Pathway to carbon neutral – whole orchard recycling in almond orchards (AL21000)

This project explored whole orchard recycling (WOR) as a sustainable alternative to the traditional practice of burning almond trees during orchard redevelopment. Almond trees store large amounts of carbon in their trunks, branches and roots throughout their lifecycle. When orchards reach the end of their commercial life, burning is commonly used to clear debris and reduce pathogens, but this method releases significant carbon into the atmosphere. WOR offers a more environmentally friendly approach by mulching the removed trees and incorporating the material into the soil, sequestering carbon, and enhancing soil organic matter, fertility, and supporting the establishment and productivity of new orchards. 

The trial, conducted in a commercial orchard near Merbein in Victoria’s Sunraysia region, compared WOR with compost amendments and a control treatment over three growing seasons. The existing orchard was removed and processed into chips using a horizontal grinder. Wood chips were broadcast onto the soil surface, then tree mounds drawn up, moving the bulk of the chip material into the tree row. As an alternative treatment, an almond hull and shell-based compost (compost) was applied to the soil and mounded in a similar manner. The control treatment was mounded, but without any organic amendments.  

Trees in the WOR treatment initially grew more slowly but caught up by the second season, showing good tree growth and light interception and no yield penalty compared to other treatments. Recycled trees or compost increased soil organic carbon by 30 per cent, although other soil physical properties remained largely unchanged by the end of the three-year study. Trees in WOR-treated soils showed reduced nitrate concentrations in the water sampled from below the root zone and only temporarily elevated carbon dioxide and nitrous oxide greenhouse gases, which normalised after the first year. 

By the third season, there were no differences in nut yield between treatments, which aligns with findings from California suggesting that productivity improvements from WOR may take up to seven years. Overall, WOR supported healthy tree growth and orchard establishment without yield penalties or adverse environmental effects through nitrate leaching or greenhouse gas emissions. Soil organic carbon levels were elevated by whole orchard recycling, but improvements in other soil parameters were not observed.