Increased soil temperature and atmospheric N deposition have no effect on the N status and growth of a mature balsam fir forest

[摘要] Nitrogen (N) is a major growth-limiting factor in boreal forest ecosystems.Increases of temperature and atmospheric N deposition are expected to affectforest growth directly and indirectly by increasing N availability due tohigher rates of N mineralization. In order to understand the potentialimpacts of these changes, a mature balsam fir stand in Québec, Canada,was subjected during three consecutive growing seasons (2009–2011) to (i)experimentally increased soil temperature (4 °C) and earliersnowmelt (2–3 weeks) as well as (ii) increased inorganic N concentration inartificial precipitation (3 × current N concentrations using¹⁵NH₄-¹⁵NO₃). Soil inorganic N was measured using buriedion-exchange membranes (PRS™ probes) and standard soil extractions.Dendrometers were used to monitor the variations in diameter growth andneedles were analyzed annually for N to assess the nutritional response oftrees. Results from the second (2010) and third (2011) year of treatment arereported.

After three years of treatment, there was no significant increase in soilnitrate (NO₃) or ammonium (NH₄) availability either in theorganic or in the mineral soil as measured with standard soil extractions.Similar results were obtained with ion-exchange membranes, except forNH₄ in the forest floor, which increased by an average of 54% overthe two years. No effect of treatments were observed on needle N or diametergrowth, but an 8-day earlier peak in diameter growth was measured inheated plots in 2010.

We attributed the limited effects of our treatments to the acute soilcompetition for available N at the site. As a result, the projectedmodifications of the forest N cycle and concomitant increased forest growthdue to an earlier snowmelt, increased soil temperature and N depositionshould be considered with caution in similar cold N-poor ecosystems.

[学科分类] 地球化学与岩石