The effect of roots and ectomycorrhizal fungi on carbon cycling in forest soils

Though the input of labile substrates into the rhizosphere by roots is known to promote decomposition of both soil organic matter (SOM) and surface litter, the presence of ectomycorrhizal (ECM) fungi living in symbiosis with plant roots has been shown to coincide with decreased litter decomposition rates in some systems. In a series of field experiments, techniques including forest girdling and soil trenching were used to exclude roots and ECM fungi in order to investigate the mechanisms controlling litter decomposition in forest soils.

Soil trenching was carried out in combination with litter bag incubations, and measurements of soil CO2 flux in a 20 year-old Pinus contorta stand. The use of mesh in-growth collars allowed the influence of ECM fungal hyphae on litter mass loss, and their contribution to soil CO2 flux, to be established separately to that of roots. A specialised irrigation system allowed moisture effects caused by root/ECM hyphal water uptake to be investigated. Neither the presence of roots, nor ECM fungi had any influence on litter decomposition, and soil temperature was the only factor found to correlate with litter mass loss.

The exclusion of roots and ECM hyphae led to increased utilisation of a simple substrate, 13C-labelled glucose. Results of incubations of four substrates, varying in structural complexity and nitrogen (N) content, suggested that the rapid utilisation of simple substrates by r-strategist microorganisms might be suppressed in the presence of ECM fungi. Though N content appeared to have a positive influence on substrate decomposition, the results were not significant.

In contrast, when forest girdling was used in a nearby Tsuga heterophylla stand to exclude plant-assimilate C supply to the soil, a significant reduction in the rate of litter mass loss was observed.

The results presented in this thesis indicate a potentially large role of ECM fungi in controlling decomposition in forest soils, and the mechanisms underlying their influence require further investigation.