Nitrogen phosphorus exports from artificially drained dairy pastures in the Hauraki Plains, NZ
International Interdisciplinary Conference on Land Use and Water Quality (LuWQ2017), The Hague, The Netherlands
Stenger, R., Barkle, G., Moorhead, B., McKelvey, T.
This work forms part of the MBIE-funded Transfer Pathways Programme, which targets quantification of the pathway-specific transfers of nitrogen (N) and phosphorus (P) from the land to receiving waters, taking lag times and attenuation potentials of the specific pathways into account. Artificial drainage is one of the shallowest and fastest transfer pathways from the paddock to surface waters. N and P lost from the root zone can reach surface waters essentially un-attenuated, as the P-adsorption and denitrification potentials for N removal existing in the subsurface are largely bypassed. We report here on the N and P exports via subsurface drainage measured during the first season of our monitoring programme at two Hauraki Plains field sites with relatively high land use intensities. Subsurface drains at both dairy farms are installed in approx. 0.7 m depth and both field sites have similar ‘catchment areas’ (Waharoa 3.9 ha and Tatuanui 4.3 ha). The field sites have only relatively small differences in soil profile properties, but more pronounced differences in the characteristics of the underlying deposits. As a result, the partitioning of the total water and nutrient flows between shallow lateral flows through the subsurface drains and vertical recharge into the underlying deeper groundwater system is distinctly different between the two sites. At the Waharoa site, the soil profile becomes saturated from the “top down” due to low permeability zones within the soil profile, with drainage discharge beginning eight weeks earlier (mid-May) than at the Tatuanui site (mid-July). A shallow groundwater table seasonally rising into a relatively permeable soil zone is suspected to cause the wetting up of the soil profile from the “bottom up” at Tatuanui. The initial nitrate-N concentrations in Tatuanui drainage water were low (< 1 mg/l NO3-N). However, two weeks later nitrate-N concentrations had increased to nearly 9 mg/l NO3-N coinciding with peak drainage flow rates of 10 l/s. The initially low nitrate-N concentrations are considered to be due to mixing occurring below the depth of the drainage pipes of the leachate draining from the soil zone with underlying reduced shallow groundwater. This reduced groundwater is due to decomposing peat material residing below the mineral soil in approx. 1 to 10.5 m depth. Once the groundwater table has risen to the depth of the subsurface drainage pipes, the water percolating through the soil zone in response to excess rain is directly intercepted by these drainage pipes. The strong increase in nitrate-N concentrations is thought to be a result of the soil zone leachate entering the drains before any significant mixing with the underlying reduced groundwater can occur. At Tatuanui nitrate-N 2 represented 76% of the total-N discharged in the drainage waters over the 2016 drainage season, while NH4-N and organic N made up the remainder in nearly equal proportions. At the Waharoa site, nitrate-N made up 86% of the total-N exported, however NH4-N only contributed approx. 1%, with organic N contributing the remaining 13%. The highest total-P concentrations at Tatuanui occurred in the early and mid-part of the drainage season at the peak flows in the drainage hydrograph. Again, this is considered to be due to the influence of the reduced groundwater residing in the peat layer underlying the mineral soil at the Tatuanui site. Once the drainage was dominated by soil leachate, the total-P concentrations were generally low (< 0.004 mg/l). At Tatuanui and Waharoa, dissolved-P represented approx. 58% of the total-P measured, and dissolved reactive P accounted for 27% of total-P. As the subsurface system is considered to be effectively sealed at the Tatuanui site, with very little groundwater flow occurring, the artificial drainage pathway is considered to be almost exclusively the pathway for contaminant export from this site. In contrast, at the Waharoa site it appears that the shallow groundwater is probably an important contaminant export pathway additionally to the artificial drainage pathway.