N2 fixation and cycling in Alnus glutinosa, Betula pendula and Fagus sylvatica woodland exposed to free air CO2 enrichment

We measured the effect of elevated atmospheric CO on atmospheric nitrogen (N ) fixation in the tree species Alnus glutinosa growing in monoculture or in mixture with the non-N -fixing tree species Betula pendula and Fagus sylvatica. We addressed the hypotheses that (1) N fixation in A. glutinosa will increase in response to increased atmospheric CO concentrations, when growing in monoculture, (2) the impact of elevated CO on N fixation in A. glutinosa is the same in mixture and in monoculture and (3) the impacts of elevated CO on N cycling will be evident by a decrease in leaf δ N and by the soil-leaf enrichment factor (EF), and that these impacts will not differ between mixed and single species stands. Trees were grown in a forest plantation on former agricultural fields for four growing seasons, after which the trees were on average 3.8 m tall and canopy closure had occurred. Atmospheric CO concentrations were maintained at either ambient or elevated (by 200 ppm) concentrations using a free-air CO enrichment (FACE) system. Leaf δ N was measured and used to estimate the amount (N ) and proportion (%N ) of N derived from atmospheric fixation. On average, 62% of the N in A. glutinosa leaves was from fixation. The %N and N for A. glutinosa trees in monoculture did not increase under elevated CO , despite higher growth rates. However, N fixation did increase for trees growing in mixture, despite the absence of significant growth stimulation. There was evidence that fixed N was transferred from A. glutinosa to F. sylvatica and B. pendula, but no evidence that this affected their CO response. The results of this study show that N fixation in A. glutinosa may be higher in a future elevated CO world, but that this effect will only occur where the trees are growing in mixed species stands.