The mean annual N concentration is below the EU guideline value in all circum- stances. However the concentration does fluctuate, peaking above the guideline value on all treatments and even above the MAC on the lighter soils. As the concentration does not peak above the WHO limit for drinking water (22.6 mg NO−3-N L−1) there should be minimal risk to public health from these occasional
high concentrations, but there may still be regulatory consequences if these peaks above the MAC are present in drinking water.
is dependent on the characteristics of the aquifer and the travel time for water between the farm and the abstraction point. The concentration of water removed from a large, slow moving aquifer should reflect the mean annual concentration leaving the farm, so should be below the EU guideline value. However if there is a short travel time between the farm and the abstraction point, peak concentrations may be evident in drinking water, and there is a possibility that the drinking water NO−3 concentration may exceed the MAC.
Denitrification may also occur below 1 m, and remove some of this NO−3. This means that whether the NO−3 concentrations in drainage water predicted in this study are “acceptable” or not is largely dependent on aquifer characteris- tics. In some circumstances a farm running 2.94 cows ha−1 on a Clonakilty soil may cause drinking water NO−3 concentrations to peak above the MAC, and re- quire DCD or other measures to mitigate this. However on a different aquifer, or with a greater distance between the farm and the abstraction point, the peak concentrations leaving a similar farm may not appear in drinking water.
As this model only predicts the NO−3 concentration in drainage water leaving a farm, which may not represent the resultant concentration in drinking water, it is impossible to relate these results to a blanket stocking rate that would satisfy the EU regulations for drinking water.
The results of this trial must be considered in conjunction with local hydrology if they are to be meaningfully compared to regulatory drinking water limits.
5.6
Conclusions
The simple model presented here predicts values that are reasonable and match actual measured values for N loss from grazed pasture in the literature.
The model therefore appears useful in its present form to predict N losses from dairy farms on soils similar to the three tested here, and with further refinement of the data used in it (such as the addition of values for the effect of DCD on spring and summer urine) should become more accurate.
Nitrate levels in drainage from a dairy farm may peak above the EU guideline value, and even occasionally above the WHO guideline value (the EU MAC). They do not peak above the WHO limit for drinking water (22.6 mg NO−3-N L−1).
The mean annual NO−3 concentration was below the EU guideline value in all circumstances.
Nitrate concentrations and total losses were considerably lower on the Rath- angan soil than the lighter Clonakilty and Elton soils in all circumstances. Peak NO−3 concentrations leaving the Rathangan soil never exceeded the MAC.
This trial only yields the concentrations of NO−3 in drainage water, not in abstracted drinking water. It is important to not assume that if the water leaving the farm occasionally peaks above the MAC, drinking water will also breach the MAC - as that is entirely dependent on aquifer characteristics and will vary from location to location.
The results suggest that if there are persistently higher levels of NO−3 in an aquifer than the EU guideline value, or if the concentration is occasionally peaking above the WHO limit, it is likely that at least some of this NO−3 has not originated from dairy pasture with sensible rates of nitrogen fertiliser. There may be an alternative source that needs to be identified, which could be excessive levels of inorganic or organic fertiliser, arable land, a concentration of septic tank systems, industrial contamination, or another reason. If this is the case, restricting stocking rates will do little to clean up the aquifer, and will actually divert resources and distract the attention of both regulators and the public from solving the real issue. In future if limits for NO−3 in particular sensitive environments are defined, the model outlined here will be able to define maximum stocking rates with and without DCD within these environments.
reducing:
• Peak NO−3 concentrations where these are of concern in particular aquifers.
• Mean annual NO−3 concentrations where particularly low concentrations are required for sensitive ecosystems.
• Total NO−3 loss to save money on fertiliser, or achieve greater efficiency where fertiliser rates are restricted.
Conclusions
6.1
Findings
This trial is the first work on leaching losses from grassland with the application of DCD in Ireland. The results suggest that in many circumstances N losses may already be low enough to not require either regulation or the use of nitrification inhibitors, provided fertiliser application rates are comparable to those used in this trial. However where N losses are considered excessive, DCD shows great potential to reduce these losses.
Nitrate-N losses from the lysimeters were lower from the Rathangan soil than the lighter Clonakilty and Elton soils. Annual NO−3-N leaching losses from urine patches were 16 – 100 kg NO−3-N ha−1 from the Rathangan, and 163 – 233 kg NO−3-N ha−1 from the Clonakilty and Elton soils. There was little difference in loss between the Clonakilty and Elton soils.
Nitrate losses increased with fertiliser and urine application. DCD reduced total NO−3-N losses from urine patches by 40 - 50% when reductions were significant, and also reduced NO−3-N concentrations in leachate.
The N2O emissions factors recorded for urine-N (0.4 - 1.1 % on the light soils,
7 - 9 % on the Rathangan) are comparable to those recorded by other researchers. The emissions factors for the light soils were below the IPCC default value of 2%, and the Rathangan emissions factor was above it. The emissions factor for the Rathangan soil was halved when DCD was applied.
DCD also increased herbage N content, and in some treatments increased total
herbage yield.
The urine deposition frequency and pasture coverage by urine recorded in the GPS trial were comparable to those recorded by other researchers. The urine deposition frequency of 0.359 urine patches per cow grazing hour provides a flexible figure that can be used to predict urine deposition on pastures under a wide range of grazing regimes.
The model for field-scale losses presented in this thesis is simple, but yields results that are comparable to field measurements by many researchers. In its present form it appears to predict NO−3 losses with reasonable accuracy. With further refinements to the input data (such as the addition of N loss values from spring- and summer-applied urine with DCD, or more soil types) it should become more accurate.
Nitrate-N losses increased with stocking rate. At 2.94 cows per hectare, the highest stocking rate, annual field N loss was below 34 kg NO−3-N ha−1, mean concentrations were below 5.65 mg NO−3-N L−1(the EU guideline value for drinking water), and the worst-case-scenario autumn peak concentration did not exceed 21.55 mg NO−3-N L−1 (above the EU Maximum Allowable Concentration (MAC) but below the World Health Organisation (WHO) drinking water limit).
All these values were reduced when DCD was applied. DCD was calculated to reduce the total annual field loss of N by 21%, this being a very conservative estimate of the true reduction in N loss with DCD for reasons outlined previously. DCD also reduced peak losses from the Clonakilty soil to below the MAC.
The N losses recorded were comparable to other research in Western Europe and New Zealand. DCD reduced these losses by a considerable amount, although the reductions were not as large as previously found in New Zealand. DCD has the potential to be an effective mitigation option for N losses from Irish pastoral agriculture, and may become more effective with further refinement to the DCD application regime for Irish farming conditions.