediment transport in high severity burned areas correlated with initial discharge peaks from an adjacent stream, while moderate burn severity sites had considerably more of the >2 mm fraction transported than high burn severity sites. The δ 13 C and δ 15 N values and 13 C-nuclear magnetic resonance analyses indicated that sediment eroded from moderate severity burn areas included fresh organic matter that was not as significantly affected by the fire, whereas sediments from high severity burn areas were preferentially enriched in PyC. Our results indicate that along a single hillslope after the Rim Fire, burn severity acted as a primary control on PyC transport postfire, with slope angle likely playing a secondary role. The preferential erosion of PyC has major implications for the long-term persistence of PyC within the soil system.
Geoderma, 2018
Physical and chemical stabilization, environmental conditions, and organic matter composition all... more Physical and chemical stabilization, environmental conditions, and organic matter composition all play vital roles in determining the persistence of soil organic matter (SOM). Fundamentally, SOM stability depends on the balance of microbial bioenergetics between the input of energy needed to decompose it (i.e., activation energy; E a) and the net energy gained (i.e., energy density; ED) from its decomposition. This relationship is complicated in soils by chemical and physical protection mechanisms, which require additional energies to overcome for decomposition to occur. In this study, we analyze the bioenergetics of soil density fractions, which vary in their degrees of organic matter-mineral association, and show that the relationship of ED and E a has the ability to provide information about relative differences in SOM chemical composition and stability. Our results demonstrate distinct bioenergetic signatures between particulate, light (free and occluded) fractions versus mineral-associated, heavy fractions isolated from soil samples collected at two depths from a climosequence along an elevation gradient in the Sierra Nevada, California. While there were no significant differences in ED and E a within each fraction across climates, the light fractions (LF) were characterized by larger ED and E a values, whereas the heavy fractions (HF) were characterized by smaller ED and E a values. Combined with CHN analyses, we conclude that SOM in HF pools is likely comprised of relatively simple organic compounds that have long turnover rates because of chemical association with soil minerals, whereas the LF pools are comprised of more chemically complex molecules with low chemical reactivity and high E a .
Fire Ecology, 2019
Background: Fire plays an important role in controlling the cycling and composition of organic ma... more Background: Fire plays an important role in controlling the cycling and composition of organic matter and nutrients in terrestrial and aquatic ecosystems. In this study, we investigated the effects of wildfire severity, time since fire, and site-level characteristics on (1) concentration of multiple solutes (dissolved organic carbon, DOC; total dissolved nitrogen, TDN; dissolved organic nitrogen,
JGR-B, 2019
The lateral destination and potential decomposition of soil organic matter mobilized by soil eros... more The lateral destination and potential decomposition of soil organic matter mobilized by soil erosion depends on factors such as the amount and type of precipitation, topography, the nature of mobilized organic matter (OM), potential mixing with mineral particles, and the stabilization mechanisms of the soil OM. This study examined how the relative distribution of carbon (C) and nitrogen (N) in different OM fractions varied in soils from eroding slopes and in eroded sediments in a series of low-order forested catchments in the western Sierra Nevada, California. We found that precipitation amount played a major role in mobilizing OM. More than 40% of the OM exported from these forested catchments was free particulate OM, or OM physically protected inside relatively less stable macroaggregates, compared to OM inside microaggregates or chemically associated with soil minerals. Years with high amounts of precipitation generally transported more mineral-associated OM, with lower C and N concentrations, while sediment transported in drier years was more enriched in unprotected, coarse particulate OM derived from surficial soils. When incubated under the same conditions, sediment C (from material captured in settling basins) produced 72-97% more CO 2 during decomposition than soil C did. Our results suggest that without stabilization through burial or reconfigured organomineral associations, this sediment OM is prone to decomposition, which may contribute to little to no terrestrial CO 2 sink induced from erosion in these Mediterranean montane forest ecosystems.
