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

determined using a digital-color reader (DCR) (www.solvita.com) [37]. These values were then used to calculate a SHI according to Haney [36]: SHI ¼ 1 day CO2 C C : N þ ðWEOC 100 þ WEON 10Þ Plant analysis Yield and leaf analysis. Almond kernel yields (Nonpareil) were measured by mechanical harvest on September 2017. Gross and dry kernel weights (~3% moisture) were measured on subsamples from 6 trees per treatment plot. Twenty almond leaves per tree were sampled in mid-July, dried at 60˚C, ground, sieved through 2-mm mesh and then sent to the University of California Division of Agriculture and Natural Resources (UC ANR) Analytical Laboratory for analysis of accumulated nutrients. Total Kjeldahl Nitrogen (TKN) was determined according to Horneck and Miller [38]. Phosphorus, K, S, Ca, Mg, Zn, Fe, Mn, Cu, B, Cl, Na, Al, and Mo were analyzed using nitric acid/hydrogen peroxide microwave digestion method and ICP-AES [19]. Tree water status. Tree water status was measured weekly using midday stem water potential (SWP) with a pressure chamber (Soilmoisture Equipment, Santa Barbara, CA, USA). Two lower-canopy, fully expanded, shaded leaves were measured after enclosing the leaves in bags made of plastic film and aluminum foil for at least one hour before measuring. Stomatal conductance (mmol m-2s-1) was measured on June 28 at solar noon using a leaf porometer (model SC-1, Decagon Devices, Inc., Pullman, WA, USA) on six randomly chosen leaves per tree in each plot (n = 72). Photosynthetically Active Radiations (PAR) intercepted by the tree

canopy was measured as an indicator of tree growth using a utility vehicle at solar noon (model 610 Mule; Kawasaki Heavy Industries, Tokyo) fitted with a mobile PAR measurement platform [39] on August 25. Aerial thermal images were acquired on June 28 in clear conditions between the hours of 12:00 and 13:00 to minimize effects of shadowing on the images. These images were collected at 70 meters above ground level with a metrically calibrated Zenmuse XT infrared thermal camera, mounted on a DJI Inspire 1 unmanned aerial vehicle. The Zenmuse thermal camera was equipped with a 9Hz FLIR microbolometer sensor, capturing 640 × 512 pixel frames (digi- tal images) from a 640/30 fps from an active feed, with a sensitivity (NEdT) of 50 mK at f/1.0 in the spectral range of 7.5–13.5 μm, and a lens with an field of view of 69˚ × 56˚. The aerial images were later processed in Pix4D Mapper Pro and ArcGIS software applications to pro- duce a 9.2 cm spatial resolution orthomosaic map of the canopy temperature, segmenting each almond tree from surrounding soil pixels within the entire 2.37-acre orchard, to test whether there were significant temperature differences between the irrigation and burn vs. grind treat- ment areas. Statistical analysis Data were analyzed using the PROC MIXED procedure of SAS [40]. Soil and irrigation treat- ments and the interaction of these factors were analyzed as fixed effects, and blocks and the interaction of blocks with main effects as random effects. The assumptions of ANOVA were

tested, and transformations were applied where necessary to achieve normality of residuals. Weekly measurements of SWP and neutron probe readings were analyzed with repeated mea- sure covariance structure [41]. When ANOVA showed significant differences (P � 0.05), com- parisons of means were made using the Tukey’s range test. Principal components analysis (PCA) was conducted using the JMP software [42]. Pearson correlation coefficients were cal- culated using the PROC CORR procedure to determine correlations between almond yield PLOS ONE Whole orchard recycling PLOS ONE | https://doi.org/10.1371/journal.pone.0229588 March 27, 2020 6 / 22