The soil invertebrate contribution to nitrogen mineralisation differs between soils under organic and conventional dairy management

Organic management aims to promote soil biological activity. To test whether organic management stimulates soil biological activity, invertebrates (macrofauna, mesofauna and microfauna) were collected from four paired commercial organically and conventionally managed dairy farms on different soil types (Allophanic, Pallic, Recent and flooded Recent). Food webs were constructed and rates of invertebrate-mediated N mineralisation calculated. The organic dairy operations used fewer nutrient inputs and had lower stocking rates than their paired conventional farms. This translated into lower calculated pasture production and less available plant litter entering the soil food web. Despite the lower plant litter inputs into the organic system, earthworm biomass was higher (particularly in the Recent and flooded Recent soils), suggesting that under conventional management the physical condition of the soil, as influenced by stock treading pressures, was more important for invertebrate activity and their influence on N mineralisation than was food supply. Nitrogen mineralisation was higher in organic systems, with earthworms contributing the most (24-98 kg N/ha/year). As the physical loading on the soil increased under conventional management, the ability of the soil to provide soil services (i.e. N mineralisation and litter decomposition) became compromised. Organic management on four soils stimulated biological activity by reducing the treading pressure on the soil and highlights the need to consider the influence of management practices on the faunal environment (food availability and physical condition) to understand the impacts of organic management and the role of fauna in N mineralisation.