SOIL-NITROGEN SUPPLY TO RICE - CROP SEQUENCE EFFECTS

The effect of cropping sequence on soil nitrogen (N) supply to a rice crop was investigated using an in situ incubation technique in a direct drill system on a red-brown earth soil in south-eastern Australia. The crop sequences were (i) a rice crop in each of the previous 4 summers, (ii) rice then 4 seasons of annual pasture (long pasture phase), (iii) rice, 2 winter cereal crops then 2 seasons of annual pasture (short pasture phase), and (iv) rice then 4 winter cereal crops. This study was undertaken in the fifth year of the crop sequence experiment, when all sequences had returned to rice. Within the rice crop, fertilised (160 kg N/ha) and unfertilised plots were established on the burnt stubble portion of the main crop sequence plots. Nitrogen uptake in unfertilised plots ranged from 79 kg N/ha in continuously cropped rice treatments to 165 kg N/ha in short pasture phase treatments. Application of 160 kg N/ha at permanent flood increased N uptake to 207 kg N/ha for the short pasture treatment. Crop biomass and tillering varied with cropping sequence and increased with fertiliser nitrogen application. Crop sequence had little effect on soil mineral N content during the growing season. However, total soil N mineralisation during the season varied with both crop sequence and fertiliser application. The continuous rice sequence mineralised 119 kg N/ha, whilst the long pasture phase sequence mineralised 246 kg N/ha. Fertiliser application increased mineralised N to 267 and 337 kg N/ha for continuous rice and short pasture phase treatments, respectively. Nitrogen mineralisation rate peaked (4 kg N/ha. day) some 40-50 days after permanent flood, coinciding with panicle initiation and the period of high N demand in the rice crop. Increased N availability after panicle initiation resulted in significantly higher grain yields. This work demonstrates that both the magnitude and timing of N supply affects the grain yield of the rice crop. Nitrogen supply is affected by the previous crop sequences. Practical implications are that pasture phase length of highly clover-dominant pastures could be reduced (from 4 to 2 years) and still provide similar contributions of N to succeeding rice crops; that continuous rice growing might achieve high yields similar to rice in rotation with legume pastures with the judicious application of fertiliser N; and that these N fertiliser applications may have to be quite high to achieve grain yields similar to rice in rotation with legume pastures.