N_2 fixation of nodules and N absorption by soybean roots associated with ridge tillage on poorly drained upland fields converted from rice paddy fields(Soil Chemistry and Mineralogy)

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Sowing on elevated ridges is effective in reducing wet injury of soybean plants cultivated in upland fields converted from rice paddy fields. Therefore, we investigated the effect of ridge tillage (RT) on soybean N accumulation properties. We compared the amounts of plant N associated with N_2 fixation of nodules and from soil and fertilizer in the RT treatment with amounts in conventional tillage (CT) in two fields in 2002-2003. Both fields were upland fields converted from rice paddy fields (Typic Hydraquents). The main difference between the fields was the presence of a field underdrain. The amounts of Rb and K accumulated in the shoots were also determined to estimate soybean root distribution. The grain yields with RT increased in both fields from 106% to 129% compared with CT. Increased pod number and seed weight were the major factors responsible for the yield increase. ANOVA indicated that RT significantly increased the activities of both N_2 fixation of nodules and N absorption by roots until R1 (flowering stage). The ratio of Rb and K accumulated in the shoots indicated that with RT, the root distribution was more abundant in the superficial layers compared with CT. Thus, RT reduced wet injury during the rainy season that over-lapped the flowering stage. Nitrogen accumulation from N_2 fixation until the R7 stage with RT was significantly higher than that with CT. We concluded that RT was effective in increasing N_2 fixation of nodules in poorly drained upland fields converted from rice paddy fields.

N_2 fixation of nodules and N absorption by soybean roots associated with ridge tillage on poorly drained upland fields converted from rice paddy fields(Soil Chemistry and Mineralogy)

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