Neutron imaging of exchange flow between biopore and matrix for Bt versus C horizons

Abstract

The surface coating of biopores plays a pivotal role in mediating mass exchange between the biopore and the soil matrix, ultimately governing preferential flow in structured soils. However, quantitative experiments of the flow exchange dynamics in the soil around an earthworm burrow (drilosphere) are limited. Here, we developed an experimental setup to mimic preferential flow within a biopore while monitoring the water distribution by neutron radiography. The objective was to quantify the evolving water content distribution in the biopore with surrounding drilosphere samples from Bt and C horizons. We hypothesize that the distribution of clay and organic material across biopore surfaces in Bt and C horizons impacts the biopore coating material formation (i.e., composition and thickness), thereby influencing its role in controlling mass exchange during preferential flow events. The experimental setup was proposed for in situ quantification of the preferential flow dynamics in an earthworm burrow (biopore) passing through Bt and C horizons of a Luvisol developed on glacial till. The neutron radiographies were obtained at the NEUtron Transmission Radiography facility of the Paul Scherrer Institut, Villingen. The volumetric water content was quantified by calibrating gray scale values from radiography images with defined water contents using stepped aluminum cuvettes attached to the samples; image analysis was used to determine sorptivity at the biopore–soil matrix interface. The water exchange flow was found to be smaller for coated biopore surface in the more clayey Bt horizon, as compared to the coarser textured carbonaceous subsoil C horizon, where higher sorptivity intensified mass exchange. The method most realistically captured the dynamics of the vertical preferential flow process in a biopore, including lateral exchange flow, by determining the water sorptivity of the drilosphere. In contrast to mini‐infiltrometer data, biopore–matrix exchange flow was here more restricted for Bt in contrast to carbonaceous C horizon. Since both horizons showed the presence of coating material on their burrow walls, the results suggest further analysis of their contrasting properties and structures.