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

204signifi cant correlation (P ≤ 0.001) with grain yield from PR, althoughits correlation with that from TSP was signifi cant only at P ≤ 0.05. Allthe correlation coeffi cients, although statistically signifi cant, representedweak relationships between grain yield and P use effi ciency.Nitrogen fixation in soybean genotypesNitrogen fi xation in soybean varied greatly among the genotypesand was infl uenced by P source and location (Fig. 3). However, TSPapplication improved %Ndfa signifi cantly at Shika and Davié but not atFashola; RP application did not improve %Ndfa signifi cantly at any ofthe locations. The range of %Ndfa by soybean genotypes was generallywider under low P conditions (27–53%) compared with high P (TSP)conditions (38–58%). The soybean genotypes TGm1420 and TGm1293consistently derived relatively higher %Ndfa across the locations. Also,differences between the best and poorest N fi xers within a P sourcewere signifi cant at all locations but the ranking of genotypes was notat all consistent across the locations. Strikingly, the improved genotypeTG×1456–2E had high grain yields at Fashola and Davié under lowand high P conditions, and under high P conditions at all locations, butnevertheless had relatively low %Nfda values at all locations.N nutrition and export in grainThe total N in biomass (at the R 3.5growth stages) and in grain acrosssoybean genotypes increased signifi cantly with P fertilizer application.The total N in soybean genotypes under low P conditions ranged from26.6 to 43.0 kg ha -1 at Shika, from 39.2 to 64.9 kg ha -1 at Fashola, andfrom 41.4 to 81.4 kg ha -1 at Davié (Table 4). Increases in biomass totalN in response to RP and TSP ranged from 51 to 171% at Shika, from–2 to 36% at Fashola, and from 17 to 48% at Davié. Nitrogen uptakein grain of soybean genotypes TGm1566 and TG×1456–2E wassignifi cantly higher than in TGm1251 at all P levels at Shika, but a fewsoybean genotypes (e.g., TG×1456–2E, TGm1566, and TGm0944)demonstrated the capacity to improve their N nutrition with RP as Psource at Shika and Davié. None of the genotypes demonstrated thiscapacity at Fashola.The total N exported in grain of soybean genotypes was greaterthan that derived from the atmosphere (Ndfa) in most genotypes,resulting in negative N balances in all soybean plots (Fig. 3) exceptat Davié. There, three soybean genotypes (TGm1420, TGm0944,and TGm1360) gave positive N balances in all the P treatments. Thenegative N balances were generally highest at Fashola, with or withoutP application, but were signifi cantly increased with P application acrosssoybean genotypes at Shika. Surprisingly, the improved soybeangenotype TGm1456–2E was among those with the highest negativeN balances in all three locations (Fig. 3).