Volunteer biomass production between multipurpose tree hedgerows after two years of fallow in southern Cameroon

The ability of multipurpose hedgerow tree species to out-compete undesired regrowth during fallow phases was examined. Biomass and spatial distribution of grass and broad leaf volunteers was measured after two years of fallow, in two alley cropping systems planted at six m interrow distance, at the Humid Forest Ecoregional Centre Research Station, Mbalmayo, southern Cameroon. The two experiments had been continuously cropped for five and six years previously. In the experiment cropped for six years, the presence of Senna spectabilis [(DC.) Irwin and Barnaby] hedgerows reduced the biomass of the volunteer regrowth from 9.2 to 4.3 Mg ha⁻¹. Tillage during the previous cropping phase increased the broad leaf biomass from 3.0 to 4.4 Mg ha⁻¹, reduced the biomass of grasses from 3.4 to 2.7 Mg ha⁻¹ but had no effect on the total volunteer biomass. Volunteer biomass was significantly lower within 1.5 m of the S. spectabilis hedgerows than at positions further away. In the experiment cropped for five years, S. spectabilis reduced the volunteer regrowth biomass significantly. Two other hedgerow species, Dactyladenia barteri [(Hook f ex Oliv.) Engl.] and Flemingia macrophylla [(Willd.) Merrill] had no effect on the total amount of volunteer regrowth but did reduce volunteer biomass within 0.5 m of the hedgerows. S. spectabilis caused a stronger reduction of volunteer biomass than D. barteri and F. macrophylla at almost all distances from the hedgerows. The competitiveness of D. barteri and F. macrophylla is insufficient and their growth habit is unsuitable to out-compete undesired species in this alley cropping system. This revised version was published online in June 2006 with corrections to the Cover Date.     

Root distribution ofDactyladenia (Acioa) barteri andSenna (Cassia) siamea in alley cropping on Ultisol. I. Implication for field experimentation

Root observations were carried out on a typic Paleudult in the humid forest zone of south eastern Nigeria in an alley cropping trial usingDactyladenia (Acioa) barteri andSenna (Cassia) siamea as hedgerow trees and the interrow space planted to maize/cassava intercrop. Rooting depth ofD. barteri andS. siamea exceeded 1.6 m. Lateral root propagation ofS. siamea was 15 m, and ofD. barteri was 5 m from the hedgerows. The whole no-tree control plot was within the range of roots of the adjacent hedgerows. Rooting density and depth ofS. siamea in the no-tree control plot was generally higher than of cassava.S. siamea and cassava root density were inversely correlated. Assuming radial symmetry of root propagation, water and nutrients were available from an area 6.1 and 2.3 times larger than the allocated plot size ofS. siamea andD. barteri, respectively. Data obtained in alley cropping trials, not considering lateral root propagation, can be invalidated through exploitation of the no-tree control treatment and nutrient acquisition by hedgerow species from a larger area than allocated, thus underestimating and overestimating the performances of the respective treatments. Possible alternative measures for avoiding root interference are discussed but no good solution can be given.     

Effects of no-tillage and alley cropping on soil properties and crop yields in a Typic Kandiudult of southern Cameroon

The effects of no-tillage and alley cropping withCassia spectabilis hedgerows on soil properties and crop yields in a Typic Kandiudult of the humid forest zone of Central Africa were studied over the period 1990–1992 in southern Cameroon. The experimental treatments were no-tillage and hand tillage, both of which were either alley cropped withC. spectabilis hedgerows at interhedgerow spacings of 6 m or not alley cropped (control). A maize + cassava intercrop was planted in all plots at the commencement of each growing season.No-tillage had no significant effects on soil physical properties except to decrease soil temperature. In comparison to hand tillage, no-tillage increased soil organic C and total N in both years and pH in 1991. Mean organic C and total N with no-tillage were 1.77% and 0.174%, respectively, whereas with hand tillage were 1.35% and 0.145%, respectively. Notillage also resulted in a greater proportion ofC. spectabilis roots occurring in the topsoil. Alley cropping caused significant reductions in dry season soil temperature, surface seal formation and cassava root growth, and increases in exchangeable Ca, effective CEC and water infiltration compared with non-alley cropped controls. Infiltration rate at 2 h after commencing measurements were greater by 75% with alley cropping. Lowest maize cob and cassava tuber yeilds were observed when no-tillage was combined with alley cropping whereas highest yields occurred with no-tillage alone.     

