The overall goal of Berhe Biogeochemistry Lab research is to improve our understanding of mechanisms that regulate organic matter (OM) persistence in the soil system to improve our understanding of the role of soil in regulating the earth’s climate. Specifically, our research focuses on the effect of physical perturbations in the environment on SOM dynamics. Our work has been notable in the integration of biogeochemical, pedogenic, climatic, and geomorphological approaches to derive an improved representation of mechanisms that regulate organic matter (OM) persistence in the soil system. 
Climatic controls on deep soil organic matter stock and stability
As part of the Southern Sierra Critical Zone observatory, we are studying how climate controls the amount, nature, stability, and stabilization mechanisms of soil organic matter along the Western Sierra Climosequence. Through this work, we are also collaborating with Craig Rasmussen’s group at the University of Arizona to develop a cross-CZO collaborative project to investigate climatic controls and deep SOM dynamics.
Vulnerability of carbon buried in Paleosols
In this project, we are studying how and to what extent carbon buried in deep soil layers, in paleosols (deep eolian deposits in Nebraska), can be rendered susceptible to decomposition and dissolution under different global change scenarios. Furthermore, we are determining the chemical composition of OM in these deep paleosols, and establishing their mechanisms of stabilization (in particular the role of Calcium and cation bridging).
Phosphorous dynamics along the Sierra Nevada and White Mountain Climosequences
In this project, we are studying how climate controls the stock speciation of phosphorus in soil along two climatically distinct climosequences: Western Slopes of the Sierra Nevada and the White Mountains in the western US. This work combines traditional approaches to phosphorus studies in soil (i.e. Hadley fractionation schemes) with advanced spectroscopy (P-XANES and 31P-NMR) to determine how depth and climate (as mediated by soil weathering) determine phosphorus speciation in Mediterranean systems).
Source of eroded organic matter in low-order catchments in the southern part of the Sierra Nevada Mountains
As part of the Southern Sierra Critical Zone Observatory, we have been investigating the effect of soil erosion on the dynamics of organic matter. In this larger project, we seek to get a better mechanistic understanding of the mechanisms that control the fate and stability of soil organic matter in eroding, fire-affected systems of the Sierra Nevada.
Role of erosion on terrestrial carbon sequestration and mechanisms of soil organic matter stabilization
Soil erosion can lead to terrestrial carbon sequestration as long as i) eroded soil carbon is, at least partially, replaced in the eroding slopes and/or ii) deposited soil carbon is protected from decomposition in depositional settings. In response to the need to provide more field data, a study is being conducted in La Rogativa watershed, south-eastern Spain, by the research group of Carolina Boix-Fayos. Part of the study we are collaborating in seeks to quantify the rate of eroded soil organic carbon replacement by the production of new photosynthate and mean residence time of soil organic matter in eroding vs. depositional settings to determine if eroded SOM is stabilized upon deposition. The findings of this study will be helpful in furthering our understanding of SOC dynamics in eroding watersheds.
Role of Fire and Pyrogenic matter on dynamics of SOM
Our work in this area mostly focuses on interactive effects of fire and erosion (that routinely overlap in space and time) on soil OM and Pyrogenic carbon (PyC) dynamics, especially on upland forest ecosystems. Our team established a multidimensional approach that combines fieldwork in the Sierra Nevada Mountains of California and Nevada with the latest advances in spectroscopy, elemental and isotopic analyses, and other physicochemical techniques. So far, our work in this area has made notable contributions that have provided improved insights into how the interaction of fire with erosion controls molecular to macro-scale properties in the soil, and stock and persistence of fire altered (pyrogenic) materials and bulk SOM
The effect of fire on soil aggregation and physical stability of soil organic matter
Fire has significant impact on the dynamics of organic matter in soil and soil health. Over the last couple of decades, our understanding of how fires affect the chemical composition of organic matter and lead to the accumulation of fire-altered (pyrogenic) matter in soil has improved. However, there is still very little published data on how combustion temperature affects chemical and physical properties of topsoil and its implication on mineral-OM association in soil that have important implications for formation and stability of aggregates, and physical protection of OM from decomposition. In this project, we are determining how combustion temperature affects the chemical reactivity of soil, stability of soil aggregates, and physical protection of organic matter from decomposition.
Interactive effect of Fire and Erosion on the persistence of soil organic matter
Fire changes soil organic matter amount, composition, and several soil physical and chemical properties. Fire and erosion routinely overlap in time and space. But our understanding of how soil erosion affects the lateral transport of pyrogenic carbon and its implications for determining residence times of bulk SOM as well as pyrogenic C is incomplete. In this study, we are investigating the interactive effects of fire and erosion on SOM dynamics in the forested upland ecosystem in Southern Lake Tahoe.
