Nitrogen Use Efficiency of taro and sweet potato in the humid lowlands of Papua New Guinea

Root crops are an important staple food in the Pacific region. Yields are generally low and inorganic fertilizers are deemed an option to increase root crop production. The effects of inorganic N fertilizers on upland taro (Colocasia esculenta (L.) Schott) and sweet potato (Ipomoea batatas (L.) Lam.) were quantified with the aim to investigate relationships between inherent soil fertility, N uptake, N application rates and crop yield. The research took place on a sandy, Typic Tropofluvents in the humid lowlands of Papua New Guinea. Five levels of fertilizer N (0,100, 200, 300 and 400 kg ha¿1) were given in split applications. The yield of marketable taro corms was not affected by N fertilizer but non-marketable corm yield doubled at high N fertilizer rates. High N applications yielded 8¿11 Mg ha¿1 more taro tops. Marketable and non-marketable sweet potato yield was negatively affected by N fertilizers. High N applications yielded 26 Mg ha¿1 more vines than the control treatment. Nitrogen fertilizer significantly reduced the harvest index in both crops. When no fertilizer was applied, the total N uptake of taro was 32.0 kg ha¿1 of which 9.7 kg was taken up in the marketable corms. At 400 kg N ha¿1 the total N uptake was 67.5 kg ha¿1 of which 23% was taken up by the marketable corms. Uptake of N in the marketable sweet potato tubers was less than 11 kg ha¿1 and for most treatments more N was taken up in the non-marketable tubers than in the marketable yield. Up to 156 kg N ha¿1 was taken up with the sweet potato vines. Despite the negative effect of N on sweet potato yield, sweet potato had a higher N use efficiency than taro due to a higher above-ground biomass production. The N fertilizer recovery was 25% for the sweet potato but only 9% for the taro indicating considerable N losses, likely caused by leachin