University of California Merced, California, United States
Abstract: Carbon (C) storage within soils, and the potential for increased C sequestration with management, can play a pivotal role in climate mitigation and agricultural adaptation. California has an estimated ~ 2 million bearing acres of orchards, providing ample acreage to experiment with increasing C storage with various soil health practices. It is important to assess the amount of soil C in both surface (0-30 cm) and subsoil (30 -100 cm) in orchards as a baseline for understanding their response to sustainable soil health management. Deep soil C, which we define as soil below 30 cm in agricultural systems, can be influenced by land management practices but is poorly characterized within orchards. Our study aims to address the great uncertainty of deep soil C storage in various orchard types across the Central Valley. Our approach was to conduct a large baseline soil sampling campaign in June of 2021, as part of a collaborative NRCS Conservation and Innovation Grant (CIG) project. Our study spanned across 8 sites (five almond orchards, and one each of peach, walnut, and pistachios) in the Central Valley of California. Samples were collected at five depth intervals to one-meter depth, analyzed for total soil C concentration, then converted to soil C stocks via two methods: equivalent soil mass and bulk density. The two methodologies are compared to assess the associated biases and errors related with sampling procedure and calculations. We hypothesized that orchard soils would have relatively low C content which would result in small soil C pools. We expected the strongest indicator of soil C pool size would be management and site history, followed by soil texture, tree type, and then tree age. Sites already implementing soil health practices will have a higher soil C pool size than those managed conventionally. Preliminary results show that the average soil C content of surface soils (0-30 cm) across the Central Valley is 0.97 % (SE 0.03), and sharply decreases to 0.34% (SE 0.34) at 1 m. From these data, we will then evaluate the potential of these soils to sequester more C, based on their assumed saturation point based on soil texture and type. The very low content of soil C in orchards provides an exciting opportunity to increase soil C content with soil health practices such as compost with the co-benefits of increased ecosystem resilience to drought.
