N. Sanginga

Integrated Soil Fertility Management: Operational Definition and Consequences for Implementation and Dissemination

Traditional farming systems in Sub-Saharan Africa depend primarily on mining soil nutrients. The African green revolution aims to intensify agriculture through the dissemination of integrated soil fertility management (ISFM). This paper develops a robust and operational definition of ISFM based on detailed knowledge of African farming systems and their inherent variability and of the optimal use of nutrients. The authors define ISFM as a set of soil fertility management practices that necessarily include the use of fertilizer, organic inputs and improved germplasm, combined with the knowledge on how to adapt these practices to local conditions, aimed at maximizing agronomic use efficiency of the applied nutrients and improving crop productivity. All inputs need to be managed in accordance with sound agronomic principles. The integration of ISFM practices into farming systems is illustrated with the dual-purpose grain legume–maize rotations in the savannas and fertilizer micro-dosing in the Sahel. Finally, the dissemination of ISFM practices is discussed.

Biological nitrogen fixation in trees in agro-ecosystems

The integration of trees, especially nitrogen fixing trees (NFTs), into agroforestry and silvo-pastoral systems can make a major contribution to sustainable agriculture by restoring and maintaining soil fertility, and in combating erosion and desertification as well as providing fuelwood. The particular advantage of NFTs is their biological nitrogen fixation (BNF), their ability to establish in nitrogen-deficient soils and the benefits of the nitrogen fixed (and extra organic matter) to succeeding or associated crops.The importance of NFTs leads to the question of how we can maximise or optimize their effects and how we can manage BNF and the transfer of nitrogen to associated or succeeding plantings. To be able to achieve these goals, suitable methods of measuring BNF in trees are necessary. The total nitrogen difference (TND) method is simple, but is better suited for low than high soil N conditions. The acetylene reduction assay (ARA), although sensitive and simple, has many technical limitations especially for NFTs, and the estimates of BNF have generally been very low, compared to other methods. For NFTs, the 15N techniques are still under development, but have already given some promising results (e.g., has been used to measure large genetic variability in BNF within different NFTs).Various factors affect BNF in trees. They include the age of trees, the microbial component, soil moisture, temperature, salinity, pH, soil N level and plant nutrient deficiencies. Some of the factors, e.g. temperature, affect the symbiosis more than plant growth, and differences in the effects of these factors on BNF in different NFT genotypes have been reported. These factors and research needs for improving BNF in trees are discussed.

Phosphorus use efficiency and nitrogen balance of cowpea breeding lines in a low P soil of the derived savanna zone in West Africa

Differences in growth, nodulation and arbuscular mycorrhizal fungi (AMF) root infection among recent cowpea breeding lines from IITA were examined at low and high P levels in pot (94 lines) and field experiments (43 lines) at Fashola in the derived savanna zone of Nigeria. Based on their growth performance, these lines were subdivided into 5 groups: (i) poor performance under low and high P conditions; (ii) good performance under low P and poor performance under high P; (iii) intermediate performance under high and low P; (iv) good performance under high and low P conditions; and (v) good performance under high P and poor performance under low P. About 42% of the breeding lines (18 out of 43 lines tested) had the same grouping for the field and pot experiments. Eight cowpea lines (4 P-responders and 4 non-P-responders) were selected from the first experiment for subsequent studies on the effect of P supply (0, 20, 40 and 60 kg P ha-1) on P uptake, P use efficiency, dry matter production, N-fixation, AMF infection and N balance. Dry matter production, shoot/root ratio, total shoot N, and total N-fixed of the non-P-responder line, IT81D-715, were strongly related to P uptake efficiency. The P-responder IT81D-849 had a significant (95%) correlation between AMF and P-use efficiency. The cowpea lines fixed on average 22 kg N ha-1, which was 70% of the plant total N. The N balance based on the difference between the amount of N2 fixed and N exported through the harvest, ranged between −10.6 kg N ha-1 and +7.7 kg N ha-1. Based on its adaptability to grow in low P soils and overall positive N balance, the cowpea line IT81D-715 should be recommended for cultivation when P is the limiting factor.

