EFFECT OF SOIL TREATMENT WITH CEREAL STRAW AND METHOD OF CROP ESTABLISHMENT ON FIELD PEA (PISUM-SATIVUM L) N-2 FIXATION

The effects of soil incorporation with cereal straw (nil, 2.5, 5 and 1 0 t straw ha(-1)) and direct drilling on the proportion and amount of pea N derived from biological N fixation were investigated in three fi eld experiments. Fixed N was determined by N-15 dilution using barley as a reference plant. The three sites were on acidic, red clay-loams i n the cropping zone of southeastern Australia. Seasonal plant availabl e soil N, as determined by the N accumulated in barley, was 31, 56 and 158 kg N hdl, for the three sites. Incorporated straw reduced soil ni trate at sowing by 10-50 kg N ha(-1) (0-30 cm), and 5 or 10 t straw ha (-1) reduced barley uptake of N by 10-38 kg N ha('1). However, reducin g plant available soil N was generally ineffective for increasing the N fixed by pea. Fixed N increased only at the site with the least plan t-available N, and only one-third of the increase could be attributed to lower soil N uptake by pea. There was no evidence that direct drill ing pea increased fixed N by decreasing crop uptake of soil N. It is p roposed that a lower requirement for soil N by pea as compared to barl ey, and availability of mineral N beneath the soil layer treated with straw, minimise the effectiveness of straw incorporation for increasin g the N fixed by pea.