Segregation of nitrogen use between ammonium and nitrate of ectomycorrhizas and beech trees

Here, we characterized nitrogen (N) uptake of beech (Fagus sylvatica) and their associated ectomycorrhizal (EM) communities from NH4+ and NO3-. We hypothesized that a proportional fraction of ectomycorrhizal N uptake is transferred to the host, thereby resulting in the same uptake patterns of plants and their associated mycorrhizal communities. N-15 uptake was studied under various field conditions after short-term and long-term exposure to a pulse of equimolar NH4+ and NO3- concentrations, where one compound was replaced by N-15. In native EM assemblages, long-term and short-term N-15 uptake from NH4+ was higher than that from NO3-, regardless of season, water availability and site exposure, whereas in beech long-term N-15 uptake from NO3- was higher than that from NH4+. The transfer rates from the EM to beech were lower for N-15 from NH4+ than from NO3-. N-15 content in EM was correlated with N-15 uptake of the host for (NH4+)-N-15, but not for (NO3-)-N-15-derived N. These findings suggest stronger control of the EM assemblage on N provision to the host from NH4+ than from NO3-. Different host and EM accumulation patterns for inorganic N will result in complementary resource use, which might be advantageous in forest ecosystems with limited N availability.