Based on the results of the soil aeration quantifiers at CT, forced aeration by air injection can improve the soil aeration status during Ag-MAR.However, based on the same quantifiers, the results from KARE and NSL show a minor impact of air injection.This discrepancy can be explained by the SDI system that delivered the injected air,emitter density, and the location of the soil sensors relative to the emitters.The ad-hoc SDI that we installed at KARE and NSL was suffering from the “chimney-effect,” a well-known phenomenon in air sparging where air flows quickly upward to the soil surface through large pores without penetrating laterally into smaller pores.Suggested methods to overcome the “chimney effect” include increasing the injection depth and pulse-mode injection.At CT, the SDI was already established for several years and emitter density was higher compared with KARE and NSL.In addition, soil sensors at CT were installed closer to the emitters compared with KARE and NSL, and therefore were closer to the zone of influence of the emitter.As the injected air does not spread homogenously in saturated soil, the ZOI comprises an ensemble of discrete air channels extended from the injection point upwards, separated by zones fully saturated with water, which together form a parabolic plume shape.The ZOI depends on the soil type, air-injection depth, and flow rate.Generally, fine soils, deep injection depths, and high flow rates increase the ZOI compared with coarse soils, shallow injection depths, and low flow rates.In this study , these parameters were given by the soil texture at each site and the design of the installed SDI system.For an injection depth of 30 cm in fine sand,cannabis vertical farming the ZOI has the range of several centimeters and up to ∼50 cm , which explains why at KARE and NSL the effect of air injection was barely detected by the soil sensors.
Given this ZOI, even for a well-established SDI system, it is still questionable whether the density and depth of the emitters at KARE and NSL can support significant aeration through air injection.On the other hand, in an orchard originally planted with SDI, active roots will be found within the air-injection ZOI, as root density is expected to be higher next to the emitters.Our DO measurements at NSL and CT demonstrate that forced aeration by air injection can increase the DO in saturated soils.A similar approach is used in hydroponics and soilless media where active aeration of the nutrient solution is a common growing practice to avoid the decrease of DO below 3–4 mg L−1, which inhibits root growth.However, it is still unclear whether air injection can support root respiration of woody perennials under complete water-logged conditions.In this study, air was injected into the soil after the water content reached saturation.However, as discussed above, under these conditions the spreading of the injected air is limited, and O2 diffusion rates are significantly reduced.Alternatively, injection of air during the infiltration stage and/or during the drainage stage, might be more effective in reducing and enhancing soil O2 depletion and recovery rates, respectively.A similar approach resulted in increased yield of greenhouse cucumbers under both SDI and furrow irrigation, aerated by air injection after the irrigation cycles ; however, further study is needed to evaluate the benefit of this approach in Ag-MAR.Our soil O2 results agree with the results of forced aeration air-injection experiments conducted in barrels with peppers and at a citrus orchard.Our DO results agree with studies that used aerated irrigation through SDI in several crops.However, most aeration studies report only plant indices for assessing the impact of active aeration.The impact of air injection on plant indices was not staThistically different at all sites.However, this finding is based on only one growing season with air-injection treatment.At CT, Ag-MAR flooding was applied daily, generating flood/drainage cycles that probably reduced the flood-related stress of the cover crops.
