Prior crop and residue incorporation time affect the response of paddy rice to fertiliser nitrogen

Crop residues are an important source of nitrogen (N) for rice (Oryza sativ a L.). The objective of this research was to determine how the supply of mi neral N from different prior crops or fallow might affect the growth and yi eld of rice. The study also tested whether N use by rice might be improved by timing the application of inorganic fertiliser N to supplement the N min eralised after prior crops. Experiments consisted of fallow, or cereal or l egume crops in the dry-season followed by wet-season rice; and fallow, or c ereal or legume crops in the wet-season followed by dry-season rice. Urea a t one-third of the rate required for optimum rice yield was applied at 3 ti mes during the rice crop: sowing, permanent flood, and/or panicle initiatio n. The prior fallow and crop treatments significantly influenced the growth and yield of rice crops. After a fallow, the pattern of soil N mineralisat ion promoted vegetative growth but was limiting during grain-filling. In co ntrast, after a cereal crop, rice vegetative growth was limited but grain-f illing was promoted. Legume prior crops promoted both vegetative and grain growth. The benefits derived from growing the cereal or legume crops before rice, in terms of replacing fertiliser N, were dependent on the time at wh ich fertiliser N was applied to the rice crop. In particular, legume crops frequently nullified the rice growth responses to fertiliser N. The results demonstrated that fallow and prior crops can alter the amount and timing o f mineral N supply to a rice crop. Farmers should consider including a legu me crop in rotation with rice because legumes supply N, which increases ric e yield and reduces the requirement for fertiliser N. Cereal crops also con tribute N, although farmers who use a cereal rotation should monitor the so il and crop N status during early rice growth, and supply extra fertiliser N to alleviate N deficiency.