Phosphorus Uptake and Balance in a Soyabean-Maize Rotation in the Moist Savanna of West Africa

This study investigated the effect of maturity class on phosphorus (P) balance in a soyabean-maize rotation in the West African moist savanna.F our soyabean varieties of different maturity classes were grown with or without P fertilizer followed by a maize crop. Soyabean stover P content averaged 1.77 kg ha−1 compared with 5.13 kg ha−1 in the grain. The late soyabean variety TGx1670-1F accumulated a significantly higher P in the grain (6.56 kg ha−1), and stover (2.57 kg ha−1) than the others. While P harvest index averaged 79%, P application increased grain P by 63-81% and stover P by 100% or more. When either soyabean grain or grain+stover was exported, P balance was negative and was not statistically different for varieties when no P was applied. At 30 or 60 kg P ha−1, P balance was negative but significantly lower in TGx1670-1F compared with other varieties. In creasing P rate applied to soyabean significantly (p 0.01) increased maize grain P by 35-66% in the second year. When P was exported only in soyabean grain, cumulative P balances after maize grain harvest (with no P or 30 kg ha−1 applied previous year) were not significantly different for previous soyabean crops. At 60 kg ha−1, however, P balance in previous TGx1670-1F plot was significantly lower than for other varieties. A further export of soyabean stover reduced P balance. Significant residual P effect was observed emphasizing the need to focus P fertilizer application in the cropping system rather than on the single crop. Also with more P in soyabean grain, a reduction in the extent of P depletion will be achieved by returning soyabean stover to the field after threshing

Arbuscular mycorrhizal fungi (AMF) respond to rhizobial inoculation and cropping systems in farmers fields in the Guinea savanna

It has been difficult to explain the rotation effect based solely on N availability in maize-soybean cropping systems in the moist savanna zone of sub-Saharan Africa. Although arbuscular mycorrhizal fungi (AMF) can contribute to plant growth by reducing stresses resulting from other nutrient deficiencies (mainly P) and drought, their role in the maize/soybean rotation cropping systems in the Guinea savanna has not yet been determined. Pot and field experiments were conducted for 2 years using 13 farmers' fields with different cropping histories in two agroecological zones (Zaria, northern Guinea savanna and Zonkwa, southern Guinea savanna) in Nigeria. We quantified the influence of cropping systems and rhizobial inoculation on plant growth, mycorrhizal colonization and diversity of promiscuous soybean and maize grown in rotation. The relationships between these variables and selected soil characteristics in farmers' fields were also examined. Percentage mycorrhizal colonization in promiscuous soybean roots ranged from 7% to 36%, while in maize it varied between 17% and 33%, depending on fields and the previous cropping history. A large variation was also observed for mycorrhizal spores, but these were not correlated with mycorrhizal colonization and did not appear to be influenced by rotation systems. Soybean mycorrhizal colonization was higher (13% increase) in Zonkwa, but not in Zaria, if the preceding crop was maize and not soybean. These differences were related to the soil P concentration, which was positively related to mycorrhizal colonization in Zonkwa but negatively to this parameter in Zaria. The previous crop did not affect mycorrhizal colonization of maize in both locations. Soybean cultivars inoculated with rhizobia had a higher mycorrhizal colonization rate (25%) and more AMF species than maize or uninoculated soybean (19%). Maize grown in plots previously under inoculated soybean also had higher percentage mycorrhizal colonization than when grown after uninoculated soybean and maize. Four AMF genera comprising 29 species were observed at Zaria and Zonkwa. Glomus was the dominant genus (56%) followed by Gigaspora (26%) and Acaulospora (14%). The genus Sclerocystis was the least represented (4%

Soybean maturity and environmental effects in savanna systems; I: dry matter accumulation

Growing soybean varieties with high residue yield may help to sustain the soil organic matter (SOM) content when recycled. Replicated field trials were conducted in four Guinea savanna sites in Nigeria to study the dry matter partitioning in six soybean genotypes and evaluate them for biomass production. The varieties were early TGx1485-1D and late TGx1670-1F in Trial 1 and early TGx1485-1D, TGx1805-2E and TGx1681-3F, medium TGx1809-12E and late TGx923-2E and TGx1670-1F in Trial 2. On average, the proportion of total dry matter accumulated in soybean plant parts was 42% in grain, 36% in stover, 12% in leaf litter and 11% in roots and nodules. While maturity class had no significant effect on the grain yield of soybean, significantly higher dry matter accumulation of roots and nodules, leaf litter, and stover was observed in medium and late varieties compared with early varieties (P < 0.05). Thus, medium and late varieties would be better able to sustain the SOM content than early varieties when the residues are recycled. Strongly acid soils in a high rainfall environment limited soybean biomass production and potential to maintain SOM

Effect of soybean on subsequent maize grain yield in the Guinea savanna zone of West Africa

