Separating the effects of defoliation and dairy cow treading pressure on the abundance and diversity of soil invertebrates in pastures

Abstract

The impact on soil invertebrates of changes in the physical environment and feed availability in intensive pastoral systems is difficult to predict given the number of interacting factors. This study explores the influence of a change in defoliation regimes (pastoral fallow, cut and carry, cow grazing), feed type and availability to ruminants (pasture and pasture supplemented with maize) and dairy cow stocking rates (3, 4 and 5 cows ha −1) on the abundance and diversity of soil invertebrates (macrofauna, mesofauna and microfauna). Samples were collected from two depths (0–7.5 and 7.5–15 cm) on a well-structured loamy Andosol soil in two seasons (autumn and winter). Treatments allowed an independent assessment of the influence of changes in food sources to the soil food-web from that of physical disturbance from livestock treading. There was a clear distinction in habitable pore space between the fallow and other treatments. The high porosity under fallow (28% under fallow vs. 11–13% under other treatments) appears to be an important factor, increasing faunal abundance and biomass, including macrofaunal predators and Oribatida. Large inputs of litter under fallow benefited epigeic earthworms and resulted in a dominance of bacterial-feeding nematodes (Nematode Channel Ratio 0.97), reflecting the still high quality litter from previous management. Grazed pastures had higher abundances of plant-feeding and plant-associated nematodes as they were stimulated by increased root growth (abundances up to 1.2 × 10 6 ind m −2). While there was little difference in total earthworm abundance between treatments, anecic A. longa became progressively more abundant under higher cow stocking rates as food availability and physical pressure increased (18 ind m −2 vs. 128 ind m −2, under 3 and 5 cows ha −1, respectively). Initial earthworm functional diversity is important and we suggest that anecic earthworms can substitute for epigeic earthworms in intensively managed pastoral systems by incorporating litter, as well as being important ecosystem engineers.