The main reason for this was that breaking the application into smaller hydraulic loadings resulted in O2 concentrations to recover to background atmospheric conditions faster than the larger allat-once application in scenario S1. In fact, the O2 concentration differed slightly between S2 and S3. Because O2 inhibits denitrification, we conclude that these conditions resulted in the different denitrification capacity across application frequency and duration. In summary, we find that larger amounts of water applied all-at-once increased the denitrification capacity of the vadose zone while incremental application of water did not. However, NO3 – movement to deeper depths was slower under S2 and S3. Because initial saturation conditions impact nitrogen leaching, we also simulated the impact of wetter antecedent moisture with 15% higher saturation levels than the base case simulation for the ERT profile. Simulated profiles of liquid saturation, NO3 – , NO3 – :Cland acetate for the simplified ERT stratigraphy under wetter conditions are shown in Figure 10. Model results demonstrate that the water front moved faster and deeper into the soil profile under initially wetter conditions for all three scenarios. Within the shallow vadose zone , across AgMAR scenarios, O2 concentrations were similar initially, but began differing at early simulated times, with lower O2 under wetter antecedent moisture conditions than with the base-case simulation. In addition, both oxygen and nitrate concentrations showed significant spatial variation across the modeled column. Notably,fodder system nitrate concentrations were 166% higher in the preferential flow channel compared to the sandy loam matrix under wetter conditions, while only 161% difference was observed under the base case simulation .

Nitrate movement followed a pattern similar to water flow, with NO3 – reaching greater depths with the wetter antecedent moisture conditions. Under S1, however, at 150 cm-bgs, NO3 – decreased more quickly under the wetter antecedent moisture conditions due to biochemical reduction of NO3 – , as evidenced by the decrease in NO3 – :Clratio, as well as by dilution of the incoming floodwater. In the wetter antecedent moisture conditions, 39%, 31%, and 30% of NO3 – was denitrified under S1, S2, and S3, respectively. For S1, where water was applied all at once, more denitrification occurred in the wetter antecedent moisture conditions, however, the same was not true of S2 and S3 where water applications were broken up over time. This could be due to the hysteresis effect of subsequent applications of water occurring at higher initial moisture contents, allowing the NO3 – to move faster and deeper into the profile without the longer residence times needed for denitrification to occur. Thus, wetter antecedent moisture conditions prime the system for increased denitrification capacity when water is applied all at once and sufficient reducing conditions are reached, however, this is counteracted by faster movement of NO3 – into the vadose zone. Simluations from our study demonstrate that low-permeability zones such as silt loams allow for reducing conditions to develop, thereby leading to higher denitrification in these sediments as compared to high permeability zones such as sandy loams. In fact, the homogenous silt loam profile reported the maximum amount of denitrification occurring across all five stratigraphic configurations . Furthermore, the presence of a silt loam channel in a dominant sandy loam column increased the capacity of the column to denitrify by 2%. Conversely, adding a sandy loam channel into a silt loam matrix decreased the capacity of the column to denitrify by 2%. These relatively simple heterogeneities exemplify how hot spots in the vadose zone can have a small but accumulating effect on denitrification capacity . Note that differences in denitrification capacity maybe much greater than reported here because of increased complexity and heterogeneity of actual field sites when compared to our simplified modeling domains.

Another observation of interest for silty loams is the prominence of chemolithoautotrophic reactions and Fe cycling observed in these sediments. In comparison, sandy loam sediments showed persistence and transport of NO3 – to greater depths. A reason for this is that oxygen concentration was much more dynamic in sandy loams, rebounding to oxic conditions more readily than in silt loams, even deep into the vadose zone . Dutta et al. found similar re-aeration patterns in a 1 m column experiment in a sand dominated soil, with re-aeration occurring quickly once drying commenced. Even with the presence of a limiting layer, defined by lower pore gas velocities and higher carbon concentration, a sandy loam channel acted as a conduit of O2 into the deep vadose zone maintaining a relatively oxic state and thus decreasing the ability of the vadose zone to denitrify. In systems with higher DOC loadings to the subsurface, oxygen consumption may proceed at higher rates creating sub-oxic conditions in the recharge water and more readily create reducing conditions favorable to denitrification in the subsurface . We note here that microbial growth, which was not modeled in this study, could also affect the rates of O2 consumption and re-aeration, which could lead to underestimation of O2 consumption . Overall, denitrification capacity across different lithologies was shown to depend on the tight coupling between transport, biotic reactions as well as the cycling of Fe and S through chemolithoautotrophic pathways. Under large hydraulic loadings , overall denitrification was estimated to be the greatest as compared to the lower hydraulic loading scenarios . The main reason for the higher denitrification capacity was the significant decline in O2 concentration estimated for this scenario, whereas such conditions could not be maintained below one meter with lower hydraulic loadings under scenarios S2 and S3. However, nitrate was also transported deeper into the column under S1 as compared to S2 or S3. Tomasek et al. found the reverse in a floodplain setting, where intermittent indundation with flood water, comparable to our S2 and S3 contexts, resulted in higher rates of denitrification in the zone that was always inundated, due to priming of the microbial community and pulse releases of substrates and electron donors.

