Growth of sugarcane under high input conditions in tropical Australia. III. Accumulation, partitioning and use of nitrogen

Abstract

Leaf nitrogen (g N m −2 leaf) is an important determinant of crop radiation-use efficiency. It is not known to what extent sugarcane can maintain high leaf N over its long growth duration. This study analyses the accumulation of N in two contrasting cultivars of sugarcane (Q117, Q138) under plant and ratoon crop conditions, and the partitioning of N to the various plant components, including the leaf. The crops were grown for 15 months under irrigated conditions in the same season and received 34.4 g N m −2 as fertiliser over the first 100–120 days of the season. Higher early N accumulation by the ratoon crop was associated with higher early biomass production. However, maximum N accumulation was unrelated to maximum biomass accumulation, with the plant crop (25.9 g N m −2) accumulating more than the ratoon crop (21.3 g N m −2), which resulted in widely varying values for biomass/N ratio. N accumulation ceased later in the plant (200 days) than in the ratoon (150 days) crop, and this occurred 100–140 days before maximum biomass. More work is needed to determine if this is due to exhaustion of soil N supply, reduced root activity, or a lowered crop N requirement. Leaf N was maintained above 1.2 g N m −2 for 300 days of the 450 day season. Decline in leaf N below 1.2 g N m −2 at the end of the season was associated with loss in leaf and total crop N accumulation, and apparently unrelated to the timing of cessation in biomass accumulation. Throughout the season, leaf N was higher in the plant than the ratoon crop (averaged over cultivars), and Q117 versus Q138 (averaged over crop classes), however these differences could not explain crop-class differences in RUE. Cultivar differences in leaf N were due to higher N accumulation because specific leaf area and partitioning of biomass and N to leaf were unaffected by crop class or cultivar. The biomass/N ratio varied with cultivar and crop class, and increased with crop age.