Vand- og nitratbalance i lavbundsjord

Abstract

Projekt omhandler nitrat udvaskning i lavbundsjord, da drænvandsundersøgelser foretaget på lavbundsjord og højbundsjord har indikeret, at nitrat koncentrationen generelt er lavere på lavbundsjord end højbundsjord. I projekt vil der undersøges, hvilke forhold der kan være med til, at give en højere reduktion af nitrat. Undersøgelsen tager udgangspunkt i en lavbundsjord beliggende i Nordjylland nær Brønderslev. Der er taget jordprøver, der er analyseret for kvælstof her blev fundet store nitrat og ammoniums koncentrationer i rodzonen (0-1m). I kvælstofprofilerne reduceres porevandskoncentrationerne i alle 3 måneder ved den vandmættet zone. Der opnås en reduktion op til 75 % i porevandskoncentrationen af nitrat ved overgangen til den mættet zone , hvilket bevirker, at der må være iltfrie forhold omkring grundvandsspejlet, ellers ville der ikke ske denitrifikation. Porevandskoncentration reduceres med 7,5 mgN/l over 90 dage, hvilket giver en 0’ordens denitrifikationsrate på 8,33E-02 mgN/ldag. Kvælstofkoncentrationerne blev også undersøgt i drænudløbene, hvor koncentrationen af nitrat var meget mindre end porevandskoncentration fra rodzone. I programmet GMS opbygges en model over dræntværsnittet med forsøgsbestemt hydraulisk ledningsevne på 8,54E-05 m/s og en porøsitet på 0,44 cm^3/cm^3. Modellen beregner ved partikeltracking den gennemsnitlige strømningstid i den mættet zone til dræn. Ved at antage en 0’ordens denitrifikationsrate udregnes denitrifikationsraten til at ligge i intervallet 1,62E-02 mgN/ldag til 9,4E-3 mgN/ldag.

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.

A master thesis from Aalborg University

Vand- og nitratbalance i lavbundsjord

[Water and nitrate balance in lowland soils]