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Abstract
The amount of plant nitrogen (N) uptake from the soil and fixed N in the herbage of clover species were measured using a 15N dilution technique on mowing trials at 3 lime rates (0, 5000, and 10 000 kg/ha) at a low rainfall site (Matapiro soil) near Hastings over 5 years and at a high rainfall site (Mangatea soil) near Te Kuiti over 4 years. At the low rainfall site, measurements were also made over 5 years on a grazing trial at 2 lime rates (0, 7500 kg/ha). The dominant clover species was subterranean clover on the low rainfall site and white clover on the high rainfall site. Overall, the average proportion of clover N that was fixed from the atmosphere (PNfix) was about 0.8 of total N in clover. The average annual amount of fixed N in clover herbage was 35 kg/ha in the grazing trial on the Matapiro soil, 65 kg/ha on the mowing trial and 105 kg/ha in the trial on the Mangatea soil. Over all harvests and lime treatments, the amount of N fixed could be estimated as 0.04 × clover yield (kg DM/ha) on the high rainfall site and 0.046 × clover yield on the low rainfall site. The average annual amount of plant N uptake from the soil by grass, clover, and weeds was 150 kg/ha in the grazing trial and 160 kg/ha in the mowing trial on the Matapiro soil, and 240 kg/ha in the trial on the Mangatea soil. In the first 3 years, lime increased grass N uptake by an average of 24 kg/ha/yr over all trials. In the mowing trials, grass N uptake increased during the first 3 years by an average of 16 kg/ha when 5000 kg/ha of lime was applied, and by 33 kg/ha when 10 000 kg/ha was applied. This increase was attributed to an increase in net N mineralisation. However, the increase in net N mineralisation due to liming was a short term effect as lime had no significant effect on grass N uptake after 3 years.
