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

• Published in: Rapid Communications in Mass Spectrometry
• Main Authors: Brunet, F., Potot, C., Probst, A., Probst, J.‐L.
• Format: Article in Journal/Newspaper
• Language: English
• Published: Wiley 2011
• Subjects: Carbonic acid
• Online Access: http://dx.doi.org/10.1002/rcm.5050; https://api.wiley.com/onlinelibrary/tdm/v1/articles/10.1002%2Frcm.5050; https://onlinelibrary.wiley.com/doi/full/10.1002/rcm.5050

The isotopic signature of Dissolved Inorganic Carbon (DIC), δ 13 C DIC , 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 NH 4 + , 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, δ 13 C DIC is intermediate between the δ 13 C of soil CO 2 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 δ 13 C DIC values range from −17.1‰ to −10.7‰ in Montoussé stream waters. We also measured the δ 13 C 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 δ 13 C DIC 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 δ 13 C DIC and the molar ratio NO 3 – /(Ca 2+ + Mg 2+ ). Consequently, when the contribution of nitrate increases, the δ 13 C DIC increases towards the calcareous end‐member. This new isotopic result provides evidence for the direct influence of nitrogen fertilizer inputs on weathering, CO 2 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 ...