Impact of residue quality on the C and N mineralization of leaf and root residues of three agroforestry species

A laboratory incubation experiment with 15N labeled root and leaf residues of 3 agroforestry species (Leucaena leucocephala, Dactyladenia barteri and Flemingia macrophylla) was conducted under controlled conditions (25 C) for 56 days to quantify residue C and N mineralization and its relationship with residue quality.No uniform relation was found between the chemical composition of the above and below residues. The leucaena and dactyladenia roots contained more lignin (8 and 26% respectively) and less N (2.0 and 1.0% respectively) than the respective leaves (2 and 13% lignin and 2.9 and 1.4% N, respectively), whereas the differences between the lignin and N contents of the flemingia leaves and roots were not significant (4.6 and 3.0% lignin and 2.63 and 2.68% N, respectively). The leucaena leaves contained more polyphenols than the roots (6.4 and 3.6%), while the polyphenol content of the leaves and roots of the other residues was similar (5.0 and 5.1% for dactyladenia and 4.0 and 3.5% for flemingia).Three patterns of N mineralization could be distinguished. A first pattern, followed by residues producing the highest amounts of CO2, showed an initial immobilization of soil derived N, followed by a net release of both soil and residue derived N after 7 days of incubation. A second pattern, followed by the flemingia leaf residues which produced intermediate amounts of CO2 and had an intermediate quality, showed no significant immobilization of soil derived N, and significant mineralization of residue N. A third pattern, followed by both low quality dactyladenia residues, showed a low release of residue derived N and a continued inmobilization of soil derived N.Residue C mineralization was significantly (p<0.05) correlated with the residue lignin content, C-to-N ratio, and polyphenol-to-N ratio. The proportion of residue N mineralized (immobilized) after 56 days of incubation was significantly correlated with the residue N content (p<0.01) and the C-to-N ratio (p<0.05). The relations were quadratic, rather than linear. The ratio of the proportion of residue N mineralized (immobilized) over the proportion of residue C mineralized after 56 days was highly significantly correlated with the lignin content (p<0.01) and C-to-N (p<0.001), lignin-to-N (p<0.01), polyphenol-to-N (p<0.01) and (lignin+polyphenol)-to-N ratios (p<0.01) in a linear way. This indicates that due to the low availability of the residue C, relatively less N is immobilized for the very low quality residues ((lignin+polyphenol)-to-N ratio: 29.7) than for the residues with a relatively higher quality ((lignin+polyphenol)-to-N ratios between 3.3 and 12.5).

Management of biological N2 fixation in alley cropping systems: estimation and contribution to N balance.

Alley cropping is being widely tested in the tropics for its potential to sustain adequate food production with low agricultural inputs, while conserving the resource base. Fast growth and N yield of most trees used as hedgerows in alley cropping is due greatly to their ability to fix N2 symbiotically with Rhizobium. Measurements of biological N2 fixation (BNF) in alley cropping systems show that some tree species such as Leucaena leucocephala, Gliricidia sepium and Acacia mangium can derive between 100 and 300 kg N ha-1 yr−1 from atmospheric N2, while species such as Faidherbia albida and Acacia senegal might fix less than 20 kg N ha-1 yr-1. Other tree species such as Senna siamea and S. spectabilis are also used in alley cropping, although they do not nodulate and therefore do not fix N2. The long-term evaluation of the potential or actual amounts of N2 fixed in trees however, poses problems that are associated with their perennial nature and massive size, the great difficulty in obtaining representative samples and applying reliable methodologies for measuring N2 fixed. Strategies for obtaining representative samples (as against the whole tree or destructive plant sampling), the application of 15N procedures and the selection criteria for appropriate reference plants have been discussed.Little is known about the effect of environmental factors and management practices such as tree cutting or pruning and residue management on BNF and eventually their N contribution in alley cropping. Data using the 15N labelling techniques have indicated that up to 50% or more of the trees N may be below ground after pruning. In this case, quantification of N2 fixed that disregards roots, nodules and crowns would result in serious errors and the amount of N2 fixed may be largely underestimated. Large quantities of N are harvested with hedgerow prunings (>300 kg N ha-1 yr-1) but N contribution to crops is commonly in the range of 40–70 kg N ha-1 season. This represents about 30% of N applied as prunings; however, N recoveries as low as 5–10% have been reported. The low N recovery in maize (Zea mays) is partly caused by lack of synchronization between the hedgerow trees N release and the associated food crop N demand. The N not taken up by the associated crop can be immobilized in soil organic matter or assimilated by the hedgerow trees and thus remain in the system. This N can also be lost from the system through denitrification, volatilization or is leached beyond the rooting zone. Below ground contribution (from root turnover and nodule decay) to an associated food crop in alley cropping is estimated at about 25–102 kg N ha-1 season-1. Timing and severity of pruning may allow for some management of underground transfer of fixed N2 to associated crops. However many aspects of root dynamics in alley cropping systems are poorly understood. Current research projects based on 15N labelling techniques or 15N natural abundance measurements are outlined. These would lead to estimates of N2 fixation and N saving resulting from the management of N2 fixation in alley cropping systems.

