Isotopic and microbiologic evidence of greenhouse gases transformation mechanisms in groundwater

One of the major global challenges of this century is to find the balance between the intensive agricultural production and the environmental damage that it causes by contributing to climate change and deterioration of water resources and soils. Agriculture accounts for up to one third of anthropogenic emissions of greenhouse gases (GHGs) which exacerbate the climate change (increase in nitrous oxide (N2O), methane (CH4), and carbon dioxide (CO2)) and lead to the depletion of stratospheric ozone layer (N2O).
Recent studies have suggested that in agricultural areas groundwater systems might be the significant sources of GHGs emissions, especially N2O, to the atmosphere due to intensive application of nitrogen containing inorganic and organic fertilizers used to increase soil fertility. However, the dynamics of N2O, CH4 and CO2 in aquifers is still poorly characterized due to the insufficient insight into kinetics and controls of processes regulating their production, transport and consumption. That is why, it is important to obtain more information regarding functional zones controlling fate of GHGs in subsurface. This knowledge is important for constraining the GHGs budgets, understanding the mechanisms behind climate change and developing mitigation measures to stop the rise of concentrations of N2O, CH4 and CO2.
In this context this study focuses on evaluating the potential role of aquifers affected by the agricultural activities as a source of GHGs emission to the atmosphere and improving the understanding of the impact of the spatial heterogeneity of subsurface media on the dynamics of N2O production and consumption processes. In this project advanced techniques and methods from hydrogeological, isotope and microbiological fields were used for investigation of the actual subsurface conditions and analysis of their impact on production and consumption of N2O in groundwater.
The study was divided into two stages: 1) regional investigations and 2) local-scale explorations. The main aim of the regional survey was to examine the distribution and accumulation of GHGs in different parts of the studied aquifer across its lateral and vertical dimensions and to obtain better information regarding the hydrogeochemical conditions of the subsurface. Meanwhile, the local scale investigations were focused on the occurrence of biochemical stratification in the same aquifer and analysis of its impact on N2O dynamics. It aimed to identify and quantify the rates of N2O production/consumption processes using data obtained from ambient groundwater and laboratory designed experiments. Since N2O production and consumption processes can proceed through abiotic and biotic pathways, the measurements of the activity of the microorganisms that accomplish biotic N transformations were conducted to obtain more information about N2O dynamics.