Characteristics of the Soil Matrix of Macro-Aggregates Based on Computed Micro-Tomography Images.

Computed micro-tomography has allowed for the non-destructive analysis of soil aggregates in three dimensions.  Most previous work has focused on pore structure, hydraulic properties, particulate organic matter and plant roots, with little focus on the soil matrix itself.  The soil matrix is the mineral components of soil, which is believed to be an important player in soil carbon protection and carbon sequestration.  This study used micro-tomographic images to look at spatial characteristics determined from geostatistical analyses of the soil matrix from two different long term managements, biologically based and conventional management, of a corn-soybean-wheat rotation as well as a primary succession/old field treatment.  Results indicate that, while spatial correlation between treatments are similar, conventional tillage has much less spatial variability than biologically based and/or primary succession, which have similar amounts of spatial variability.  This variability is believed to be correlated to pore structure.  Matrix material around image identified pores were isolated in micro-tomographic images and the average gray scale values relative to the average whole image gray scale value were investigated to see if pore structure differences were responsible for the observed differences in variability.  Gray scale values roughly correspond to the average atomic number in a voxel with lighter elements (C, N) appearing darker than heavier elements (Fe, Si).  Average gray scale values indicated increased lighter elements closest to pores which progressively decreased in concentration as one moved further out from pores.  No difference between average gray scale value for any of the three treatments was observed, except at the furthest distance measured (143-205 μm) where biologically based and primary succession contained much less lighter elements than conventional tillage.  Since micro-porosity was not different between the treatments, it is hypothesized that this difference is due to differences in dissolved organic matter migration into the soil matrix.