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

199of BNF to the N balance of the soil was estimated at soybean poddingstage (R 3.5) using the equation proposed by Peoples and Craswell(1992) as follows:Net N balance = Nf– N gwhere N fis the amount of N fi xed (total N in biomass at R 3.5× %Ndfa)and N grepresents the total N in grain at harvest.Grain yield and total N and P exported in grain were measured atharvest maturity stages of the crops. Subsamples of harvested plantmaterials (shoot and grain) were oven dried at 65 o C for 7 days fordry weight determination. The dried samples were then ground andanalyzed for total N and P by hot acid digestion (Novozamsky et al.1983). Colorimetric analysis was done on a Technicon autoanalyzer,using the indophenol blue method (Searle 1984) for N and theascorbic acid method (Murphy and Riley 1962) for P. Soybeangenotypes were classifi ed into P use and response effi ciency groupsas follows: effi cient responder (high yield at both low and high P),ineffi cient responder (low yield at low P and high yield at high P),effi cient nonresponder (high yield at low P but low yield at high P),and ineffi cient nonresponder (low yield at both low and high P) basedon grain yield at low and high P, as described by Gerloff (1977).Low yield refers to grain yields that were signifi cantly lower than theoverall mean of low P treatments. High yield refers to grain yields thatwere signifi cantly higher than the overall mean of high P treatments.Genotypes with grain yields not signifi cantly different from the overallmean were classifi ed as intermediate. Phosphorus accumulation inbiomass, P export in grain, VAM colonization of roots, P utilizationquotient (grain yield/total P in grain), and relative P response effi ciency(grain yield at high P/grain yield at low P) were used to assess the useof grain yield as a suitable criterion for P effi ciency and to explain themechanisms underlying P effi ciency in soybean.Residual effects of the soybean genotypes and P applied in 2001and 2002 on the subsequent maize crop were measured in 2002 and2003. In 2003, the previous 0 P plots were divided into two; one halfreceived 15 kg P ha -1 (15P) as a fresh addition of TSP while the otherhalf did not. Weeds that had grown in the previous crop cycle duringthe fallow period were carefully cleared with a cutlass, ensuring thatremaining legume residues were not dislodged from their originalplots. Soil samples (0–10 cm) from the individual soybean and maizeplots were taken before maize was planted and analyzed for pH,Olsen P content, and P fractions. Four seeds of maize (Oba Super I)were sown at 75 cm × 25 cm spacing within the previous soybean andmaize plots; the seedlings were later thinned to one/stand at 2 WAP.