Benchmarking nitrous oxide emissions in deciduous tree cropping systems

Nitrous oxide (N₂O) emissions contribute 6% of the global warming effect and are derived from the activity of soil-based microorganisms involved in nitrification and denitrification processes. There is a paucity of greenhouse gas emissions data for Australia’s horticulture industry. In this study we investigated N₂O flux from two deciduous fruit tree crops, apples and cherries, in two predominant growing regions in eastern Australia, the Huon Valley in southern Tasmania (Lucaston – apples and Lower Longley – cherries), and high altitude northern New South Wales (Orange – apples and Young – cherries). Estimated from manual chamber measurements over a 12-month period, average daily emissions were very low ranging from 0.78 g N₂O-N ha^–1 day^–1 in the apple orchard at Lucaston to 1.86 g N₂O-N ha^–1 day^–1 in the cherry orchard in Lower Longley. Daily emissions were up to 50% higher in summer (maximum 5.27 g N₂O-N ha^–1 day^–1 at Lower Longley) than winter (maximum 2.47 g N₂O-N ha^–1 day^–1 at Young) across the four trial orchards. N₂O emissions were ~40% greater in the inter-row than the tree line for each orchard. Daily flux rates were used as a loss estimate for annual emissions, which ranged from 298 g N₂O-N ha^–1 year^–1 at Lucaston to 736 g N₂O-N ha^–1 year^–1 at Lower Longley. Emissions were poorly correlated with soil temperature, volumetric water content, water filled porosity, gravimetric water content and matric potential – with inconsistent patterns between sites, within the tree line and inter-row and between seasons. Stepwise linear regression models for the Lucaston site accounted for less than 10% of the variance in N₂O emissions, for which soil temperature was the strongest predictor. N₂O emissions in deciduous tree crops were among the lowest recorded for Australian agriculture, most likely due to low rates of N fertiliser, cool temperate growing conditions and highly efficient drip irrigation systems. We recommend that optimising nutrient use efficiency with improved drainage and a reduction in soil compaction in the inter-row will facilitate further mitigation of N₂O emissions.