The significance of endophytic and phyllospheric N 2 fixation in forest trees: evidence from stable ( 15 N) and radioactive ( 13 N) tracer studies

Key message The proportional contribution of endophytic and phyllospheric diazotrophs to tree N nutrition can be quantified by 15 N techniques. Abstract Observations of vigorous growth of Pinus spp. on unreclaimed gravel mining sites, together with measured net nitrogen (N) gains of 50 kg N ha –1 y –1 in sand culture, indicate the importance of alternative N sources when soil organic matter (SOM) cannot meet plant demand. Non-nodulated trees can respond positively to inoculation with N 2 fixing bacteria under conditions of low N supply, suggesting that non-nodular symbiotic N 2 fixation can make a significant contribution to tree nutrition, a well-established pathway for cereals, forage grasses and some industrial crops in N-limited environments. We reviewed the literature where non-isotopic approaches and techniques based on stable ( 15 N) and radioactive ( 13 N) tracers were used to estimate the relative contribution of endophytic and phyllospheric N 2 fixation to the N economies of non-nodulated trees. The principal avenue of enquiry has so far involved determination of the response of trees to inoculation with diazotrophic bacteria. There is a need to obtain in situ measurements of the reliance of non-leguminous trees on endophytic and phyllospheric N 2 fixation under a range of environmental and edaphic conditions. Current methodological obstacles have to be overcome to meet this challenge. Future research directions are suggested.