Rising atmospheric CO<sub>2</sub> reduces nitrogen availability in boreal forests.

Foremost, the highly significant effect of atmospheric CO 2 on tree-ring δ 15 N that we observed, using a well-constrained sampling methodology that minimizes isotopic effects associated with tree ageing and N translocation across tree rings 38 , provides robust evidence that negative δ 15 N chronologies are an indicator of ecosystem oligotrophication in response to rising CO 2 (refs. 2 , 4 , 16 , 25 ), rather than a symptom of changing Nr deposition patterns 3 , 6 .

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