The interactions between trees and ground vegetation for organic nitrogen uptake via ericoid and ectomycorrhizal fungi

The boreal forest soils are globally significant reservoirs of carbon (C). In the organic layer of boreal forest soil, humic compounds form the most stable C pool. In these soils, growth limiting nutrient is often nitrogen (N) that is tightly incorporated into soil organic matter (SOM). Therefore, the release of N from SOM is a key factor regulating the productivity of boreal forest ecosystems.
When the amount of atmospheric CO2 increases, it accelerates the plant photosynthesis rate and increases the C flow to the soil. Recently, it has been shown that the plant-derived, easily utilizable, C accelerates the decomposition of the older and more slowly degradable SOM, i.e. humus, through a phenomenon called the priming effect. It has also been suggested that the availability of N is one of the main factors affecting the rate of priming. Moreover, the ectomycorrhizal fungi, which get most of their carbohydrates from the host plants, have been shown to be involved in the SOM decomposition and priming.
The focus of this work was to study the effect of plant-derived C on the structure and enzymatic activities of the microbial community in boreal forest humus. For that, a three-year field experiment was set up in Hyytiälä forestry field station in southern Finland. In the experiment, bags (N=24) filled with sieved (4mm mesh size) and homogenized humus were buried between organic and mineral soil horizons. The below-ground C fluxes were controlled with three different mesh sizes of the bags, 1µm, 50µm and 1mm. The different mesh sizes prevent the entrance of plant-derived C into the bags, exclude root-derived C from the bags but allow fungal hyphae to penetrate, or allow penetration of both fungal hyphae and fine roots, respectively.
Enzymes are to be recovered from humus bags by filter centrifugation. Enzyme activities of acid phosphatase, N-acetylglucosaminidase, β-glucosidase, β-glucuronidase, β-xylosidase, cellobiohydrolase, and leucine amino peptidase are to be measured with fluorometric assay and the Laccase activities with the colorimetric assay. The changes of the microbial community structure in the humus bags will be observed with phospholipid fatty-acid analysis (PLFA; performed by Luke). The C and N contents and mass losses will be analysed.