Nitrogen transfer and dry matter production in soybean and sorghum mixed cropping system at different population densities

Field experiments were conducted to examine the effects of mixed cropping of soybean (Glycine max L. cv. Kurosengoku) and sorghum (Sorghum bicolor Moench cv. Yukijirushi) on dry matter production, dinitrogen-fixing activity of soybean nodules and possible nitrogen transfer from soybean to sorghum at four spacing levels, namely 12.5 × 12.5 cm; 17.7 × 17.7 cm; 25 × 25 cm; and 50 × 50 cm. Dry matter production was consistently greater in mixed cropping than in mono-cropping, mainly due to the enhancement of sorghum growth regardless of spacing. Dinitrogen-fixing activity estimated by the acetylene reduction method in soybean decreased with the increase in the plant population density and with mixed cropping. Sorghum growth was closely related to the N accumulated in the plant. N-transfer from soybean was evident and was greatest at a 12.5 × 12.5 cm spacing. N-transfer from soybean to sorghum calculated on the assumption that there is no difference in the absorption ability of soil nitrogen between sorghum and soybean accounted for 32 to 58% of the total nitrogen in sorghum. Although the dinitrogen-fixing activity was reduced at a 12.5 × 12.5 cm spacing, N-transfer was greatest at this spacing. This finding suggests that the relative proximity of the root systems of associated grass/legume is essential for ensuring N-transfer from the legume to the grass. N-transfer becomes significant when other N sources are limited.