Accumulation and partitioning of nitrogen, phosphorus and potassium in different varieties of sweet sorghum

This study investigated changes in accumulation and partitioning of nitrogen (N), phosphorus (P), and potassium (K) with harvest dates of early, middle, and late maturity sweet sorghum varieties in 2006 and 2007 in North China. All the varieties exhibited an obvious trend of decrease in concentrations of N, P and K in aboveground plants from elongation to 60 days after anthesis (DAA). The reduction in nutrient concentrations was found in the order of K (14.5-4.5gkg^-1)>N (13.3-7.4gkg^-1)>P (2.40-0.96gkg^-1). Conversely, N, P, and K accumulation significantly increased from elongation to anthesis, and continued to increase until 40 DAA. The accumulation of N, P, and K at maturity (40 DAA) was 128-339kgha^-1, 30-75kgha^-1 and 109-300kgha^-1, respectively. Between elongation and anthesis, the middle and late maturity varieties had a higher ratio of N (50-82%), P (55-83%), and K (62-88%) accumulation than the early varieties (51-64% for N, 40-62% for P, and 55-75% for K). Sweet sorghum exhibited only one important K uptake stage from elongation to thesis according to the accumulation ratio (percentage of the nutrient accumulated at a given stage relative to that at physiological maturity) and rate (kilogram of nutrient accumulated per day per hectare). The stage from anthesis to grain maturity was the second important N and P uptake period. During the delay harvest period between 40 and 60 DAA, the early varieties exhibited significant increases in N accumulation; and the late varieties exhibited the reverse. P accumulation did not decrease significantly, whereas K accumulation decreased for all varieties in both years. Although of the N and P concentrations in straw were significantly lower than in grains, the N, P and K accumulation in straw was 2.2-9.3, 1.7-7.7, and 8.1-30.5 times higher than in grains, respectively. The concentrations of N and P in leaves were higher than in stems after anthesis. We found significantly higher accumulation of P and K in stems than in leaves, with a comparable N accumulation. The findings are helpful to make a fertilization regime recommendation for sweet sorghum production as a bioethanol crop in North China. It also suggests a further genetic improvement for optimizing nutrient use.