title: Response of canopy nitrogen uptake to a rapid decrease in bulk nitrate deposition in two eastern Canadian boreal forests.
abstract: A few studies have reported a recent and rapid decline in NO3(-) deposition in eastern North America. Whether this trend can be observed at remote boreal sites with low rates of N deposition and how it could impact canopy uptake (CU) of N remain unknown. Here we report trends between 1997/1999 and 2012 for precipitation, throughfall N deposition as well as inorganic N CU for two boreal forest sites of Quebec, Canada, with contrasted N deposition rates and tree species composition. NO3(-) bulk deposition declined by approximately 50% at both sites over the studied period while no change was observed for NH4(+). As a result, the contribution of NH4(+) to inorganic N deposition changed from ~33% to more than 50% during the study period. On average, 52-59% of N deposition was intercepted by the canopy, the retention being higher for NH4(+) (60-67%) than for NO3(-) (45-54%). The decrease in NO3(-) bulk deposition and the increase in the NH4(+):NO3(-) ratio had important impacts on N-canopy interactions. The contribution of NH4(+) CU to that of total inorganic N CU increased at both sites but the trend was significant only at Tirasse (lowest N deposition). At this site, absolute NO3(-) CU significantly decreased (as did total N CU) during the study period, a consequence of the strong relationship (r(2) = 0.88) between NO3(-) bulk deposition and NO3(-) CU. Our data suggest that N interactions with forest canopies may change rapidly with changes in N deposition as well as with tree species composition.
authors.affiliation: Direction de la Recherche Forestière, Ministère des Ressources Naturelles, 2700 rue Einstein, Sainte-Foy, QC, G1P 3W8, Canada.
authors.forename: D
authors.initials: D
authors.lastname: Houle
authors.forename: C
authors.initials: C
authors.lastname: Marty
authors.forename: L
authors.initials: L
authors.lastname: Duchesne
mesh_terms: D000393:Air Pollutants / Q000032:analysis*; D000397:Air Pollution; D064751:Ammonium Compounds / Q000032:analysis; D002170:Canada; D002980:Climate; D004784:Environmental Monitoring; D009584:Nitrogen / Q000378:metabolism*; D009589:Nitrogen Oxides / Q000032:analysis; D018515:Plant Leaves; D011792:Quebec; D066188:Taiga; D014197:Trees / Q000378:metabolism*
chemical_list: D000393:Air Pollutants; D064751:Ammonium Compounds; D009589:Nitrogen Oxides; D009584:Nitrogen