Elucidating controls of the variability of deep soil bulk density

Abstract

Information on the variability of bulk density (BD) in deep soil is critical to understand the hydrological dynamics of the deep unsaturated zone (USZ), but studies have mainly focused on quantifying BD in shallow soils (<1 m). The objective of this study was to quantify the spatial variability of soil BD in deep USZ and their controls at multiple scales. Three undisturbed soil cores (98.0, 18.4, 22.8 m) were collected from three land-use areas (grassland, orchard, and farmland) from the Changwu tableland of the Chinese Loess Plateau; then, BD, magnetic susceptibility, particle size and organic carbon were determined in the laboratory. The spatial patterns and scale- and location-specific controls of BD variability were quantified using traditional statistics and wavelet analyses. BD distribution in the deep USZ was consistent across the cores indicating the minimal influence of land-use. For the vertical distribution of the USZ BD, the coefficient of variation (CV) in the upper 0–40 m was higher (8%) compared to the broadly steady BD (CV = 3%) in the lower 40–90 m depth. BD with depth mainly exhibited two-type variations: an exponentially increasing trend at the large scale (13–30 m) and a cyclical trend at the medium scale (3–13 m). While soil depth was the major factor contributing to BD variability at the large scale, magnetic susceptibility contributed at the medium scale and exhibited a stronger effect than organic carbon and particle size composition. This information on the deep USZ BD is distinctly different than that of the shallow soil from previous studies and enhanced our understanding of BD spatial patterns with depth and identified the dominant factor(s) to improve predictability. This can provide an objective evaluation of soil characteristics, accurate estimation of soil water, nutrients and carbon stocks and fluxes, and the simulation of related hydraulic processes in the deep USZ.