COMPARISON OF LEGUME-BASED CROPPING SYSTEMS AT WARRA, QUEENSLAND .2. MINERAL NITROGEN ACCUMULATION AND AVAILABILITY TO THE SUBSEQUENT WHEATCROP

Mineral nitrogen release following legume-based cropping systems for r estoring the fertility of a Vertisol and the yield response and N upta ke of subsequent wheat crops was studied. Legume phases of pastures, i ncluding a 4 year grass+legume ley, and lucerne and medic leys (simila r to 1 year) were terminated in October 1988 or 1989 and rotated with wheat. Chickpea-wheat rotations matched those of lucerne and medic ley s. Mineral N accumulations during a subsequent fallow period were dete rmined by core sampling to 1 . 5 m in October, February and May. Grain yield and N uptake of wheat enabled comparisons of the fertility rest orative effects of the various systems relative to continuous wheat cr opping. Averaged for two fallow periods, increases in mineral N down t o 1 . 2 m depth were 93, 91, 68, and 37 kg/ha following grass+legume, lucerne and medic leys, and chickpea, respectively, compared with the continuous wheat treatment. Wheat yields were generally lower in 1989 (1 . 85-2 . 88 t/ha) than in 1990 (2 . 08-3 . 59 t/ha) following all l eys and crops due to seasonal conditions. There was a grain yield incr ease of 0 . 11 and 0 . 52 t/ha in 1989 and 1 . 23 and 1 . 26 t/ha in 1 990 following lucerne and medic leys, respectively and 0 . 85 t/ha in 1990 following a 4 year grass+legume ley. Following chickpea there was a yield increase of 0 . 81 and 1 . 36 t/ha in 1989 and 1990 respectiv ely. Nitrogen uptake by wheat was increased by 40 and 49 kg/ha in 1989 and 48 and 58 kg/ha in 1990 following lucerne and medic leys respecti vely and 63 kg/ha in 1990 following a 4 year grass+legume ley. Followi ng chickpea N uptake by wheat was increased by 27 and 32 kg/ha in 1989 and 1990 respectively. Grain protein concentration of wheat was subst antially higher following all pasture leys (11 . 7-15 . 8%) than follo wing wheat (8 . 0-9 . 4%) or chickpea (9 . 4-10 . 1%). Therefore, ther e was substantial evidence of the effectiveness of pasture leys in soi l fertility restoration, as reflected in mineral N, yield response and N uptake by subsequent wheat crops.