Effects of Phosphorus-Solubilizing Bacteria and Vesicular-Arbuscular Mycorrhizal Fungi on the Growth of Tree Species in Subtropical-Tropical Soils

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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 estimatied 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 12.512.5 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
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