Water and nitrate balance in lowland soils

Student thesis: Master Thesis and HD Thesis

  • Tommy Bistrup Knudsen
This project deals with nitrate leaching in low-level land on the basis of drain water analysis’ carried out on low- and high-level lands, which has indicated that the nitrate concentration is generally lower on the low-level land than the high-level land. In this project, it will be investigated which circumstances that exist to achieve a greater nitrate reduction.
The analysis is based on a low-level land situated in North Jutland near Brønderslev. The land was divided into two parcels from which there have been taken soil samples in a depth of 1.25 meters in five intervals. The sampling was conducted in November, January and March. Hereafter, the samples were analyzed for nitrogen (ammonium $NH_4^+-N$ and nitrate $NO_3^--N$), and hereby it was possible to determine the N-min contents (inorganic nitrogen) in the root zone (0-1 meters). The concentrations were, respectively, 176,6 kgN/ha and 93,2 kgN/ha, whereof about 30 % was nitrate that can potentially be leached.
It can be seen from the nitrogen profiles, how the nitrate concentration is reduced by water saturation in the three months, November, January and March. A reduction up to 75 % is reached in the pore water concentration of nitrate at the transition to the saturated zone. This indicates deoxidized circumstances around the ground-water table, because there would not otherwise be a denitrification. The pore water concentration was reduced with 7,5 mgN/l in 90 days, which results in a zero-order denitrification rate at 8,33E-02 mgN/ldag. Therefore, it is indicated that there is not a lack of substrate (active organic matter).
By comparing the root zone model, N-LES, with the measured pore water concentrations, it is apparent that the model overestimates the concentrations of nitrate in the root zone. There are many uncertainties connected with the use of this model to describe leaching of nitrate on a specific location.
The nitrogen concentrations were also examined in the drain outlets, and here it was found that the concentration of nitrate was much lower than the pore water concentration from the root zone. The average leaching of nitrate in drain outlets is 0,69 mgN/l. Through the program GMS (Groundwater Modeling System), a model, of the drainage cross-section with a test-determined hydraulic conductivity of 8,54E-05 m/s and a porosity of 0,44 cm^3/cm^3, was constructed. The model estimates, through particle-tracking, the average time-flow from the saturated zone to the drain. By assuming a zero-order denitrification rate, the denitrification rate is calculated to be in the interval 1,62E-02 mgN/ldag to 9,4E-3 mgN/ldag.
Publication date25 Jun 2015
Number of pages62
ID: 213943022