Consequently, lower NO3 – concentration and lower NO3 – :Clratio are predicted in the silty loam vadose zone as compared to the sandy loam column. It is interesting to note that while greater NO3 – loss and denitrification are predicted for the silty loam vadose zone, carbon concentration associated with the shallow vadose zone are comparatively lower than for the sandy loam column. Moreover, the calculated pH is lower and iron concentrations are higher in the silt loam profile below the top meter when compared to the same depths within the sandy loam column . This suggests that chemolithoautotrophic reactions could be more important for these finer textured sediments. While both heterotrophic and chemolithoautotrophic reactions would be expected to result in a pH decrease , the greater decline in pH and concomitant increase in Fe+3 concentration suggests the importance of Fe and S redox cycling associated with the chemolithoautotrophic reactions in silty loam sediments . Evolving from these steady state conditions, scenario S1 suggests that denitrification is enhanced as floodwater infiltrates into the silt loam column. Model results indicate that saturation increases to 80% from 1 to 4 m depths and O2 decreases from 2.1 x 10-4 mol L-1 to 1.7 x 10-4 mol L -1 , resulting in 43% of the NO3 – being denitrified for this scenario . In comparison to the homogeneous profiles, the sandy loam with silt loam channel stratigraphy has higher calculated water contents and slightly lower O2 concentration within and surrounding the silt loam channel than the homogenous sandy loam column under steady state conditions . Calculated NO3 – concentrations are also similar between the homogenous sandy loam column and SaSi case,flower display buckets except for within and below the silt loam channel where lower NO3 – concentration was predicted .
For scenario S1, water content for the SaSi case increased in a manner similar to the homogenous sandy loam, except for within the silt loam channel, which increased from 60 to 81%. Figure 4 further demonstrates that the infiltrating floodwater resulted in an increase in NO3 – concentration between 1 and 3 m within the sandy loam textured soil, but a decrease elsewhere. Within the channel itself , lower nitrate and NO3 – :Clratio are predicted, suggesting higher rates of denitrification . Overall, the model results indicate that an average of 37% of the NO3 – concentration is denitrified in the SaSi case 60 days after flooding, with 35% denitrification occurring in the sandy loam matrix and 40% occurring within the silt loam channel. Furthermore, the silt loam channel has lower carbon and higher Fe+3 concentrations similar to the homogenous silt loam column again suggesting the importance of both heterotrophic and chemolithoautotrophic denitrification in these finer textured sediments. In comparison to the SaSi case, calculated water saturation and O2 profiles were markedly different between the homogenous silt loam column and the silt loam with sandy loam channel under steady state conditions . In particular, the sandy loam channel has lower calculated water content than the homogenous silt loam column . Further, greater gas flux within the channel resulted in 11-19% higher O2 concentration that penetrated deeper into the vadose zone as compared to the homogeneously textured column. NO3 – concentration are also estimated to penetrate deeper into the vadose zone in the SiSa case due to the high permeability of the sandy loam channel .
While carbon concentration also penetrated deeper in the vadose zone in the SiSa case, higher calculated O2 concentration did not allow for comparable rates of denitrification below 1 m in this case as observed in the homogenous silt loam profile. This is further confirmed by the lower NO3 – :Clratio, which indicates that transport processes dominate biogeochemical fluxes within this column . With scenario S1, the calculated water content increased to 48% saturation while the O2 concentration remained the same within the channel. The high permeability channel allowed for NO3 – to move faster and deeper into the vadose zone. Overall, calculated denitrification was lower in the SiSa case as compared to the homogeneous textured column. In the simplified ERT stratigraphy, similar patterns were observed such that high permeability channels transported water, O2, and NO3 – faster and deeper into the subsurface than low permeability regions . As a result, concentration profiles showed significant variability across the modeled domain even under steady state conditions. For example, the calculated O2 and NO3 – concentrations are an order of magnitude lower in the shallow vadose zone below the limiting layer than within the preferential flow channel. Higher NO3 – :Clratio within the channel further confirms that preferential flow paths transport higher quantities of dissolved aqueous species without their being impacted by other processes such as denitrification . Other interesting trends are shown by carbon and Fe+2 concentrations within the modeled column. Dissolved carbon in particular is predicted to have a lower concentration in the preferential flow channel and the matrix surrounding the channel than below the limiting layer. In contrast, the Fe+2 concentration is estimated to be higher in the matrix surrounding the preferential flow channel and below the limiting layer . For scenario S1, model results indicate that NO3 – moved through the preferential flow path faster and deeper into the profile, while the limiting layer acts as a denitrification barrier as evidenced by the decrease in NO3 – :Clratio.
The highest denitrification was estimated to occur in the matrix adjacent to the preferential flow channel , followed by intermediate nitrate reduction below the limiting layer and far away from the channel , while the lowest denitrification was estimated to occur within the channel itself . The confluence of higher amounts of C and NO3 – moving into a reduced zone could be the reason that the matrix surrounding the preferential flow channel has higher denitrification rates, while the regions further away from the preferential flow channel have lower amounts of microbially available C and NO3 – . In contrast, residence times are too short in the channel to allow for reducing conditions to develop. The ability of the entire vadose zone to denitrify would depend on the overall surface area of preferential flow paths to the rest of the surrounding matrix in the zone of flooding. Overall, we find that low permeability zones alone or embedded within high flow zones demonstrate highest denitrification rates across all soil profiles. Because the ERT column more closely approximates the heterogenity of our agricultural field site,flower bucket we use this column to demonstrate the impact of hydraulic loading and application frequency on nitrogen fate and dynamics. Simulated profiles of liquid saturation, NO3 – , NO3 – :Cland acetate for the simplified ERT stratigraphy for scenarios S2 and S3 are shown in Figure 9 and A3. It is interesting to note that AgMAR ponding under scenarios S2 and S3 resulted in fully saturated conditions to persist within the root zone only. In comparison, the 68 cm all-at-once application for scenario S1 resulted in fully saturated conditions to occur at even greater depths of 235 cm-bgs . This resulted in the NO3 – front moving deeper into the subsurface to depths of 450 cm-bgs under S1 compared to 150 cm-bgs for scenarios S2 and S3 . Much lower concentrations of NO3 – were found at 450 cm-bgs in scenarios S2 and S3 compared to S1 . Thus, larger amounts of water applied all-at-once led to NO3 – being transported faster and deeper into the profile. Surprisingly, model results indicate 37% of NO3 – was denitrified with scenario S1, while 34% and 32% of NO3 – was denitrified in scenarios S2 and S3, respectively. For scenarios S2 and S3, denitrification was estimated to occur only within the root zone. This was confirmed by NO3 – :Clratio that did not show any reduction with depth for these scenarios. A reason for this could be that acetate was not estimated to occur below the root zone, preventing electron donors from reaching greater depths for denitrification to occur. In contrast, model results for S1 indicate that acetate was leached down to 235 cm-bgs below the limiting layer. Overall, model results indicate that NO3 – did not move as fast or as deep in scenarios S2 or S3; however, the ability of the vadose zone to denitrify was reduced when the hydraulic loading was decreased. The main reason for this was that breaking the application into smaller hydraulic loadings resulted in O2concentrations 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, 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 .