Alley cropping on an Ultisol in subhumid Benin. Part 1: Long-term effect on maize, cassava and tree productivity

In southern Benin, West Africa, two alley cropping systems were studied from 1986 to 1992. Yield development was followed in a maize and cassava crop rotation vs. intercropping system, with alleys of Leucaena leucocephala (Lam.) de Wit and Cajanus cajan (L.) Millsp. vs. a no-tree control, with and without NPK fertiliser. Without alleys, NPK fertilisation maintained high yield levels of 2–3 t maize dry grain plus 4–6 t ha^–1 cassava root DM in intercropping, 3–4 t ha^–1 maize and 6–10 t ha^–1 cassava in solercropping. Without NPK, final yields seemed to stabilise at about 1 t maize plus 2 t cassava in intercropping and twice as much in each solecrop. Alley cropping induced significant yield increases by about 50% with both tree species in unfertilised, intercropped maize, and with Cajanus in fertilised, solecropped cassava. In monetary terms, the NPK-fertiliser response of stabilised yields was significant for all treatments except the solecropped Leucaena alleys. It is concluded that on Ultisols with low nutrient status in the upper rooting zone, alley cropping with low-competitive tree species may improve food crop yields but the greatest monetary output is achieved by intercropping with mineral fertiliser independent of the presence or absence of an agroforestry component.     

The potential of alley cropping im improvement of cultivation systems in the high rainfall areas of Zambia II. Maize production

Theee trials to evaluat the potential of alley cropping in maize production on the low fertility, acidic soils in Northern Zambia are described. Leucaena leucocephala, Gliricidia sepium, Sesbania sesban, Albizia falcataria, Fleminga congesta, and Cassia spectabilis, were grown in alley crops with hybrid maize and soybean. All trials received recommended rates of P and K fertiliser; N fertiliser was applied at three rates as a subplot treatment. One trial received lime before establishment.Only in the limed trial was there a significant improvement in maize yields through alley cropping; when no N fertiliser was applied, incorporation of Leucaena leucocephala prunings resulted in an increase of up to 95% in yields, with a smaller improvement being produced by Flemingia congesta. There was a significant correlation between the quantity of prunings biomass applied and the proportional increase in maize yields over the control treatment. It is suggested that the lack of effect of most of the tree species on crop yields was due to low biomass production.An economic analysis showed that alley cropping with limed Leucaena was only profitable when fertiliser costs were high in relation to maize prices. However, lime is both expensive and difficult to obtain and transport for most small scale farmers in the region, and is therefore not a practical recommendation. It is suggested that future alley cropping research should focus on screening a wider range of tree species, including other species of Leucaena, for acid tolerance and higher biomass production.     

Performance of a maize/cassava intercrop in tilled and no-till Senna spectabilis alley cropping on an Ultisol in southern Cameroon

An intercrop of maize and cassava was cultivated for six consecutive years from 1990/91 to 1995/96. This paper reports on crop performance from the fourth to the sixth year in alley cropping with Senna spectabilis and in a no-tree control, with and without tillage. Maize grain yield increased significantly from 1993 to 1994 (P = 0.0032) and from 1994 to 1995 (P = 0.019). Tillage only had a significantly positive effect on maize grain yield in 1993. Alley cropping had no significant effect on maize grain yield in any year. Cassava root yield was neither affected by tillage nor by alley cropping in any year. Neither the cumulative yields over the three years nor the sum of maize grain and cassava root yield were significantly affected by alley cropping or tillage. No significant interactions were found between cropping system and tillage system.This revised version was published online in November 2005 with corrections to the Cover Date.     

