Reactive Modeling of Denitrification in Soils with Natural and Depleted Organic Matter

Abstract

Nitrogen fertilizers used in agriculture often cause nitrate leaching towards shallow groundwater, especially in lowland areas where the flat topography minimize the surface run off. In order to introduce good agricultural practices that reduce the amount of nitrate entering the groundwater system, it is important to quantify the kinetic control on nitrate attenuation capacity. With this aim, a series of anaerobic batch experiments, consisting of loamy soils and nitrate-contaminated groundwater, were carried out using acetate and natural dissolved organic matter as electron donors. Acetate was chosen because it is the main intermediate species in many biodegradation pathways of organic compounds, and it is a suitable carbon source for denitrification. Sorption of acetate was also determined, fitting a Langmuir isotherm in both natural and artificially depleted organic matter soils. Experiments were performed in quadruplicate to account for the spatial variability of soil parameters. The geochemical code PHREEQC (version 2) was used to simulate kinetic denitrification using Monod equation, equilibrium Langmuir sorption of acetate, and equilibrium reactions of gas and mineral phases (calcite). The reactive modeling results highlighted a rapid acetate and nitrate mineralization rate, suggesting that the main pathway of nitrate attenuation is through denitrification while calcite acted as a buffer for pH. However, in the absence of acetate, the natural content of organic matter did not allow to complete the denitrification process leading to nitrite accumulation. Reactive modeling is thought to be an efficient and robust tool to quantify the complex biogeochemical reactions which can take place in underground environments.

Development of a Fully Coupled Biogeochemical Reactive Transport Model to Simulate Microbial Oxidation of Organic Carbon and Pyrite Under Nitrate‐Reducing Conditions

“…Nitrite accumulates in our experiment for both heterotrophic and autolithotrophic denitrification, a phenomenon, which has been observed in a multitude of studies (e.g., Bosch et al, ; Carrey et al, ; Mastrocicco et al, ; Parmentier et al, ). It was observed that the rates for nitrate and nitrite reduction are affected differently by specific environmental conditions.…”

“…Some of those models do not consider in detail microbial growth (Antoniou et al, 2013;Engelhardt et al, 2014), while others account for microbial growth and decay (Clement et al, 1997;Rodríguez-Escales et al, 2016). Some investigations are based on laboratory studies (Clement et al, 1997;Mastrocicco et al, 2011;Molins et al, 2015;Rodríguez-Escales et al, 2016), while others analyze field investigations (Engelhardt et al, 2014;Lee et al, 2009). A special case is the thermodynamic approach which was employed by André et al (2011) for their laboratory studies using thermodynamic factors similar to Monod factors and which account for changes in the thermodynamic equilibrium of the redox reaction.…”

In Search for the Missing Nitrogen: Closing the Budget to Assess the Role of Denitrification in Agricultural Watersheds

“…According to the origin and texture of the soil matrix, organic matter content varies from <2% for silty loam to 28% for peat [33]. The shallow unconfined aquifer, whose thickness ranges between 2 and 7 m, is fed by rain infiltration and the surface water system; thus, the water table is usually very proximal to the ground surface [34]. Agriculture is the dominant land use with ~80% of the watershed area classified as utilized agricultural land (UAA).…”

Nitrate bioreduction in redox-variable low permeability sediments

“…Reaction 2 is the overall reaction that merges intermediate reactions involving N2O (Table S2). These overall reactions have been used widely for mass and electron balance in modeling nitrate bioreduction in wastewater treatment engineering and agricultural systems (Ahn, 2006;Mastrocicco et al, 2011;Matějů et al, 1992;McCarty et al, 1969;Rittmann and McCarty, 2001;U.S. EPA, 1993).…”