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

Abstract

The potential for nitrate (NO 3 – ) 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 NO 3 – 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 NO 3 – –N were calculated from soil solution NO 3 – –N concentrations (measured in ceramic suction cups installed at 600 mm) and drainage volumes. Cumulative NO 3 – –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 NO 3 – –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 NO 3 – –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 NO 3 –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 NO 3 – leaching within the acceptable range for arable cropping in New Zealand.