Characterizing soil surface structure in a temperate tree-based intercropping system using X-ray computed tomography

Abstract

The objective of this work was to characterize differences in the soil surface (top 3.5 cm) microstructure, as influenced by four tree species, within a temperate tree based intercropping (TBI) system. Soils adjacent to walnut (Juglans nigra), poplar (Populus spp.), red oak (Quercus rubra), Norway spruce (Picea abies), as well as three types of ground cover [row crop, willow (Salix spp.), and perennial grass tree rows] were analyzed. X-ray computed micro –tomography (µCT) was employed to evaluate soil void phase characteristics, as well as heterogeneity of soil matrix radiodensity. X-ray µCT identified void phase parameters were not affected by tree species due to confounding effects caused by perennial vegetation and mixed leaf litter inputs.. A positive correlation was found between traditionally measured soil bulk density and bulk X-ray radiodensity (rs = 0.53, p < 0.01) and a negative correlation between mean intra-aggregate X-ray radiodensity and soil organic carbon (rs = −0.48, p = 0.03). It was determined, through the use of geostatistics, that there were no distinct or consistent anisotropic structures, in directional semivariograms, evident for the various species. However, the semivariograms revealed greater variability, correlated with less directional anisotropy within the tree row as compared to cropping alley soils. It was interpreted that processes within soils in the tree rows were leading to a homogenous type of structure, and that soils under row crops exhibited a greater tendency for destruction of surface structure, leading to more directional anisotropy (trends).