I’m going to restart the blog after the Christmas break by discussing acidification, another environmental issue caused by human interference in the nitrogen cycle. Soil acidification is one probable mechanism linking nitrogen deposition to plant diversity loss, a topic already blogged on earlier this month. I’m hoping this post should go some way to explaining the processes occurring to produce the acidification effect and the damage that is currently being caused as a result.
Atmospheric nitrogen deposition can result in soil acidification directly as a result of acid deposition (nitric acid) and indirectly through processes and reaction in soil and water (Stevens et al., 2010). This indirect acidification is produced by nitrogen deposition from airborne sources, such as nitrogen oxides (produced by fossil fuel burning for power and transport), or ammonia (produced as a result evaporation from manure used in agricultural processes). Some statistics indicate that the quantity of these compounds being emitted into the atmosphere has in fact been falling over the last few decades, land based emissions of nitrogen oxides in Europe have fallen by over 30% from 1980-2000, but this has been largely offset by increased emissions at sea. European ammonia emissions from agriculture did fall by 25% between 1990 and 2000, possibly due to changes in handling of manure (statistics are from the Air Pollution and Climate Secretariat).
Atmospheric nitrogen deposition can result in soil acidification directly as a result of acid deposition (nitric acid) and indirectly through processes and reaction in soil and water (Stevens et al., 2010). This indirect acidification is produced by nitrogen deposition from airborne sources, such as nitrogen oxides (produced by fossil fuel burning for power and transport), or ammonia (produced as a result evaporation from manure used in agricultural processes). Some statistics indicate that the quantity of these compounds being emitted into the atmosphere has in fact been falling over the last few decades, land based emissions of nitrogen oxides in Europe have fallen by over 30% from 1980-2000, but this has been largely offset by increased emissions at sea. European ammonia emissions from agriculture did fall by 25% between 1990 and 2000, possibly due to changes in handling of manure (statistics are from the Air Pollution and Climate Secretariat).
There are several papers indicating the damage caused to vegetation as a result of acidification. One such paper is from Horswill et al. (2008), whose experiments in highly nitrogen and acid rain polluted regions of the UK found that Nitrogen deposition treatments caused grassland soils to lose 23 to 35% of their total available bases (Ca, Mg, K, and Na) and treated areas became acidified by 0.2 to 0.4 pH units. This study provided the first definitive evidence that nitrogen deposition depletes base cat ions from grassland soils.
The problem of acidifying pollutants is a serious one, and it is necessary to take a broad geographical approach to understand the solutions and dangers. Nitrogen and sulphur oxides can be transported thousands of kilometres by the air, this means that some countries are net exporters of these pollutants and others net importers.
There are two main factors that determine which areas are affected by acidification: 1) the amount of acid deposition and 2) the resistance of the soil to pH change. The limits to what a particular area can tolerate in terms of acidifying pollutants are called critical loads, in order to make sure that critical loads in Europe are not exceeded it is necessary to reduce emissions of sulphur dioxide and nitrogen compounds in parts of Europe by 80-90% compared with 1990 levels (Air Pollution and Climate Secretariat).
There are two main factors that determine which areas are affected by acidification: 1) the amount of acid deposition and 2) the resistance of the soil to pH change. The limits to what a particular area can tolerate in terms of acidifying pollutants are called critical loads, in order to make sure that critical loads in Europe are not exceeded it is necessary to reduce emissions of sulphur dioxide and nitrogen compounds in parts of Europe by 80-90% compared with 1990 levels (Air Pollution and Climate Secretariat).
Fig 1. 'Map indicating the deposition of hydrogen ions that sensitive ecosystems across Europe can tolerate without being acidified.' (Air Pollution and Climate Secretariat)
It seems that although progress has been made on reducing the production of the nitrogen and sulphur pollutants that cause acidification, significantly more action is required if we are to prevent continuing damage to the vulnerable ecosystems shown above. Reducing the human input of nitrogen compounds to the atmosphere is a key part of this, requiring changes in the way we use and generate energy for our homes, industries and transport.
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