Impact of fertilization with pig slurry on the isotopic composition of nitrate retained in soil and leached to groundwater in agricultural areas
Margalef-Marti, Rosanna | Llovet, Alba | Carrey, Raúl | Ribas, Angela | Domene, Xavier | Mattana, Stefania | Chin-Pampillo, Juan | Mondini, Claudio | Alcañiz, Josep Ma | Soler, Albert | Otero, Neus
The isotopic composition of N and O of nitrate (NO₃⁻) is usually employed to trace its sources of pollution in groundwater. In agricultural areas, the amount of NO₃⁻ that reaches the aquifers after fertilization is controlled by different transformation processes that can affect the nitrogen species isotopic composition. Aiming to address the reliability of using isotope tools to trace sources of groundwater NO₃⁻, the goal of this study was to check the effect of fertilization on the isotopic composition of N compounds retained and leached from soils. The concentration and isotopic composition (δ¹⁵N and δ¹⁸O) of ammonium (NH₄⁺), NO₃⁻ and nitrite (NO₂⁻) was characterized after the application of pig slurry in lysimeters containing either soil under fallow (LF) or the same soil continuously cropped and fertilized (LC) during the previous six years. Results showed that the leached NO₃⁻ isotopic signature did not directly reflect the isotopic composition of the applied pig slurry. Just after fertilization, nitrification led to lower δ¹⁵NNO₃ values in soil extracts and leachates (e.g. from +5.9 ± 0.9‰ to +3.8 ± 3.1‰ in soil extracts of LF lysimeters). These values increased after complete nitrification (+11.5 ± 1.3‰) towards the δ¹⁵Nbᵤₗₖ of pig slurry (+19.6 ± 0.5‰). Later on, due to soil organic matter and plant debris mineralization and subsequent nitrification, values decreased towards the initial δ¹⁵NNO₃ of soil but remained above them (+8.6 ± 1.0‰). Both LF and LC experiments showed a similar trend and the latter ones allowed to reinforce that long-term fertilization with pig slurry can increase the soil δ¹⁵NNO₃. Concerning the δ¹⁸O of NO₃⁻ from soil extracts and leachates, it mainly depended on the δ¹⁸O of irrigation water and oxygen, after nitrification of NH₄⁺ from pig slurry. Therefore, studies aiming to trace groundwater NO₃⁻ pollution sources in rural areas by using an isotopic approach should consider the fertilization history of each setting. Also, analyzing the δ¹⁵Nbᵤₗₖ of soil is recommended, since it could mask the isotopic signature of the N applied through fertilization.
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