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

and soil health parameters as well as plant water status measurements [40]. The HYPROP data evaluation software (UMS GmbH) was used to fit the parameters of a model for the soil water retention q(h) and unsaturated hydraulic conductivity K(h) to the evaporation. We tested van Genuchten unimodal [43] and Durner bimodal retention model equations [44,45] and evalu- ated model fit quality with the corrected Akaike Information Criterion (AIC) [46]. Smaller AIC values indicate a better estimation quality of the model. Data were analyzed with the PROC MIXED model to compare soil treatments and means were separated using Tukey’s range test. Results Tree nutrition, yields, and productivity Trees following WOR significantly out-yielded those in the burn treatment nine years after establishment (+19%, 400 kg ha-1; Fig 1A), which led to increased irrigation water use effi- ciency (IWUE) compared to the burn treatment (1.25 kg m-3 vs 1.04 kg m-3; Fig 1C). Higher kernel yields have been consistently observed in the grind treatment since 2014 (S1A Fig). Fur- thermore, the accumulated yields indicated a 15% kernel yield increase in the grind treatment compared to the burn (S1B Fig). Trees grown under WOR accumulated slightly higher leaf N content compared to the burning practice regardless of the irrigation treatments; yet, accumu- lation of other minerals was not significantly affected by the treatments (S1 Table). Soil carbon pools The grind treatment had higher SOM content (+ 0.45%) than the burn treatment nine years

after WOR implementation (P = 0.001; Table 1). A proportional increase was observed in SOC of the grind compared to the burn treatment (Table 1). The total accumulation of soil C in the grind treatment was 5.2 tons per hectare (0–15 cm) higher than in the burn treatment (Table 1). Labile pools of SOC, such as POxC (Table 1), MBC (Table 2), and WEOC (Table 2), also increased relative to the burn treatment (+66, +47, and +26%, respectively). The distribu- tion of C and N within aggregate fractions differed between the soil treatments, with a higher relative proportion and total weight of SOC occluded within large macroaggregates after grinding (35 vs 28% after burning, Fig 2A). Additionally, a larger proportion of TN was bound to minerals in the clay and silt fractions of the grind treatment (+ 34%) than in the burn treat- ment (Fig 2B). Soil chemical properties Soil TN (+23%), Mn (+33%), Cl (+46%), and Na (+24%) contents were significantly greater following WOR implementation relative to the burn experiment (Table 1). Observed increases in TN content of the grind treatment did not result in increased NH4+ nor NO3- contents. No significant differences were observed in other chemical properties, including pH, CEC, and EC (Table 1). Aggregation, compaction, and hydraulic properties As the proportion of stable large macroaggregates increased (Fig 3A), WOR significantly enhanced aggregate stability (MWD) compared to the burn treatment (Fig 3B). Bulk density and compaction of topsoil layers (Table 1) were also significantly reduced under WOR (5 vs

16%). Water retention curves showed that the addition of woody biomass into the orchard soil had a significant effect on water retention in the topsoil. Soils were able to retain higher mois- ture (%VWC) at matric potential from 0 to ~10 kPA. The recycled treatment was able to store PLOS ONE Whole orchard recycling PLOS ONE | https://doi.org/10.1371/journal.pone.0229588 March 27, 2020 7 / 22