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

Jonathan Millett, Douglas Godbold, Andrew R. Smith and Helen Grant
Oecologia
Vol. 169, No. 2 (June 2012), pp. 541-552
Published by: Springer in cooperation with International Association for Ecology
Stable URL: http://www.jstor.org/stable/41500053
Page Count: 12
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N₂ fixation and cycling in Alnus glutinosa, Betula pendula and Fagus sylvatica woodland exposed to free air CO₂ enrichment
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Abstract

We measured the effect of elevated atmospheric CO₂ on atmospheric nitrogen (N₂) fixation in the tree species Alnus glutinosa growing in monoculture or in mixture with the non-N₂-fixing tree species Betula pendula and Fagus sylvatica. We addressed the hypotheses that (1) N₂ fixation in A. glutinosa will increase in response to increased atmospheric CO₂ concentrations, when growing in monoculture, (2) the impact of elevated CO₂ on N₂ fixation in A. glutinosa is the same in mixture and in monoculture and (3) the impacts of elevated CO₂ on N cycling will be evident by a decrease in leaf δ¹⁵N 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 CO₂ concentrations were maintained at either ambient or elevated (by 200 ppm) concentrations using a free-air CO₂ enrichment (FACE) system. Leaf δ¹⁵N was measured and used to estimate the amount (N dfa ) and proportion (%N dfa ) of N derived from atmospheric fixation. On average, 62% of the N in A. glutinosa leaves was from fixation. The %N dfa and N dfa for A. glutinosa trees in monoculture did not increase under elevated CO₂, despite higher growth rates. However, N₂ fixation did increase for trees growing in mixture, despite the absence of significant growth stimulation. There was evidence that fixed N₂ was transferred from A. glutinosa to F. sylvatica and B. pendula, but no evidence that this affected their CO₂ response. The results of this study show that N₂ fixation in A. glutinosa may be higher in a future elevated CO₂ world, but that this effect will only occur where the trees are growing in mixed species stands.

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