LAUSR

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 [Prunus dulcis (Mill.) D. A. Webb] orchard trials on Hanford fine sandy loam 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 (0-15 cm) without impacts on N leaching potential. However, this practice drastically reduced nitrate leaching potential by 52%, ten years after biomass 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. This article is protected by copyright. All rights reserved.