Fate of nitrogen in pig slurry applied to a New Zealand pasture soil

A a-year lysimeter study was conducted to determine the fate of nitrogen in pig slurry applied to a moderately fertile, semi-free-draining pasture soil in the Canterbury Plains of New Zealand. Pig slurry was applied annually for 2 years in autumn, at 3 rates of 0, 200, and 400 kg N/ha to 12 large soil lysimeters (4 at each rate), 800 mm in diameter by 1200 mm deep. Slurry applied in Year 1 was labelled with N-15 and a mass balance obtained at the end of the experiment. The mass balance showed that over the 2 years following application of N-15-labelled slurry, 8-19% was lost in the leachate, 20% was removed in the cut pasture, 15-26% was lost via volatilisation, 14-18% remained in the roots and soil, and approximately 30% was lost by denitrification. The high denitrification loss was attributed to (i) a large soil concentration of nitrate supplied from nitrification of the ammonium-N in the slurry; (ii) a readily oxidisable source of carbon supplied in the slurry; and (iii) transient anaerobic conditions produced by textural discontinuities and impeding layers within the soil profile. The fate of applied nitrogen between years was affected by the pattern of water inputs (rainfall and irrigation) and the resulting effect on drainage. Concentrations of inorganic nitrogen in the leachate from the 200 kg N/ha.year treatment were found to be consistently below 25 mg N/L, but those from the 400 kg N/ha.year treatment were considerably higher (c. 65 mg N/L) and persisted for a prolonged period. The latter N concentration represented a significant loss of nitrogen over the study period and may be of environmental concern.