Effects of whole-orchard recycling on nitrate leaching potential in almond production systems

Authors: JAHANZAD, EMAD - University Of California, Davis; BREWER, KELSEY - University Of California, Davis; Poret-Peterson, Amisha; CULUMBER, MAE - University Of California Agriculture And Natural Resources (UCANR); HOLTZ, BRENT - University Of California Agriculture And Natural Resources (UCANR); GAUDIN, AMELIE - University Of California, Davis

Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/13/2022
Publication Date: 7/3/2022
Citation: Jahanzad, E., Brewer, K., Poret-Peterson, A.T., Culumber, M., Holtz, B.A., Gaudin, A. 2022. Effects of whole-orchard recycling on nitrate leaching potential in almond production systems. Journal of Environmental Quality. 51(5):941-951. https://doi.org/10.1002/jeq2.20385.
DOI: https://doi.org/10.1002/jeq2.20385

Interpretive Summary: Inefficiencies in fertilization and irrigation in California’s agricultural production regions has led to high nitrate levels in groundwater. At the same time, a practice called whole orchard recycling provides an opportunity for perennial crop growers who replant orchards to conserve soil and water resources. Whole orchard recycling reincorporates tree biomass into the soil prior to planting a new orchard. This study examines the benefits and tradeoffs of this practice for soil nitrogen cycling and retention using laboratory-based soil column incubation experiments. Nitrogen pools, including microbial biomass and inorganic nitrogen in soil and leachate, were traced and microbial community shifts characterized by the abundance of key nitrogen cycling genes. In the short-term, whole orchard recycling led to nitrogen immobilization into biomass without impacts on nitrogen leaching potential. In the long-term, this practice considerably reduced nitrate leaching potential by 52% without increasing nitrogen immobilization. This study highlights the potential of whole orchard recycling to meaningfully mitigate nitrate loss into groundwater.

Technical Abstract: Inefficient nitrogen (N) fertilization and irrigation have led to unhealthy nitrate levels in groundwater bodies of agricultural areas in California. Simultaneously, high commodity prices and drought have encouraged perennial crop growers to turnover less productive orchards, providing opportunities to recycle tree biomass in situ and use high carbon (C) residues to conserve soil and water resources. While climate change adaptation and mitigation benefits of high C soil amendments have been shown, uncertainties remain regarding the benefits and tradeoffs of this practice for N cycling and retention. We used established Almond orchard trials with short-term and long-term biomass recycling legacies to better understand the changes in N dynamics and retention capacity associated with this practice. In a soil column experiment, labeled N fertilizer was added and traced into various N pools, including microbial biomass, and inorganic fractions in soil and leachate. Shifts in microbial communities were characterized using abundance of key N cycling functional genes regulating nitrification and denitrification processes. Our findings showed that, in the short-term, biomass recycling led to N immobilization within the orchard biomass incorporation depth zone without impacts on N leaching potential. However, this practice drastically reduced nitrate leaching potential by 52% ten years after residue incorporation without increase in N immobilization. Although timing of these potential benefits as a function of microbial population and C and N biogeochemical cycles still need to be clarified, our results highlight the potential of this practice to meaningfully mitigate nitrate discharges into groundwater while conserving soil resources.