Growth responses of cereal crops to organic nitrogen in the field

Okamoto M, Okada K, Watanabe T, Ae N

SOIL SCIENCE AND PLANT NUTRITION 49 (3): 445-452 JUN 2003

Abstract: Growth responses of rice (Oryza sativa), sorghum (Sorghum bicolor), pearl millet (Pennisetum glaucum), and maize (Zea mays) to applications of the organic form of nitrogen (N) were examined under field conditions in 2000-2001. Nitrogen was applied at the rate of 15 g m^-2 in the form of inorganic N (IN, urea) or organic N (ON, a mixture of rice bran and straw). Organic N application had no significant effect on the concentration of soil inorganic N (ammonium and nitrate), but increased the concentration of neutral phosphate-buffer-extractable organic N in the soil at the early growth stage. The shoot dry weight and N uptake of crops in the ON treatment were similar to those in the IN treatment for pearl millet and maize, but were significantly higher than in the IN treatment for rice and sorghum at the early growth stage. The higher proportion of organic N in the xylem sap of rice and sorghum in the ON treatment compared with the IN treatment also suggests that these two crops could take up the increased amount of soil organic N more efficiently than could maize, for which there was no difference between the treatments. Although pearl millet showed similar results to those of rice and sorghum in xylem sap analysis, the utilization of absorbed organic N was considered inefficient because of the adequate N status in the plant, based on the high concentration of nitrate N in the plant tissues. This is the first study to demonstrate the possibility that sorghum can utilize organic N in the soil more efficiently than can pearl millet. The possible forms of organic N in the soil absorbed by crop roots, such as protein and 8,000-9,000 Da molecular weight compounds, are discussed.

Keywords: inorganic nitrogen; organic nitrogen; pearl millet (Pennisetum glaucum); sorghum (Sorghum bicolor); xylem sap