Numerous studies have shown a shift in both species presence and relative abundance in ectomycorrhizal (ECM) communities of forests undergoing high levels of nitrogen deposition. Very few of these studies have been in deciduous forests. None of them have looked at different horizons separately. Forests approaching nitrogen saturation may shift from nitrogen limitation to base-cation limitation or co-limitation. Some studies have observed a relative increase in mycorrhizal colonization in the mineral layer in N-deposition affected forests, possibly reflecting such a shift in nutrient demands. Begun in 1988, the Harvard Forest Long Term Ecological Research nitrogen enrichment experiment is the oldest continuous nitrogen fertilization experiment in North America. The high N treatment is approaching nitrogen saturation and has reduced productivity. We investigated how heavy inorganic N fertilization at the Harvard Forest LTER site has affected the ECM community in the mixed deciduous forest. The treatments are 0, 50, and 150 kg/ha/yr N (as NH₄NO₃). We sampled the organic and mineral soil separately. DNA was extracted from individual tips, amplified (ITS) and sequenced to species or genus. The host species for each root tip was identified using RFLP. Fine roots were collected from both the mineral and organic layer to assess mycorrhizal colonization intensity measured as percent root length colonized using a gridline intersect method.

Results/Conclusions

Overall ECM diversity was not affected by nitrogen fertilization. There was a clear shift in species composition with increasing nitrogen fertilization. The genera Cennoccocum and Thelephora show consistently reduced abundance with N fertilization, while species in the genera Lactarius, Russula, and Amanita demonstrated variable responses to N fertilization. In all three N treatments, the organic horizon and mineral layer had distinct ectomycorrhizal communities with less than 25% of species being found in both horizons. There does not seem to be any N level X horizon effect in terms of species presence. Colonization intensity was not significantly affected by N fertilization. Different tree species had distinct ECM communities with significant overlap between tree species. The ECM communities on different tree species were affected to varying degrees by N fertilization. While the high N treatment is clearly nearing N saturation, the ectomycorrhizal community has not shifted its relative abundance to favor heavier colonization in the mineral soil. This is the first published study which demonstrates that ectomycorrhizal communities on different tree species are differently affected by nitrogen fertilization.