Airborne nitrogen rewires the forest underground

Fertilising a forest does not have to arrive on a spreader. Nitrogen oxides and ammonia emitted by transport, industry and agriculture move through the atmosphere and then reach the soil with rain and dust. For an ecosystem adapted to low nutrient availability this is a lasting change of conditions. Trees may initially grow faster, but their relationships with ectomycorrhizal fungi become poorer and allocate carbon differently.

Long-term studies from the Netherlands compared fruiting body records with a period in which nitrogen deposition first rose and then fell thanks to emission limits. After air quality improved, some ectomycorrhizal fungi showed signs of recovery. This was not, however, a simple return to the starting state. Different groups responded at different rates, and soil history, acidification, the tree stand and local moisture continued to affect the outcome.

The mechanism is not that nitrogen is "a poison for every fungus". An excess changes competition. A plant receiving more readily available nitrogen may allocate fewer sugars to mycorrhizal partners, and species adapted to poor soil lose out to those tolerating fertility. At the same time, leaching of cations and acidification act on roots and microorganisms. The number of fruiting bodies of one species is therefore not a standalone measure of the whole process.

In forest conservation the conclusion is practical: a site of a rare webcap or tooth fungus will not survive on a picking ban alone. It needs reduced pressure at the scale of the landscape and the atmosphere. Monitoring fungi can show the response to air quality policy, but it requires many years, fixed plots and weather data. Improvement after emission cuts is possible, but the ecological lag has to be counted in years or decades.

Recovery stalled after 2010

An analysis of 1×1 km grid observations corrected with detection models found that 43 of 75 ectomycorrhizal species declined in 1965–1985, while 46 increased in 1994–2013. The multispecies indicator changed by −3.3% and +3.4% per year respectively, most strongly among nitrogen-avoiding species. This aligns strongly with reduced deposition but remains a fruit-body analysis, not direct mycelial measurement. New CBS data change the ending: after peaking in 2010, the index fell from 177.42 to 134.30 in 2024, although it remained 34% above 1994. CBS links the renewed fall to stalled deposition reductions and dry summers; recovery was neither permanent nor complete.