Morphological characterization of solute flow in a brown earth grassland soil with cranefly larvae burrows (leatherjackets)

Cranefly larvae, often known as leatherjackets, are a well known horticultural pest, damaging garden lawns, plants and sporting venues. They burrow into the soil and feed on the roots of plants. However, little is known about the hydrological role of leatherjackets. Such information is needed by modellers to improve water and solute models. This paper investigates flow through leatherjacket burrows in grassland fields. It compares infiltration rates, patterns of dye tracer and pore size distributions in a grassland system with and without leatherjackets. It also compares these distributions to those in an adjacent cropped tillage system which was largely devoid of leatherjackets. Dye tracer distributions show the leatherjacket burrows to act as active preferential flowpaths for water during percolation. A significant relationship between leatherjacket density and infiltration rate was found which took the form of an exponential curve. Leatherjackets were predominately found in the upper 30 cm of the grassland where there was a significantly greater volume of pores that were greater than 2 mm than in the arable soil and where visible macropores, including those open to the surface were significantly more common, covered a significantly larger cross-sectional area and were more circular in cross-sectional area. Leatherjacket burrows significantly reduced the spread of water into the surrounding soil and thereby acted as effective conduits for water to bypass the soil matrix in the upper 30 cm of the grassland. Mean dye coverage was greater at between 30 and 36 cm in the grassland soil profile, where leatherjacket burrows ended, than any other parts of the soil profile deeper than 8 cm. Such dye distributions were rare in the arable soils where very few leatherjackets were present. In some regions fallow grassland is increasing in area in response to environmental policy. Therefore, since infiltration significantly increased with leatherjacket density, consideration of their role in macropore flow will be necessary to understand the full range of hydrological and water quality implications of conversion from arable to fallow grassland management.