Earthworms and nitrogen applications to improve soil health in an intensively cultivated kiwifruit orchard

Abstract Integrated approaches which simultaneously consider how intensification affects soil biota and the processes they regulate assist in developing sustainable management practices. Therefore, in this study, we investigated the effect of introducing an anecic earthworm ( Lumbricus friendi ), in combination with two nitrogen applications either via biological fixation (clover, +EC) or cattle manure (+EM) on the chemical properties of an horticultural soil (C release, N mineralisation, soil pH, cations and P availability) collected from a kiwifruit orchard where the fruit is intensively produced using conventional agriculture practices. The laboratory incubation study also included two controls with and without earthworms (+E and −E, respectively) replicated 15 times. Results from destructive sampling across 5 sampling occasions showed that the addition of an organic fertiliser had a positive effect on the earthworm biomass, C and N mineralisation, and nutrient availability (namely Mg, K and assimilable P), suggesting that the organic matter provided was very accessible to microorganisms and earthworms. In contrast, the inclusion of a N-fixing legume did not have similar positive effects on the health of these agricultural soils, which could be a reflection of an unsuccessful nodulation and P limitation. Importantly, up to 85% of the total amount of nitrogen leached from these treatments consisted of dissolved organic nitrogen, which represents a significant N pool and could have economic and environmental implications, but could be controlled by synchronising application rates with crop demands. These results suggest that the combined effects of earthworms and animal manures supply the soil with a good source of both nutrients and microbial populations and provide a more sustainable management of these agro-ecosystems than conventional agriculture.