Quantifying the potential contribution of soil carbon to orchard carbon footprints

Increasing soil carbon is promoted to growers as a means of increasing soil function and mitigating climate change. However, the importance of soil carbon is not yet fully acknowledged in life cycle assessment guidelines. Because of their perennial nature and deep rooting systems, orchards have the potential to increase subsoil carbon stocks below 0.3 m depth. Quantifying soil carbon stocks at the orchard scale, however, requires sampling protocols that incorporate the spatial variability of carbon in orchards. We have investigated soil carbon stocks to 1 m depth in apple orchards in Australia and New Zealand and found that total carbon stocks were maintained over four to 12 years. Orchard floor management practices can create a spatial distribution of shallow soil carbon stocks, and we observed a trend for decreasing surface-soil carbon stocks under the alley. Additionally, we determined soil carbon stocks to 9 m depth in a New Zealand kiwifruit orchard and calculated an increase of 190 t C ha-1 after 30 years compared with that in the nearby pasture soil that was the antecedent land-use. This increase in soil carbon equates to 42% of the carbon footprint of New Zealand kiwifruit on United Kingdom supermarket shelves. Therefore, accurately quantifying soil carbon stock changes under orchard systems presents a potential opportunity to reduce horticultural carbon footprints.