Natural aeration refers to the replenishment of O2 to the root zone from the atmosphere during the drainage period.It depends on the drainage class of the soil because O2 supply to the root zone is possible only after some critical air contenThis reached.Note that the critical air content represents the connectivity between the root zone and the atmosphere by air-filled pores.In the context of Ag-MAR, natural reaeration of the soil after flooding can be controlled by considering the soil parameters, the water application duration, and the crop tolerance to saturation.Forced aeration refers to intentional oxygenation of the root zone by several methods such as air injection, air bubbles, H2O2 , and solid peroxides.These methods are currently not used in commercial agriculture, although some of them have shown positive results in previous studies.Among these methods, air injection through subsurface drip systems might have higher potential in terms of O2 delivery and implementation costs, because it uses the in situ subsurface drip system.Previous forced aeration studies with air injection have focused on its impact on improving crop yield, nutritional value, and water use efficiency , but it has not been studied in Ag-MAR applications as a method to protect yield lost due to prolonged flooding.Monitoring soil physical–biogeochemical processes during MAR has been extensively studied ; however, since Ag-MAR is a relatively new technique in the MAR toolbox, to date only a few studies have monitored these processes in actual agricultural fields during Ag-MAR.Most Ag-MAR studies have focused on developing soil suitability guidelines , regionalscale aquifer storage estimations ,vertical grow rack water availability analysis , hydro-economicanalysis , and benefits evaluation using numerical modeling.

Among the few Ag-MAR field studies that exist, the soil aeration status, which may impair the implementation of future Ag-MAR projects, has been largely neglected.Dahlke et al.estimated soil aeration status using Eh measurements during 3 d of Ag-MAR in an alfalfa field on a well-drained gravelly sandy loam.The Eh values were closely correlated to water content and Eh was quickly returned to preflooding aerobic conditions when water application ceased.The report of Bachand et al.is the only work that examined the impact of flooding on soil O2 during Ag-MAR, which was studied in three almond, walnut [Juglans regia L.], and pistachio [Pistacia vera L.] orchards all located on well-drained soils.Bachand et al.calculated O2 depletion and recovery rates based on soil O2 and water content measurements in the root zone and suggested a few best management guidelines for growers:avoid standing water for more than 3–4 d,reduce time with water saturation above 74%, and plan Ag-MAR flood duration based on past, soil-specific flood irrigation guidelines.The applied water amounts in these demonstrations were relatively conservative , and therefore in the current study, we sought to explore soil aeration during Ag-MAR with higher hydraulic loads.The goal of this study is to quantify the soil aeration status during Ag-MAR experiments and to test air injection as a technique for improving soil aeration during continuous flooding, thus reducing the risk of root and crop damage due to anoxia.For this purpose, three field experiments were conducted: one at a cover-crop field, and two at almond orchards, all located in the Central Valley, California, USA.The experiments were used to compare natural aeration and forced aeration by air injection through the subsurface drip system during Ag-MAR.In the following, we first explain the method we chose to quantify soil aeration and the methodology of the experiments.Next, we present the results of the Ag-MAR field experiments.Finally, we discuss the impact of forced aeration during flooding and implications for Ag-MAR projects.We used soil O2 concentration and Ehmeasurements to quantify soil aeration status during Ag-MAR.

Using these aeration quantifiers in combination allows assessing the soil aeration status during both aerobic and anaerobic conditions.We set a soil O2 threshold of 5% as a lower bound, since O2 concentrations below that are considered inadequate for root function.In all O2 measurements, we assume an equilibriumin the bulk soil between the gas and liquid phases.Redox potential is a useful soil aeration quantifier in waterlogged soils where O2 levels are low.Generally, Eh above and below 300 mV indicates aerobic and anaerobic conditions, respectively.The Eh values of 300 to −50 mV indicate moderately reducing conditions, which are dominated by facultative reducing microbes.In this range, O2 is the preferred electron acceptor in cellular respiration, followed by NO3 −, Mn4+, and Fe3+.The Eh below −50 mV indicates highly reducing conditions where SO4 2− and CO2 are the electron acceptors.Note that Eh is a qualitative aeration quantifier, as it measures the mixed potentials of the soil and therefore cannot be used for identifying a specific redox couple.As such, it is only useful for indicating trends over time of more reducing or oxidizing conditions.At each site three treatments were tested:flooding with air injection by subsurface drip irrigation ;flooding without air injection; and control.The treatments were divided by bermsto prevent flooding of adjacent plots.Each treatment comprised a row of 11–12 almond trees at KARE and NSL, or four beds of cover crop along 80 m, at CT.Plot area, including all three treatments, was 1,500, 940, and 1,440 m2 at KARE, NSL, and CT, respectively.In each treatment, one to four profiles were installed with soil sensors at 15-, 30-, and 50-cm depth.Soil sensors measured volumetric water content , temperature, gas-phase soil O2, and Eh.To complete the Eh measurements, a commercial Ag/AgCl reference electrode was placed in a salt bridge that was installed at a depth of 30 cm at each profile.Redox potential readings in the field were corrected to standard Eh by adding ∼210 mV.Soil O2 readings in the soil were temperature and pressure corrected as recommended by the manufacturer.Note that these galvanic-cell O2 sensors are diffusion based, and when its membrane is clogged , it will measure zero O2 concentration, although a pore water sample from the same location might show higher DO concentration.Still, these sensors are widely used in soil studies and our experience under flooded conditions shows that this issue is more prominent, as expected, in clayey soils.All sensors readings were taken every 1 min, and 10-min mean values were recorded with data loggers.

