Nitrate leaching losses under a legume-based crop rotation in Central Canterbury, New Zealand

Abstract

Nitrate leaching was investigated in a lightly grazed pasture soil and under 4 crops (white clover, peas, wheat, and wheat) grown in a typical legume-based crop rotation in Canterbury, New Zealand. The movement of soil nitrate, as shown by periodic soil sampling and data on crop nitrogen (N) uptake, rainfall, and soil moisture contents, was used as an indirect measure of leaching losses. Some N fertiliser was used during the 2 wheat cycles — application rates were based on soil tests and known wheat requirements. Most soil N replenishment occurred during the legume (white clover) phase of the rotation. Approximate estimated annual nitrate-N leaching losses were: pasture 5 kg/ha white clover 10 kg/ha peas 90 kg/ha wheat 60 kg/ha and second-year wheat 35 kg/ha. N fertiliser applications in spring did not contribute significantly to nitrate leaching losses. Losses were derived from nitrification of organic N in soil organic matter. The extent of these losses was associated with declining amounts of nitrate-N in the soil profile under successive crops following ploughing in of the N-rich white clover residues. Leaching losses occurred mainly during late winter- early spring. © 1985 Taylor & Francis Group, LLC.