Summary: Final Report of the ERTDI-funded project 2000-LS-18.104.22.168-M2
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Much of the intensive dairy farming in Ireland is located on free-draining unsaturated soils overlying important aquifers. This study was undertaken to measure the concentrations of nitrate-nitrogen (NO3-N) in soil pore water following land spreading of fertiliser nitrogen (N), dirty water and farm slurry to grassland on a free draining sandy loam overlying a limestone aquifer. Three rates of fertiliser-N, dirty water and farm slurry, corresponding to high, medium and low, were applied. Chemical fertiliser-N, dirty water and farm slurry were applied at the same times as those applied on an adjoining intensively managed dairy farm. Ten experimental field plots, including one control plot, were laid out following a geotechnical survey to establish soil depth and characteristics. There was no evidence of cracks, fissures or macropores below the topsoil (0.3 – 0.4 m thick). The plots were instrumented to 3 m depth with (i) porous ceramic suction samplers to sample the soil pore water at various depths; (ii) tensiometers to measure the pressures of the soil pore water and (iii) neutron probe access tubes to estimate soil water contents at different depths. Pore water samples were taken weekly during the summer and twice weekly during the winter and analysed for NO3-N; pore water pressures and soil water content were measured at the same time as the soil pore water samples were withdrawn. Over the two year period of measurement, the NO3-N concentrations in the pore water of the control, the low N fertiliser (174 kg /ha.) and medium N fertiliser (286 kg /ha.) were low, in general. However, there was a tendency for higher NO3-N concentrations in the pore water of the medium fertiliser treatment in spring following late September and late January - early February N applications. The high N fertiliser (387 kg /hectare) gave rise to high NO3-N in the soil pore water. The NO3-N concentrations in the soil pore water generally showed no response to summer and winter applications of low (10 mm) dairy waste water applications. Similarly, medium (25 mm) and high (50 mm) rates of dairy wastewater in summer had little effect on NO3-N concentrations in the soil pore water. However, applications of medium (25 mm) and high (50 mm) rates in winter increased the NO3-N concentrations of the soil pore water to high levels. None of the slurry applications of low (15 m3/ha), medium (30 m3/ha) and high (45 m3/ha) had a significant effect on soil pore water NO3-N concentrations. LEACHN modelling gave fair agreement between measured and modelled NO3-N concentrations in the soil pore water following low, medium and high dirty water applications. High initial NO3-N concentrations in the soil pore water at the start of the study at depths greater than 1.5 m declined to low levels over an 18 to 24 month period and are continuing to decline in 9 of the 10 study plots. This indicates that high NO3-N concentrations in soil decline quickly under Irish recharge conditions. There was very good agreement between the calculated depth of travel of NO3-N solutions in the soil assuming piston flow (also called plug) and the measured depth of travel. This agreement was obtained at 10 locations for 13 events, confirming the absence of by-pass or macropore flow in the soil at the experimental site.