Depth-differentiated, multivariate control of biopore number under different land-use practices

Earthworms and (tap-)roots impact the soil structure by creating large biopores, affecting infiltration capacity, seepage, nutrient cycling, and soil aeration. Despite the importance of biopores for the functions of soils and the fact that several hundreds of biopores >2 mm in diameter may occur on one square meter of soil, knowledge on the interdependence of soil properties, land-use intensity, and biopore number is still rudimentary. In this study, we investigate the linkage of the number of biopores (>2 mm i.d.) with the earthworm community, root biomass, and soil properties, including pH, water content, soil organic carbon (SOC), as well as the land-use intensity (pasture vs. cropland) as a function of the soil depth (15, 30 and 50 cm). Hypothesized causal relationships among these factors were analyzed by piecewise structural equation modelling (SEM). We found various and novel linkages between roots, earthworms, biopores, and soil properties depending on soil depth. In topsoil (at 15 cm depth), roots directly affected the number of small-sized biopores, and anecic earthworms were related to medium-sized biopores. These effects diminished with depth. We identified land-use intensity as the factor preponderating the relations between biopores, root biomass, and earthworm number in the topsoil horizons, thereby masking other interactions among variables. This appeared as high multicollinearity among variables in the SEM of the topsoil. Land-use intensity effects were found to impact the whole soil profile but decreased with soil depth. To further elucidate the single effects of soil properties on biopore-forming biota and number of biopores in the topsoil, we excluded land-use intensity as a variable in subsequent analyses. Biopores increased with soil pH and soil water content but decreased with increasing SOC. Based on our SEM analysis, we conclude that the occurrence, frequency, and persistence of biopores are the consequence of intricate interdependencies between earthworm communities, roots, and site-specific soil properties, governed by land-use intensity.