Testing numeric and logarithmic scales for hydraulic-energy indices and functions to assess physical earthworm effects on soil structure

Sufficient soil physical quality (SPQ) is needed to support the provision of multiple ecosystem services including food and fibre production, cycling of nutrients, cycling and storage of carbon, filtration of water and a habitat for biodiversity. Several SPQ indices are used to ascertain the SPQ of a soil and are often derived from the soil water retention curve (SWRC), built with multiple volumetric water content (θ) points across the pressure head (h) range, from saturation to oven-dried soil. The integral area under the SWRC has been used as an indicator for evaluating SPQ. The present study compared five hydraulic-energy SPQ indices and two cumulative hydraulic-energy functions based on the SWRC integral calculated on θ[log10(h)] basis and θ(h) in terms of their ability to track and refine soil structural information over time. For this, both approaches were applied to the SWRC data developed to examine the relationship between SPQ and earthworm abundance under dung pat and non-dung pat treatments from deposition to decay and beyond at a grassland field site in Ireland. Results showed that plant available water content (PAWC) and drainable porosity (DP) can be examined with either numeric (θ(h)) and logarithmic (θ[log10(h)]) scales and outcomes were not significantly different. Although the absolute hydraulic-energy SPQ indices of aeration and water retention presented distinct results, they both resulted in the same information with respect to soil structure status and this was consistent across numeric or logarithmic scales. However, the relative hydraulic-energy indices of aeration, water retention and air-water revealed more information with respect to water retention and drainage processes than when calculated on a logarithmic basis. Over the study duration, it was shown that more than 85% of the integral energy was used for storing water in the PAWC range and was dominant over the aeration process regardless of the treatment imposed. The results also indicate high and medium effect sizes for some parameters suggesting that a larger population or sample size could enhance the correlations and significance above those found in the present study.