Historical soil compaction impairs biogeochemical cycling in restored tidal marshes through reduced groundwater...

Tidal marshes are often restored on compact agricultural soil that limits tidally induced groundwater dynamics and soil aeration after restoration. We hypothesized that impaired soil aeration affects biogeochemical cycling and leads to altered porewater nutrient concentrations in restored tidal marshes. We studied soil hydraulic properties, groundwater dynamics and porewater nutrient concentrations (nitrogen, phosphorus and dissolved silica) over the course of one year in a natural and a restored freshwater tidal marsh in the Scheldt estuary, Belgium. From measured groundwater levels, we calculated the soil saturation index (the proportion of time the soil is saturated at a certain depth). The aerated zone generally extends over a deeper soil profile in the natural marsh compared to the restored marsh, where the former agricultural subsoil has a higher compaction rate and lower hydraulic conductivity. The soil saturation index was negatively correlated with nitrate (ρ = -0.21, p < 0.001) and positively correlated with ammonium (ρ = 0.32, p < 0.001). Concentrations of phosphate (ρ = 0.43, p < 0.001) and dissolved iron (ρ = 0.44, p < 0.001) were positively correlated to the soil saturation index, suggesting retention of phosphate on iron oxides in well aerated zones, which are more abundant in the natural marsh. The depth profile of soil hydraulic properties and soil aeration is very site specific, even within the same marsh, suggesting the need for a pre-restoration assessment of soil hydraulic properties to determine where and which design measures are required to optimize nutrient cycling in newly restored tidal marshes.