Heterogeneities in the redox dynamics and nitrate reduction of artificially drained soils in different landscapes

Publikation: Bog/antologi/afhandling/rapportPh.d.-afhandling

  • Maria Isabel Senal
One of the strategies to decrease nitrogen (N) loading from agricultural activities is the implementation of a more targeted, site-specific nutrient management strategies that utilizes the natural N attenuation in the soil. The strategy however requires new and hydro-geochemical mapping at a fine scale for its effective implementation. In a coupled groundwater-surface water catchment model of agricultural catchment in Denmark, incorporation of artificial drainage discharges into the model improves the capability of the model to characterize local heterogeneity, groundwater flow and nitrate reduction estimates in the saturated zone. This study aims to contribute on understanding the N loading from artificially drained agricultural areas, particularly on the description of the heterogeneity and N reduction mechanisms in the unsaturated zone. This study initially describes the magnitude and profiles of redox potential values (Eh), different forms of N (nitrate (NO3ˉ); ammonium (NH4⁺); total inorganic nitrogen (TIN); total nitrogen (TN); total organic nitrogen (TON)), potential denitrification (PD) rates, and various physico-chemical soil properties to establish the magnitude and relative patterns of the different conceptualized redox environments and to elucidate possible NO3ˉ reduction mechanisms. The relative patterns between the conceptualized redox environments characterized at scales of less than 120 m were then upscaled at the scale of the drainage catchment to delineate possible NO3ˉ reduction hotspots (NRH). Field sampling campaigns were conducted between February to August 2017 wherein Eh and water samples were collected from Pt-tipped redox probes and piezometers, respectively. The Pt-tipped redox probes and piezometers were installed in transects along assumed groundwater flow paths based on topography and variations in apparent electrical conductivity (ECa). Concentrations of the different forms of nitrogen and organic carbon from the collected water samples were supplemented by measurements of PD rates, Fe (total Fe, amorphous, and dissolved), organic matter (OM) and clay content in the soil. Inverted ECa (EC) values obtained through electromagnetic induction (EMI) were also utilized as it provided high-resolution data for approximation of the clay and soil water content in the soil. Four soil combinations (SC) were generated based on the average ECa and topographic slope values of the piezometer transects to represent the redox environments. Soil classifications classified as having relatively high ECa has significantly lower Eh, NO3ˉ, and TN concentrations. The Eh, NO3ˉ and TN concentrations indicated the presence of intense nitrate reduction in areas classified as having high ECa yet the PD rates does not fully support this scenario. The PD rates were only significantly highest in the upper 50 cm which then becomes almost undetectable at the deeper depths. This is in contradiction with the observations from Eh, NO3ˉ and TN concentrations wherein the values reduces further at depths slightly above the tile drains (~150 cm). A comparison of the C:N ratios between areas classified as having high and low ECa values demonstrated that N immobilization is a probable N removal mechanism in areas with high ECa,. The lower C:N ratios in areas classified as having low ECₐ indicate the N mineralization is a more dominant process. Hence, the NO3ˉ and TN concentrations were less in areas classified as having high ECa. The high clay content in areas with high ECa stabilizes the OM in these areas and makes them less susceptible for mineralization. In addition, high clay content enhances the presence of anoxic microsites, which are sites for enhanced reduction processes. Other biogeochemical processes could also be present in the study site yet the data to support these were inconclusive. In particular, the concentrations of amorphous Fe was significantly correlated with the PD rates yet the lack significant correlation of PD rates with dissolved Fe does not sufficiently support the presence of Fe2+ mediated NO3ˉ reduction. Nonetheless, these observations introduce potential themes for further research on the nutrient biogeochemistry in the area. The high resolution of EC values and its capability to elucidate possible nitrogen reduction mechanisms from the SCs makes it suitable for upscaling the observations at the drainage catchment scale. Clustering of the EC values generated three classes wherein the class with the lowest average of EC values has significantly lower Eh and NO3ˉ concentrations. Areas classified as such have the potential to be NRH which comprises approximately 26% of the total area of the drainage catchment. This study demonstrated that ECa and topographic slope, two parameters that are quite easy to obtain, could be a potential mapping tool for NRH in other artificially drained areas. The methods and analyses presented were effective in characterizing patterns and potential mechanisms in the fine scale, as required for more targeted, site-specific nutrient strategies. It was also an attempt to contribute to the understanding of the heterogeneity in geochemical processes and to support hydrogeochemical modelling efforts in the unsaturated zone.
ForlagAarhus Universitet
Antal sider166
StatusUdgivet - mar. 2021

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