Climate imprints on tree-ring δ15N signatures of sessile oak ( Quercus petraea Liebl.) on soils with contrasting water availability

Abstract In the present study we investigated long-term climate imprints (160 year period) on tree-ring δ 15 N signatures of sessile oak ( Quercus petraea ) at sites with contrasting water availability (i.e. Cambisols vs. Regosols in Luxembourg, Central Europe, with 175 mm and 42 mm available water capacity, respectively). We hypothesized that tree-ring δ 15 N signatures constitute a sensitive indicator to long-term shifts in climatic conditions. Our findings revealed a close positive correlation between winter and spring temperatures and tree-ring δ 15 N signatures. These relationships were stronger for Cambisol than for Regosol sites. If entire chronologies were considered, peaks in annual mean temperatures closely corresponded with peaks in tree-ring δ 15 N signatures, with both annual mean temperatures and δ 15 N signatures reaching their maxima within the last two decades. In addition, we found a weak but significant impact of February precipitation on δ 15 N signatures, but only for Cambisols. We hypothesize that these findings are attributable to climate- (particularly temperature-) mediated nitrification rates in forest soils. As nitrification is a strongly fractionating process that produces 15 N-depleted nitrate and higher isotopic ratios for ammonium in the topsoil, increased nitrification leads to 15 N-enriched pools of N inorg in the upper soil horizons and therefore higher δ 15 N signatures in plant tissues. Weaker correlations at Regosol sites were likely related to dryer and more acidic site conditions, both of which may reduce nitrification rates. Comparisons of oak and beech trees in the study area point to species-specific trajectories of wood nitrogen isotopes, likely related to the partitioning of ammonium and nitrate among species. In conclusion, tree-ring δ 15 N signatures may serve as an integrator of terrestrial N cycling and as such constitute a valuable tool in the identification of spatial and temporal patterns of N cycling in relation to environmental changes. Due to the mediating effects of the isotopic composition of the respective N sources, analyses of the isotopic composition of airborne N loads would support the interpretation of wood δ 15 N patterns, particularly in areas that are subject to high N pollution.