Carbon Distribution within Intact Soil Macro-Aggregates.

Soil carbon is important not only to agriculture, as it is vital to soil health, but also as the major terrestrial sink of atmospheric CO₂.  Increasing soil carbon positively improves soil aggregation, soil cation exchange, soil water holding capacity, and soil drainage in agricultural soils, while minimizing climate change.  It is believed that most of the carbon in soils is protected and stored within soil aggregates.  However, how and where this protection and storage takes place within the aggregates themselves is not completely understood.  One reason for this is that studying soil aggregates usually involves destructive measurements which limit the level of understanding that can be achieved regarding intra-aggregate physical structure.  Computed microtomography (µCT) is a non-destructive technique that creates 3D images of soil aggregates at resolutions of several microns.  Gray scale values of the images correlate to intra-aggregate structural components.  Mineralogy, pore/void space, soil organic matter (SOM), and particulate organic matter (POM) are the major components that affect the gray scale values.  Previous results have shown that by combining traditional techniques, such as carbon measurements, with μCT gray images, it is possible to correlate soil carbon within an aggregate to the aggregate’s physical features, such as pore size.  Our research continues this line of investigation by looking at where existing and fresh carbon inputs are being added/protected within soil aggregates.  The study is based on using stable carbon isotopes and the inherently different isotopic signatures of C3 and C4 plants to identify existing (C4 plant) and freshly added (C3 plant) carbon inputs. The isotope data can then be correlated to pore characteristics and POM features identified from the µCT aggregate images. Results of these and additional analyses will be presented and discussed.