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

In a previous paper, we reported that prior crops either increased or decre ased 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 h ave 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 o r legume crops, or fallow, followed by dry-season rice. Urea at one-third o f 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 sup plied 4.1 to 6.5 g N/m(2) to the rice crop, depending on the season. Rice a lso 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 upt ake was the major contributor to heavier yield. The relationship between gr ain yield and crop N content was mostly linear, and thus physiological effi ciency of N use for rice grain production was essentially constant across t he range of environments provided by fertiliser N and cropping system treat ments in this study. In experiments where fertiliser N was applied, there w ere small effects of prior cereal and legume cropping treatments on physiol ogical efficiency. In contrast, without fertiliser N application, physiolog ical efficiency was increased by prior cereal and legume crops, which likel y 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 wit h a fallow treatment.