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 nitrog en in pig slurry applied to a moderately fertile, semi-free-draining p asture soil in the Canterbury Plains of New Zealand. Pig slurry was ap plied 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 s lurry, 8-19% was lost in the leachate, 20% was removed in the cut past ure, 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; a nd (iii) transient anaerobic conditions produced by textural discontin uities and impeding layers within the soil profile. The fate of applie d nitrogen between years was affected by the pattern of water inputs ( rainfall and irrigation) and the resulting effect on drainage. Concent rations of inorganic nitrogen in the leachate from the 200 kg N/ha.yea r treatment were found to be consistently below 25 mg N/L, but those f rom 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 m ay be of environmental concern.