Contrary to other oxidation processes, nitrification also occurs below hummocks (Figure 6), because

here ammonium, released during turnover of organic material due to aerobic respiration, reacts with oxygen to form nitrate. Nevertheless, even for nitrification the areas showing the most intensive turnover rates are preferentially located below depressions where upwelling water rich in ammonium gets in contact with atmospheric oxygen. As mentioned before, the location of oxidation and reduction hot spots in the virtual wetland models are strongly correlated with the surface micro-topography as illustrated in Figure 7. For each of the models surface micro-topography is displayed in plan view. A binary classification into areas with higher relative elevation (red areas) and local depressions (blue areas) is used for the micro-topography models, whereas the surface of the planar reference model is displayed in graduated colors. Directly to the right of the plots showing the model surface, plan views of high process activity (hot spots) are shown in black, evaluated based on the maximum process activity across the vertical extent of the model at each location in the 2D horizontal domain (again for sulfate reduction as an example). A strong spatial correlation between hot spots and wetland topography can be seen for the micro-topography models. In close proximity to the stream channel (X > 18 m) also surface depressions can be zones of infiltration because of the steep hydraulic gradients towards the adjacent stream channel. Under these conditions depressions are no longer characterized by upwelling of reduced groundwater, which suppresses oxidation but in turn fosters reduction processes (Plots for the other implemented processes, which are shown in the supplement Figure A8-A12).

Although the formation of hot spots was generally found in both micro-topography models (Figure 8 A and B) it is significantly more pronounced in the model with larger mean length of the structures.

The main reason for that are the more pronounced shallow flow cells that develop in the model with coarser micro-topography. In the model with finer micro-topography (ml-0.25m) hot spots are less pronounced (the relative difference in reaction rates between the hot spot and its surrounding area is smaller) and spatially more dispersed. This model represents a transition to the planar reference model (Figure 8 C), where almost the entire surface area of the model shows infiltration and upwelling conditions are restricted to the zone between X = 9 m and 16 m. As a result biogeochemical process patterns are more uniform and less patchy. The characteristic patterns of hot spots are not only visible along the main direction of subsurface flow as shown in the transects but also in 3D, which is shown in Figure 9 (plots for the mean length 0.25 m model and the planar reference are shown in the supplement Figure A13 and A14).

STUDY 2

Figure 6: Results of the biogeochemical simulations exemplarily shown for nitrification of the micro-topography scenario with the mean length 0.5 m. PHREEQC simulations were performed along the flow paths shown in A. Results were interpolated into the 2D cross sections. B shows the age distribution in years of subsurface flow derived from backward particle tracking. C represents process activity of nitrification (kinetic rate in mol/Ls). Concentrations in mol/L for ammonium, nitrate and oxygen are shown in D, E and F respectively.

STUDY 2

Figure 7: Plan view of the micro-topography and the planar reference models. Micro-topography is depicted in two categories, red for hummocks and blue for hollows. For the planar reference model elevation classes are shown reflecting the linear slope of the surface. Black areas on the right represent areas of preferential sulfate reduction (hot spots) relative to their surroundings. In general hot spots for reduction processes preferentially form below hummocks and hot spots for oxidation processes below hollows (as shown in the supplement).

STUDY 2

Figure 8: Comparison of simulated sulfate reduction hot spots for the two different micro-topography scenarios (A and B) and the planar reference (C).

Figure 9: Fence plots showing the zones of preferential sulfate reduction for the whole 3D domain of the mean length 0.5 m model (3D plots for the mean length 0.25m and the planar reference case are shown in the supplement).

STUDY 2

4 Discussion

4.1 Hydrological controls of hot spot formation

Our model scenarios have demonstrated that the basic mechanism of hot spot formation is the same for both micro-topography models. In young, freshly infiltrated subsurface water, electron acceptors are abundant and anaerobic respiration can proceed as soon as the infiltrating water reaches zone 3 where oxygen resupply is assumed to be negligible (τ_resupply<<τ_depletion). After depletion of oxygen, denitrification, iron(III) – and sulfate reduction are sequentially initiated for infiltrating water, triggered by the high availability of electron acceptors. Reductive hot spots are generated below infiltration areas located preferentially underneath hummock structures. Initially, below hummocks, infiltrating water passes the unsaturated zone (zone 1) where resupply of oxygen is assumed to occur instantly (τresupply>>τdepletion), here aerobic respiration is the only active process. As water infiltrates deeper (zone 2), resupply of atmospheric oxygen is assumed to significantly slow down (reduced diffusivity) and anaerobic processes are being initiated. Denitrification, iron(II) and sulfate reduction become dominant in the the part of the saturated zone (zone 3) where the resupply of oxygen is cut off. On the other hand, upwelling zones, where older and already reduced groundwater rises into superficial layers, are characterized by inactivation of reduction processes. Here denitrification, iron(III) and sulfate reduction are inhibited because electron acceptors are not available. Oxidation processes however, are triggered for upwelling areas because here reduced water gets in contact with atmospheric oxygen, which is supplied to zone 2 by diffusion through the saturated pore space.

Upwelling of subsurface water preferentially occurs below local depressions. Whether oxidation hot spots can be generated below a depression or not depends on the amount of surface water stored within the superficial depression. If a surface depression is filled with too much water (depth of surface ponding > 0.25 m) diffusive penetration of atmospheric oxygen is inhibited and hence the formation of hot spots for oxidation processes is suppressed. In contrast, very pronounced hot spots for oxidation processes can be found below depressions with upwelling groundwater and low ponding depths. Here the saturated pore space is located within zone 2 where diffusion of atmospheric oxygen still exceeds depletion (as opposed to zone 3) and where oxygen can penetrate into shallow layers where it gets in contact with upwelling water carrying high concentrations of reduced species. How fast turnover of reduced species in oxidation hot spots occurs, depends on the availability of oxygen in zone 2, which is controlled by the amount of surface water being stored in the superficial depression.

If surface ponding depths are very low (< 0.05 m) availability of oxygen is assumed to be very high within zone 2 resulting in fast turnover of reduced species and very pronounced local oxidation hot spots. With increasing surface ponding (0.05 m - 0.25 m) oxygen availability drops rapidly resulting in slower turnover rates and less pronounced oxidation hot spots. The availability of oxygen below depressions is therefore mainly controlled by the dynamics of surface water storage, which was found

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