NITRATE ACCUMULATION UNDER PEA CROPPING AND THE EFFECTS OF CROP ESTABLISHMENT METHODS - A SUSTAINABILITY ISSUE

Grain legume-cereal rotations are unsustainable on acid soils because they promote acidification of surface soil through nitrate leaching. T wo field experiments were conducted on red, clay-loams in the cropping zone of central western New South Wales to determine whether soil min eral N concentrations during crop growth are higher under pea than bar ley, and whether the nitrate concentration under pea crops can be decr eased by ammending soil with cereal straw before sowing. Significantly higher mineral N, particularly nitrate, was found under pea than unde r barley, as early as 6 weeks following autumn sowing, and also in spr ing. The pea effect represented an increase of up to 23 kg N/ha of min eral N (0-30 cm). It is proposed that the source of higher nitrate con centration under pea may be residual soil nitrate not utilised by pea, or nitrate derived from the mineralisation of pea roots or exudate. T he increase in soil nitrate during pea growth contributes to greater p ostharvest soil mineral N and higher wheat yields after pea, but also increases the risk of soil acidification. Soil ammendment with cereal straw was partially effective in reducing nitrate concentration under pea, but a more effective treatment is required.