Climate imprints on tree-ring delta 15N signatures of sessile oak (Quercus petraea Liebl.) on soils with contrasting water availability

• Published in: Ecological indicators Vol. 45; pp. 45 - 50
• Main Authors: Haerdtle, Werner, Niemeyer, Thomas, Fichtner, Andreas, Li, Ying, Ries, Christian, Schuldt, Andreas, Walmsley, David, von Oheimb, Goddert
• Format: Journal Article
• Language: English
• Published: 01.10.2014
• Subjects: Availability; Correlation; Indicators; Nitrification; Oak; Quercus petraea; Signatures; Soils; Wood; ANE, Europe; Luxembourg; Europe
• ISSN: 1470-160X
• DOI: 10.1016/j.ecolind.2014.03.015

In the present study we investigated long-term climate imprints (160 year period) on tree-ring delta 15N signatures of sessile oak (Quercus petraea) at sites with contrasting water availability (i.e. Cambisols vs. Regosols in Luxembourg, Central Europe, with 175mm and 42mm available water capacity, respectively). We hypothesized that tree-ring delta 15N 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 delta 15N 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 delta 15N signatures, with both annual mean temperatures and delta 15N signatures reaching their maxima within the last two decades. In addition, we found a weak but significant impact of February precipitation on delta 15N 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 15N-depleted nitrate and higher isotopic ratios for ammonium in the topsoil, increased nitrification leads to 15N-enriched pools of Ninorg in the upper soil horizons and therefore higher delta 15N 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 delta 15N 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 delta 15N patterns, particularly in areas that are subject to high N pollution.