Effects of Soil Moisture On Macropore Geometry.

Macropores—large voids that can alter soil hydrology—include interpedal pores, faunal burrows, large interstitial pores, and tillage fractures. These large pores (>0.03 mm) are dynamic and can respond rapidly to changes in soil moisture content. The goal of this work was to understand the response of macropore geometries to decreasing volumetric soil water content (θ_v). We used multistripe laser triangulation (MLT) scanning technology to quantitatively characterize macropores. A 6.5 cm diameter by 1 m depth core was taken from a Grundy Series (fine, smectitic, mesic Oxyaquic Vertic Argiudoll) at the University of Kansas Field Station. This core was divided into several depth sections. Each section was vertically split into mirrored half sections by inserting a flat spatula into the sample oriented along the long axis of the core. The cores were wrapped in cheesecloth and allowed to saturated in a few cm of tap water for several days.  Half sections of the core were then periodically scanned as the soil dried over the course of a week. We simultaneously took samples for θ_v determination from the corresponding half sections. Macropore geometries were obtained by projecting missing data points from the MLT scan onto a 2D plane. The resulting polygons were analyzed using ImageJ for average area, perimeter, width, circularity, and feret diameter. The shape of the moisture depth profile remained relatively constant throughout the drying process. Macropore geometries experienced the most amount of change within the first few days of drying. Area, perimeter, width, and feret diameter increased significantly across all depths as the soil dried. As θ_v  decreased from ~0.3-0.1, we observed little change in average area and perimeter. Circularity dropped throughout the profile as θ_v decreased with the largest change observed between 0 and 24 hours, after which circularity became less dependent on θ_v.