Inorganic nitrogen fertiliser can be managed to maximise the nitrogen uptake and yield of a rice crop grown after a fallow, however, the nitrogen benefits or disadvantages of growing a cereal or legume crop instead of the fallow are not known. This study questions whether the timing of nitrogen released from the soil and crop residues has an impact" on the nitrogen nutrition of rice crops. Because soil nitrogen and the nitrogen in crop residues is rarely adequate for the rice crop, inorganic nitrogen fertiliser was applied at different times (planting, permanent flood or panicle initiation) to maximise yield, and to see whether nitrogen uptake, and its use for rice production, could be improved.
Three field experiments were conducted between 1988 to 1990 on a cracking clay soil in north Queensland. Wet-season rice was grown after dry-season maize or chickpea crops, and dry-season rice was grown after wet-season maize
or soybean crops, or a lablab green manure. The control was a weed-free fallow. The soil nitrogen dynamics, nitrogen uptake by the rice crop and the efficiency of nitrogen use for rice grain production were compared.
Cereal and legume crop residues provided a maximum of about 4-7 g nitrogen m-2 for the following rice crop, which was similar to the amount of soil nitrogen which was mineralised during a fallow. However, some of the soil, legume residue and inorganic nitrogen was mineralised, nitrified and denitrified before the rice crop was permanently flooded, and consequently was of limited benefit to the rice crop. Late incorporation of the legume residue reduced the amount of nitrogen lost. The nitrogen in maize residue was slowly mineralised in both the wet and dry-seasons and was protected from loss. The maize residue also immobilised some soil and inorganic nitrogen.
In both seasons, soil nitrogen was taken
up by the rice crop after permanent flood, but the amount taken up declined between an thesis and maturity. The amount of nitrogen taken up from cereal and legume residues increased between permanent flood and maturity. The rice crop took up about the same proportion (0.2-0.5) of residue nitrogen as inorganic nitrogen applied at planting and panicle initiation, however, more of the inorganic nitrogen applied at permanent flood was taken up (0.6-1.0).
The timing of nitrogen uptake affected the growth pattern of the rice crop. Soil nitrogen promoted the growth of rice tops but was insufficient for grain-filling, maize residue nitrogen limited tops growth but promoted grain-filling, and chickpea, soybean and lablab residue nitrogen promoted both tops and grain growth. The responses to inorganic nitrogen were similar to soil and residue nitrogen, such that inorganic nitrogen at planting and permanent flood promoted tops growth, and inorganic nitrogen at
permanent flood and panicle initiation promoted grain growth.
Legume residue nitrogen increased rice yield during all seasons, whereas. maize residue nitrogen increased dry-season rice yield only. The yield benefits from residue nitrogen ranged from 20-200 g m-2 , and were generally correlated to increases in the nitrogen content of the rice crop. The rice crop used nitrogen very efficiently to produce grain (53 g grain g-1 crop nitrogen), however, soil, residue and inorganic nitrogen was used with different efficiency.
The nitrogen nutrition of a rice crop after a fallow was unbalanced because nitrogen was lost before permanent flood and the rice crop was nitrogen deficient during grain-filling. Maize or legume crops promoted rice growth and nitrogen use. With maize residues, inorganic nitrogen was required during early rice crop, whereas with legume residues, some of the inorganic
nitrogen applied at permanent flood and panicle initiation was used ineffectively.