Nitrogen fixation and N contribution by promiscuous nodulating soybeans in the southern Guinea savanna of Nigeria

Attention is being paid to improving the N2 fixation of promiscuous nodulating soybeans in an attempt to develop sustainable cropping systems in the moist savanna. There is however a dearth of reliable estimates of N2 fixation by these promiscuous soybeans and hardly any quantitative information is available on their residual N benefits to subsequent cereal crops grown in the southern Guinea savanna zone. The15 N isotope dilution method was used to assess symbiotic N2 fixation and response to inoculation and N contribution of three IITA promiscuous and two Brazilian soybean lines grown in the field at Mokwa (southern Guinea savanna) for two years. Rhizobial inoculation increased total N and grain yield of early maturing cultivars IAC 100 and TGX 1456–2E but did not affect the late maturing cultivar TGX 1660–19F. Both fixed N (Ndfa) and N derived from the soil were the major sources of N accounting for 84 and 75 kg N ha-1 or 46 and 43%, respectively, of the plant total N. A line effect was, however, apparent with the late maturing line TGX 1660–19F deriving on the average 126 kg N ha-1 or 52% of plant total N from N2 fixation compared to the early maturing line IAC 100 with 37 kg N ha-1 or 38%. Total N accumulated and amounts of N2 fixed were low during early growth (V2/V3 and R1/R2 stages), but increased rapidly after this period to reach the maximum at R3/R4 and then dropped after R3/R4. The proportion of Ndfa, however, increased with the growing period. At the physiological maturity (R8), N2 fixed accounted for an average of 70% of total N accumulated in the seeds. Roots accumulated about 13% while leaves and stems had 53 and 32% of the entire plant N at R3/R4, respectively. It was estimated after grain removal, that soybean growth led to a net contribution of an average of 18 kg N ha-1 to soil N. However, the N contribution ranged from - 8 to 43 kg N ha-1 depending on the soybean cultivars and inoculation treatment.