Nitrogen input in forests: a fertilizer to cut both ways

Changes in environmental conditions, correlated with site quality and stand characteristics, can substantially affect the growth of forests and thus their carbon sequestration. It is therefore critical to understand the effect of these driving forces in order to assess how forest ecosystems respond to climate change.

Researchers led by Sophia Etzold of the Swiss Federal Institute for Forest, Snow and Landscape Research examined annual increment data from 1995-2010 in Europe. The data came from 100,000 coniferous and deciduous trees in 442 even-aged pure stands from 23 European countries. Same-aged stands account for about 70% of the forest area in Europe. Of these, 30% are stocked with only one tree species. The research group studied how European forests responded to changes in 11 variables, including stand characteristics, climatic conditions, air and soil quality, and their mutual influences.

Too much of a good thing

The scientists found that, regardless of the different environmental influences, stand density and age had the greatest impact on forest growth. Nitrogen also had a positive effect on growth in large areas. However, this could turn into a negative effect if nitrogen inputs were too high (between 24 and 30kg N/ha/year), because high nitrogen levels worsen site conditions, e.g. by altering nutrient concentrations in leaves resulting in a change in soil pH.

According to the authors of the study, nitrogen deposition has at least as strong an impact on tree growth as climate change.

Comment

The study by Etzold et al. (2020) shows clearly that forests are influenced by many different factors. Just like CO₂ depositions, over-fertilization of the environment with nitrogen is an anthropogenic influence that must be reduced by appropriate measures. Road traffic is responsible for 13% of total nitrogen emissions (nitrogen oxides), while more than 60% of nitrogen inputs come from conventional agriculture, more precisely from mineral and organic fertilizers. When soils are over-fertilized, excess nitrogen is generated, which is mainly dispersed through the atmosphere (ammonia and nitrous oxide) and soil water (in the form of nitrates). In recent years, ammonia emissions from agriculture exceeded the nationwide limits of Germany by almost a third. The EU directive on reducing national emissions of certain air pollutants calls on Germany to cut its ammonia emissions by nearly a third by 2030 compared with 2005 levels. High nitrogen pollution not only damages forest growth, it also leads to the loss of biodiversity through eutrophication and acidification. In addition, nitrogen pollutes our drinking water and is harmful to health.