Winter Nitrate Leaching under Different Tillage and Winter Cover Crop Management Practices

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 vulgareL.], wheat [Triticum aestivumL.], and pea [Pisum sativumL.]). 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– 1in 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.