Nitrous oxide emissions following dairy shed effluent application beneath Kunzea robusta (Myrtaceae) trees

Abstract

Agriculture contributes more than a third of anthropogenic nitrous oxide (N₂O) emissions globally. In New Zealand, land application of dairy shed effluent contributes to the 90% of N₂O emitted from agricultural soils. Novel strategies are urgently required to mitigate N₂O production to ensure New Zealand's dairy-based economy is environmentally sustainable. Species of Myrtaceae, including Kunzea spp. (kānuka, white tea-tree) have previously been shown to produce antimicrobial compounds which extend to the soil. It is possible that these may inhibit the microbes involved in biological nitrification and denitrification which could thereby suppress N₂O production. Therefore, in this work we aimed to test whether irrigation of effluents to stands of Kunzea spp. could minimize resulting N₂O emissions. This study investigated soil inorganic N and N₂O emissions following the application of dairy shed effluent to soil beneath 5-yr-old K. robusta compared with bare soil. Following effluent application, N₂O emissions beneath K. robusta were reduced by 80% relative to bare soil, but nitrate-N was five-fold higher than bare soil, sufficiently available for denitrification. The drier, more aerated soil associated with K. robusta may have constrained denitrification. Application of DSE (50 kg N ha⁻¹) to K. robusta produced 0.133 kg N₂O–N ha⁻¹ during the experimental period; equivalent to the lower range of emissions measured following comparable applications to grazed dairy pastures in New Zealand (0.13-1.08 kg N ha⁻¹). The environmental benefits of reduced N₂O emissions warrant further investigation on the effect of Myrtaceae on the soil N cycle worldwide.