Future studies examining the impact of AgMAR on denitrification should include processes such as mineralization to see if the same behavior would be observed. It seems that there may exist a threshold hydraulic loading and frequency of application that could result in anoxic conditions and therefore promote denitrification within the vadose zone for different stratigraphic configurations, although this was not further explored in this study. In another study, Schmidt et al. found a threshold infiltration rate of 0.7 m d-1 for a three hectare recharge pond located in the Pajaro Valley of central coastal California, such that no denitrification occurred when this threshold was reached. For our simulations, we used a fixed, average infiltration rate of 0.17 cm hr-1 for our all-at-once and incremental AgMAR scenarios, however,fodder system for sale application rates can be expected to be more varied under natural field settings. Our results further indicate that the all-at-once higher hydraulic loading, in addition to causing increased levels of saturation and decrease in O2, resulted in leaching of DOC to greater depths in comparison to lower, incremental hydraulic loading scenarios . Akhavan et al. 2013 found similar results for an infiltration basin wherein 1.4% higher DOC levels were reported at depths down to 4 m when hydraulic loading was increased. Because organic carbon is typically limited to top 1 m in soils , leached DOC that has not been microbially processed could be an important source of electron donors for denitrification at depth. Systems that are already rich in DOC within the subsurface are likely to be more effective in denitrifying, and thus attenuating, NO3 – , such as floodplains, reactive barriers in MAR settings, or potentially, organically managed agroecosystems .. This finding can also be exploited in agricultural soils by using cover crop and other management practices that increase soluble carbon at depth and therefore remove residual N from the vadose zone . While lower denitrification capacity was estimated for scenarios S2 and S3, an advantage of incremental application was that NO3 – concentration was not transported to greater depths. Thus, higher NO3 – concentration was confined to the root zone. If NO3 – under these scenarios stays closer to the surface, where microbial biomass is higher, and where roots, especially in deep rooted perennial systems such as almonds, can access it, it could ultimately lead to less NO3 – lost to groundwater. While there is potential for redistribution of this NO3 – via wetting and drying cycles, future modeling studies should explore multi-year AgMAR management strategies combined with root dynamics to understand N cycling and loading to groundwater under long-term AgMAR. Simulation results indicate that wetter antecedent moisture conditions promote water and NO3 – to move deeper into the domain compared to the drier base case simulation. This finding has been noted previously in the literature, however, disagreement exists on the magnitude and extent to which antecedent moisture conditions affect water and solute movement and is highly dependent on vadose zone characteristics. For example, in systems dominated by macropore flow, higher antecedent soil moisture increased the depth to which water and solutes were transported . In a soil with textural contrast, where hydraulic conductivity between the topsoil and subsoil decreases sharply, drier antecedent moisture conditions caused water to move faster and deeper into the profile compared to wetter antecedent moisture conditions.

In our system, where a low-permeability layer lies above a high permeability layer , the reverse trend was observed. Thus, a tight coupling of stratigraphic heterogeneity and antecedent moisture conditions interact to affect both NO3 – transport and cycling in the vadose zone, which should be considered while designing AgMAR management strategies to reduce NO3 – contamination of groundwater. Furthermore, dry and wet cycles affect other aspects of the N cycle that were not included in this study . Specifically, the effect of flood water application frequency on mineralization of organic N to inorganic forms should be investigated to assess the full N loading amount to groundwater under AgMAR.The value of California’s fruit, nut, and vegetable crops was $20 billion in 2009, almost 60% of the state’s farm sales of $35 billion. California dominates U.S. production of these crops and currently accounts for about half of the U.S. fresh vegetable production and about half of total fruit production. Many of these fruits and vegetables are labor intensive; labor costs for fruit and vegetables average 42% of variable production costs. Over half of the state’s hired farm workers are unauthorized, and most move on to non-agricultural employment within a decade of beginning to work in the fields. The California produce industry depends on a constant influx of new, foreign-born labor attracted by wages above those in their countries of origin, primarily Mexico.Enforcement of immigration laws has increased recently in two major ways. First, the U.S. government has erected fences and vehicle barriers on a third of the 2,000 mile Mexico–U.S. border to deter unauthorized entries. Second, the Immigration and Enforcement Agency that enforces immigration laws inside the United States has begun to audit more of the I-9 forms completed by newly hired workers and their employers. After these audits, employers are asked to inform workers whose data do not match government records to clear up discrepancies. Most workers instead quit, which has prompted some farm employers to invest in housing in order to hire legal H-2A guest workers . H-2A workers must be paid at least the so-called Adverse Effect Wage Rate , which in 2011 is $10.31 an hour in California, higher than the state and federal minimum wages. AEWRs were established in the 1960s to prevent the presence of legal foreign workers from depressing the wages of U.S. farm workers. Immigration reform could also raise farm labor costs by legalizing currently unauthorized farm workers and encouraging farm employers to turn to H-2A guest workers if legalized workers find non-farm jobs, which could raise labor costs. Efforts to enact immigration reform between 2005 and 2007 failed, but in his 2011 State of the Union speech, President Obama urged Congress to try again. He said: “I know that debate will be difficult. I know it will take time. But tonight, let’s agree to make that effort.” This paper reviews the three most likely adjustments in the fruit and vegetable industry to higher labor costs: mechanization, imports, and labor aids.U.S. production of fresh-market fruit and vegetables has increased in the last two decades—up 12% for fresh fruit and 41% for fresh vegetables .