Today, precipitation contains considerably more nitrates and ammonia than it did before the Second World War. Road traffic and shipping are the main sources of nitrogen oxide emissions, while ammonia deposition occurs as a result of ammonia evaporation from animal manure.
Airborne nitrogen compounds boost the load to the marine environment. Air pollutants currently account for roughly a third of the volume of nitrogen reaching the Baltic Sea.
Nitrogen deposition also affects vegetation on land. Forest trees are growing more rapidly than before and ‘nitrogen-loving’ plant species are spreading at the expense of others.
Soil nitrogen stored in forest land has gradually accumulated over the millennia. Historically, the annual increment of nitrogen has been from one to two kilos of nitrogen per hectare. Nowadays, the annual figure is roughly one kilo per hectare in the forests of Norrland and nearly 10 kg/ha in those of south Sweden.
Thanks to regulations on spreading animal manure, catalytic conversion of vehicle exhausts and other measures, it has been possible to reduce Swedish emissions of nitrogen oxides and ammonia in recent years. But the decrease is slow.