NATURAL-ABUNDANCE OF N-15 IN FORESTS ACROSS A NITROGEN DEPOSITION GRADIENT

Chronic atmospheric nitrogen deposition can alter the rate of internal nitrogen cycling and increase the magnitude of N leaching losses in f orested ecosystems, As fractionation of nitrogen in favour of the ligh ter N-14 occurs during various transformations associated with N-enric hment and nitrogen loss, it has been proposed that the N-15 Signal of vegetation may provide a useful tool in evaluating the past and curren t N status of forested ecosystems. A series of coniferous forests acro ss a European nitrogen deposition gradient within the NITREX project p rovided an opportunity to test the relationships between nitrogen supp ly from atmospheric deposition and the relative N-15-enrichment of veg etation to soil, across a large geographical area. Most delta(15)N val ues for above-and below-ground tree components, soil at four depths, b ulk precipitation and/or throughfall water and soil solution or outflo w water values were within those observed elsewhere except for a few n otable exceptions. There was a significant positive relationship betwe en the delta(15)N enrichment of the tree foliage relative to the soil horizons (or the enrichment factor), and nitrogen flux in the throughf all if Aber forest, N. Wales, was excluded from the regression analysi s. An unusually high enrichment factor at the Aber site indicated that a the high rate of N cycling at the site was in excess of that predic ted from current N deposition. This was attributed to the effect of pl oughing and tree planting on the relatively N- and clay-rich mineral h orizons at Aber compared to other sites. Highly significant relationsh ips (P < 0.01) between enrichment factors and parameters describing in ternal rates of N cycling, such as litterfall N flux and nitrification rates in upper soil horizons, supported this conclusion. There appear s to be a strong link between the rate of N cycling and the delta(15)N enrichment factor, rather than N deposition or nitrate leaching per s e. These results confirm the potential use of the delta(15)N enrichmen t factor to identify sites influenced by nitrogen deposition. However, consideration should be taken of other site characteristics and land management practises which also influence soil N dynamics and N cyclin g. (C) 1998 Elsevier Science B.V.