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The potential for nitrate (NO₃–) to leach is enhanced following cultivation of pastoral land, due to rapid mineralization of labile soil organic matter (SOM). In a 7-yr field trial in New Zealand, we examined the impacts of tillage intensity [intensive (plowing to 20 cm), minimum, or no-tillage] and winter cover crops (forage rape) on NO₃– leaching following cultivation of permanent (sheep [Ovis aries]-grazed) pasture to grow arable crops (the rotation included barley [Hordeum vulgare L.], wheat [Triticum aestivum L.], and pea [Pisum sativum L.]). Permanent pasture and permanent fallow (maintained plant-free using herbicides, i.e., not cultivated or fertilized) treatments were included as controls. Losses of NO₃––N were calculated from soil solution NO₃––N concentrations (measured in ceramic suction cups installed at 600 mm) and drainage volumes. Cumulative NO₃––N leached over 7 yr ranged from 20 to 428 kg N ha– ¹, with least N lost under pasture. Residual soil mineral N in autumn accounted for ∼30% of the variability in leaching. Nitrate leaching under arable crops generally increased rapidly as winter rainfall (range 78–352 mm yr– ¹) increased. Winter cover crops were effective in reducing NO₃––N leaching losses, particularly in drier winters when about 50% less N leached where cover crops were grown. On average, annual leaching was only 10 to 18 kg N ha– ¹ in the presence of cover crops. Tillage had relatively little influence on leaching, though use of minimum tillage for autumn cultivation resulted in significantly (P < 0.001) less NO₃––N leaching than either intensive or no tillage. Largest leaching losses were recorded in the unfertilized, permanent fallow where there was no plant sink for NO₃–N derived from SOM mineralization. Growing a crop during the winter period, combined with good N management practices to minimize pre-winter soil mineral N, provides the best option to keep NO₃– leaching within the acceptable range for arable cropping in New Zealand. |
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