A national study of denitrification dynamics across English saltmarshes and relationships with potential drivers

The natural capital approach advocates for the value of the natural environment for both people and the economy. A fundamental aspect of this approach is building a robust evidence base to evaluate the state of natural habitats and their capacity to provide essential ecosystem services. Building the evidence base helps us better understand and value the environment and its contributions to people. Concerns over the health of coastal and estuarine habitats are becoming increasingly prominent, especially with pollution affecting water and sediment quality. Excess nutrients entering the marine environment from multiple terrestrial sources, for example agriculture, domestic sewage and industry, are a particular concern given they harm biodiversity and contribute to algal blooms. It is believed that natural coastal ecosystems, such as saltmarshes, can contribute to the remediation of these pollutants potentially at a far lower economic cost than industrial treatment; in other words, coastal habitats can provide a ‘nature-based solution’ to a pressing socio-environmental issue. This critical nutrient remediation ecosystem service provided by coastal habitats can be achieved through processes such as denitrification – the transformation of nitrate to environmentally benign dinitrogen gas. While saltmarshes are believed to perform denitrification, there is a notable lack of evidence in England about the magnitude of this process and any potential differences among saltmarshes. This research represents the first step in addressing these knowledge gaps. It provides benchmarks for denitrification process rates across twelve intact saltmarshes located within six estuaries in England, at one point in time for a given marsh. Using laboratory incubations of intact saltmarsh sediment cores collected between August and November 2024, this research shows that saltmarshes on the southern and eastern coasts of England denitrify at far greater rates (an average of 441 μg N m-2 hr-1) than those on the northwest coast (an average of 188 μg N m-2 hr-1). Furthermore, across marshes, upper marsh vegetation communities denitrified, on average, at a rate 140% greater than that found in pioneer/low and low-mid marsh communities. Substantial variation in mean denitrification rates across marsh zones did exist though: from 43 to 1037 μg N m-2 hr-1. Additionally, although most cores indicated that denitrification would go to completion i.e. to dinitrogen gas, some cores showed gaseous emissions of an intermediate compound arising during incomplete denitrification: nitrous oxide. This is a concern since nitrous oxide is a potent contributor to climate change. This new research is particularly significant for the Environment Agency (EA) due to pressing concerns over water quality. The EA monitor key coastal habitats, including saltmarshes, under the Water Environment Regulations (WER). This monitoring helps to give saltmarshes a classification status, so it is clear which marshes are ecologically healthy and which saltmarshes are in poor health and need management or intervention to help restore them. However, these classifications are a snapshot and generally don’t give information on what is happening within the saltmarsh sediment given this requires more specialised research. Therefore, the information on denitrification processes within twelve 7 of 76 saltmarshes across England could give an insight into what is happening within the sediment and can be compared to most recent saltmarsh classification results to see if these results follow any national trends. Furthermore, the potential implications of incomplete denitrification within saltmarsh offers insight into the extent to which saltmarsh systems can mitigate climate change. Indeed, this research emphasizes the need to consider a suite of greenhouse gas fluxes within saltmarsh systems, as well as carbon sediment and biomass stock changes when considering their climate mitigation potential. Once baseline data on denitrification rates in England's saltmarshes are further established through seasonal surveys and in restoring, as well as intact, marsh contexts, it can guide future management efforts, including incentivising restoration and the creation of new saltmarsh habitats. The evidence contained within this report can provide a basis for advocating for nature-based solutions using natural capital assets to deliver the ecosystem services that enhance the wellbeing of people and the planet. As detailed in schemes like the Water Industry National Environment Programme, there was a recommendation to enhance the natural environment while also addressing environmental challenges faced by coastal habitats. An example of this enhancement could involve using saltmarsh systems to offset harmful levels of available nitrogen added into estuaries through water treatment works. Empirical data on how saltmarshes process nitrates could also help inform restoration initiatives through frameworks like Environmental Land Management schemes and in the future may be useful to Biodiversity Net Gain and Marine Net Gain. These data can also contribute to nutrient units within the Saltmarsh Code and give an insight in to how different saltmarshes process nutrients, which is important for schemes such as Nutrient Neutrality, administered by Natural England. Through its Land Sea Interface project, the EA has adopted a source-to-sea approach to address the disconnect in monitoring, assessment, management, and decision-making across terrestrial, coastal, and marine habitats. Land-based pressures are often managed without considering their effects on estuarine, coastal, and marine natural capital assets. By addressing this disconnect through the EA’s research-led strategy, the source-to-sea approach promotes cohesive and impactful management practices. Such practices are crucial for the success of conservation and restoration projects in coastal and estuarine areas, thus achieving outcomes to benefit people and the planet.