Role of metal oxides in Stabilization and distribution of soil organic matter
Nano-sized sesquioxides have tremendous potential to react, aggregate, and get transported in the presence of soil organic carbon. Metal oxide availability, concentration, and phases exert strong control on the behavior of organic matter in the soil, as well as that of phosphorus and contaminants such as arsenic. We are studying the sorption and desorption kinetics of crystalline and poorly crystalline metal oxides with organic matter to get important insights into the general fate of natural and synthetic nanocrystals in the environment and ways nano-sized sesquioxides can affect the destabilization of soil organic carbon.
Methods in Soil Organic Matter Biogeochemistry
Ultrasonication: The application of ultrasonic energy is an effective method of dispersing soil aggregates that range in size over multiple orders of magnitude (µm to mm). This technique is used to analyze relationships between the amount of applied energy, aggregate size class distribution, and organic matter turnover in soils. Application of increasing amounts of ultrasonic energy, to break up progressively small and more tightly bound aggregates, likely leads to the creation of artifacts that affect the interpretation of experimental findings. However, the type and magnitude of the created artifacts have not been thoroughly investigated. In this study, we are quantifying how the application of varying amounts of ultrasonic energy affect different soil minerals and organic matter in different types of soils.
Experimental artifacts from common soil techniques: Soil sample pretreatment and soil organic matter (SOM) fractionation procedures have become common in terrestrial biogeochemistry research. Experimental conditions during sample pretreatment and fractionation of SOM affect the characteristics of separated organic matter (OM). However, the effect of these treatments on the nature of the soil and SOM, and their implications for the explanatory power of the measured data are rarely considered. In this project, we are compiling information on method-related modifications of soil and SOM characteristics during air drying, rewetting, application of ultrasound energy, and sodium pyrophosphate (Na4P₂O₇) extraction. Here we aim to develop recommendations for adaptation of the sample-specific experimental designs to reduce method-related artifacts and aid in the interpretation of experimental data.
Deep soil CO2 fluxes
In this study, we are monitoring the spatial and temporal variability of the soil gaseous and soluble organic matter (OM) fluxes, in near-surface vs. deep soil layers. In this project, we are using continuous long-term measurements of soil gaseous and soluble element fluxes (being conducted in the student farm at UC Davis) to determine how new vs. old photosynthate is incorporated into soil organic matter at different depths and fluxes associated with it. This study is expected to yield important insights regarding (1) the distribution, chemical composition, reactivity, and stabilization of new vs. old organic matter along a soil profile, and (2) the relationship between the temporal dynamics of dissolved organic matter concentration and chemical composition in deep soil layers and soil CO₂ production at depth vs. near the soil surface.
Coupled biogeochemistry and hydrology in high elevation meadows
High elevation meadow stability has traditionally been viewed as a product of two parameters, geological stability, and biological stability. Hydrologic conditions in meadows exert strong control on the rate of organic matter decomposition/storage where the amount and composition of SOM can have important implications for the amount of water stored in the meadow. However, there is currently a lack of significant understanding of the mechanisms by which the SOM in meadows retains water and contributes to the biogeochemical stability of high elevation meadows. It is important to
close this knowledge gap because meadows play a fundamental role in the filtering, storage, and slow release of water in snowmelt-dominated watersheds in the Sierra Nevada and beyond, and their stability expected to be sensitive to changes in climate. This research examines the interconnected role that SOM quality/quantity and hydrology play on meadow stability.
PAST RESEARCH PROJECTS
- Decomposition of organic substrates in deep vs. near-surface soil layers along topographic gradients (Berhe, 2012)
- Effect of litterbags on the rate of organic substrate decomposition in deep vs. near-surface soils along a topographic gradient (Berhe, 2013)
- Mechanisms of soil organic matter stabilization in eroding vs. depositional landform positions (Berhe et al 2008, Berhe et al, 2012)
- Effect of changing amount and timing of rainfall on soil organic matter dynamics (Berhe et al, 2012)
- Using biochar to recover nutrients from dairy waste (Sarkhot et al, 2012, 2013 JEQ)
- Role of soil erosion in terrestrial carbon sequestration in naturally eroding watershed (Berhe et al, 2007, 2008)
- Effect of landmines on land degradation (Berhe 2007)
- Soil organic matter biogeochemistry in permafrost under anticipated climate change (Waldrop, et al 2010)
For information about our work or to inquire about joining our group please contact Prof. Asmeret Asefaw Berhe at aaberhe(at)ucmerced(dot)edu.