Regulating the aeration conditions of the soil during Ag-MAR can be achieved by natural aeration and by forced aeration methods.Controlling the flood/drainage duration is an efficient and inexpensive way to ensure adequate soil aeration during Ag-MAR.However, maintaining even flooding in a field scale Ag-MAR project is a major challenge due to surface irregularities and soil heterogeneity, which is found in most agricultural fields.For these reasons, careful inspection of the potential Ag-MAR site must be made before initiating Ag-MAR project, and a small-scale pilot is recommended.In many cases, nonuniform infiltration is expected that will lead to nonuniform soil aeration conditions and can result in hypoxic or anoxic zones.As observed at the NSL site, these zones are a limiting factor for implementing large-scale Ag-MAR projects and may benefit from a local forced aeration treatment.Hence, air injection, even for a short duration of a few hours, can be beneficial and easily implement in field-scale Ag-MAR projects.Implementing forced aeration by air injection requires agricultural fields with pre-installed SDI, which are less abundant in some crops.In California, SDI is commonly used in annual crops and to a lesser extent in perennial crops.When SDI is unavailable other forced aeration methods can be used.For example, we are currently testing the application of peroxide fertilizers during Ag-MAR, because O2is released when solid peroxides react with water.An example of O2 response to an application of CaO2 was shown in this study for one tree at the end of the experiment at NSL.The main limitations of this aeration method are its high costs and the limited control of its decomposition rate.Plants have developed various defence strategies against toxic heavy metals, including complexation and chelation with strong ligands, and compartmentation into specific Thissues, cells and cellular organelles.Accumulation of metals inside trichomes, specialized cells located at the surface of leaves, is common and was documented, for example, in Brassica juncea L., Alyssum lesbiacum , Cucurbita moschata , Nymphaea sp., and Arabidopsis halleri and thaliana.
Heavy metals can also be excreted at the top of this hair-like appendage, but this process is less common and generally observed in halophyte species, such as Armeria maritima ssp., Silene vulgaris , Avicennia marina,and Atriplex halimus L..Tobacco detoxifies Zn and Cd by producing micrometer-sized Ca/ Zn and Ca/Cd-containing grains at the top of trichomes, similarly to halophytes.Biomineralization processes can be biologically induced or biologically controlled.In the first case, the living organism modifies the physico-chemical conditions of its environment, so as to induce mineral precipitation near or at its surface.The organism has little control over the type and shape of minerals, which generally have heterogeneous morphology, composition, and structure.In biologically controlled biomineralization, nucleation, crystal growth, and the shape and size of crystallites can be controlled by bio-molecules.The production of grain precipitated by tobacco plants is considered to be biologically induced, but the formation mechanism remains unclear.The Ca/Zn grains produced under Zn and Zn + Ca exposures were 20–150 lm in diameter and polycrystalline aggregates of sub-micrometer crystals with some amorphous material.The crystals were composed dominantly of – substituted calcite.Aragonite and vaterite, the two other CaCO3 polymorphs, amorphous CaCO3 and Ca oxalatemonohydrate and dihydratesecondarily occurred, generally as an admixture of calcite.Other possible species included Zn complexed to organic compounds, Zn-containing silica and Zn phosphate.The proportion of Zn-substituted calcite relative to other Zn species and the density of trichomes increased with Ca, and in total more Zn was excreted.As with Zn, trichomes produced 10–150 lm Ca/Cd grains when the plant roots were in contact with cadmium.Cd exposure retarded tobacco growth and doubled the density of trichomes per unit leaf area.Tolerance to metal toxicity was enhanced by adding Ca, which stimulated the production of grains.Because Cd and Ca form complete solid solutions in carbonates, as a result of their oxidation state and similar ionic radii ,cannabis drying racks tolerance to Cd toxicity is probably linked to the production of calcium carbonate, but in a form and a manner as yet unknown.In this study, the nature of the Cd precipitates was investigated by growing tobacco plants in hydroponics in the presence of low and high Ca concentrations.The morphology, chemical composition, and crystalline nature of the Ca/Cd grains, and Cd speciation were characterized