Orchard recycling improves climate change adaptation and mitigation potential of almond production systems

Resistance to deficit irrigation We tested the significance of changes in soil health parameters for tree and yield resistance to water shortages. Deficit irrigation resulted in a decrease in soil moisture in both treatments; however, the difference was more pronounced in the burn treatment (Fig 6). We observed a significant interaction between irrigation and soil treatments (P = 0.05), where the burn plots had a lower soil water content than the grind treatments under deficit irrigation, especially in the topsoil and at depth (Fig 6). We observed no significant interaction between soil treatment and irrigation on yields (P = 0.25, Fig 1B). However, stem water potential (SWP) measure- ments showed that trees in the grind plots maintained less negative value than those in the burn treatment (-14.11 vs -16.22 bar, respectively) over the course of the season (Fig 7A). Trees were significantly less water stressed on the last day of the deficit irrigation period in the grind treatment, as shown by significantly less negative SWP and higher stomatal conductance (Fig 7B and 7C). The grind treatment also assisted trees in their recovery post stress (Fig 7B). No changes in plant nutrition, canopy temperature and PAR interception were detected between treatments (S2 Table). Yields and soil health parameters We examined correlations between soil health indicators and tree water status and yields (Table 3). Indicators of soil health improvement were positively correlated with almond yields. Significant positive correlations were observed between yield, soil N and C content (TN, SOC,

and SOM) and C cycling (BG). Among soil physical properties, improvements in infiltration were significantly corrected with higher yields while bulk density and soil compaction trended negatively with yields. Total C content and SOM levels were also positively correlated with some orchard resiliency indicators (Table 3) such as soil water content and tree water status (SWP), where less negative SWP values indicate more favorable plant water status (Table 3). Discussion The purpose of this experiment was to evaluate long term effects of WOR adoption on almond orchard functioning and climate change mitigation and adaptation services. We found that in situ biomass recycling prior to replanting orchards can help sequester a significant amount of C in the soil while improving yields and mitigating the impacts of acute water shortages on tree water stress. Although this practice can be costlier and challenging operationally on the short term, it offers a promising strategy for long term improvement in soil health and reduce risks of crop failure of orchard cropping systems. Biomass recycling provides substantial mitigation potential The adoption of WOR is characterized by the reincorporation of large quantities of high C:N woody biomass (ratio of ~160) into orchard top soils. We found that the recycled orchard soils Table 2. Impact of WOR on microbial parameters and Soil Health Index determinants. Treatment MBN (μg N g soil-1) MBC (μg C g soil-1) Respiration (ppm) WEON (ppm) WEOC (ppm) C:N SHI Grind 7.91 192 57.7 4.98 56.69 11.38 6.13

Burn 6.97 150 32.7 4.58 44.99 9.81 4.24 P value 0.4113 0.0490 0.002 0.065 0.012 0.056 0.019 MBN and MBC, Microbial biomass carbon and nitrogen, respectively; WEON, water extractable organic nitrogen; WEOC, water extractable organic carbon; C:N, carbon to nitrogen ratio of soil; SHI, soil health index. https://doi.org/10.1371/journal.pone.0229588.t002 PLOS ONE Whole orchard recycling PLOS ONE | https://doi.org/10.1371/journal.pone.0229588 March 27, 2020 10 / 22