Effects of time of urea application on combine-sown Calrose rice in south-east Australia. III. Fertiliser nitrogen recovery, efficiency of fertilisation and soil nitrogen supply

A combine-sown crop of Calrose rice was grown on an alkaline self-mulching grey clay soil. Prilled urea (50 kg N ha-1) was broadcast at four different times: sowing, before permanent flood, after permanent flood and panicle initiation. Field microplots received 5 atom % 15N-labelled urea at the same times. Plant recovery of 15N applied at sowing was only 3.8%, with 80% unaccounted for. The data are consistent with the poor performance of rice fertilised at sowing (Part I) and the disappearance of mineral N from the top 20 cm of soil during the flushing period (Part II). Plant recovery of 15N applied after permanent flood was also relatively low (28%) with a loss of 45%. Even where fertiliser was used most efficiently in producing more grain (application before permanent flood and at panicle initiation), substantial losses were incurred (25-40%), with plant recoveries up to 41%. The lack of significant amounts of excess 15N in the soil at depths below 10 cm was consistent with the suggestion that leaching was not an important loss mechanism and that losses may be reduced by deep placement of the fertiliser (Part II). The majority of the nitrogen in the plant tops was derived from soil nitrogen, even where fertiliser nitrogen was used most efficiently, thus emphasising the importance of the rate and pattern of release of the soil N supply. Fertilisation increased soil nitrogen uptake by up to 7 kg N ha-1.