Since 2020, the report has included a metric against which progress can be measured towards the UK government’s Clean Air Strategy target (CAS) seeking “to reduce damaging deposition of reactive forms of nitrogen by 17% over England’s protected priority sensitive habitats by 2030”.
The Trends Report 2021 says that the reasons for the increase are “being investigated”.
The report also notes that whilst ammonia gas emissions - formed from a nitrogen compound - have shown a slight decrease in recent data, particularly at the “most polluted sites”, they have increased at many less-polluted ones resulting in an overall increase.
This is reported as being due to a combination of factors, including “increasing temperatures causing greater ammonia emissions than the inventory predicts”.
Emissions of air pollutants have a wide range of effects in the environment. Poor air quality can affect human health, while deposition of air pollutants back to earth can affect ecosystems, changing biodiversity and reducing water quality.
Excess nitrogen in the atmosphere can produce pollutants which can harm the health of forests, soils and waterways.
The CAS nitrogen target only applies in England - but the nitrogen levels at protected sensitive habitats in the devolved administrations has also been measured for comparison.
Those figures show that the deposition of nitrogen in protected sites has increased across all countries, particularly in Northern Ireland which records a 8.8% increase between 2016-18, Scotland a 0.9% increase, and Wales 0.5%.
Elsewhere in the report there is more positive news.
In UK habitats at risk from acidification, the report shows the exceedance of critical loads has fallen by more than a third, from 77.3% in 1996, to 40.4% in 2018.
The report states that exceedance of critical load indicates that an ecosystem is at risk from potential harmful effects.
The largest drop has been seen in Scotland - a trend seen through the report - where the acidity critical load has fallen by two thirds. The smallest reduction is seen in Northern Ireland.
There’s also been an overall drop in exceeded levels of nitrogen in habitats considered at risk from eutrophication, where the load fell from 75% in 1996 to 58.9% in 2018.
However, the report points out that almost 100% of the area of unmanaged beech woodland has overly high levels of nitrogen - even if the amount by which it exceeds the critical load has decreased.
The report also shows that critical amounts of nitrogen have been exceeded in more than 80% of the areas of six habitats sensitive to nitrogen - calcareous grasslands, unmanaged beech woodland, unmanaged oak woodland, other unmanaged woodland, managed conifer and managed broadleaved woodland.
With regards to acidity levels at designated sites, the positive news is that the percentage of special areas of conservation and special protection areas in the UK which exceed critical loads of acidity for one or more features has decreased substantially.
Nonetheless, the report states that roughly 50% of designated sites in Scotland and more than 70% of designated sites in other UK nations are still currently exceeding acidity critical loads. This is with the exception of sites of special scientific interest in England, where 57% of them exceeded critical acidity loads.
A DEFRA spokesperson said: “The government is firmly committed to improving the UK's air quality. The Trends Report shows the success policies have had in reducing the impacts of key air pollutants on many sensitive habitats in the UK since the mid-1990s.
“However, it also highlights that there is more to do. The Clean Air Strategy sets out the action that government will take to reduce harmful emissions further and to reduce deposition on sensitive habitats.”
The spokesperson also said that dispersion and deposition of air pollutants depends on several factors, adding that further analysis is being carried out to identify the relative contribution of the different possible factors, and that data from additional years will be needed to confirm whether this represents a long-term trend.