Coupling Computed Micro-Tomographic Images and Stable Isotopes to Elucidate the Relationship between Soil Carbon and Soil Structure.

Soil carbon is an important soil resource. It improves soil health, which boost plant productivity, but also stores atmospheric carbon, which mitigates global climate change. Soil structure and soil pores, specifically, control the transport of microbes, air and water throughout the soil. This in turn affects the physical protection of soil carbon within soil aggregates. Plant roots both influence and are influenced by soil pore structure. However, the relationship at aggregate scale between pores, roots and soil carbon is yet to be fully understood. This study uses stable carbon isotopes from an enrichment study to “track” the input of carbon into the system from plant roots and the subsequent utilization of this carbon during incubation coupled with pore size distribution data from computed micro-tomographic (μCT) images. Soil was obtained from two treatments (conventional and cover crop-based) of a long-term experiment in southwest Michigan. Cereal rye (Secale cereale L.) was planted in boxes with collected soil, either intact or sieved through a 1 mm sieve, and pulse-labeled with enriched CO₂; areas inaccessible to plant roots were created in each box using 35 μm mesh enclosures. Collected soil samples were split into two groups. One half were not incubated and had μCT images and δ¹³C measurements taken, while the other half had μCT images taken prior to and after incubation and δ¹³C measurements taken after incubation only. Differences in total carbon were strictly management based. However, when looking at the fraction of new carbon, root presence was the only significant factor prior to incubation, while both root presence and intact or destroyed structure was an effect after incubation. This indicates that while long term management affects the overall carbon content, soil structure may control usage of new carbon.