In addition to the continuous monitoring, following the method of Friedman and Naftaliev , air samples were extracted with a 100-ml syringe from perforated 100-ml plastic bottles that were buried inside the soilat depths of 15, 30, and 50 cm.Air samples were measured onsite with an O2 flow-through sensor.In several cases where the plastic bottles were filled with pore water, samples were extracted using a syringe and measured onsite for DO with an optic sensor.At CT, dedicated pore-water samplers were installed at depths of 15 and 30 cm and used for routine manual measurements of DO.The detailed setup of the three sites is shown in Figure 2.Both almond sites were regularly irrigated with surface micro-sprinklers, and therefore a dedicated SDI was installed for the air injection treatment as follows: holes were augured to 30-cm depth using a 5-cm-or 2.5-cm-diam.hand auger and then a 4-mm polyethylenetube was inserted inside, and the holes were back filled with a soil-bentonite slurry.This was done carefully using an outer rigid pipe as a guide, to prevent soil clogging of the tubes.The tubes were connected to drippers that were connected to an on-surface lateral line , which delivered the injected air.In order to mimic an SDI system of a commercial orchard, we used a configuration of two lateral lines, each at a distance of 90 cm from the tree trunks, with drippersat a depth of 30 cm, spaced 120 cm apart.Note that the horizontal distance between the buried emitters centerline and the soil sensors was in the range of 40–60 cm.The cover crop site was rainfed, and air injection was based on an SDI system that was already installed in the soil for several years.The horizontal distance between the buried emitters centerline and the soil sensors was in the range of 0–15 cm.Each experiment started by flooding the plots with ground wateror surface water using flood-irrigation-gated pipesor sprinklers.Applied water volumes were measured by water meter and doppler flow meter.At NSL, the continuous discharge was not measured but applied water volume was estimated manually several times during the experiment using a graduated measuring bucket.Water was applied continuously for a few hours and up to1 d,cannabis grow racks depending on the infiltration rate of each site.When O2 concentrations started to decline, air was injected through the SDI with a pressure-regulated air compressor , which kept the absolute pressure at the range of 160–200 kPa.Air injection was ceased a few hours and up to 2 d after water application ceased.At the end of the experiment at NSL, we conducted a preliminary test of aeration using CaO2 powder, which reacts with water to produce O2 and H2O2.A 600 g of CaO2 powder was scattered on the soil surface, covering an area of 9 m2 around one tree only , and a small amount of water was sprayed over it.Gross almond yield was collected per tree for all varieties at KARE and for the Nonpareil variety at NSL at the end of August after the recharge season.At CT, plant height, dry root length, and dry root weight of bell beans were measured before and after the flooding experiment.Plant sampling included careful excavation of the roots using a shovel, measurement of stem height in the field, washing roots in the laboratory, and measuring dry root weight and vertical root length.From each treatment 12 bell beans plants were sampled before and after the flooding experiment, and a total of 68 plants were analyzed.Root weight was normalized to root vertical length and plant height, to reduce the sampling error due to natural variability in root and plant morphology, and averages values were calculated for each treatment.Data were analyzed using ANOVA performed with the software R.A one-way ANOVA was applied to the manually collected data and differences between means were determined with Tukey’s test.Continuous data collected with sensors were not analyzed with ANOVA due to heteroscedasticity and lack of independence of the high-resolution continuous measurements.For these measurements, we report the summary staThistics and compare the distribution of differences of the soil aeration quantifiers between the treatments with and without air injection.Water was applied continuously at all sites according to the site-specific infiltration rate, in order to maintain a ponding depth of few centimeters.

To avoid flooding of adjacent plots, the water supply was decreased or stopped occasionally during the experiment.Although maintaining even flooding within the flooded treatments was a difficult task at all sites , it was practically impossible at the NSL site due to a combined effect of poor soil drainage and plot slope.Nevertheless, an estimated total water amount per area of 0.76, 0.065, and 2.9 m, was applied at KARE, NSL, and CT, respectively.Note that these estimations are averages that are based on the total plot area.Although at KARE flooded area and total plot area were almost identical, at NSL and CT, it represents lower and upper bounds for the total applied water, due to smaller and larger effective flooding area, respectively.Flooding of the plots led to the expected trend of increasing soil water content and decreasing O2 and redoxlevels, whereas at the control treatments, high soil aeration status was observed at relatively low θw.An example of the soil aeration status is shown in Figure 3, where for each site one profile from each treatment at 30-cm depth is presented.For the air injection treatments, the impact of air injection on soil aeration status can be detected as an increase in O2 and Eh levels during air-injection periods; it is limited at KARE and NSL, but consistent at CT.The distribution of the continuous measurements of all sensors at all depths is summarized in Figure 4, where results are grouped according to treatment and period.The period before air injection was defined as the time before the first air injection started, the period during air injection includes active air injection and the times between air injection cycles, and the period after air injection starts when the last injection ends.Accordingly, in all flooded treatments , aeration status decreased below the soil-aeration lower bounds for few hours at CT, a few days at KARE, and up to several days at NSL.At NSL, hypoxic to anoxic, and anaerobic conditions were mainly observed in the air-injection treatment, likely as a compounding effect of plot slope and specific poor-drainage conditions where the soil sensors were located.