UC Davis

The use of organic matter amendments (OMAs) is an ancient practice for improving agricultural soils, but our understanding of the microbial mechanisms by which benefits to soil ecosystems occur remains incomplete. The 1.6 million acres of almond orchards in California need effective and sustainable management practices that promote soil as a living ecosystem. This thesis examines OMAs in the context of emerging microbiome science, our current understanding of relevant soil processes as affected by OMAs, and the challenges involved in modeling nutrient cycling through the microbiome. Results from two research trials conducted in 2019-2021 to evaluate three soil health practices in almond agroecosystems are presented. First, a field trial explored the effects of an almond hull and shell amendment and off-ground harvest on soil C and N and soil microbial biomass in almond orchards. A 210-day incubation trial further evaluated the same amendment on soils with differing management histories by utilizing soil that had previously received a green waste compost OMA for three years. Results highlight the suitability of the almond hull and shell amendment as a soil health treatment, finding that it increased microbial respiration and microbial biomass without significantly affecting nitrogen immobilization. Soil with a history of past OMA application displayed higher microbial respiration, dissolved organic carbon, and net N mineralization than soil without that history when receiving a new amendment of almond hulls and shells. Decomposition of amendment residue progressed at nearly identical rates in the field trial and the incubation trial and exhibited characteristics comparable to forest litter layers. These findings add to our understanding of the processes by which OMAs impact the soil microbiome and our ability to utilize these processes to achieve specific outcomes.