Whole Orchard Recycling Could Benefit Soil Quality and Provide Additional Carbon Sequestration

Posted February 13th, 2016

California almond growers are examining new ways to deal with tree biomass produced by the removal of old orchards other than cogeneration burning. While sending the biomass to cogeneration converts the wood to electricity, returning the wood back to the soil would return the nutrients and energy back to the soil ecosystem.

Thus, one concept getting closer attention is whole orchard recycling – grinding up entire almond orchards and incorporating the tree biomass into the soil, returning nutrients, improving the soil quality, and potentially sequestering the carbon contained in the wood chips.

The potential benefits of whole orchard recycling on soil quality and subsequent trees replanted in that orchard have been the subject of Almond Board-funded research by Dr. Brent Holtz of the UC Cooperative Extension farm advisor in San Joaquin County and will be discussed and demonstrated at a grower field day next week.

Using a 50-ton “Iron Wolf” rototiller capable of grinding standing whole trees and incorporating chips into the soil, peach trees in an old orchard were shredded and incorporated in a plot, with trees in another plot burned and the ashes spread on the soil surface. Almond trees planted in both plots were fertilized normally. By the third year, the nutrients were significantly greater where trees were ground up and incorporated into the soil. By the 6th year, the whole orchard chip incorporation treatment resulted in increased organic matter, soil carbon storage, nutrients and microbial diversity, including beneficial fungi, as well as increased water-holding capacity of the soil.

An almond Life Cycle Assessment (LCA) study released last year by UC Davis showed that almond trees accumulate and store significant amounts of carbon in the woody biomass during the average 25 year lifespan of an orchard.   If the woody biomass is incorporated into the soil at the end of the orchard’s life, that carbon in the wood is only gradually released by soil microbes, extending the carbon sequestration. Thus, orchard recycling could contribute to reducing the carbon footprint of producing almonds.    

“This first research trial of some 6 years provides some very intriguing results indicating a potentially sustainable solution to orchard biomass, though additional work is needed about its impacts under variable orchard and soil conditions to prove its feasibility for all California almond orchards,” said the Almond Board’s Dr. Gabriele Ludwig, Director, Sustainability & Environmental Affairs. “The Almond Board will continue to invest in research opportunities to prove this technique.”