The uptake and use of nitrogen by paddy rice in fallow, cereal, and legume cropping systems

In a previous paper, we reported that prior crops either increased or decreased the yield of paddy rice (Oryza sativa L.) and altered its response to fertiliser N. We considered that rice yield responses to prior crop might have reflected the uptake of crop residue N and the efficiency of its use to produce grain. Experiments consisted of dry-season grain or legume crops, or fallow, followed by wet-season rice (cv. Lemont); and wet-season grain or legume crops, or fallow, followed by dry-season rice. Urea at one-third of the rate required for optimum rice yield was applied at 3 stages of rice crop growth: sowing, permanent flood, and/or panicle initiation. Soil N supplied 4.1 to 6.5 g N/m2 to the rice crop, depending on the season. Rice also recovered 0 to 0.25 of the N in the residue of a prior maize crop and 0.23 to 0.57 of the N in grain legume residues or a legume green manure crop; the fraction was greater if fertiliser N was not applied. Increased N uptake was the major contributor to heavier yield. The relationship between grain yield and crop N content was mostly linear, and thus physiological efficiency of N use for rice grain production was essentially constant across the range of environments provided by fertiliser N and cropping system treatments in this study. In experiments where fertiliser N was applied, there were small effects of prior cereal and legume cropping treatments on physiological efficiency. In contrast, without fertiliser N application, physiological efficiency was increased by prior cereal and legume crops, which likely resulted from a greater congruence between the N demand of the rice crop, and the N supply from the soil and incorporated residue, when compared with a fallow treatment.