After the air-injection period , all sites except NSL showed a similar recovery in soil aeration status in all flooded treatments , which is another indication of the local drainage issue at NSL.The average impact of air injection varies among sites.The impact was small at CT compared with the flooded treatment without air injection, whereas no impact and even negative impact was observed at KARE and NSL.However, the wide variability in O2 data in the air-injection treatments at KARE and NSL implies that air injection may improve the aeration status for a limited time.The fast increase in soil O2 at the end of the experiment at NSL is the result of the CaO2 preliminary test , which demonstrates the potential of this technique for improving soil O2 during Ag-MAR.The soil sensors measurements presented in Figures 3–5 provide continuous high-resolution temporal data, but with limited spatial resolution.A complementary spatial description of the soil aeration status was obtained by the manual measurements of soil O2.At NSL, these measurements show results that differed from the continuous measurements , indicating that air injection improves average O2 levels by ∼1% compared with the flooded treatment without air injection, at 15 cm and at 30-cm depth , whereas no impact was observed at 50-cm depth.At CT, the manual soil O2 measurements are consistent with the continuous measurements, showing improvement of up to ∼2% O2 at all depths.For the experiments at the almond orchards, gross yield was measured during the harvest season after the Ag-MAR experiments.At KARE, the Nonpareil was the only variety that showed a higher average yield in the air-injection treatment than the no-air injection treatment , whereas all other varieties showed no staThistical difference between treatments.At NSL,cannabis grow system the average yield of the air-injection treatment was lower than the no-air injection treatment , but this yield reduction is correlated with annual precipitation, which implies a drainage issue in the specific row of the air treatment.At CT, plant indices measured before and after the experiment showed an average decrease for the no-air injection treatment, whereas an increase was observed for the air injection and the control treatments.
Changes were not staThistically different based on a significance level of α =.05 , but the difference between the no-air injection and the control treatments after flooding is significant at the significance level of α =.1.The water application results demonstrate the degree of suitability of the selected sites for Ag-MAR projects in terms of infiltration rates.Both the KARE and CT sites are suitable for Ag-MAR, as their infiltration rate is greater than 1 m in a few days , whereas the low infiltration rate measured at NSL makes the site unsuitable for Ag-MAR.Indeed, the NSL site was selected in this study to demonstrate the difference among Ag-MAR sites with different soil texture, SAGBI class, and hydraulic properties.The soil at NSL has a SAGBI rating of moderately good to poor , which illustrates the role that SAGBI has as a first approximation planning tool for locating potential Ag-MAR sites.However, the final selection of an Ag-MAR site should also consider the growers’ experience.Apart from soil suitability for Ag-MAR, technical constraints and agronomic constraints should all be considered when designing an Ag-MAR project.Naturally, well-drained soils, which are more suitable for Ag-MAR, are also well aerated.This characteristic is manifested in the flood-drainage intervals that allow reaeration of the soil during drainage, even without the use of forced aeration.Conversely, poorly drained soils are unsuitable for AgMAR, even if forced aeration can maintain adequate soil aeration status, due to the low infiltration rates and insufficient deep percolation that these soils achieve, which does not promote recharge of large amounts of water to the groundwater.Ideally, O2 and Eh measurements are complementary soil aeration quantifiers for aerobic and anaerobic conditions, respectively.In this study, the duration of soil saturation was shorter than 48 h at all sites , and therefore root zone residence time was relatively short, and aerobic conditions prevailed most of the time.Besides the duration of saturation, soil aeration status is also affected by soil respiration, which differed among the sites.Based on the O2 measurements of the control treatments , the lowest soil respiration rates were observed in the almond experiment during the winter , followed by higher rates in the cover crop during the spring , whereas the highest rates were observed in the almonds during the summer.
The increase in soil respiration with increasing temperatures is well documented, and generally soil respiration increases by a factor of two to three for a temperature increase of 10 ˚C.The relationship between O2 and Eh measurements is complex, as sometimes high O2 levels may coexist with low Eh levels, and vice versa.Some of this complexity can be explained by the lag of Eh behind O2 in response to flooding.Data showing this Eh-O2 lag can be found in a handful of studies , but Blackwell was the only one who explicitly related this lag to the soil volume over which the measurement is integrated.Black well concluded that O2 measurements are more affected by large pores compared with redox measurements that are measured by a small-size electrode.Some of our data support this explanation as indicated by the Eh minima, which lags up to 20 h behind the O2 minima, which might correspond to the lag in draining small pores vs.larger pores during soil drainage.Another explanation for the lag in Eh readings is related to slow reaction kinetics and mixed potentials, where the latter is inherent in most measurements done with redox electrodes.Hence, to obtain an unbiased result of the soil aeration status during Ag-MAR, a paired measurement of O2 and Eh is needed even in relatively well-drained soils with short flooding events.Based on our O2 and Eh measurements and the results of previous studies , denitrification will probably occur during Ag-MAR even in well-drained soils.This may promote N2O emissions as well as prevent NO3 − leaching to groundwater.The Eh results from KARE demonstrate that during an Ag-MAR event the soil may reach anaerobic conditions even in well-drained soils.At KARE, moderately reducing conditions were observed for up to 2 d , which can promote sequential reduction reactions according to the thermodynamic theory.However, in oxic terrestrial soils, some of these reactions may occur simultaneously; for example, reduction of NO3 − followed by concurrent reduction of Mn4+, SO4 2−, and Fe3+.Although these chemical species are stable under aerobic conditions, upon flooding they might undergo reduction and might get released from the solid phase into the soil solution, thereby changing the bio-geochemical state of the soil and increasing the risk of groundwater contamination.
