Quigley, M. Y. 2019. Contribution of soil pores to the processing and protection of soil carbon at micro-scale. Dissertation, Michigan State University, East Lansing, MI.
oil carbon has the potential to increase crop yield and mitigate climate change. As the largest terrestrial carbon stock, gains and losses of soil carbon can have a great impact on atmospheric CO2 concentrations. Additionally, many beneficial soil properties for agricultural sustainability are tied to soil carbon. This makes understanding the mechanics of soil carbon vital to accurate climate change modeling and management recommendations. However, current soil carbon models, relying on bulk characteristics, can vary widely in their results and current recommendations for improving soil carbon do not work in all circumstances. Micro-scale processes, the scale at which carbon protection occurs, are currently not well understood. Improving the understanding of micro-scale processes would improve both climate models and management recommendations.
Carbon processes at micro-scale are believed to occur in diverse microenvironments. However, it is soil pores that, through transport of gasses, water, nutrients and microorganisms, may ultimately control the formation of these microenvironments. Therefore, understanding the relationship between soil pores and carbon is potentially vital to understanding micro-scale carbon processes. To understand the relationship between soil pores and carbon I employed computed microtomography (μCT) to obtain pore information and stable carbon isotopes to track carbon. I investigated the spatial variability of soil carbon within the soil matrix of different soil managements and how pores of different origin contributed to this variability to explore the effect of management and pore origin on the creation of microenvironments. Then I investigated the effect of pore size distribution on carbon addition during growth of cereal rye (Secale cereale L.) and usage during a subsequent incubation using natural abundance stable carbon isotopes. I investigated the role of management history on the effect of pore size distribution during new carbon addition and usage using enriched stable carbon isotopes.
I found managements that build carbon have higher spatial variability of grayscale values in μCT images than managements that lose carbon. This variability is related to the amount of biological pores, due to their larger range of influence as compared to mechanical pores (123 μm vs. 30 μm), which would impact variability greater. The influence of biological and mechanical pores on adjacent carbon concentrations was found to be independent of management. Pores of 15-40 μm range were associated with carbon protection after incubation, matching previously reported results, indicating a universal mechanism for carbon protection, possibly related to fungi, in these pores. From both natural abundance and enriched stable carbon isotope studies, I found that 40-90 μm pores are associated with large gains of new carbon during rye growth, but large losses of new carbon in the subsequent incubations.
I found important relationships between pore origin, pore sizes, and carbon, specifically, that biological pores exert more influence on the carbon concentrations adjacent to them than mechanical pores. A technique to measure this influence using osmium staining of organic matter and grayscale gradients of images was developed. I found that 40-90 μm pores are important avenues of carbon addition, but also are associated with carbon losses. However, the reasons for these easy gains and losses is yet unclear, requiring further research, but it is believed to be associated with small plant roots.
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