using scanning electron microscopy coupled with energy dispersive X-ray microanalysis , micro-focused X-ray diffraction , and micro-focused Cd LIII-edge X-ray absorption near edge structure spectroscopy.The Cd species were identified by principal component analysis , and their proportions in grains quantified by linear least squares combination fit of the l-XANES spectra.Solid and aqueous Cd-containing standards were prepared and analyzed by Cd LIII-edge XANES spectroscopy.Synthesis of Cd-phosphate 4 4) and Cd-oxalate , and the preparation of Cd2+aq and Cd organic compounds, including Cd-pectin, Cd-citrate, Cd-malate, Cd-cell wall , and Cd-cysteine, were described previously.Commercial powders of CdS, CdSO4, CdCl2, and CdCO3were purchased from Sigma–Aldrich, and their purity and crystallinity verified by XRD.In addition, Cd-containing calcite and vaterite were synthesized at room temperature by a protocol modified from Paquette and Reeder and Reeder.Solid ammonium carbonate was introduced into a 50-mL Falcon tube floating in a sealed glass reactor containing 500 mL of 10 mM CaCl2 and 1.8 M NH4Cl.The second salt was used as a background electrolyte to provide a high ionic strength.Initial pH was 4.9.The gradual decomposition of ammonium carbonate produced NH3 and CO2, which dissolved into the solution, increasing pH and alkalinity.The supersaturation of the unstirred solution led to the nucleation and growth of CaCO3 crystals.Continuous sublimation of NH3 buffered the solution near pH 7.9.After 13 days, the reactor contained rhombohedral crystals of calcite and spherical particles of vaterite attached to the surface of the glass.At this time, the CaCl2–NH4Cl solution was spiked slowly for 7 days with 0.1 M CdCl2 to a total concentration of 100 lM Cd or 10 lM Cd.
During this period, crystals continued to grow and Cd was incorporated as a Ca substituent in vaterite and calcite.Because Mg occurs in all grains produced by tobacco , -substituted calcite -calcite and – substituted vaterite -vaterite also were synthesized by co-adding 100 lM CdCl2 and 100 lM MgCl2 to a CaCl2–NH4Cl solution after 13 days and for 7 days.After 20 days, the particles from the three experiments were collected, rinsed with deionized water, and handpicked on the basis of their morphology.The distribution in sizewas independent of the morphology.SEMEDX, XRD and micro-focused X-ray fluorescence analyses showed that the rhombohedral crystals were pure Cd-containing or -containing calcite, and the spherical particles pure Cd-containing or -containing vaterite.Several tens of calcite and vaterite grains from the 100 lM Cd experiments were digested at 200C with pure HCl in Teflon bombs, and Cd concentrations analyzed by inductively coupled plasma atomic emission spectrometry.The Cd10-calcite and Cd10-vaterite crystals could not be analyzed by ICP-AES due to the limited supply of material.Cd-sorbed calcite was prepared following the protocol described in Elzinga and Reeder.Briefly, 0.1 g of powdered calcite was controlled by XRD and equilibrated at ambient pressure and temperature in 1 L ultrapure water for 1 month.The suspension stabilized at pH 8.2 was spiked with 2 lmol Cd from a 0.01 M CdCl2 solution.The pH remained constant for the 2 days of equilibration, at which time the suspension was filtered, and the solid rinsed and dried for XANES measurements.The two-dimensional XRD patterns were calibrated with alumina and integrated to one-dimensional patterns using fifit2D software.The stoichiometry of the Mg-, Mn-, Zn-, and Cd-substituents in calcite crystals was estimated by refining the unit cell parameters a and c over the 10–33 2h angular range using the Ufit software , and applying the Vegard law.The end-members for Vegard law calculations were calcite, magnesite , rhodocrosite , smithsonite , and otavite.The unit cell parameters a and c of the substituted vaterite crystals were refined but the stoichiometry of the substituents could not be estimated, because of the lack of metal carbonates isomorphic to vaterite.The l-XANES spectra were calibrated using the first inflection point of Cd metal set at 3538 eV, then pre-edge background subtracted with a linear polynomial and post edge normalized with a linear or quadratic polynomial using the Athena software.The normalized spectra were analyzed by principal components analysis using the beamline 10.3.2 LabView based software.This numerical linear algebra analysis allows estimating the number of species required to describe the dataset, provided the number of species is smaller than the number of spectra and their fractional amounts vary in the dataset, and identifying their nature from a library of model compounds by target transformation.