NatureToday | Nitrogen deposition and the role of fungi in forest vitality

We have known for about thirty years that significant changes occur in the species richness and number of fruiting bodies of ectomycorrhizal fungi (species that live with tree roots) and that these changes can be attributed to nitrogen deposition. Not all outdoor mushrooms are equally sensitive to nitrogen. In general, this sensitivity is more or less constant at the level of genera of mushrooms. Field observations and nitrogen fertilization experiments in Europe and North America show very similar transformations. Thistle fungi are the most sensitive (and most degradable), and canopy fungi and knight fungi also show great sensitivity. Only a few species are more or less insensitive to nitrogen deposition, such as the zigzag edging, champignon, yellow and white russula, chestnut boletus, and wrinkled milk mushroom.

Negative effects on external fungi

The effect of nitrogen deposition is not limited to the above-ground part of the fungus, the fruiting body. The number of root tips and biomass of the fungal network is also negatively affected by nitrogen deposition. It is therefore important to ask about the effects of this decline on the health of the trees and the vitality of the forest. Decreased external roots decrease the tree’s ability to take up phosphate, and as nitrogen uptake increases, this leads to an imbalance in the ratio of each of the essential nutrients. Observations of different tree species across Europe also point to this perturbed nutrient balance. Decreased external fungal infection also results in decreased absorption of nutrients such as potassium, magnesium, and calcium.

Excess nitrogen, that is, more nitrogen than the tree can absorb or can be stored in the humus stock in the soil, leaches, with primarily negative consequences for ground and surface water quality. But nitrogen leaching also leads to acidification and loss of nutrients potassium, magnesium and calcium, reduces phosphate availability and increases aluminum availability, which discourages root growth.

More nitrogen-sensitive ectotrophic fungi often build large underground webs that are also important for water absorption. As a result, nitrogen amplifies the harmful effects of dehydration. Since the trees themselves also respond to nitrogen deposition by developing a relatively smaller root system, there is a strengthening effect from fewer roots and fewer external roots.

Accumulation of droppings and external fungus

Not only outdoor mushrooms are negatively affected by nitrogen. Changes also occur in the decomposers of litter and wood. Species that are highly efficient at breaking down the most complex chemical compounds are in decline, while opportunistic species are less affected or even benefited from nitrogen. Changes in the species composition of litter decomposers are accompanied by changes in enzyme production, which leads to a delayed decomposition of more complex compounds and the subsequent accumulation of litter, which at the same time also entails a disruption of the cycle of nutrients in litter and humus.

Small blood from the stem of the fungus.  The lesser blood stem mushroom is a saprophytic fungus that suffers from nitrogen deposition

Of course, the accumulation of garbage also leads to carbon sequestration and thus to a potential decrease in the amount of greenhouse gases CO2, but this effect is too small to really contribute much to reducing the rate of climate change. Hence this accumulation of rubbish has a negative effect on the ectopic fungus, creating a negative spiral, from which recovery is very difficult. At road edges, where litter is more easily blown away or deliberately removed by humans, the decline of fungal mushrooms is conspicuously less; Hence the tree-covered roadsides become sanctuaries (refugees) for thorn mushrooms, curtain mushrooms, and knight mushrooms. Shell tracks in the forests can counteract the effect of acidification in the first meters around the track, and there too we often find more mushrooms. But because of the increased traffic on bike paths in nature reserves, shell paths are often replaced with wider concrete paths that prevent acidification to a lesser extent.

Revitalizes mushrooms and forests

The negative effects of nitrogen on the effects of exogenous bacteria extend through the tree (which provides sugars to exogenous fungi in exchange for nutrients) and through the soil. Research indicates that the effect through the soil is far more important than the effect through the tree. Since nitrogen remains in the soil for a long time, we must bear in mind that the nitrogen problem will leave a long-term legacy.

The question then is how much can be invested in nature management to achieve mushroom recovery if nitrogen deposition is significantly reduced. The first observations and theoretical models indicate a kind of tipping point. When there is little nitrogen deposition, the forest remains in a nitrogen-poor state, but once there is a lot of nitrogen in the soil, the availability of nitrogen remains high, even if deposition decreases. Therefore the efforts required to restore the outer mushrooms and decomposers must be very large and long-term. Otherwise, feedback in a forest ecosystem leads to a state of limited recovery not only for fungi, but for all organisms that depend on them, starting with trees. However, if nitrogen loads remain high, outdoor trees will continue to perform poorly with an increased likelihood of being replaced by trees and shrubs that form another type of arboreal mycelium, such as mountain ash, blackberry, cherry, and maple. These mycorrhiza do not form mushrooms.

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Text: Thom Kuiper, Dutch Mycological Society
Images: Martijn Oud (main image: bronze-stained porcini mushroom); Dam Nico Henk Heusser

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