Groundwater dynamics in a restored tidal marsh are limited by historical soil compaction

Abstract

In places where tidal marshes were formerly embanked for agricultural land use, these marshes are nowadays increasingly restored with the aim to regain important ecosystem services. However, there is growing evidence that restored tidal marshes and their services develop slowly and differ from natural tidal marshes in many aspects. Here we focus on groundwater dynamics, because these affect several key ecosystem functions and services, such as nutrient cycling and vegetation development. We hypothesize that groundwater dynamics in restored tidal marshes are reduced as compared to natural marshes because of the difference in soil structure. In the macro-tidal Schelde estuary (Belgium), in both a natural and a restored (since 2006) freshwater tidal marsh, we measured depth profiles of soil properties (grain size distribution, LOI (loss on ignition), moisture content and bulk density) and temporal dynamics of groundwater levels along a transect with increasing distance from a tidal creek. X-ray micro CT-scanning was used to quantify soil macroporosity. The restored marsh has a twolayered soil stratigraphy with a topsoil of freshly accreted sediment (ranging in depth between 10 and 60 cm, deposited since 2006) and a subsoil of compact relict agricultural soil. We found that both the soil in the natural marsh and the topsoil of the restored marsh consist of loosely packed sediment rich in macropores and organic matter, whereas the relict agricultural soil in the restored marsh is densely packed and has few macropores. Our results show that groundwater level fluctuations in the restored marsh are restricted to the top layer of newly deposited sediment (i.e. on average 0.08 m depth). Groundwater level fluctuations in the natural marsh occur over a larger depth of the soil profile (i.e. on average 0.28 m depth). As a consequence, the reduced groundwater dynamics in restored tidal marshes are expected to alter the subsurface fluxes of water and nutrients, the sourcesink function and the development of marsh vegetation.

Effects of tidal re-introduction design on sedimentation rates in previously embanked tidal marshes

“…Former agricultural low-lying sites often have low permeable or even impermeable, heavily compacted top soils (due to former use of agricultural machinery, soil drainage, etc. ), which may seriously limit the colonization of vegetation or benthic invertebrates because of very poor drainage of the fresh tidal sediment deposits on top of this low permeable soil (Tempest et al, 2015;Van Putte et al, 2019).…”

Effect of vegetation cover and sediment type on 3D subsurface structure and shear strength in saltmarshes

“…μCT has been applied extensively to agricultural soils to investigate the impact of subsurface structures on crucial soil functions such as water infiltration (Jarvis et al, 2017; Katuwal et al, 2015; Müller et al, 2018; Pot et al, 2020; Tracy et al, 2015), root–pore interactions and patterns of plant growth (Hu et al, 2020; Lucas et al, 2019; Pulido‐Moncada et al, 2020). In recent years, the technique has been extended to saltmarsh substrates (Dale et al, 2019; Spencer et al, 2017; Van Putte et al, 2019); however, distinguishing roots from pores is challenging because their greyscale values overlap due to the partial volume effect (Cnudde & Boone, 2013; Helliwell et al, 2013), especially in these complex heterogeneous substrates. Indeed, saltmarshes are transitional habitats formed by a constant interplay of sediment deposition and erosional processes, and where ground cover and other soil characteristics can vary rapidly both in space and time.…”

Floodplain Inundation and Salinization From a Recently Restored First‐Order Tidal Stream

“…Soil samples at 10 and 30 cm (19 cm for the floodplain soils) were collected from the face of soil pits and soil classification (USDA Natural Resources Conservation Service, n.d.) and texture was determined. The floodplain soils are hydric Ocosta silty clay loam (USDA Natural Resources Conservation Service, n.d.). Soil texture was dominantly silty clay ( n = 5), but ranged from sandy clay loam to clay (Sengupta et al, 2019).…”