EFFECTS OF THE TIMING OF PLOWING-IN TEMPORARY LEGUMINOUS PASTURES AND2 WINTER COVER CROPS ON NITROGEN MINERALIZATION, NITRATE LEACHING ANDSPRING WHEAT GROWTH

Two field experiments at Canterbury, New Zealand during 1991-93 invest igated the effect of the timing of ploughing a 4-year-old ryegrass/whi te clover pasture and the effect of two winter cover crops on subseque nt N mineralization, nitrate leaching and growth and N uptake of the f ollowing wheat crops. Net N mineralization of organic N (of plant and soil origin) increased with increased fallow period between ploughing and leaching. The total amount of N accumulated in the profile by the start of winter ranged from 107 to 131 and from 42 to 45 kg N/ha for f allow treatments started in March and May respectively. Winter wheat ( planted in May) had no effect on mineral N contents by the start of wi nter, whereas greenfeed (GF) oats (planted in March) significantly red uced the mineral N content in one year. Cumulative leaching losses ove r the first winter after ploughing-in pasture varied markedly between years in relation to rainfall amount and distribution. Leaching losses were greater from the March fallow (72-106 kg N/ha) than the May fall ow treatments (8-52 kg N/ha). Winter wheat did not reduce leaching los ses in either year. GF oats did not reduce losses in 1991/92, but loss es in 1992/93, when major drainage events occurred late in the winter, were only c. 40% of those under fallow. Incorporation of a large amou nt (> 7 t/ha dry matter) of pasture or GF oat residue in spring depres sed yield and total N uptake of the following spring wheat, largely du e to net N immobilization which could be overcome by the application o f fertilizer N. First-year treatments had very little residual effect in the second year. Leaching losses over the second winter (mean 142 k g N/ha) were largely unaffected by the extent of first year leaching l osses. Second year leaching losses were greater than first year losses , probably due to the greater amount of mineral N at depth in the soil before the start of the second winter.