Evaluation of symbiotic properties and nitrogen contribution of mucuna to maize grown in the derived savanna of West Africa

The severity and increase of the Imperata cylindrica constraint as a weed, the decline of the traditional fallow systems as a means of soil fertility management and the lack of inorganic fertilizer appear to have created opportunities for adoption of mucuna (Mucuna pruriens) technology by smallholder farmers in some areas in the derived savanna of West Africa. What is not known, however, is the extent to which the establishment and N contribution of mucuna in these areas depend on symbiotic properties such as effective nodulation and mycorrhizal infection. Short term surveys carried out in 34 farmers arable fields located in four different sites in the derived savanna, southern Benin, West Africa, together with results of greenhouse and field experiments showed that mycorrhizal infection rate of mucuma ranged from 2 to 31% and correlated positively with nodulation and shoot dry matter production. Nodulation occurred in 79% of the fields with numbers of nodules ranging from 0 to 135 plant−1. Mucuna responded both to inoculation and N fertilizer in degraded soils but growth response depended on the rhizobia strains and mucuna varieties. Mucuna accumulated in 12 weeks about 313 kg N ha−1 as either a sole crop or 166 kg N ha−1 when mixed/intercropped with maize, respectively. Across all cropping systems it derived an average of 70% of its N from atmospheric N2 (estimates made by the 15N isotope dilution method), representing 167 kg N ha−1 per 12 weeks in the field. Mucuna interplanted with maize obtained a greater proportion of its nitrogen (74%) from fixation than did mucuna grown alone (66%) suggesting that competition for soil N influences the proportion of nitrogen fixed by mucuna. The total amount of N2 fixed per hectare was, however, reduced significantly by intercropping mucuna with maize. A preceding mucuna crop provided a maize yield equivalent to 120 kg N kg ha−1 of inorganic N fertilizer.

Utilization of rock phosphate by crops on a representative toposequence in the Northern Guinea savanna zone of Nigeria : response by Mucuna pruriens, Lablab purpureus and maize

