Weak growth response to nitrogen deposition in an old-growth boreal forest

Nitrogen (N) deposition has been enhanced globally due to the dramatic increase in anthropogenic N emissions, but the effects of N deposition on the growth of old-growth forests remain poorly understood. A three-year N enrichment experiment (0, 20, 50 and 100 kg N ha(-1) yr(-1)) was conducted in an old-growth boreal forest in Northeast China to explore its growth response to enhanced N deposition. Nitrogen enrichment significantly increased N availability but caused only a very weak stimulation of growth (<3 kg C/kg N), indicating a minor N limitation. Nitrogen enrichment did not significantly influence the mass production of needle-leaves but increased the production of woody biomass, leading to a slight improvement in growth efficiency. Nitrogen content increased and phosphorus (P) content decreased in freshly fallen needle-leaves, leading to an increase of N:P ratios. The failure of P acquisition to match the increased N uptake suggests a limitation of soil P availability. Our results support the hypothesis of progressive N saturation and indicate that N deposition may cause a very weak stimulation of carbon sequestration in old-growth forests.http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcApp=PARTNER_APP&SrcAuth=LinksAMR&KeyUT=WOS:000345097000006&DestLinkType=FullRecord&DestApp=ALL_WOS&UsrCustomerID=8e1609b174ce4e31116a60747a720701EcologySCI(E)1ARTICLEjyfang@urban.pku.edu.cn9null