Two varieties of soybean (Glycine max (L.) Merr.) and a maize (Zea mays L.) control crop were grown in replicated trials at 10 sites in the Guinea savana of Nigeria in 1993 followed by a test crop of maize in 1994 with 20 or 60 kg N ha-1 to test effect of soybean on subsequent maize grain yield. Maize grain yield increase following soybean was variable but the main effect of previous soybean crop was positive even though the soybean was not inoculated with rhizobia and the aboveground soybean residues, except litter fallen before harvest, were exported from the fields, following the present farmer practice. The yield increase following the medium duration soybean variety (TGx1660-19F) was similar to that from 40 kg N ha-1 applied 4 weeks after planting to maize preceded by maize. The increase following the early soybean variety (TGx1456-2E) was significantly (P<0.05) smaller than that following the medium duration soybean variety. Total N in the 0 to 10 cm depth of the previous TGx1660-19F plots (0.063%) was significantly greater (P<0.05) than in the previous maize plots (0.058%) when analysed for the combined sites. Thus, the effect of previous soybean crop on maize grain yield was due mostly to residual N availability. Additional research is justified to estimate how soybean crop management influences the yield of subsequent maize in the savanna zone.Deux varietes de soya (Glycine max (L) Merr.) et une varietede mas (Zea mays L.) en temps que temoin ont ete cultivees en essais repliques sur 10 sites dans la savane Guineenne du Nigeria en 1993. Ces cultures ont ete suivies d'une culture-test de mais en 1994 avec 20 ou 60 kg N ha-1. L'augmentation de la production en graine de mais precedee d'une culture de soya par rapport a la production du mais precedee d'une culture de mais etait variable. L'effet principal de la culture de soya etait positif, bien que le soya n'ait pas ete inocule et que le residu du soya aerien, a l'exception du dechet tombe avant la recolte, ait ete exporte des champs a l'example des pratiques courantes. L'augmentation de la recolte de mais precedee de la variete de soja a duree intermediaire (TGx1660-19F) etait semblable a l'augmentation obtenue par application de 40 kg N ha-1 sous forme d'ureee 4 semaines apres la plantation de mais precede par du mais. L'augmentation suivant la variete de soya precoce (TGx1456-2E) etait significativement plus faible (P<0.05) que celle suivant la varietede duree intermediaire. L'azote total une profondeur de 0-10cm dans les parcelles de TGx1660-19F (0.063%) etait significativement plus elevee (P< 0.05) que dans les parcelles temoin (0.058%) lorsque l'anae est realisee tous sites confondus. Par consequent, l'effet d culture de soya sur la recolte posterieure de mais resulte principalement de la disponibilite residuelle en azote. Des recherches additionnelles sont justifiees pour estimer comment la conduite de la culture du soja influence la recolte posterieure de mais dans la zone de savane Guineenne

Phosphorus recovery from triple superphosphate by soyabean in the Guinea savanna of Nigeria

Phosphorus fertilizer was applied at two rates to four soyabean varieties of different duration to maturity at four sites. The P rate had no significant (p > 0.05) interaction with site or variety. TGx1670-1F, one of the two late varieties, had the highest levels of total P (11 kg/ha) and apparent fertilizer P recovery (15%)

Reduction of Striga hermonthica parasitism on maize using soybean rotation

Published online: 26 Nov 2010Striga hermonthica is a serious parasite of cereals in most of the semi-arid savanna zone of West Africa, causing substantial yield loss. It has been observed that some soybean cultivars are capable of stimulating germination of S. hermonthica seed, which would reduce the seed bank in the soil. This study was undertaken to quantify the effect of a soybean crop (compared with a sorghum control) on S. hermonthica emergence in subsequent maize in three farmers' fields in northern Nigeria. Soybean cultivar TGx 1740-7F, previously identified as efficacious for S. hermonthica seed germination, was grown at four densities without P fertilizer to test the effect of increasing plant density of soybean on subsequent S. hermonthica parasitism. The effect of P as single super phosphate was tested on the two highest soybean densities. S. hermonthica parasitism on maize was significantly lower after unfertilized soybean than after the sorghum control treatment at two of three trial sites. Soybean rotation increased maize yield by approximately 90% for the three sites combined. Increasing soybean plant density did not result in lower emerged S. hermonthica. Application of P to soybean at the higher soybean densities resulted in higher root length density, lower emerged S. hermonthica on maize (P<0.15), and significantly higher maize yield. The results suggest that an efficacious cultivar of soybean reduces S. hermonthica parasitism on a succeeding maize crop and that the effect is increased by application of P to the soybean

Persistence and effectiveness of rhizobia nodulating promiscuous soybeans in moist savanna zones of Nigeria

The symbiotic performance of promiscuous soybeans depends upon the population size, effectiveness and survival of indigenous or introduced rhizobia in the field. A pot experiment was conducted using soils collected from 13 farmers' fields located in two agroecological zones (northen and southern Guinea savanna) in the moist savanna of Nigeria to determine the relationships between growth response to previous rhizobial inoculation and the indigenous rhizobial populations. At each farmer's field, soil was collected in plots which were planted the previous year (1993) to: (1) maize, (2) soybean cv. Bossier, (3) soybean cv. TGX 1456-2E and (4) soybean cv. TGX 1660-19F. The soybean cultivars TGX 1660-19F and TGX 1456-2E were either uninoculated or inoculated with an enriched population of a mixture of local rhizobial strains. Four test plants: soybean cultivars TGX 1660-19F (slightly promiscuous), TGX 1456-2E (highly promiscuous), Bossier (non-promiscuous) and cowpea (typically promiscuous) were planted in pots containing soils from the above field treatments. Previous inoculation increased shoot dry matter production by an average of 32% over the uninoculated controls in 94 of the 312 (30%) legume inoculations and farmers' field combinations while the indigenous rhizobia were more effective than the introduced ones in 103 combinations (33%) with an average of 20% increase compared to previous inoculation treatments. Previous inoculation increased biomass yield of both promiscuous and non-promiscuous soybean varieties. The response to previous inoculation treatments was farmers' fields dependent and inversely related to the numbers of rhizobia in the soil. Soil rhizobial population ranged from 0 to > 400 cells g−1 soil and response to inoculation often occurred when numbers of indigenous rhizobia were fewer than 10 cells g−1 soil. Numbers of indigenous rhizobia were generally lower or below detection limit in soils previously cropped to maize. These results indicate a relationship between rhizobia cell counts and promiscuous soybean responses, which may be used to indicate under which conditions inoculation will be beneficial to a farmer