Demand-Driven Technologies for Sustainable Maize ... - IITA

218The observed trends in grain yield, VAM colonization, total N andP in grain of maize-after-soybean across the locations, under bothlow and high P conditions, did not demonstrate a consistent anddiscernible pattern of genotype-specifi c rotation effects, even thoughmean increases as high as 97% in grain yield, 77% in grain N, and57% in grain total P were recorded in certain previous combinationsof P source, soybean genotype, and location. The increases measuredin parameters of maize-after-soybean support results of previousagronomic work conducted in the Guinea savanna zones of Nigeria.These results revealed that the yield of maize could be improved inrotation systems with soybean (Kaleem 1993; Carsky et al. 1997;Sanginga et al. 2002) as in other similar cereal–legume rotationsystems (Bagayoko et al. 2000a,b) where the yields of cereals afterlegumes were increased by as much as 80% over yields obtained forcereals grown after cereals. Similarly, we recorded reductions in grainyield, and total grain N and P with certain combinations of P source,genotype, and location as reported in some previous work (e.g., Kamhet al. 2002) on similar legume–cereal rotation systems.The increases in growth parameters of cereal following legumeshave usually been attributed to N benefi ts to the cereal throughN 2fi xation and N sparing effects (Sanginga et al. 2002) and otherrotation effects (Alvey et al. 2000; Bagayoko et al. 2000a, 2000b;Marschner and Baumann 2003). A conservative estimation of thecontribution of BNF to the soil N pool for use by the nonlegume,designated as N balance (Peoples and Craswell 1992), was used topredict the contribution of N by soybean to the cereal crop and alsopartially to account to for yield increases observed in the subsequentcereal crop. The N balances estimated in this study were generallynegative, except in few instances (e.g., TGm0944, TGm1420, andTGm1360 at Davié). The negative N balances are due to relative lowproportions of N derived from BNF (Fig. 3) and the high N contentof the soybean grain. However, this trend did not always result indecreases in grain yield and the total N exported in grain of maizeafter-soybean.This may underscore the limitation of using the Nbalance method of Peoples and Craswell (1992) as an index of Ncontribution by legumes to cereals in rotation as it does not accountfor the accumulation of fi xed N in the aboveground biomass afterthe R 3.5growth stage and ignores the belowground biomass. It mightalso be that rotational effects are more important and differ amongvarieties, independent of their N balance.We can infer from our results that residual P from the previousapplication to soybean was not adequate to meet the P demand ofmaize in rotation at Shika and Fashola, as the addition of 15 kg P ha -1resulted in enhanced maize grain yields at these locations. The low P