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

• Published in: Rapid communications in mass spectrometry Vol. 25; no. 19; pp. 2682 - 2690
• Main Authors: Brunet, F., Potot, C., Probst, A., Probst, J.-L.
• Format: Journal Article
• Language: English
• Published: Chichester, UK John Wiley & Sons, Ltd 15.10.2011; Wiley
• Subjects: Biodiversity and Ecology; Carbon; Carbonates; Carbonic acid; Chemical engineering; Chemical Sciences; Environmental Sciences; Farming; Fertilizing; Helianthus; Nitrification; Soil (material); Triticum aestivum; Weathering; France; Weathering; Nitrogenous-fertilizers; Stream waters; Dissolved inorganic carbon; Watershed; Carbone 13
• ISSN: 0951-4198; 1097-0231; 1097-0231
• DOI: 10.1002/rcm.5050

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. Copyright © 2011 John Wiley & Sons, Ltd.