Nitrogen Deposition :: Coastal and Floodplain Grazing Marsh

Effects and implications

• Increase graminoid biomass, with potentially adverse effects on forbs.

Overview: evidence, processes and main impacts

No studies have been made of N deposition effects on these habitats, and thus no quantified effects of potential modifiers are available. 

As marshes age i.e. during succession, N availability changes as organic matter that has accumulated in the sediments is released through mineralization. Thus the age of the marsh will influence the N response. Overall N deposition is likely to be less important than nutrient enrichment via fertiliser wash off into drainage channels. There may be some localized effects of ammonia from wintering wildfowl, especially large geese flocks. P availability also influences N responses in this habitat and wildfowl will provide an additional source of P and K. Grazing represents an important management tool and grazing intensity will modify any N response, such that more grazing offsets the likelihood of graminoid dominance at the expense of forbs. Changing the number of times sites are inundated by flooding i.e. decoupling the floodplain from its river via the deployment of physical barriers as a means of flood control, will also modify N impacts as inundation has the potential to both remove and deposit nutrients.

Most likely impacts would be loss of N sensitive species and increases in tall grass and graminoid biomass..

Pollutant type and risk

Indicators of N enrichment

• None available

Evidence of species specific responses

• None available

What factors modify N deposition impacts?

• Age of the marsh, older ones are less sensitive. 
• P availability: low P availability will restrict the capacity of graminoid species to increase their biomass and shade out forbs. 
• Decoupling from water sources through construction of embankments etc. Restricting the opportunity for nutrient flushing.