Nitrogen deposition effects on subalpine grassland: The role of nutrient limitations and changes in mycorrhizal abundance

To better understand how increasing atmospheric nitrogen (N) deposition may affect subalpine grassland, we carried out a nutrient addition experiment in the Swiss Alps. N addition (+N) was combined with phosphorus (P) addition (+P) to determine nutrient limitations in plant functional groups. To examine responses of arbuscular mycorrhizal fungi (AMF) and AMF effects on plant growth, in-growth cores containing local plant species (phytometers) were inserted, and in half of them the external mycelium was disrupted weekly to impede mycorrhizal functioning. At harvest, aboveground biomass and element concentrations of the established vegetation were measured, as well as phytometer shoot and root mass, and the percentage of root length colonized (%RLC) by AMF. Only productivity of grasses increased under +N and +P, while other groups showed no or negative growth responses. +P decreased %RLC in all phytometers, whereas +N increased %RLC in the most abundant grass species, and reduced the relative abundance of arbuscules to total intraradical mycelium in the other species. Weekly destruction of the external mycelium reduced %RLC in most species, but did not affect plant biomass. The results suggest that increased N deposition in such N- and P-co-limited grassland will lead to shifts in plant functional group composition due to differences in the plants’ nutrient demand, that +N will affect AMF abundance and mutualistic functioning, but that changes in AMF abundance may not considerably affect plant growth.