Competition for light between hedgerows and maize in an alley cropping system in Hawaii, USA

Successful agroforestry systems depend on minimizing tree-cropcompetition. In this study, field experiments and a simulation model were usedto distinguish between tree-crop competition for light and belowgroundcompetition in an alley cropping system. Maize (Zea maysL.) was harvested periodically in three treatments: between vertical barriers ofshade cloth, hedgerows of Flemingia macrophylla (Willd.)Merr., and sole maize. Radiation intercepted by the maize was calculated using asimulation model based on measured values for direct and diffuse light, hedgerowdimensions and leaf area, and solar trajectory. Radiation use efficiency wascalculated as biomass production per unit of intercepted radiation. Maizebiomass and yield in both the alley crop and the shade cloth treatment weregreatest in the center of the alleys. Grain yield between hedgerows was 3.5Mg ha⁻¹ (averaged across the alley), significantlyless than in the shade cloth (7.4 Mg ha⁻¹) or thesole maize (7.7 Mg ha⁻¹) treatments. Lightintercepted by the maize in the alley crop was about half that intercepted bythe maize in the sole crop. The shade cloth intercepted less light than thehedgerows because it did not have an appreciable width. Radiation use efficiencyin the three treatments was 0.75 g mol⁻¹ PAR anddid not differ significantly among treatments. Tree-crop competition wasoverwhelmingly for light. This revised version was published online in June 2006 with corrections to the Cover Date.     

Root distribution ofDactyladenia (Acioa) barteri andSenna (Cassia) Siamea in alley cropping on Ultisol. II. Impact on water regime and consequences for experimental design

Tensiometer measurements were carried out on a typic Paleudult in the humid forest zone of south eastern Nigeria in an alley cropping trial using fertilized and unfertilizedDactyladenia (Acioa) barteri andSenna (Cassia) siamea as hedgerow trees and a no-tree control. The interrow space of alley cropped and no-tree control plots were planted to maize/cassava intercrop. Water withdrawal during short dry spells and the dry season occurred fastest in the no-tree control plot and resembled the pattern in the adjacentS. siamea alley cropping. Previous root investigations showed that the whole no-tree control plot was within the range of root propagation of the adjacent hedgerow trees. Installation of a 70 cm deep root barrier led to a retarded water withdrawal in unfertilized no-tree control plots to a depth of 150 cm. In fertilized no-tree control this retardation occurred to a depth of 110 cm, while at 130 and 150 cm water withdrawal with root barriers was faster than without barriers. Results indicate thatS. siamea depleted water resources in the no-tree control plot and shortened the growing season of cassava. Restricting roots to the assigned plot size can reduce competition for water in adjacent plots even in layers below the depth of the barrier but can also induce compensative water withdrawal from layers which were not necessarily affected by the barrier. It appears that currently no standard methodology is available to conduct agroforestry trials without the risk of invalidation through root interference. Methods to determine minimum plot size in order to reduce the risk of invalidation and misinterpretation of results are suggested.     

The potential of alley cropping in improvement of cultivation systems in the high rainfall areas of Zambia I. Chitemene and Fundikila

Three trials investigating the potential of alley cropping to improve the traditional systems of cultivation, chitemene and fundikila, in the Northern Province of Zambia are described. Flemingia congesta, Tephrosia vogelii, and Sesbania sesban, were grown in association with finger millet, groundnut, cowpea, and maize in various traditional cropping sequences. The indigenous species Tephrosia vogelii and Sesbania sesban were not able to withstand repeated pruning and the long following dry season, and were replaced with Calliandra calothyrsus, and Cassia spectabilis.Over a four year period, there was no benefit by alley cropping with any of the tree species on crop yields, and yields in alley crop treatments even dropped significantly below the control treatments in the fourth year, casting doubt on the potential of alley cropping for sustainable production. There was also no consistent effect on soil chemical characteristics in any of the trials. It was suggested that this lack of beneficial response to alley cropping was due to low tree biomass production low quality of prunings, and an inappropriate cropping sequence. There was no evidence that alley cropping contributed to enhanced nutrient recycling, despite substantial localised pools of soil nutrients, particularly in the chitemene, with which recycling could potentially occur.     

Nitrogen fixation capacity in the component species of contour hedgerows: how important?

