Variability of earthworm-induced biopores and their hydrological effectiveness in space and time

Earthworms create biopores and thereby increase the susceptibility of soils to preferential flow which, on the one hand, reduces surface runoff and soil erosion, but, on the other hand, enhances vertical water and solute transport. Spatial and temporal variability in earthworm abundances might lead to spatial and temporal variability in biopore densities and even in the hydrological effectiveness of these pores. In this paper, we present a reproducible sampling design for simultaneous earthworm-biopore observations and analyze the temporal variability in earthworm abundances, biopore densities and hydrological effectiveness of biopores and its differences between grassland and arable land. During one year we performed six field campaigns where we sampled earthworms and performed dye tracer experiments in the small Luxembourgian Wollefsbach catchment (4.4 km(2)) on three arable land sites and three grasslands. We quantified how abundances of ecological groups of earthworms affect biopore densities and hydrological effectiveness. Finally, we applied piecewise structural equation modeling (piecewise SEM) to find the most probable structure of the ecohydrological system of soil moisture, clay content, earthworms, biopore densities and their hydrological effectiveness. Our results show that earthworm abundance and biopore density as well as their hydrological effectiveness vary strongly between seasons and between grassland and arable land (0-300 earthworms m(-2), 0-500 biopores m(-2), 0-100% hydrologically effective biopores). Piecewise SEMs indicate that the temporal variability in earthworm abundances, biopore densities and hydrological effectiveness is significantly affected by two-month averaged soil moisture variability. We also found a significant effect of earthworm abundances on biopore densities and their hydrological effectiveness in 3 and 10 cm depth. Based on these results we recommend the consideration of spatial and temporal variability in biopore densities and their hydrological effectiveness for the reliable representation of macropore flow and related solute transport in soil hydrological models.