The production and movement of pyrogenic carbon through the landscape

The production and movement of pyrogenic carbon through the landscape by Lacey Ann Pyle Soil carbon constitutes a substantial reservoir of the global carbon cycle and pyrogenic carbon (PyC – the product of incomplete combustion of biomolecules) is a sizeable and relatively stable fraction of total soil carbon. However, PyC storage in soils is still poorly understood. Very old PyC is often observed in soils, which has led to widespread interest in PyC as a carbon sequestration opportunity. Yet in modern short-term studies, PyC seems to break down rapidly through various mechanisms such as dissolution or mineralization. This leaves us with a conundrum on how to estimate soil PyC storage in a useful manner. Soil chronosequences provide an opportunity to exam soil PyC storage over longer time periods than experimental work allows. Presently, climate change is predicted to increase the frequency of fire outbreak which may alter the global carbon cycle, partially by increasing global production of PyC. As up to 80% of PyC produced in wildfires can remain local, increased fire frequency will cause significant perturbations to soil PyC accumulation. This thesis seeks to understand how PyC is first produced and then incorporated into soils through determination of soil residence times, with a special focus on the movement of PyC. The thermal production conditions of PyC control to a large degree the resulting properties of PyC. Properties such as aromaticity and C:N ratio contribute to the relative stability of PyC particles by making PyC more or less vulnerable to microbial attack and dissolution. In experimental production, PyC displays a strong increase in aromaticity of the end product concurrent with increases in charring intensity, or the amount of heat applied to the sample. These observations can be used to not only predict the end properties of designer PyC and compare production methods from different laboratories, but can also inform the various compositional changes that wildfire causes in soils. Progressively increasing charring intensity alters PyC, and has a significant impact on the pyrogenic nitrogen (PyN) within the end product as well. Charring results in transformation of nitrogen-containing biomolecules into more aromatic PyN structures, and isotopic fractionation occurs, resulting in PyN that is more enriched in 15N than the beginning biomass. This transformation is useful in identifying wildfire occurences in various ecosystems. A complicating factor of PyC retention in soils is actually physical – PyC has a tendency to be highly porous with a low envelope density, which gives it an enhanced vulnerability to erosion via surface water flow. High porosity and low density are important qualities in PyC and are drivers of the ecosystem services commonly observed with PyC soil additions, by, for example, increasing soil water holding capacity or providing refuges for soil biota. Within the soil column, PyC undergoes mineral and biological interactions that have an effect on the density of PyC particles, which may contribute to their retention. Over periods of a couple to 80 years, PyC particles within soils display significant increases in particle density, but little change in particle porosity. These density increases will change the particle vulnerability to redistribution over time, while the static porosity indicates that even aged PyC particles can provide some of the same ecosystem services as fresh PyC. Nuclear magnetic resonance (NMR) techniques reveal qualitative information about the chemistry of PyC in fire-effected soils, as well as conservative estimates of PyC contents in soils. Soils that have undergone fire treatment recently have shown high variability in soil organic carbon (SOC) constituents (such as lignin, lipids, or carbohydrates) over even very small distances, but over time this spatial variability decreases with one exception: PyC. PyC contents in soils, which are much higher on average after a fire, become more spatially variable with time. The implication of this is that much larger datasets are needed to study PyC for an accurate assessment that takes into account the spatial variability. In this large dataset, PyC appears to be more resistant to degradation and more vulnerable to physical redistribution than other fractions of SOC, even in soils that have very little reactive mineralogy. This could constitute a negative feedback relationship between climate, wildfires and soil carbon storage whereby production of PyC from increased wildfire occurrence results in more long term SOC storage and therefore a reduction in atmospheric carbon.