Ectomycorrhizal fungi: Their role in nitrogen retention and carbon sequestration in northern coniferous forests

Author

Summary, in English

Almost all northern forest tree species live in symbioses with ectomycorrhizal fungi (EMF). The trees allocate up to half of the photoassimilated carbon (C) through the roots to EMF. In return EMF deliver nutrients from both inorganic and organic sources in the soil, as well as water. Most of the nutrients trees need are assimilated through EMF, which thus play a potentially important role in retaining nitrogen (N) in the soil. However, it is still not clear to what extent they contribute to the total N retention capacity of northern forest soils. This thesis shows that recently produced (within 4 months) mycelium of EMF was capable to assimilate a substantial amount of inorganic N (6 kg-1 ha-1 month-1). If the total standing biomass of EMF mycelium where to be taken in to account, the total EMF N assimilation would likely be much larger. On the other hand, an abundant network of EMF mycelium was not sufficient to prevent a flush of N leaching that followed immediately after N fertilization, but it may be sufficient to retain more moderate additions of N. Leaching of N has been reported in N saturated northern forests, and further research is needed to reveal the role of N retention by EMF in relation to N assimilation by other microorganisms or directly by the tree roots, under various levels of N input in forest soil.

Northern forest soils have been reported as a major terrestrial C pool. This is typically explained by slow decomposition due to low temperatures, and recalcitrant litter from the coniferous trees that dominate these forests. A less discussed source to the C pool is the one originating from roots and root associated microorganisms. Since about half of the photoassimilated C is allocated belowground in coniferous forests, this is a potentially important process affecting the C sequestration. Previous studies have highlighted the potential of EMF as an important contributor to C sequestration. This is based on observations of substantial production of EMF mycelium, and that EMF tissue contains recalcitrant compounds. One of the conclusions in this thesis was that most of the soil organic C (SOC) in late successional stages originates from belowground inputs from roots and root associated fungi. This contrasts the focus on aboveground plant production in other studies where changes in forest C storage have been examined. Other results emphasize the important role of EMF in the build-up of soil organic matter: up to more than half a ton of EMF mycelium was produced during only four months, and the average C sequestration by EMF mycelium in a large scale field survey was calculated to more than 300 kg C ha-1 y-1.



Carbon sequestered by EMF may however be sensitive to additions of N, which typically repress EMF growth. There have been many observations of reduced EMF mycelium production after large additions of inorganic N (typically 100 kg-1 ha-1 y-1), but we show that the moderate (less than 20 kg-1 ha-1 y-1) and continuous addition from elevated N deposition in many northern forest regions is sufficient for the repressive effect to occur. Consequently, C sequestration by EMF may be reduced by N deposition. This opposes the positive effect that N addition has on C sequestration due to other processes that are based on aboveground C inputs and degradation of soil organic matter. Even though previous research indisputably show that large N additions increase the C sequestration in forest soils, the effect is lacking at moderate N inputs. Thus, an important question that still remains to be answered is how low levels of anthropogenic N deposition affect the C cycle in northern forest regions. Further, the belowground contribution to SOC is often neglected, but necessary to include for accurate modeling of the C cycle in northern forests, particularly given the evidence that belowground inputs can make a larger contribution to SOC than aboveground inputs.

Topic

  • Biological Sciences

Keywords

  • Ectomycorrhizal fungi
  • Carbon sequestration
  • Nitrogen deposition
  • Nitrogen leakage
  • Field survey
  • Boreo-nemoral forest
  • Boreal Forest
  • Ergosterol
  • Free ergosterol
  • Mesh-bags
  • Multivariate analysis

Status

Published

Research group

  • Microbial Ecology

ISBN/ISSN/Other

  • ISBN: 978-91-7473-645-8

Defence date

4 October 2013

Defence time

10:00

Defence place

Blå hallen, Ekologihuset, Sölvegatan 37, Lunds Universitet