The importance of the hydrological pathways in exporting nitrogen from grazed artificially drained land

Abstract

When considering hydrological export pathways from drained pastoral fields, the shallow groundwater underlying the artificial drainage system may fulfil an as important role as the artificial drainage system itself. However, the important split between artificial drainage and groundwater flows at a field site is usually unknown. Consequently, the effects that the groundwater system's redox status may have on the forms and total amounts of nitrogen (N) exported from the site cannot be confidently assessed. We addressed these deficiencies by investigating the export of various forms of N in the artificial drainage and the shallow groundwater beneath two dairy farming sites (Tatuanui, Waharoa) within the Piako River catchment in New Zealand. Due to the very low hydraulic conductivity of the degraded peat in the saturated zone perched on a clay aquiclude, no significant water or contaminants were exported at Tatuanui via the unconfined shallow groundwater. Accordingly, artificial drainage discharge into the receiving surface collector drain was the only export pathway for N at this site. The sealed nature of the groundwater zone resulted in the accumulation of organic-N and ammonium-N in the peaty shallow saturated zone underlying the mineral soil. In contrast, the Waharoa site featured a more mobile shallow groundwater system, which on average conveyed approximately equal volumes of water offsite as the artificial drainage system. Nitrate (NO₃-N) was at both sites the predominant form of N (76%) leached from the aerobic soil profile and discharged through the artificial drainage. As the shallow groundwater was at both sites in a reduced redox state, any NO₃-N leached from the rootzone into the saturated zone was consumed. Accordingly, even the mobile shallow groundwater at Waharoa did not convey significant NO₃-N. The NO₃-N consumption in the reduced groundwater presumably occurred due to dissimilatory NO₃-N reduction (concomitant increases in ammonium-N concentrations, with no organic material available for mineralisation in the saturated zone) and microbial denitrification (producing gaseous forms of N). The NO₃-N reductions in the groundwater resulted in the average total N exports via the shallow groundwater pathway at the Waharoa site being substantially lower (31% of the exported N) than in the artificial drainage (69%). This is despite both pathways exporting similar water volumes. Because of the NO₃-N consumption in the reduced shallow groundwater at Waharoa, over 98% of the N exported through this pathway was either as ammonium-N or organic-N.