Soil carbon management in Okanagan apple (Malus domestica) orchards

Strategies to promote capture and sequestration of CO₂ are necessary for achieving our climate mitigation goals in agriculture. Management of the orchard soil under crop trees to maximize storage of soil organic carbon (SOC), using diverse practices such as rootstock selection, application of organic mulches and cultivation of cover crops, have been proposed as low barrier approaches that remain under-studied. Increasing frequency of extreme drought events has also raised questions regarding irrigation costs/benefits associated with more intensive orchard soil management. To effectively evaluate use of orchard soil for long-term carbon storage we must also understand the resilience of newly stored SOC to future climate warming. My research investigated changes in SOC in the crop rows of a newly planted Okanagan apple (Malus domestica) orchard under diverse orchard soil management practices. I found that after two seasons, rootstock Malling 9 (M9) or Budogovski 9 (B9) did not affect total SOC content or concentration; woodchip mulch and a green mulch of woolly thyme (Thymus pseudolanuginosus) had limited effects on total SOC compared to conventionally managed bare soil, but particulate organic carbon was significantly higher under woolly thyme. I also evaluated tree water status and performance (as indicated by photosynthesis rates, and stomatal conductance) under each orchard soil management treatment during well-watered and simulated drought conditions. I found that in well-watered conditions, tree performance was similar among bare soil, woolly thyme and woodchip mulch treatments. During a week-long simulated drought, however, tree performance was better maintained under woodchip mulch but significantly reduced under woolly thyme compared to bare soil. Rootstock did not affect tree performance in either well-watered or drought conditions. Finally, I used Open Top Chambers (OTC) over two growing seasons to evaluate changes in the concentration of SOC in orchard drive rows under a simulated climate-warmed environment. I found that, while significantly altering the soil microclimate, OTC had minimal effect on total SOC pools or organic matter decay. Overall, this work has contributed to an improved understanding of orchard soil management as a sustainable climate change mitigation strategy, and the dynamics of, and methodologies used to estimate, SOC in orchard soils.