The Growth of External Ectomycorrhizal Mycelia in the Field in Relation to Host Nutrient Status and Local Addition of Mineral Sources

Abstract: A method based on ingrowth mesh bags was developed to study the production of EMM. These bags were made of nylon mesh with a mesh size of 50 mu-m, which allows colonisation by fungal mycelia but excludes the tree roots. Using this method, the production of EMM was estimated in forest stands subjected to whole-tree harvesting (WTH) and in forest stands with different nutrient status regarding potassium and phosphorus. It was concluded that WTH does not affect the production of EMM. Neither does the forest nutrient status affect the EMM production down to the low nutrient levels studied in this work (3.55 mg/g K and 1.16 mg/g P, needle content in spruce stands). The ability of EMM to utilize local mineral sources was also investigated. The EMM colonisation of mesh bags containing apatite-amended sand, creating a local phosphorus source, increased in forest stands with a poor phosphorus status. On the other hand, the EMM colonisation did not increase in biotite amended mesh bags (a potassium source) in forest stands with poor potassium status. This shows that EM fungi may play a role in ameliorating phosphorus limitation, but will probably not ameliorate potassium limitation. Fertilization with hardened wood ash has been suggested as a means of ameliorating nutrient depletion caused by WTH. The elemental content in EMM growing close to apatite and wood ash was investigated by studying the particle-induced X-ray emission from mycelial samples. The content of calcium increased in the EMM, indicating that the EMM had interacted with the apatite and wood ash. Some elements are taken up by EM fungi and transported to the host roots, but will not be taken up by the host tree and will instead accumulate in the EM roots. Rare-earth elements (REE) are examples of such elements and apatite also contains relatively high amounts of REE. The contents of light REE (La to Gd) were increased by 10 to 100 times in EM roots growing close to apatite-amended mesh bags compared with sand-filled mesh bags. In forest stands with a poor P status 6.2 mu-g light REE accumulated in EM roots outside apatite-amended mesh bags, while the corresponding figure in forest stands with an adequate P status was 4.2 mu-g. Apatite is the main source of phosphorus by weathering, and such an increase in the weathering rate could cover a quarter of the calculated P depletion induced by WTH in forests in southern Sweden, given that the amount of EM roots and EMM increase in the mineral soil in the forest.