The effect of forest nutrients status on ectomycorrhizal growth and community structure in response to minerals

Sammanfattning: Ectomycorrhiza are a symbiosis between trees and soil inhabiting fungi. The fungal hyphae ensheath the fine roots creating a mantle and grow in between root cells to create the Hartig net, where the exchange of carbon and mineral nutrients occurs. The ectomycorrhizal fungus is dependent on the host tree to receive the carbon it needs to grow. Trees with different nutrient deficiencies differ in the amounts of carbon they allocate to the roots and subsequently to the ectomycorrhizal fungi (EMF). It has been suggested that EMF are able to take up nutrients from mineral sources through the process of weathering. Presence of minerals rich in nutrients that the trees are deficient in might affect the growth of the EMF and the EMF community structure. In this thesis I used in-growth mesh bags amended with minerals to analyse how these minerals affected the growth of EMF as well and the community structure of the fungi, which grew into the mesh bags. The main results of the thesis are: - Forests limited in phosphorus (P) have elevated EMF growth due to increased carbon allocation belowground. Magnesium and potassium limitations lead to decreased EMF growth, probably due to impaired belowground carbon allocation arising from these deficiencies. - Colonisation of apatite (P mineral) by EMF increases during P limitation. Similar increased EMF colonisation was not seen when hornblende (Mg mineral) or biotite (K mineral) was present in Mg and K limited forests. This suggests that EMF growing in the field take up P by weathering of apatit in a similar way as has been found in controlled laboratory experiments. This is, however, not the case for K uptake from biotite or Mg uptake from hornblende during Mg and K limitations. - Increased colonisation of apatite minerals did not translate to changes in EMF community composition in the mesh bags, suggesting that if some of the EMF species in the mesh bags are enhancing the weathering of minerals they are not rewarded with more carbon from the host tree. - The impaired carbon allocation during Mg and K deficiency can be reversed by addition of these nutrients to the deficient soil. This also results in increased EMF growth, which enhances the nutrient uptake capacity of the trees. In this way, Mg addition should be considered as a remedy for forests suffering from low Mg status due to acid rain or due to intensive harvesting of forest residues.