From the Paddock to the Stream — Unravelling the Nitrogen Flowpaths in a New Zealand Dairying Catchment

The Toenepi catchment (15 km²) is dominated by dairying, and ranges in elevation from 40 to 130m above sea level (ASL). Most of the catchment is flat land, with some rolling and steep land occurring on the boundaries. Annual rainfall is 1132 mm and mean annual temperature is 13.3°C. Well-drained Allophanic soils dominate in the catchment in close association with granular soils of moderate permeability. Poorly drained Gley soils occur in the lowest areas adjacent to the stream and require artificial drainage. The average stocking rate is 3.0 cows ha^–1, which graze all year. The catchment export of total nitrogen through the stream had been calculated in an earlier project as 35 kg ha^–1 yr^–1. The median total nitrogen concentration in the stream was 3 mg L^–1 (1995/97). To better understand nitrogen flowpaths, we initially installed groundwater monitoring transects in seven subcatchments, which reflected the major site and landuse conditions. Monthly sampling indicated that the concentrations of inorganic nitrogen in the shallow groundwater were generally well below the concentrations measured in the stream. This result would not support the hypothesis that the majority of the nitrate in the stream is derived from groundwater. Monitoring of the nitrogen concentrations in drains indicated that artificial drainage may be a major conduit for nitrogen into the stream. Artificial drains bypass the deeper subsoil and riparian zones where denitrification is likely to occur. A mathematical groundwater discharge model is used to quantify the proportion of streamflow that can be explained by groundwater discharge in contrast to near-surface flowpaths (surface runoff, artificial drainage). Understanding the pathways through which nitrogen enters Toenepi Stream is considered a prerequisite for the development of the most effective and efficient measures to reduce the N contamination of the stream.