The availability of P from rock phosphate (RP) is often too low to demonstrate an immediate impact on cereal production. Legumes may improve the immediate availability of P from RP and eventually benefit subsequent maize crops. The ability of Mucuna pruriens (L.) var utilis (Wright) Burck and Lablab purpureus L. to use P from RP and the changes in selected plant and symbiotic properties and in the soil available P and particulate organic matter (POM) pool as affected by the addition of RP were measured for a set of soils on a representative toposequence (‘plateau’, ‘slope’ and ‘valley’ field) in the Northern Guinea savanna zone of Nigeria. At 18 weeks after planting (WAP), Mucuna accumulated significantly more N and P in the total biomass in the plots treated with RP compared to the plots without RP addition on all fields. Nitrogen accumulation of Mucuna reached 175, 177 and 164 kg N ha 21 in the treatments with RP on the ‘plateau’, ‘slope’ and ‘valley’ fields, respectively. Phosphorus accumulation of Mucuna was highest at 18 WAP in all sites and reached 10, 14 and 10 kg P ha 21 in the treatments with RP on the ‘plateau’, ‘slope’ and ‘valley’ fields, respectively. Lablab accumulated significantly more N and P at 18 WAP only on the ‘plateau’ field, but some of the potential differences in N or P accumulation may have been masked by various pests especially affecting Lablab. A highly significant negative correlation was observed between the aboveground biomass at 16 WAP and the nematode population. The addition of RP significantly increased arbuscular mycorrhizal fungi (AMF) infection of the Mucuna (from 24 to 33%) and Lablab roots (from 15 to 28%) to a similar extent in all fields. This increased AMF infection was most likely caused by specific processes in the rhizosphere of the legumes as AMF infection of the maize roots (8%) was not affected by RP addition. Increases in nodule numbers and fresh weight were site- and species-specific and highest for the ‘plateau’ and ‘slope’ fields. The number of nodules increased on average from 8 to 19 (3 plants) 21 and from 7 to 30 (3 plants) 21 for Mucuna and Lablab, respectively, after RP addition. Although nearly all the aboveground legume biomass had disappeared from the soil surface at 51 WAP, both the Olsen-P status and POM N concentration were increased by the presence of legumes. Mucuna significantly enhanced the Olsen-P content of the soil after RP addition compared to the Lablab or maize treatments on the ‘plateau’ and ‘valley’ fields. Due to the relatively high initial Olsen-P content of the ‘slope’ field (14 mg kg 21 ), differences between treatments were not significant. The N concentration of the POM pool was significantly higher under legumes than under maize on the ‘slope’ and ‘valley’ fields, and indicates incorporation of part of the legume biomass in the POM pool. The addition of RP to herbaceous legumes was observed to lead to site- and species-specific changes in the tripartite legume‐rhizobium‐ mycorrhizal fungus, driven by processes taking place in the rhizosphere of the legumes, and in the soil available P pool. A cereal following these herbaceous legumes could benefit from this improvement in soil fertility status. q 2000 Elsevier Science Ltd. All rights reserved.

Effectiveness of rhizobia nodulating recent promiscuous soyabean selections in the moist savanna of Nigeria.

Abstract The ability of soybean to nodulate with a wide range of indigenous bradyrhizobia has been used in a breeding programme since 1997 in Nigeria. As far as is known, these indigenous bradyrhizobia strains have not been tested for compatibility and effectiveness with recent selections from a breeding programme which has proceeded without input from soil microbiologists for the last 20 yr. Twenty bradyrhizobia strains isolated from soyabean and cowpea grown in Ibadan and Zaria soils in Nigeria were examined in a pot experiment for symbiotic effectiveness on two promiscuous soyabean breeding lines (TGX 1660-19F and TGX 1456-2E) and a cowpea cultivar (IT 849-92). Two bradyrhizobial isolates (R25B and IRj 2180A) had an average symbiotic effectiveness (SE) of 2.36-fold and 1.62-fold of the uninoculated control when inoculated on 1456-2E and 1660-19F, respectively. These strains, however, were less effective on cowpea having a SE of 1.20-fold. Cowpea bradyrhizobia inoculated on promiscuous soyabeans produced less than 77 nodules plant−1 compared to an average of 120 by the two best bradyrhizobia strains from soyabean. The best isolates (R25B, IRj 2180A and their mixture) and one cowpea bradyrhizobia (IRc 461) were further tested on these lines under field conditions at three sites in different agroecological zones in moist savanna (Fashola, Mokwa and Zaria) in Nigeria. Both soyabean lines nodulated with the local rhizobia, but the degree of effectiveness depended on the plant genotype and field sites. Soyabean line 1456-2E showed improved growth and yield in response to N fertilizer application indicating that in this line N2 fixation induced by the indigenous bradyrhizobial community supplied less than optimal amounts of N. The mixture of bradyrhizobial isolates R25B and IRj 2180A increased grain yield of 1456-2E by 30 and 25% at Zaria and Mokwa but failed to do so at Fashola. Grain yield of 1660-19F was not affected by bradyrhizobial inoculation and N fertilizer at any of the three sites. Thus, the need for bradyrhizobia inoculation will be determined by the degree of promiscuity of soybean lines and the effectiveness of the community of indigenous bradyrhizobia present in the site.

