1. Organic vs. inorganic N uptake
To address HP 2, we will undertake isotope labelling studies to assess plant preference for different N forms along the post-fire chronosequence. This will be achieved by directly injecting a range of 15N-labelled substrates (15NO3-, 15NH4+, 15N/13C-amino acid mixture or 15N/13C-oligopeptide mixture) into soil supporting communities of either D. flexuosa, P. schreberi or Vaccinium vitus-idea. The rate of 15N uptake and the ratio of 13C and 15N in the plant tissue of individual plant types will be used to infer intact uptake. To assess intact organic N uptake and to visually quantify the 15N/13C isotope within the plant tissues, roots will be excavated from the field and immediately fixed in glutaraldeyhde for nanoSIMS analysis.
2. Role of mycorrhizas in N transfer
To invesitgate the role of mycorrhizas in N transfer, at the field sites hyphal in-growth mesocosms will be established. At the sites, cores of feathermoss will be removed and placed in hyphal exclusion vessel or hyphal in-growth vessels. The mesocosms will be labelled with 15N gas as described in work package 1, and left in situ until the following season. At the end of the following vegetation period the rates of acetylene reduction will be determined. The total fungal biomass in the mesocosms will be determined using the phospholipid fatty acid 18.2ω6 (PLFA). To assess the uptake and transport of fixed N, hyphae will be collected from the mesocosms onto a 0.45 μm membrane and analysed by nanoSIMS as described above.