Surviving the nitrogen crisis: The case of Juniperus communis and its mycorrhizal fungi

Nitrogen deposition causes soil nutrient leaching and soil acidification. These biochemical changes can affect the soil microbial community, including mycorrhizal fungi, and subsequently the performance of plants. One of the plants that seems to suffer from high nitrogen deposition loads is the common juniper (Juniperus communis). Juniper populations are rapidly declining in North-western Europe due to a lack of rejuvenation. The aim of this thesis is to give a better understanding of how N deposition and related soil acidification affect J. communis and its symbiosis with mycorrhizae.
Using multiple experiments we showed that by altering soil nutrient status, N deposition can 1) reduce positive effects of mycorrhiza on junipers, 2) reduce mycorrhizal fungi diversity in junipers rhizosphere, 3) deteriorate nutrient status of leafs which are associated with plant growth and seed production. In the Netherlands and Flanders both the nutrient status (Ca and K) and the viable seed production are extremely low compared to the rest of Europe. Restoration interventions such as addition of lime and rock powder can restore soil pH and nutrient status and thereby improve nutrient status of juniper leaves, and improve rejuvenation of junipers. However these restoration interventions to have detrimental side effects as well. Therefore reducing nitrogen deposition, and thereby nutrient leaching and soil acidification, is necessary to restore the rejuvenation of junipers.