Soil Organic Matter Processes From the Treetops to the Clay Particle: The Role of Climate, Minerals, And Organic Matter Chemistry in Soil Carbon Accumulation and Stability

Soils store more carbon than plants and the atmosphere, yet release ten times more carbon dioxide (CO2) to the atmosphere than human activities each year. Small shifts in the balance of soil carbon storage and soil carbon emissions could make climate change less severe, by acting as a sink for the carbon plants pull from the atmosphere, or make climate change worse, by releasing even more planet-warming greenhouse gases. The processes governing whether soils store or release carbon depend on tiny soil microorganisms which convert plant carbon to soil organic matter, releasing CO2 in the process. In my dissertation, I addressed questions about soil organic matter development and the factors that determine whether carbon stays locked in the soil or is lost to the atmosphere. I investigated how climate and small-scale processes determine how much canopy soil accumulates on tree branches. I found evidence that canopy soils may lose more carbon under climate change, as it relies on fog to accumulate and may decompose faster in warmer climates. I also set the stage for future canopy soil research by defining the scales at which canopy soil properties vary. I realized that canopy soils, which consist of organic matter with no clay minerals, present an opportunity to study basic questions about SOM processes. I mixed canopy soil with clay in the laboratory to understand how clay interacts with soil organic matter chemistry to prevent soil CO2 losses and store new soil carbon. Clay reduced soil CO2 emissions while promoting the storage of new carbon that enters the soil. However, when the soil organic matter has already lost carbon through decomposition, soils with clay store less new carbon. I then investigated how soil carbon is affected by warming in natural ecosystems using an existing field warming experiment in the tropical alpine páramo. I found páramo soil CO2 emissions were unaffected by 1-2°C warming because they are more sensitive to soil moisture than temperature. Collectively, this work shines new light on the different scales at which soil organic matter processes occur, and how the factors driving soil carbon storage vs. CO2 emissions interact.