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Stable carbon isotope evidence for nitrogenous fertilizer impact on carbonate weathering in a small agricultural watershed

Brunet, Frédéric and Potot, C. and Probst, Anne and Probst, Jean-Luc Stable carbon isotope evidence for nitrogenous fertilizer impact on carbonate weathering in a small agricultural watershed. (2011) Rapid Communications in Mass Spectrometry, vol. 25 (n°19). pp. 2682-2690. ISSN 0951-4198

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Official URL: http://dx.doi.org/10.1002/rcm.5050

Abstract

The isotopic signature of Dissolved Inorganic Carbon (DIC), δ13CDIC, has been investigated in the surface waters of a small agricultural catchment on calcareous substratum, Montoussé, located at Auradé (south‐west France). The Montoussé catchment is subjected to intense farming (wheat/sunflower rotation) and a moderated application of nitrogenous fertilizers. During the nitrification of the NH4 +, supplied by fertilization, nitrate and H+ ions are produced in the soil. This anthropogenic acidity is combined with the natural acidity due to carbonic acid in weathering processes. From an isotopic point of view, with ’natural weathering’, using carbonic acid, δ13CDIC is intermediate between the δ13C of soil CO2 produced by organic matter oxidation and that of the carbonate rocks, while it has the same value as the carbonates when carbonic acid is substituted by another acid like nitric acid derived from nitrogen fertilizer. The δ13CDIC values range from −17.1‰ to −10.7‰ in Montoussé stream waters. We also measured the δ13C of calcareous molassic deposits (average −7.9‰) and of soil organic carbon (between −24.1‰ and −26‰) to identify the different sources of DIC and to estimate their contribution. The δ13CDIC value indicates that weathering largely follows the carbonic acid pathway at the springs (sources of the stream). At the outlet of the basin, H+ ions, produced during the nitrification of N‐fertilizer, also contribute to weathering, especially during flood events. This result is illustrated by the relationship between δ13CDIC and the molar ratio NO3 –/(Ca2+ + Mg2+). Consequently, when the contribution of nitrate increases, the δ13CDIC increases towards the calcareous end‐member. This new isotopic result provides evidence for the direct influence of nitrogen fertilizer inputs on weathering, CO2 consumption and base cation leaching and confirms previous results obtained using the chemistry of the major ions present in the field, and in soil column experiments.

Item Type:Article
Additional Information:Thanks to John Wiley & Sons editor. The definitive version is available at www3.interscience.wiley.com Presented at the 6th Congress of the French Society of Stable Isotopes (Société Française des Isotopes Stables, SFIS) held 26–29 October 2010 in Toulouse, France.
Audience (journal):International peer-reviewed journal
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Institution:French research institutions > Centre National de la Recherche Scientifique - CNRS
Université de Toulouse > Institut National Polytechnique de Toulouse - INPT
Université de Toulouse > Université Paul Sabatier-Toulouse III - UPS
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Deposited By: Jean-luc PROBST

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