Improved quantification of the factors controlling soil organic matter (SOM) stabilization at con... more Improved quantification of the factors controlling soil organic matter (SOM) stabilization at continental to global scales is needed to inform projections of the largest actively cycling terrestrial carbon pool on Earth, and its response to environmental change. Biogeochemical models rely almost exclusively on clay content to modify rates of SOM turnover and fluxes of climate-active CO 2 to the atmosphere. Emerging conceptual understanding, however, suggests other soil physicochemical properties may predict SOM stabilization better than clay content. We addressed this discrepancy by synthesizing data from over 5,500 soil profiles spanning continental scale environmental gradients. Here, we demonstrate that other physicochemical parameters are much stronger predictors of SOM content, with clay content having relatively little explanatory power. We show that exchangeable calcium strongly predicted SOM content in water-limited, alkaline soils, whereas with increasing moisture availability and acidity, iron-and aluminum-oxyhydroxides emerged as better predictors, demonstrating that the relative importance of SOM stabilization mechanisms scales with climate and acidity. These results highlight the urgent need to modify biogeochemical models to
To the Editor-Scientists make lists. We do this to organize, summarize, discover patterns, predic... more To the Editor-Scientists make lists. We do this to organize, summarize, discover patterns, predict future trends and understand how the world works. Some lists expand our thinking and improve our research, other lists are restrictive. Here, we argue that the '100 articles' list compiled by Courchamp & Bradshaw 1 is more restrictive than constructive. The list continues a long-standing tradition of highlighting almost exclusively work from male scientists and perpetuates a false perception that women, people of colour and people from the Global South are new to the field of ecology. In addition, the list is restrictive in classifying what ecology is, and is not. There are entire dimensions of ecology that are not represented, and the emphasis of traditional community ecology comes at the cost of population ecology, landscape ecology, ecosystem ecology, palaeoecology, behavioural ecology, physiological ecology and more 2. We write this as a form of positive intervention because in order to advance the field of ecology, we need to advance and welcome the intellectual contributions of the full breadth of past, current and aspiring ecological scientists. Innovation, creativity and problem-solving are enhanced when tackled by diverse teams 3,4. Numerous ecology studies have demonstrated that species and genetic diversity support ecosystem functioning, the effect of which is often more than the sum of the individual parts 5-7. Social interactions are similar-socially diverse teams arrive at consensus more quickly and perform tasks more effectively 8. Despite
Most of Earth's terrestrial surface is made up of sloping landscapes. The lateral distribution of... more Most of Earth's terrestrial surface is made up of sloping landscapes. The lateral distribution of topsoil by erosion controls the availability, stock, and persistence of essential elements in the terrestrial ecosystem. Over the last two decades, the role of soil erosion in biogeochemical cycling of essential elements has gained considerable interest from the climate, global change, and biogeochemistry communities after soil erosion and terrestrial sedimen-tation were found to induce a previously unaccounted terrestrial sink for atmospheric carbon dioxide. More recent studies have highlighted the role of erosion in the persistence of organic matter in soil and in the biogeochemical cycling of elements beyond carbon. Here we synthesize available knowledge and data on how erosion serves as a major driver of biogeochemical cycling of essential elements. We address implications of erosion-driven changes in biogeochemical cycles on the availability of essential elements for primary production, on the magnitude of elemental exports downstream, and on the 521 exchange of greenhouse gases from the terrestrial ecosystem to the atmosphere. Furthermore, we explore fates of eroded material and how terrestrial mass movement events play major roles in modifying Earth's climate.
Pyrogenic carbon (PyC) constitutes an important pool of soil organic matter (SOM), particularly f... more Pyrogenic carbon (PyC) constitutes an important pool of soil organic matter (SOM), particularly for its reactivity and because of its assumed long residence times in soil. In the past, research on the dynamics of PyC in the soil system has focused on quantifying stock and mean residence time (MRT) of PyC in soil, as well as determining both PyC stabilization mechanisms and loss pathways. Much of this research has focused on decomposition as the most important loss pathway for PyC from soil. However, the low density of PyC and its high concentration on the soil surface after fire indicates that a significant proportion of PyC formed or deposited on the soil surface is likely laterally transported away from the site of production by wind and water erosion. Here, we present a synthesis of available data and literature to compare the magnitude of the water-driven erosional PyC flux with other important loss pathways, including leaching and decomposition, of PyC from soil. Furthermore, we use a simple first-order kinetic model of soil PyC dynamics to assess the effect of erosion and deposition on residence time of PyC in eroding landscapes. Current reports of PyC MRT range from 250 to 660 years. Using a specific example-based model system, we find that ignoring the role of erosion may lead to the under-or over-estimation of PyC MRT on the centennial time scale. Furthermore, we find that, depending on the specific landform positions, timescales considered, and initial concentrations of PyC in soil, ignoring the role of erosion in distributing PyC across a landscape can lead to discrepancies in PyC concentrations on the order of several 100 g PyC m −2. Erosion is an important PyC flux that can act as a significant control on the stock and residence time of PyC in the soil system.
In the next decades, the influence of wildfires in controlling the cycling and composition of soi... more In the next decades, the influence of wildfires in controlling the cycling and composition of soil organic matter (SOM) globally and in the western U.S. is expected to grow. While the impact of fires on bulk SOM has been extensively studied, the extent at which heating of soil affects the soluble component of SOM remains unclear. Here we investigated the thermal transformations of water-extractable organic matter (WEOM) by examining the changes in the distribution of carbon (C) functional groups in WEOM from soils heated at low and intermediate temperatures. WEOM (<0.7 μm particle size) was extracted from topsoils (0–5 cm depth) of five soil series formed from a nonglaciated granitic bedrock and sampled along a Sierra Nevada climosequence. Soils were heated in a muffle furnace at 150°C, 250°C, and 350°C for 1 h. The extracted solution was analyzed for WEOM aromaticity, mean molecular weight, organic C (OC) concentration, and major structural components by employing optical spectrophotometry and liquid-state 1 H-NMR spectroscopy. At 150°C and 250°C, OC concentrations increased relative to the thermally unaltered samples, with losses of oxygenated functional C groups and enrichment of aliphatic C structures observed at 250°C. Conversely, OC concentration and mean molecular weight decreased as heating increased from 250°C to 350°C, whereas WEOC became more enriched in aromatic C structures. Our results suggest that low and intermediate fire intensities significantly alter the nature of dissolved organic matter exported from soils to rivers in the Sierra Nevada and beyond.
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Papers by Asmeret Asefaw Berhe