Soil O2 is negatively correlated with soil water content, as high water content reduces the soil O2 diffusivity and air permeability.At the same time O2 demand by microbial respiration increases, as it highly depends on water content.A similar negative correlation was observed in this study, but in the air injection treatment, higher O2 levels at relatively higher water contents were observed, which indicates the positive impact of air injection.At the same time, a negative impact of air injection was observed for some trees at KARE.This negative impact can be explained by the injected air that pushes the pore water towards the O2 sensors.In this case, the readings of the galvanic O2 sensors might be different from the actual DO concentration of the pushed pore water.Under flooded conditions that are expected in Ag-MAR, O2 levels will decrease sharply after the water content reaches some critical value.Bachand et al.suggested a critical value of 74% degree of saturation in their report on Ag-MAR and soil O2, although a critical value based on air content is a better quantifier, marijuana grow system as it represents an absolute value that can be compared across soils with different porosities.The term effective volumetric air content is used here because we assume it represents only the conducting gas-phase fraction.Omitting the data of air injection and drainage stage, we set the critical air content upper bound as θa,eff ≈0.09 based on the inflection points of O2 levels, identified using a smoothed moving average procedure.It is noted that identifying the critical air content based on this method can be somewhat subjective and that the critical value is subjected to the accuracy of the water content sensors.As noted by Bachand et al., reaching the critical air content is inevitable during Ag-MAR flooding, and therefore attaining hypoxic/anoxic conditions will depend on the soil O2 depletion/recovery rates and on the flooding duration.We calculated the average soil O2 depletion/recovery rates for sites KARE and CT, as each represents a potential site forAg-MAR operation.The average depletion rate is lower than 0.3% O2 h−1, hence an O2 reduction from atmospheric conditions to 5% O2will take about 2 d for the soils tested here.Average recovery rates, excluding the active injection periods, are on the same order as the depletion rates, and usually the recovery rates are higher at the beginning of the reaeration stage, and then level off over time.Our observed average depletion rates are higher than the maximum depletion rates reported by Bachand et al.by up to ∼0.1% O2 h−1.This is probably because their water application was more conservative compared with this study.Under waterlogged conditions the galvanic gas-phase soil O2 sensors might malfunction , returning low or zero O2 readings, whereas the actual DO of the pore water might be higher.An example for this condition was observed at NSL, where we measured sporadically DO in water samples that were extracted from the air samplers at depths of 15, 30, and 50 cm.In addition, we also measured a few DO samples of ponded and irrigation water.
Although the total sampling amount was relatively small , it better represents the aeration status for some trees.For example, at Tree 3W, 15-cm depth, the DO is always higher than 0.5 O2 saturation compared with 0% O2 obtained with the galvanic soil O2 sensor.These measurements also demonstrate the impact of air injection on DO, as the trend of increasing DO during air injection periods is evident for all measured trees.At CT, we were able to extract more DO samples using dedicated pore-water samplers.Dissolved O2 levels fluctuated according to water application events and for the air-injection treatment also by the duration of air injection.Under flooded conditions, it is difficult to monitor the gas phase in the soil , and therefore DO measurements might be more suitable as soil aeration quantifier in Ag-MAR.Reliable in situ DO measurements can be achieved using optic sensors; however, its spatial resolution is still limited due to the relatively high costs of these sensors.The almond yield at KARE showed no staThistical difference between the treatments, except for the Nonpareil variety that showed higher yield for the air treatment compared to the no air treatment.This result is encouraging, but it should be taken with caution as it is based on only a few trees, and it might not correlate directly with the impact of Ag-MAR flooding, because yield in almond trees is commonly determined by the nutrient and water management practices of the previous growing seasons.The impact of flooding on almond yield was not studied systemically previously, although few studies on deficit irrigation showed yield reduction either by insufficient or excess irrigation.Still, these reported yield reductions are due to over irrigation throughout the entire growing season and not due to occasional flooding.In this study, the yield results of the air treatment at NSL demonstrate the potential negative impact that Ag-MAR may have on almond yield in poorly aerated soils.Previous studies on flooding in almond seedlings , and 1-yr-old almond trees demonstrated sensitivity to water logging which can lead to tree mortality.This sensitivity, also related to root diseases such as Phytophthora, varies among different almond varieties and Prunus rootstocks.At CT, the decrease in plant indices for the no-air-injection treatment compared with the other treatments was due to a reduction in the dry root weight of the cover crop after flooding.Although these changes were not staThistically significant, they agree with several studies that showed a reduction of root weight in bell beans under water-logged soils.This reduction is related to the formation of adventitious roots and aerenchyma with higher root porosity, which helps the plant to transport O2 from the soil surface and the stem, in order to reduce flooding injury.