Epsilon Archive for Student Projects

Abstract

The boreal forest biome serves as a valuable resource, both as a source of wood, pulp and biofuel, and as a sink of atmospheric carbon (C). A large proportion of C is sequestered to the soil of the boreal biome, however belowground processes are still poorly understood. Biogeochemical processes occurring in the soil are influenced by fungal communities and activity. Different processes and microbial communities exist throughout the soil profile due to differences in physico-chemical and biological properties. Boreal forest soils are commonly podzolized, with stratified layers of organic and mineral soil referred to as horizons. With intensifying forest harvesting practices, it is important to build our knowledge of fungal community function in each horizon and establish predictions of the effects intensified forest harvesting may have on these communities. The main objectives of this microcosm-based study were to characterise the fungal communities present in each horizon and investigate how these are affected by different degrees of organic matter depletion, simulating a gradient of intensity in forest harvesting.

We used a microcosm experiment with Pinus sylvestris seedlings and reconstructed soil profiles containing decreasing amounts of organic material to simulate increasing intensification of forestry. Fungal communities were characterised by high throughput DNA sequencing and statistical analysis. The 20 most abundant fungi accounted for over 80% of the DNA sequences, and there were statistically distinct communities in the O, E and B horizons. Piloderma sphaerosporum was the most abundant species in the O horizon and Suillus bovinus the most abundant in both the mineral horizons. Fungal species richness was significantly higher in the E horizon layers with an overlying O horizon compared to systems where the O horizon was removed completely (simulating extreme loss of organic matter due to intensive biomass harvesting). Fungal species richness was not otherwise affected by the treatments during the 14 month duration of the experiment, but they caused significant changes in community structure in the E and B horizons.

Parallel studies of the same experimental system showed that plant growth was proportional to the amount of organic material and it is likely the reduced growth in systems with reduced amounts of organic material was due to reduced supply of organic N mobilised and transported to plants by mycorrhizal fungi, and lower C allocation by these small plants to their mycorrhizal symbionts. Published studies suggest that mycorrhizal fungi can access Mg from mineral weathering in the B horizon and the present study shows that the relative abundance of ectomycorrhizal fungi is high in the B horizon. Further studies of the distribution and activity of soil microorganisms in all soil horizons are therefore necessary to improve understanding of their contributions to sustainable forest growth and management.