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Thesis defended by Nathan Chin, January 29, 2025 – Institute of Earth Surface Dynamics (IDYST).
Soils across the globe contain a significant amount of carbon and nutrients, which are often derived from dead plants and plant litter. The breakdown of this plant material, known as decomposition, not only releases the nutrients required by plants and microbes living in the soil, but also releases carbon in the form of CO2, transferring carbon from the soil to the atmosphere. Therefore, factors that affect the decomposition of plant litter have huge implications for both nutrient cycling, and soil CO2 emissions, which in turn affect ecosystem health and climate change.
One significant factor that affects decomposition is the breakdown of highly complex compounds in plant litter, which require electron transfer reactions called oxidation that cleave chemical bonds and makes it easier for microbial enzymes to decompose litter. In soils this can be done by a small specific group of microbial enzymes, and more recently noted, the presence of elements that are highly reactive and facilitate these oxidation reactions. Specifically, the presence of one such metal, manganese (Mn), has been shown to correlate with decreasing soil carbon stocks and increasing rates of decomposition. However, Mn is most reactive when it is in the Mn(III) oxidation state, which is primarily facilitated through microbially-mediated transformation. Mn is also sensitive to oxygen concentrations in soils, influenced by soil moisture and precipitation, affecting its ability to be transformed into Mn(III), with recent evidence suggesting that Mn(III) may form preferentially at water interfaces or transition zones. Despite the demonstrated relationships between Mn(III) and litter decomposition, many studies on decomposition do not take into account the spatial and temporal fluctuations of oxygen that exist naturally in soils. Ignoring the importance of oxygen gradients in decomposition studies creates a poor understanding of how microbes, oxygen gradients, and Mn availability form Mn(III) and affect decomposition.
The objective of this thesis is to determine the microbial and geochemical drivers of Mn(III) formation across oxygen gradients in soils, and ultimately how that controls litter decomposition. We do this using a combination of laboratory soil incubation with fixed oxygen gradients and measurements across a soil forest transect with natural transitions in soil oxygen conditions. We found adding more Mn increases the microbial production of enzymes that directly and indirectly transform Mn(II) to Mn(III) specifically at the oxygen transition zone. In turn, this enhanced formation of reactive Mn(III) enhanced decomposition and created more CO2. The field experiment helped us verify that even across large-scale soil transects, increased formation of Mn(III), led in part by increases in fungi, also results in greater litter decomposition.
Although this research specifically highlights the role of Mn cycling in soils, it also demonstrates future work focusing on decomposition must account for how heterogeneity of oxygen conditions, and its impact on oxygen-sensitive elements that are critical in breaking down plant litter, can be a significant control on decomposition. This is especially true in the face of climate change, which will alter precipitation patterns across different ecosystems, with potential implications for decomposition across soils globally.
