Influence of dung pats on soil physical quality mediated by earthworms: from dung deposition to decay and beyond

Abstract

Soil quality determines the ability of soil to deliver ecosystem services and can be inferred from physical, biological and chemical indicators either in isolation or in combination. Earthworms are good soil-quality indicators that contribute to both chemical and physical quality by maintaining soil structure and cycling nutrients. The presence of dung pats can increase earthworm abundance locally and consequently the network of pores that they create through their burrowing activity. Inevitably this affects soil structure and consequently will have a spatially distributed effect on soil physical quality (SPQ). The aim of this field study was to examine the relationship between SPQ and earthworm abundance under dung and non-dung pat areas from deposition to decay and beyond. The present spatial and temporal study compared SPQ indicator (integral air-water energy, AWr) results with earthworm abundance across control and simulated dung pat treatments. Results showed that existing earthworm populations in this grassland were already very large (>500 individuals m–2) and SPQ (AWr) remained in the ‘very good’ category throughout the experiment. Earthworm abundance under dung pats and SPQ exhibited a significant (P = 0.05) temporal trend. In general, the time of decay of the dung pats played a role in increasing earthworm abundance and SPQ. Earthworm abundance and macropore density data formed a similar, ‘hump’-shaped dynamic over time. However, when an earthworm abundance threshold was exceeded (equivalent to about >3000 individuals m–2), the increase of SPQ under dung was attenuated and did increase further only under the control sward with high earthworm abundance. After 11 weeks, for both treatments, AWr under dung pats was capped at 0.83% and AWr under control sward peaked at 1.34%. Future work should focus on (a) further exploration of the threshold where earthworm abundance becomes detrimental for SPQ and (b) using the AWr SPQ indicator within an actual grazed trial which incorporates a gradient of soil degradation.