Nitrogen contributions from decomposing cover crop residues to maize in a tropical derived savanna.

In cover cropping systems in the tropics with herbaceous legumes, plant residues are expected to supply nitrogen (N) to non-legume crops during decomposition. Field experiments were carried out to (i) determine the effects of residue quality on decomposition and N release patterns of selected plants in cover cropping systems, (ii) relate the pattern of residue N release to N uptake by maize in cover cropping systems. To study decomposition, litter bags were used and monitored over two maize growing seasons. The residues studied were mucuna (Mucuna pruriens (L.) DC. var. utilis (Wright) Bruck), lablab (Lablab purpureus (L.) Sweet), and leaves and rhizomes of imperata (Imperata cylindrica (L.) Raueschel). Mucuna and lablab decomposed rapidly losing more than 60% of their dry weight within 28 days. In contrast, imperata decomposed slowly with only 25% of its dry matter lost in 56 days. At 28 days, mucuna had released 154 kg N ha-1 in in-situ mulch systems and 87 kg N ha-1 in live- mulch systems representing more than 50% of its N. More than 64% of N in lablab was released within 28 days amounting to 21 to 174 kg N ha-1. Imperata rhizomes mineralized 4 to 14 kg N ha-1 within 14 days, and subsequently immobilized N until 112 days whereas imperata leaves immobilized N throughout the study period. Decomposition and N release rates from the plant residues were most strongly correlated with the (lignin+polyphenol)/N ratio, N content, lignin/N ratio, polyphenol/N ratio, C/N ratio and lignin content of the residues. Relative to the controls, herbaceous legume residues increased maize dry matter yield and N uptake during the two cropping seasons. At 84 days, the maize crop had utilized 13 to 63 kg N ha-1from mucuna representing 13 to 36% of N released, whereas 16 to 25% of N released from mucuna was recovered by the maize crop at 168 days. The first maize crop recovered 9 to 62 kg N ha-1 or 28 to 35% of N released from lablab. However, at 168 days, N uptake by maize in antecedent live-mulched lablab was 32% higher than the quantity of N released, whereas imperata residues generally, resulted in net reduction of maize N uptake.

Decomposition of four Leucaena and Senna prunings in alley cropping systems under sub-humid tropical conditions: The process and its modifiers

Abstract A litterbag experiment with Leucaena leucocephala and Senna siamea residues collected during four different pruning activities was carried out in no-tree-control and alley cropping plots. Decomposition was followed for 112 days after the respective pruning dates. Other factors studied were “amount of residue” confined in the litterbags and “plot management” (cropped or bare plots). The leaf litter of the same species, collected during the different prunings had different qualities. Older Leucaena and Senna residues had a lower N content. Older Leucaena residues contained substantially more polyphenols than residues younger than 8 weeks, while 8-week-old Senna residues contained more soluble polyphenols than residues of 29 weeks. The lignin content of the Leucaena residues was greatest for the oldest prunings; while for the Senna residues no clear trend was observed. The decomposition and N release patterns of the four Leucaena and Senna pruning residues were different. The first and second prunings decomposed following first order kinetics. For the third pruning, a negative exponential regression procedure against the number of days where rainfall exceeded pan evaporation (“rainy days”) yielded a better fit than against time. Less than 10% of the dry matter of the fourth pruning decomposed after 112 days. Significant correlations were found between the decomposition rate calculated against the number of “rainy days” and the N content, the C-to-N ratio and the (lignin + polyphenol)-to-N ratio. The N release rate, calculated against the number of “rainy days” correlated significantly with the polyphenol-to-N and (lignin + polyphenol)-to-N ratios. The presence of a fully established crop increased the decomposition and N release of the residues of the second pruning in the no-tree-control plots. The amount of residues confined in the litterbags and the presence of a tree canopy had little effect on residue decomposition.