The choice of an appropriate hedgerow species is one of the most critical decisions in exploiting the value of a contour hedgerow system. The implications of hedgerow species with nitrogen (N)-fixation capacity on hedgerow-crop competition and crop productivity have been widely debated. We examined the agronomic significance of N-fixation by comparing the performance of species representing three classes of hedgerow vegetation: A nitrogen-fixing tree legumeGliricidia sepium), a non-nitrogen fixing tree (Senna spectabilis syn.Cassia spectabilis), and a forage grass (Pennisetum purpureum). The 4-year study investigated the hedgerow biomass and nutrient yields, and their relative effects on the performance of two annual crops commonly grown in alley farming systems, with emphasis on hedgerow-crop interference. The work was done on an Ultic Haplorthox (pH 4.8, organic C 1.9%, total N 0.18%).Senna produced 46% more pruning biomass on an annual basis than didGliricidia; N supplied to the alley crops was similar toGliricidia in the first year of observation, but 20–30% higher in the succeeding years. Upland rice and maize grain yields and total dry matter were unaffected by tree species, but the nitrogen-fixing tree exerted less competitive effects on the annual crops growing in adjacent rows. Grass hedgerows reduced maize yields 86% by the second year, indicating an unsustainable drawdown of nutrients and water. We conclude that hedgerow systems composed of a nitrogen-fixing tree did not exert significant advantages compared to a non-fixing tree species, and that factors other than N-fixation were more important determinants for the choice of hedgerow species.     

Effect of alley cropping with Leucaena leucocephala and fertilizer application on yield of vegetable crops

Field experiments were conducted to investigate the effect of alley cropping vegetable crops with Leucaena leucocephala (Lam.) de Wit for two seasons on a sandy loam Oxic paleustalf in southwestern Nigeria. Four vegetable crops (Amaranthus cruentus L.; Celosia argentea L.; Okra, Hibiscus esculentus L.; and tomato, Lycopersicon esculentum Mill.) were grown in control plots and in 4-m wide alleys between established Leucaena hedgerows without and with fertilizer (30 N-13 P-24 K kg ha^–1). Leucaena prunings yielded large dry biomass and nutrients during both seasons. Yield of four vegetable crops responded more to fertilizer in control than in alley cropped treatments. Better yield of vegetables in alley cropped plots was in part due to following effect of the Leucaena hedgerows. Fertilizer application increased mean yields of Amaranthus, Celosia, okra and tomato by 325, 164, 47 and 94% in control plots and by 36, 26, 4 and 20% in alley cropped plots, respectively. For both seasons, yields were not significantly different between alley cropped with and without fertilizer and the control with fertilizer treatments. Yield was least in control without fertilizer. Alley cropping with Leucaena can reduce fertilizer requirement for vegetable production. Cost and return analysis using 1988 prices indicated that alley cropping with vegetable crops can be profitable.c/o Miss Maureen Larkin, L. W. Lambourn & Company, 26 Dingwall Road, Croydon CR9 3EE, England     

Weed biomass dynamics in planted fallow systems in the humid forest zone of southern Cameroon

Interest in planted fallow systems has focused on soil fertility improvement, neglecting other potential benefits of such systems. It is important to quantify other processes responsible for crop yield increases under planted fallows, such as weed control. The suppressive potential on weeds of Flemingia macrophylla [(Willd.) Merrill] and Pueraria phaseoloides (Roxb.) Benth, planted fallows was evaluated in field trials in three villages in southern Cameroon. In each village, experiments were set up in 4–5 year-old bush fallow dominated by Chromolaena odorata (L.) R. M. King & H. Rob. and 20 year-old secondary forest. Total aboveground biomass production of P. phaseoloides was 7.45 Mg ha⁻¹, 4.2 times higher than F. macrophylla (1.78 Mg ha⁻¹ ; P < 0.05). The high biomass of P. phaseoloides resulted in a significantly greater reduction in total weed biomass compared to Flemingia macrophylla in both wet and dry seasons. In the wet season (11 and 18 MAP), there were significant fallow system × land use and fallow system × village interactions for total weeds and broadleaf weeds. P. phaseoloides in bush (0.55 Mg ha⁻¹), and P. phaseoloides at Ngoumou (0.09 Mg ha⁻¹) had the lowest total weeds in the wet seasons. After the dry season, the lowest total weed mass was consistently recorded in P. phaseoloides while the highest was in the natural regrowth. The population of grasses was always higher in the F. macrophylla system than in P. phaseoloides system throughout the wet and dry seasons. Grass biomass in the P. phaseoloides-forest LUS was the least (0.01 Mg ha⁻¹), 58 times lower than in F. macrophylla-bush (0.58 Mg ha⁻¹). Biomass production of P. phaseoloides was highly significantly correlated with total weed biomass (r = −0.64; P = 0.004) while no relationship was found between biomass production of F. macrophylla and total weed biomass (r = −0.08, P = 0.747). It was concluded that P. phaseoloides was a suitable leguminous species for weed control. But for F. macrophylla, its low biomass production coupled with a compact plant architecture compromised it as an appropriate species for weed control in a planted fallow system.     

