Our science

Research

The overall goal of our work in the Berhe Soil Biogeochemistry Lab is to understand what controls the persistence of organic matter in soil, and what that means for the role soil plays in regulating Earth’s climate. Our approach brings together biogeochemistry, pedology, climatology, and geomorphology, and it has taken us from the Sierra Nevada to Nebraska’s ancient dune fields to soil archives collected across the African continent. Below, our research is organized around four major themes.

Deep Soil Carbon and Climate Controls

More than half of the world’s soil carbon sits below the depths most studies ever sample, and for a long time that carbon was treated as effectively inert. Our work has helped overturn that assumption. We have shown that weathered bedrock in the critical zone holds a carbon pool that is often overlooked in inventories, one that can rival or exceed the amount stored in surface soil. Building on this, we have demonstrated that climate exerts strong control over how much deep soil carbon accumulates and how stable it is, using the Western Sierra Nevada climosequence as a natural laboratory. We have also studied how carbon buried in paleosols, ancient wind-deposited soils in Nebraska, becomes vulnerable to decomposition under different global change scenarios, with a particular focus on the role calcium plays in binding organic matter to minerals. This body of work culminated in an invited synthesis for the Annual Review of Ecology, Evolution, and Systematics and, more recently, a Tansley Review examining how shifting precipitation patterns reshape deep soil carbon storage and stability.

Fire, Erosion, and Pyrogenic Carbon

Fire and erosion routinely overlap in time and space, particularly in the fire-prone landscapes of the Sierra Nevada, yet their combined effect on soil carbon has rarely been studied as a single process. Our lab has worked to change that. Following the 2013 Rim Fire in Yosemite, we showed that the erosion of fire-altered carbon was controlled jointly by burn severity and slope, and our subsequent work established that the timing of pyrogenic carbon’s first contact with water after a fire is what largely determines whether it persists or is lost from the system. We have also examined how combustion temperature itself changes soil aggregate stability and the physical protection organic matter receives from minerals. This work sits alongside our long-running interest in how erosion, more broadly, can act as a mechanism of carbon sequestration when eroded soil is replaced by new plant inputs or when deposited carbon is protected in the landscapes where it settles.

Mechanisms of Soil Organic Matter Stabilization

Understanding why some organic matter survives in soil for centuries while other organic matter is lost in months requires getting at the mineral and molecular scale. Our group studies how metal oxides, particularly the iron and aluminum oxides common in weathered and tropical soils, bind with organic carbon and regulate its fate. Work using archived soil samples from across sub-Saharan Africa showed that these oxide minerals exert a dominant influence on carbon persistence at continental scale, and our follow-up work has extended this to understanding how long carbon actually stays put once stabilized. We have also studied how phosphorus speciation shifts with climate and depth along Sierra Nevada and White Mountain climosequences, and we have taken a hard look at our own field’s methods, documenting how common laboratory procedures like ultrasonic dispersion and chemical fractionation can introduce artifacts that shape the conclusions researchers draw from their data.

Landscape to Global Scale Synthesis

Soil carbon dynamics do not stop at the plot boundary, and much of our work has pushed toward understanding controls that operate across landscapes and around the world. Using the Merced River chronosequence, we showed that as soils age and their minerals weather, the shifting mineral assemblage and associated microbial communities create feedbacks that shape carbon storage and climate sensitivity, with intermediate-aged soils offering the greatest capacity for carbon protection. That same chronosequence, combined with others from around the globe, has helped reveal how belowground biodiversity and whole ecosystem structure change systematically as soils develop. Closer to home, we study how the coupling of hydrology and biogeochemistry sustains high-elevation meadows in the Sierra Nevada, ecosystems that filter and store water for entire watersheds. And through collaborative synthesis efforts, including our contributions to the International Soil Radiocarbon Database and an invited review in Nature Reviews Earth & Environment, we work to bring landscape and global perspectives together into frameworks the broader research community can build on.

In depth

Specific areas of past and ongoing research

Dynamics and vulnerability of deep soil organic matter and weathered bedrock carbon
Carbon buried in paleosols and other deep, buried soil layers
Climatic controls on soil organic matter along elevation and precipitation gradients
Fire effects on soil aggregation, mineral–organic matter associations, and pyrogenic carbon
Interactions between fire and erosion in forested, fire-affected landscapes
Role of soil erosion in terrestrial carbon sequestration
Metal oxide (iron and aluminum) controls on organic matter and nutrient stabilization
Phosphorus speciation and cycling across climate gradients
Soil development over geologic time and its effects on carbon, nutrients, and biodiversity
Continental-scale controls on soil organic carbon across sub-Saharan Africa
Hydrology and biogeochemistry coupling in high-elevation meadows
Soil management, composting, and biochar strategies for climate change mitigation
Methodological artifacts in soil fractionation, pretreatment, and dispersion techniques
Deep soil CO₂ and dissolved organic matter fluxes
Contributions of root systems, including deep-rooted perennials, to soil carbon storage

For more on our current projects or to inquire about joining the group, contact Prof. Asmeret Asefaw Berhe at aaberhe@ucmerced.edu.