OOS 29-5 - Organic matter accumulation in biomass-amended agricultural soils

Enhancing soil carbon (C) stocks, which represent the balance between C input and loss, has strong potential to reduce climate change effects and build resilience into agroecosystems by improving soil health. Such an approach is particularly needed in California’s Central Valley, one of the most productive agricultural regions in the United States, which is threatened by interacting stressors of higher temperatures, extreme heat waves, and water scarcity. Nut tree orchards (e.g., almonds, walnuts, and pistachios) occupy nearly 2.5 million acres of land in the Central Valley. Many of these orchards have soils with low organic C contents (e.g., <2%) and thus the potential to hold substantially more C. A climate-smart practice called biomass recycling (BR) or whole orchard recycling offers a strategy to build soil organic matter (SOM). BR is the on-site grinding of trees into wood chips and their subsequent incorporation into the soil prior to replanting orchards or other crops. In the first year after BR, carbon dioxide emissions are higher in recycled soils than in unamended soils; these emissions approach rates similar to unamended soils within three years. Modeling of soil C dynamics in conventionally-managed and recycled orchards predicts BR will yield higher soil organic C over a 20-year life span of an orchard. In this work, we characterize SOM, soil physicochemistry, and microbial communities in recycled and conventionally-managed orchards of various ages to test the hypothesis that wood chip amendment leads to greater SOM accumulation by initially shifting microbial communities towards fungal dominance and lower microbial carbon use efficiency. Over time, we expect C derived from wood chip degradation to be processed through active microbial pools with greater efficiency and enter the stable C pool through higher microbial necromass formation. Results indicate recycled soils have higher microbial biomass, greater activity of enzymes involved in carbon degradation, different microbiome compositions and predicted functions, and higher predicted growth rates derived from genomic signatures than unamended soils. Also, these differences between recycled and unamended soils change over time. When combined with analysis of SOM pools, we expect to acquire an integrated understanding of how soil C accumulates under BR as recycled orchards age.