Dynamics of Fertilizer Nitrogen Applied to Sweet Sorghum (Sorghum bicolor (L.) Moench) in the Semi-Arid Tropics

Although sweet sorghum is in focus as a promising multi-purpose crop in semi-arid tropical regions, information on optimum nitrogen (N) management remains rather limited. To determine and understand the N dynamics in sweet sorghum-soil ecosystem, we evaluated the N recovery of applied fertilizer in the Alfisol field during the 2009 and 2010 rainy seasons. Sweet sorghum was grown under six N rates (0, 30, 60, 90, 120 and 150 kg N ha1) with split application (at 0, 30 and 60 days after sowing) and N concentration and N recovery efficiency (NRE) were measured. In addition, to determine the fate of basal and topdressed N, 15N-labeled urea was applied at each application timing within main plots of 90 kg N ha1. There was no significant difference in NRE calculated by the difference method between six N rates and N uptake rose with increasing N rate. The total dry weight and sugar yield increased at rates up to 90 kg N ha1 and higher N rates did not significantly affect sorghum productivity. As a total of basal and topdressing applications, 33% of the applied 15N-labeled urea was absorbed by the sorghum plant, while 36 and 30% remained in the soil and unaccounted for, respectively. The NRE calculated by the isotopic method was about 39 and 37% for topdressings at 30 and 60 days after sowing, respectively. The distribution ratio of absorbed N in leaves was higher for basal applications and topdressing at 30 days after sowing, while that in grains was higher for topdressing at 60 days after sowing, compared to the other application timings. In contrast to topdressings, the NRE was very low (13%) for basal N application: about 70% of the basal applied N remained in the soil at the physiological maturity. The results imply that the NRE might be increased by improving the ways in which basal N is applied. Discipline: Biofuel/Soils fertilizers and plant nutrition Additional key words: 15N-labeled urea, fertilizer application timing, nitrogen recovery efficiency This study was part of a research project titled “Development of sustainable soil fertility management for sorghum and sweet sorghum through effective use of biological nitrification inhibition (BNI)” and was funded by a grant from Japan’s Ministry of Agriculture, Forestry and Fisheries (MAFF). Present address: 5 National Agriculture and Food Research Organization (NARO), Tohoku Agricultural Research Center (Morioka, Iwate 020-0198, Japan) *Corresponding author: e-mail [email protected], [email protected] Received 22 April 2014; accepted 26 February 2015. Introduction Sweet sorghum (Sorghum bicolor (L.) Moench) is a promising multi-purpose crop, since its grain can be used as food, the stalks to produce ethanol and bagasse and the stripped leaves for fodder and fuel. Compared to grain sorghum, which has been widely grown in semi-arid tropical regions, sweet sorghum is characterized by more biomass