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

depth of 0–15 cm, which corresponds to the woody biomass incorporation depth. Only the middle row trees (Nonpareil) were used for sampling and field measurements and trees at the beginning and at the end of each tree row were not sampled to avoid edge effects. Three root zone samples per plot were taken and composited (n = 12) on the same day to a depth of 0–15 cm for biological analysis. Field-moist soil was sieved to 2 mm and stored at 4˚C prior to assays of microbial biomass and enzyme activities. For soil chemical properties, five soil cores per plot were sampled composited (n = 12) and dried in a forced-air oven at 50˚C prior to analysis. Three separate undisturbed soil cores were also taken from each plot (n = 36, not composited) and stored at 4˚C until processing for aggregate stability and fractionation tests. In addition, five soil cores were collected from each plot (n = 60) for bulk density (BD) measurements. Soil analysis Chemical properties. Soils were analyzed in house at the University of California Divi- sion of Agriculture and Natural Resources Analytical Laboratory. The pH was determined using a saturated paste method [17]. Electrical conductivity (EC) was measured according to the method described by Rhoades [18]. Sodium (Na), Ca, Mg, K, B, S and micronutrients (Zn, Mn, Fe, Cu, and Mo) were measured using Inductively Coupled Plasma Emission Spectros- copy (ICP-AES) [19]. Cation exchange capacity (CEC) was determined based on the method described by Rible and Quick [20]. Soil nitrate, extractable ammonium, and Cl were measured

using Flow Injection Analysis Colorimetry (QuikChem, Lachat Instruments, Milwaukee, WI, USA) [21]. Total N and C were determined using the combustion method (ECS 4010, Costech Analytical Technologies Inc, Valencia, CA; USA). Soil C stock was calculated as the product of soil C concentration, soil sample bulk density, and sampling depth [22–24]. Soil organic mat- ter (SOM) was measured using the Loss-On-Ignition Method [25]. Permanganate Oxidizable C (POxC) was measured using the procedure described by Weil et al. [26]. Physical properties. Soil texture was determined using the hydrometer technique [27]. Soil compaction, defined as resistance, was measured using a soil cone penetrometer (Dickey- John, Auburn, IL, USA). Bulk density (BD) was measured according to Blake and Hartge [28]. Briefly, volume of soil (cm3) was calculated by subtracting volume of rocks from total soil vol- ume. Volume of rocks was determined by submerging the rocks in a graduated cylinder with water and recording the difference before and after submerging (1 mL = 1 cm-3). The below equation was used to calculate BD: Soil BD (g cm-3) = oven dry weight of soil / volume of soil Soil water-stable aggregation and C and N content of different aggregate fractions were measured on undisturbed cores. Large particles were removed through sieving moist soil (8mm) and water-stable aggregates were separated by wet-sieving of a 40g subsample into four aggregate size fractions: >2000 μm (large macroaggregates), 250–2000 μm (small macroaggre-

gates), 53–250 μm (microaggregates), and <53 μm (silt and clay fraction) according to a proto- col modified from Elliot [29]. A vibratory sieve shaker with rainfall simulator (Fritsch Analysette 3 Pro, Pittsboro, NC, USA) was used. Aggregate fractions remaining on each sieve were oven-dried at 50˚C and weighed. Carbon and N content of different aggregate fractions were measured using the combustion method. Mean weight diameter (MWD), a weighted- average index of aggregate stability, was calculated according to the following equation [30]: MWD ¼ P4 i¼1Pi � Si where Si is the average diameter (μm) for particles in that fraction and Pi is the weight percent- age of the fraction in the whole soil. PLOS ONE Whole orchard recycling PLOS ONE | https://doi.org/10.1371/journal.pone.0229588 March 27, 2020 4 / 22