Geochemical processes affecting reactive nitrogen in a clay till, hill slope field system

Rasmus Jakobsen, Anne Lausten Hansen, Klaus Hinsby and Jens Christian Refsgaard

 

Geological Survey of Denmark and Greenland (GEUS), Copenhagen, Denmark

 

The geochemical processes, with focus on reactive nitrogen, occurring in the clay till of a hill slope used for wheat production for for >5 yr was assessed through water samples from 25 cm screens inserted by hand augering. Thirty samplers were distributed randomly in an ~0.5 ha area and sampled 4 times during a year. Another set of 21 samplers made up a 135 m long, 2D transect in the direction of the general groundwater head gradient established from piezometers in the vicinity. This set was sampled 3 times during a year. Both sets were supplemented by a set of 12 suction cups inserted at 0.5, 1, 1.5 and 2.0 meters below surface (mbs), providing water samples from the unsaturated zone and the uppermost groundwater. The level of the water table fluctuates from 0.5-1 mbs during the wet season to 3 mbs during dry summers. Samples were analyzed for main cations and anions as well as ammonium, nitrite and water isotopes.

Data from the suction cups indicate that the water infiltrating to the groundwater has a fairly constant nitrate concentration around 1 mM at 2 mbs. This is in spite of a clear seasonal variation above 2 mbs related to uptake. Here concentrations go from 10 µM at the end of the growing season to slightly above 1 mM in February, indicating that reactive nitrogen must be transported to 2 mbs via macropores. Low concentrations (~1 µmol/l) of ammonium in the upper 2 m indicate that this zone contains enough oxygen for efficient nitrification. Dissolved organic carbon goes from ~0.6 mM at 0.5 mbs to ~0.1 mM at 2 mbs.

In the till system, deeper samples are generally more reduced, but the depth at which nitrate disappears varies from 3-5 mbs. Plotting data for sulfate and total inorganic carbon as a function of decreasing nitrate concentrations indicates a zone rather than a sharp front, where nitrate reduction occurs via pyrite oxidation. Nitrate reduction by organic matter does not show due to interfering carbonate dissolution. Below 2 mbs an increase in the ammonium concentration indicates release from oxidation of organic matter, given that there are no clear signs of cation exchange. Assuming that the oxidizing organic matter is derived from wheat roots and a typical C/N ratio for wheat roots, it appears that approximately 20% of the nitrate could be reduced by organic matter. Inverse modelling using PHREEQC on samples constructed from averages samples at 0.5 mbs, 2 mbs and 5-6 mbs also indicate that organic matter could be responsible for around 20% of the denitrification between 2 and 5 mbs. In addition, the inverse modelling indicated that potassium in the infiltrating water is removed by the formation of illite and that primary silicates are dissolving by weathering processes.

Data from the 2D cross-section show variations in the nitrate and ammonium distribution as well as the position of the redox interface that appear to correlate with the content of stable water isotopes. It seems that the nitrification goes slower in a zone where the stable isotopes indicate that infiltration is focused, apparently related to a flat central part in the 2D transect. Perhaps locally increased infiltration limits the access of oxygen, limiting nitrification. It could imply that the depth to the reducing zone where nitrate disappears relates to small-scale variations in the topography. A quantification of the described processes in the saturated zone of the 2D transect is underway using the reactive transport code PHAST in which PHREEQC is used to handle the biogeochemical reactions in the system.