Artificial subsurface drainage is necessary to enable viable agricultural production on imperfectly to very poorly drained soils. These soils make up approx. 40% of land used for dairying in NZ. However, artificial drainage also provides a “short-circuiting” pathway for unattenuated delivery of nutrients from paddocks to streams. There is a particular knowledge gap concerning the extent of vertical nutrient recharges into the shallow groundwater underlying artificially drained land.
To determine the lateral and vertical water and N and P fluxes, and the fate of nutrients recharged vertically into shallow groundwater from two drained dairy farms (Tatuanui and Waharoa) on the Hauraki Plains (Waikato).
Artificial drainage flows were continuously monitored, and flow-proportional samples analysed for N and P over two drainage seasons. Sub-soil coring permitted the controls on the drainage hydrology to be understood, with shallow wells installed to monitor water table dynamics. Depth profiling, using a dual packer system, allowed for monitoring of N and P and redox status through the shallow groundwater.
A water balance over the drainage seasons confirmed the soil coring results that the Tatuanui site was hydraulically sealed in the subsurface and no vertical recharge and contaminants were being exported through this pathway. In contrast, between 39 and 46% of the rainfall over the drainage season recharged vertically into the shallow groundwater at Waharoa.
The predominant form of N lost via artificial drainage was nitrate-N (72-86% of total N). The cumulative annual mass exported of each form of N (nitrate, ammonium, organic) at each site increased linearly from the origin with the amount of annual drainage, except for nitrate-N at Waharoa.
The average nitrate-N concentrations measured in the groundwater at both sites were all less than 0.2 mg NO3-N/L. Concomitantly, indicators for redox conditions showed the shallow groundwater to be strongly reduced at both sites. These results indicate that at both sites any vertically recharged nitrate-N is likely to be denitrified in the shallow groundwater.
To understand and quantify the export of nutrients from artificially drained land it is essential to also consider the role of the shallow groundwater beneath the installed drainage. Where soil hydraulic properties and the landscape position favour vertical recharge into the underlying shallow groundwater, the redox status of this zone becomes crucial for the fate of nitrate-N.
This work was supported by MBIE-funded ‘Transfer Pathways’ and ‘Critical Pathways’ programmes and co-funded by Waikato Regional Council and DairyNZ.