Benchmarking nitrous oxide emissions in deciduous tree cropping systems

Abstract

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 0.78gN2O-Nha-1day-1 in the apple orchard at Lucaston to 1.86gN2O-Nha-1day-1 in the cherry orchard in Lower Longley. Daily emissions were up to 50% higher in summer (maximum 5.27gN2O-Nha-1day-1 at Lower Longley) than winter (maximum 2.47gN2O-Nha-1day-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 298gN2O-Nha-1year-1 at Lucaston to 736gN2O-Nha-1year-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.