Abstract

Environmental context. Anthropogenic nitrogen inputs have significant effects on terrestrial and aquatic ecosystems, the extent of which can be traced by using the natural stable isotopic composition of nitrate to integrate the sources of nitrogen and the biological processes of their production. In ecosystems, nitrates are transported by diffusion in water and advection of water masses, but these physical processes have not been characterised in terms of isotopic fractionation. We report experiments demonstrating that physical transport processes have a negligible effect on the isotopic composition of dissolved nitrate.Abstract. We experimentally investigated the effect of the physical process of transport (diffusion and advection) on the isotopic composition of nitrate (d 15 N and d 18O). Strict diffusion of nitrate in water was studied using a modified Richter apparatus. The combination of diffusion and advection processes was followed by elution of nitrate solution onto silica gel column. No significant isotopic fractionation was observed. Throughout the last decades, increasing anthropogenic nitrogen inputs have significantly affected the nitrogen cycle in continental ecosystems. As a consequence, increasing nitrate contamination of surface waters and groundwaters has emerged as a major problem in many agricultural areas of the world. [1] Moreover, the effect and the fate of the excess nitrogen in the environment become a critical question. The use of isotopic biogeochemistry is a potentially powerful tool to allow a better characterisation of nitrate-nitrogen cycling dynamics in the environment. [2][3][4] The measurement of the naturally occurring stable isotopic composition of nitrate (i.e. d [2] In ecosystems, complex systems involve generally sequential processes. The reaction between substrate and product can be limited by one step of transformation that has a lower reaction rate than the other steps. Isotopic fractionation (e) cannot therefore be totally expressed; there are 'apparent' isotopic effects (e app ) that depend on the limiting process. However, only denitrification and nitrification processes are considered to generate isotopic fractionation, affecting the isotopic composition of nitrate, by enriching the substrate. As the soil is negatively charged, nitrates move freely through the soil environment by diffusion and advection, therefore diffusive fractionation could occur as demonstrated for other solute molecules.[5] Nevertheless, when the transport of nitrate is strictly limiting and its availability of nitrate is low, the biological fractioning-specific processes do not occur. In that case, the strong biological isotopic fractionation related to the higher level of energy needed to break or form chemical bonds involving 15 N rather than 14 N (primary isotope effects) is replaced by the isotopic effect due to diffusion depending on the mass difference between the isotopes. In spite of their major role, the influence of physical processes, such as diffusion and advection, on nitrate isoto...