Density of macropores as related to soil and earthworm community parameters in cultivated grasslands

Macropore density is a key parameter to predict the leaching of contaminants from the root zone. The aim of this study was to assess the influence of intrinsic soil properties and earthworm characteristics on macropore density at different depths for five typical, Danish soil types under the same agricultural management (third year of pasture). Macropores were marked on a transparent sheet (0.7 × 1.0 m) placed horizontally for the three main horizons of each soil type in eight plots along the local gradients in soil texture. Macropores were then counted on each sheet by digital image analysis. Particle size distribution was determined for the same soil horizons for each plot. The earthworm population was characterised for each plot by hand-sorting soil samples (0.5 × 0.5 × 0.3 m). Results show that the macropore density: 1) was highest for the soil types developed on glacial deposits, which were the soils with the highest clay content, 2) in the topsoil (Ap horizon) was related to the fresh biomass of anecic earthworms across all soil types and textural gradients, 3) in the B horizon was related to the clay content, 4) in the C horizon, was directly related to the soil type, and 5) that for all soil types developed on moraines, the macropore density was highest in the B horizon, indicating an accumulation of macropores with time. Our results indicate that estimates of the macropore density in agricultural soils need to consider both short-term factors (agricultural management effects on the development of earthworm communities), and long-term factors (soil intrinsic properties). Further work is needed to estimate the risk of macropore flow.

Research Highlights
► Macropore density in the topsoil was directly related to anecic earthworm density.
► Macropore density in the B-horizons was directly related to clay content.
► The long-term macropores were related to the geology.
► The short-term macropores were related to the actual earthworm population.
► The use of graphical models eliminated the spurious relations between soil variables.