Macropore formation
The process of macropore formation refers to the creation of large channels in the soil either by roots, earthworms (burrows) or ants/termites (nests). The resulting macropores can be a range of different sizes (0.05 mm to > 6 mm) depending upon the size of the biotic actor that created it, but in general in the field macropores more than 2 mm in diameter are measured[1]. Macropores have a great influence on the function of Water Regulation and Purification. Not only do macropores control the rate of movement of water through the soil[2] but they also support infiltration where the macropores come to the surface of the soil such as in the case of burrows made by anecic earthworms[3]. The regulation of water through the profile is not only dependent on the number of macropores present, but also the connectivity of the macropores, known as macropore architecture. The architecture of the macropore network really determines the rate of water percolation on the larger landscape scale [4],[5].
The measurement of macroporosity is generally a physical assessment of the soil structure where the presence of macropores are counted or mapped. This can be completed in the field through visual assessment where earthworm burrows of > 2 mm in diameter (visible to the eye) are counted and mapped and compared to earthworm abundance and species diversity or ecological groups present[6] (see description on earthworms). In the lab macropores in soil columns can be assessed using imaging techniques such as x-ray tomography where the macropores are mapped digitally[7]. Tracer experiments using dyes have also been applied to measure the conductivity of macropores in the field to assess the impact on water infiltration and percolation[8]. The tomography and tracer experiments are considered research methodologies, due to the specialised equipment needed.
[1] van Schaik L et al. 2014. Linking spatial earthworm distribution to macropore numbers and hydrological effectiveness. Ecohydrology 7: 401-408.
[2] Jarvis NJ. 2007. A review of non-equilibrium water flow and solute transport in soil macropores: principles, controlling factors and consequences for water quality. European Journal of Soil Science 58: 523– 546.
[3] Spurgeon DJ et al. 2013. Land-use and landmanagement change: relationships with earthworm and fungi communities and soil structural properties. BMC Ecology 13:46.
[4] Beven K & Germann P. 2013. Macropores and water flow in soils revisited. Water Resource Research 49: 3071–3092.
[5] Schneider A-K et al. 2018. Variability of earthworm-induced biopores and their hydrological effectiveness in space and time. Pedobiologia 71:8-19.
[6] Pérès G et al. 2010. Relationships between earthworm communities and burrow numbers under different land use systems. Pedobiologia 54: 37–44.
[7] Capowiez Y et al. 2000. Evolution of burrow systems after the accidental introduction of a new earthworm species into a Swiss pre-alpine meadow. Biology and Fertility of Soils 31: 494–500.
[8] Flury M et al. 1994. Susceptibility of soils to preferential flow of water: a field study. Water Resource Research 30: 1945–1954.