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 sativa L.). The objective of this research was to determine how the supply of mineral N from different prior crops or fallow might affect the growth and yield 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 mineralised after prior crops. Experiments consisted of fallow, or cereal or legume crops in the dry-season followed by wet-season rice; and fallow, or cereal or legume crops in the wet-season followed by dry-season rice. Urea at one-third of the rate required for optimum rice yield was applied at 3 times during the rice crop: sowing, permanent flood, and/or panicle initiation. The prior fallow and crop treatments significantly influenced the growth and yield of rice crops. After a fallow, the pattern of soil N mineralisation promoted vegetative growth but was limiting during grain-filling. In contrast, after a cereal crop, rice vegetative growth was limited but grain-filling 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 which 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 of mineral N supply to a rice crop. Farmers should consider including a legume crop in rotation with rice because legumes supply N, which increases rice yield and reduces the requirement for fertiliser N. Cereal crops also contribute N, although farmers who use a cereal rotation should monitor the soil and crop N status during early rice growth, and supply extra fertiliser N to alleviate N deficiency