Anthropogenic induced climate change has raised global sea levels and caused an amplification of coastal flooding events. 

An increase in ‘storminess’ (storm surges, flooding and the encroaching of seawater inland) is predicted to have two major consequences. The first is the erosion and submergence of coastal wetlands, which play key roles as flood defences, and are unique habitats to wildlife. The second is the repeated flooding of low-lying coastal agricultural land.

Salinity is an environmental stress that at high enough levels can impair the metabolic functions of living organisms. In soils, as salinity increases changes in water availability and an increase of ions can suppress plant growth, alter plant community dynamics and disrupt below-ground processes of microbial communities.

Fundamentally, increasing abiotic stress causes a knock-on effect, perturbing the biotic communities that reside in the focal habitat. The consequences of seawater flooding are multifaceted. Changes caused by an influx of metal and chloride ions alter the physical properties of the soil, and the submergence of the habitat fosters anoxic conditions, ultimately disturbing microbial composition, activity and functioning. If microbial communities are damaged through stress, then functioning of key ecosystem processes (nutrient cycling, organic matter storage and decomposition) are affected. It is therefore highly pertinent to understand the consequences of seawater flooding on different terrestrial soils.

Our research aimed to elucidate how saltwater inundation of saltmarshes and coastal agricultural soils impacted microbial community function and structure. We created a mesocosm experiment to address the questions: (1) Does flooding duration significantly impact the functional recovery of agricultural soil-associated microbial communities? (2) Does preadaptation to stress alter the resistance and resilience of a microbial community?

Our mesocosm approach simulated seawater flooding on a naturally occurring saltmarsh-terrestrial pasture gradient near Southport, UK with varying degrees of previous exposure to saltwater. Using the mesocosm set-up allowed us to monitor changes in soil environmental parameters (pH, metal concentration, conductivity) and microbial functioning (metabolic activity and degradative enzymes) and composition (16S rRNA gene sequencing).

The soils used consisted of two saltmarsh sites (low and high) previously exposed to varying frequencies of repeated seawater inundation and a coastal agricultural (pasture) site that had not been previously exposed to seawater. Soil mesocosms were exposed to seawater submersion for durations of 0, 1, 96 and 192 hours to test resistance to the stress. After draining the seawater, the samples were analysed again after a subsequent 14-day recovery period.

We found that environmental characteristics shifted in the mesocosms as physiochemical properties of pasture soils were shown to become more that like of saltmarshes. These included a rise in pH and conductivity, and a more similar composition of metal ions in the soil. Further, we monitored the microbial abundance (cell counts), community functioning (enzyme activity) and overall activity (metabolic potential) and found that these measures all increased during the flooding, but all returned to a lower level after recovery. Interestingly, there appeared to be shift to higher-energy communities utilising more labile carbon as the environment became more stressed (longer flooding duration). Pasture communities previously unexposed to saltwater were shown to have higher resilience and tolerate short inundations with functioning returning to prior state after one hour; however, extended saltwater flooding duration significantly altered functioning and structure thereafter. Conversely, communities from saltmarsh sites demonstrated a higher resistance, retaining function following prolonged exposure and higher resilience to seawater inundation.

When we investigated changes in community composition, we were surprised to find that although there were significant differences between the sites (in particular the high level of species dominance in the saltmarsh sites compared with the pasture site), the composition was not impacted by flooding duration. This is at odds with the functioning data that suggested there was some degree of re-structuring of the communities during the flood. Our results imply that whilst flooding disrupts how the microorganisms perform functions, they themselves remain stable in the community.

It has been previously established that saltmarsh sites act as natural sinks, accumulating heavy metals in vegetation and sediment. Encroachment of saltwater onto agricultural lands could cause a spreading of heavy metals deposited on agricultural soils. Previous studies have shown the effect of increased salinity and ionic sodium (Na+) linked to the mobility of heavy metals (Cr, Cu, Co, Pb and Zn) increasing toxicity in soils that could not only damage potential crop production but, as shown here, alter the functioning of the community.

The ingress of salinity, combined with extended waterlogging, is an increased stress factor on soil communities. This forces the community to utilise resources and energy for defence mechanisms to prevent lysis due to osmotic pressure and rapidly decreasing oxygen levels. The significant increase in pH observed within mesocosms has been suggested to disrupt soil aggregate stability. The disruption in aggregates can be potentially damaging to organic matter within the soils impacting microbial community function within the ecosystems. Soils will be more susceptible to soil erosion and runoff during storm surges due to the alteration in composition. Erosion of the topsoil leads to a loss of fertile land and affects water-holding capacity, leading to a worsening effect of flooding and threatened food security.

The study we conducted has shown the potential implication of saltwater exposure caused by sea-level rise on wetlands and coastal agricultural land. This has furthered our understanding of salinity ingress on microbial communities, and our ability to predict future changes in community composition and functioning. These results suggest that communities previously exposed to flooding have increased protection against seawater inundation, with pasture soils significantly impacted even after a short flooding duration. As extreme weather events are increasing in frequency and intensity across the planet, subsequent saltwater intrusion events will rapidly accelerate.