Nitrophobic ectomycorrhizal fungi are associated with enhanced hydrophobicity of soil organic matter in a Norway spruce forest

Abstract

In boreal forests an important part of the photo assimilates are allocated belowground to support symbiosis of ectomycorrhizal fungi (EMF). The production of EMF extramatrical mycelium can contribute to carbon (C) sequestration in soils, but the extent of this contribution depends on the composition of the EMF community. Some species can decrease soil C stocks by degrading soil organic matter (SOM), and certain species may enhance soil C stocks by producing hydrophobic mycelia which can reduce the rate of SOM decomposition. To test how EMF communities contribute to the development of hydrophobicity in SOM, we incubated sand-filled fungal-ingrowth mesh bags amended with maize compost for one, two or three growing seasons in non-fertilized and fertilized plots in a young Norway spruce (Picea abies) forest. We measured hydrophobicity as determined by the contact angle and the C/N ratios in the mesh bags contents along with the amount of new C entering the mesh bags from outside (determined by C3 input to C4 substrate), and related that to the fungal community composition. The proportion of EMF species increased over time to become the dominant fungal guild after three growing seasons. Fertilization significantly reduced fungal growth and altered EMF communities. In the control plots the most abundant EMF species was Piloderma olivaceum, which was absent in the fertilized plots. The hydrophobicity of the mesh bag contents reached the highest values after three growing seasons only in the unfertilized controls plots and was positively related to the abundance of P. olivaceum, the C/N ratios of the mesh bag contents and the amount of new C in the mesh bags. These results suggest that some EMF species are associated with higher hydrophobicity of SOM and that EMF community shifts induced by fertilization may result in reduced hydrophobicity of soil organic matter, which in turn may reduce C sequestration rates.