Effects of the timing of ploughing-in temporary leguminous pastures and two winter cover crops on nitrogen mineralization, nitrate leaching and spring wheat growth

Abstract

SUMMARYTwo field experiments at Canterbury, New Zealand during 1991–93 investigated the effect of the timing of ploughing a 4-year-old ryegrass/white clover pasture and the effect of two winter cover crops on subsequent N mineralization, nitrate leaching and growth and N uptake of the following 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 fallow treatments started in March and May respectively. Winter wheat (planted in May) had no effect on mineral N contents by the start of winter, whereas greenfeed (GF) oats (planted in March) significantly reduced the mineral N content in one year.Cumulative leaching losses over 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 fallow treatments (8–52 kg N/ha). Winter wheat did not reduce leaching losses in either year. GF oats did not reduce losses in 1991/92, but losses in 1992/93, when major drainage events occurred late in the winter, were only c. 40% of those under fallow.Incorporation of a large amount (> 7 t/ha dry matter) of pasture or GF oat residue in spring depressed yield and total N uptake of the following spring wheat, largely due to net N immobilization which could be overcome by the application of fertilizer N.First-year treatments had very little residual effect in the second year. Leaching losses over the second winter (mean 142 kg N/ha) were largely unaffected by the extent of first year leaching losses. 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.