Weed seedbank characteristics of arable fields under different fallow management systems in the humid tropical zone of southeastern Nigeria

The composition and pattern of weed flora in arable fields are determined by their seedbank structure; but the influence of fallow management practices on weed seedbank structure is presently unknown. The objective of this study was to investigate weed seedbank characteristics and weed population dynamics in arable fields in natural and planted-fallow systems. The study plots were at Mbaise, a densely populated area of southeastern Nigeria, where farmers regenerate their exhausted soils by maintaining planted fallows of Dactyladenia barteri (Hook. F. ex Oliv.) Prance & F. White, and at Umuahia, a less-densely populated area in the same region, where farmers depend on natural bush fallow for soil regeneration. The effect of three years of fallow on the weed flora of arable fields in the two fallow management systems differed remarkably. The first flush of weeds on fields that were cultivated after three years of planted D. barteri fallow (Mbaise) consisted of 80% broadleaf weeds, 7% grass weeds and 13% sedges. On the other hand, the first flush of weeds on the natural bush fallow fields (Umuahia) of the same fallow duration as the D. barteri fallow system consisted of 17% broadleaf weeds, 70% grasses and 13% sedges. Three years of planted fallow caused 36% decrease in weed seedbank at Mbaise relative to the cropped field while the same duration of natural bush fallow caused a 31% increase in weed seedbank at Umuahia. These results show that the planted D. barteri fallow system has a higher potential to reduce weed pressure in smallholder agriculture than the natural bush fallow system and may explain in part why farmers in this humid forest zone have adopted the practice. This revised version was published online in June 2006 with corrections to the Cover Date.     

Alley cropping with Leucaena leucocephala (Lam.) De Wit and Acioa barteri (Hook.f.) Engl.

The effect of alley cropping with seven combinations of Acioa and Leucaena hedgerows and a control (no hedgerow) treatment on sequentially cropped maize and cowpea was studied in 1985 and 1986. The trial was carried out on an Alfisol in the humid zone of southwestern Nigeria. Hedgerows were established in 1983, using 4 m inter-hedgerow spacing and pruned to 25 cm height during cropping. Highest dry matter, wood, and nutrient yields of prunings of Acioa and Leucaena hedgerows were obtained with sole cropping. Growing Leucaena and Acioa in the same hedgerow suppressed dry matter production and nutrient yield of Acioa more than of Leucaena owing to the latter's faster growth. Total pruning dry matter yield was reduced as the proportion of Acioa increased in the combination. Leucaena prunings had higher nutrient yield than Acioa. Under 22-month old uncut hedgerows, weed biomass declined in the presence of Leucaena, either alone or in combination with Acioa. Weed weight under sole Leucaena hedgerows was about a third of that in the control plot. There was no significant effect of alley cropping on weed biomass, although alley cropping with Acioa and Leucaena hedgerows resulted in the dominance of broadleaf weeds while the control had a mixture of broadleaves and grasses. Alley cropping with various combinations of Acioa and Leucaena hedgerows increased maize and cowpea yields compared to control. Nitrogen application in both years increased maize grain yield. Mean yield increase due to N application in both years was highest in the control (47.2%) followed by the sole Acioa hedgerow (25.2%) and less in hedgerows with Leucaena. The results of observations over two years do not show any advantage for the tested Leucaena and Acioa combinations on maize and cowpea crops as compared to the sole hedgerows. B.T. Kang (IITA), c/o Miss Maureen Larkin     

Incidence and Severity of Bean Rust (Uromyces appendiculatus) in Alleys between Leucaena Hedgerows in Kenya

