NITROGEN MINERALIZATION, NITRATE LEACHING AND CROP GROWTH AFTER PLOWING-IN LEGUMINOUS AND NON-LEGUMINOUS GRAIN CROP RESIDUES

A field experiment was conducted in Canterbury, New Zealand to investigate the effect of six leguminous and non-leguminous grain crops on soil N fertility over a 12 month period (March 1989 to March 1990). All crops had an overall negative N balance during their growing season. A greater amount of soil N was removed by barley, rape and lupins (104-119 kg N/ha) than by field beans, field peas or lentils (50-74 kg N/ha). Net N mineralization was measured in all treatments between residue incorporation and the start of winter. With the exception of the lupins, accumulation of mineral N in the soil profile before the start of winter drainage was greater following leguminous (mean 124 kg N/ha) than non-leguminous crops (mean 80 kg N/ha). Cumulative apparent leaching losses over the autumn/winter were largely a reflection of the mineral N content of the profile before the start of drainage. Excluding lupins, leaching losses declined in the order fallow > legumes > non-legumes (110 > 72 > 37 kg N/ha respectively). The anomalous results for the lupins were attributed to the incorporation of a large amount of woody residues after harvest which may well have resulted in extensive net N mineralization occurring later in the autumn. Over a 12 month period, all treatments showed a decline in N fertility (110-160 kg N/ha), although compared with barley, the total loss of soil N was 10-40 kg N/ha less following leguminous crops. Growth of the following spring wheat test crop was affected by the preceding crop. Grain yield, grain N yield and total N yield were significantly related to the mineral N content of the soil at the end of leaching, and to a measure of net N mineralization during the growing season of the test crop.