Denitrification dynamics and relationships with potential drivers across English saltmarshes, seagrass beds, and mudflats: capturing national variation in space and time

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 (e.g. agriculture, industry and domestic sewage), are a particular concern given the potential harm to biodiversity and their contribution to algal blooms. It is believed natural coastal ecosystems, such as saltmarshes, seagrass beds and mudflats, can help remediate 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 coastal habitats, and saltmarshes in particular, are believed to perform denitrification, there is a notable lack of evidence in England regarding the magnitude of this process and any potential differences among and within habitats. This research represents the first step in addressing these knowledge gaps. It provides benchmarks for denitrification process rates through three integrated projects: (i) across twelve intact saltmarshes located within six estuaries in England, at one point in time for a given saltmarsh; (ii) in a saltmarsh in each of two English estuaries (the Ribble (north west England) and the Blackwater (south east England)) across seasons; and, (iii) across twelve seagrass beds and twelve mudflats located across six coastal sites in England, at one point in time for a given habitat. Using laboratory incubations of intact sediment cores, the national saltmarsh survey (August – November 2024) shows 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 saltmarshes, upper saltmarsh vegetation communities denitrified, on average, at a rate 140% greater than that found in pioneer/low and low-mid saltmarsh communities. Substantial variation in mean denitrification rates across saltmarsh zones did exist though: from 43 to 1037 μg N m-2 hr-1. Seasonal variation in denitrification was high, with idiosyncratic patterns between saltmarshes and vegetation zones although there was a clear pattern that complete denitrification increased at the high saltmarsh in the Ribble from an average of 106 μg N m-2 hr-1 in summer to an average of 3134 μg N m-2 hr-1 in late winter. The national sampling of seagrass beds and mudflats (December 2024 – March 2025) showed similar and generally low values of denitrification between these habitats (seagrass: 239 μg N m-2 hr-1; mudflat: 258 μg N m-2 hr-1), with variability related to coastal location. Most cores, in all habitats and seasons, indicated that denitrification would go to completion i.e. to dinitrogen gas, but some cores, especially in saltmarsh, showed gaseous emissions of an 8 of 131 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 and seagrass beds, under the Water Environment Regulations (WER). This monitoring helps give habitats a classification status, so it is clear which habitats are ecologically healthy and which 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 sediment given this requires more specialised research. Therefore, the information on denitrification processes within each of twelve saltmarshes, mudflats and seagrass beds across England could give an insight into what is happening within the sediment and can be compared to most recent habitat classifications to see if these results follow any national trends. Furthermore, the potential implications of incomplete denitrification within these systems offers insight into the extent to which these habitats can mitigate climate change. Indeed, this research emphasizes the need to consider a suite of greenhouse gas fluxes within these 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 and seagrass beds are further established in restoring, as well as intact contexts, preferably across seasons, it can guide future management efforts, including incentivising restoration and the creation of new habitats. The evidence contained within this report can provide a basis for advocating for nature-based solutions that enhance the wellbeing of people and the planet. The Water Industry National Environment Programme recommended enhancing the natural environment while also addressing environmental challenges faced by coastal habitats. An example of enhancement could involve using saltmarsh systems to offset harmful levels of available nitrogen added into estuaries through water treatment works, though implications for biodiversity require addressing. Empirical data on how coastal habitats 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 habitats 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.