Natural abundance of 15N in sweet potato, pumpkin, sorghum and castor bean: possible input of N2-derived nitrogen in sweet potato

Two combinations of plant species, sweet potato (three cultivars) and pumpkin, and sweet sorghum (three cultivars) and castor bean were grown separately in three plots of alluvial soil from June to September 1996. The shoots (leaves plus stems) of sweet potato and pumpkin, and the whole tops (leaves plus stems and grains) of sweet sorghum and castor bean were harvested twice, once in August and once in September in order to analyze their natural abundance of ¹⁵N (δ¹⁵N). The δ¹⁵N values of two of the varieties of sweet potato harvested in September were significantly lower than those of pumpkin, while δ¹⁵N values of sweet potato and pumpkin harvested in August, as well as those of sweet sorghum and castor bean harvested in August and September, did not significantly differ. The lower δ¹⁵N values observed in the September-harvested sweet potato may indicate that as much as 40% of the N intake of this species is derived from dinitrogen. This species is known to have a high ability to take up N from undefined sources. Received: 23 February 1997