The effects of inter-row spacing of Leucaena leucocephala (Lam.) de Wit in an alley cropping system on the incidence and severity of rust (Uromyces appendiculatus (Pers.) Unger) on intercropped beans (Phaseolus vulgaris L.) and their yield were examined over 2 years (1993 and 1994) at Chepkoilel Campus in Kenya. Each experiment consisted of three randomized blocks with treatments of three alley widths (2 m, 4 m and 8 m) and a treeless control with two intra-row spacings of Leucaena (0.5 m and 1.0 m). Hedgerows were coppiced at 1.0 m height and pruned subsequently at 2–3 months intervals. No fertilizer was applied but Leucaena loppings were incorporated as green leaf manure. Rust on beans was assessed at three growth stages in each season, using the Centro International de Agricultura Tropical (CIAT) scales. Microclimate was monitored in treatments, in 1994. Bean growth and yield were also measured. Rust increased with increasing alley width and was less severe in bean rows adjacent to hedgerows. Bean yield was highest in the treeless control plots and declined with decreasing alley width. Beans in 2 m alleys had significantly lower (p < 0.05) yields than 8 m alleys and treeless control plots. Bean growth was greatly modified in 2 m alleys and close to hedgerows. Light availability and diurnal temperature increased with alley width but relative humidity and leaf wetness duration decreased. Proximity to hedgerows also had marked effect on microclimate. The changes in yield, rust incidence and severity were examined in relation to microclimate, inoculum survival and dispersal.     

Effect on maize growth of the interaction between increased nitrogen availability and competition with trees in alley cropping

Maize growing next toErythrina hedgerows had 44% lower biomass (p<0.01) and 35% lower N content (p<0.1) than maize growing in the middle of the alleys. Maize growing next toGliricidia hedgerows had the same biomass but 56% higher N content (p<0.1) than maize growing in the middle of the alleys. However these differences did not develop until 2 months after sowing of the maize.Spatial variability in soil nitrogen mineralization and mulch nitrogen release did not explain any of the differences in growth or N uptake of the maize with respect to distance from the trees. It is hypothesized that the slower growth of the maize next to theErythrina trees after 2 months is due to increasing light and/or nutrient competition from the trees as the trees recover from pollarding. The apparent lack of competition fromGlirigidia may be due to different rates of regrowth or different shoot and root architecture.A theoretical model is described demonstrating that if a crop is to take advantage of the higher nutrient availability under alley cropping it must complete the major part of its growth before the trees recover significantly from pollarding, and start competing strongly with the crop.     

Shading effects of alley cropped Leucaena leucocephala on weed biomass and maize yield at Mtwapa,Coast Province,Kenya

Reductions of up to 90% in weed biomass was observed under Leucaena leucocephala alley cropping with varying tree between (2,4 and 8 m) and within-row spacing combinations when compared to crop-only control. An increase of 24 to 76 % in maize yields of alley cropped plots compared to the crop-only control was also recorded. The 2m alley widths closed canopy faster than the 4 and 8m, and hence realized highest weed biomass reduction during the short-fallow period between two cropping seasons. At the end of the short fallow period, substantial fuelwood (up to 8 t ha ^–1) was realized.     

Green manures for maize—bean systems in eastern Uganda: Agronomic performance and farmers' perceptions

Researchers worked with farmers in eastern Uganda to develop alternatives for soil management using crotalaria (Crotalaria ochroleuca), mucuna (Mucuna pruriens var. utilis), lablab (Dolichos lablab), and canavalia (Canavalia ensiformis) as green manures in short-term fallows. The participatory research was part of a community-based approach for systems improvement. Grain yields of maize (Zea mays) and bean (Phaseolus vulgaris) following one season of crotalaria fallow were 41% and 43%, respectively, more than following a two-season weedy fallow. Grain yields of maize following a one-season fallow with mucuna and lablab were 60% and 50% higher, respectively, as compared with maize following maize. Maize and bean yield were more, although effects were small, during the second and third subsequent seasons, indicating probable residual effects of the green manures. Mucuna and lablab were successfully produced by intersowing into maize at three weeks after sowing maize, although the yields of the associated maize crop were reduced by 24% to 28%. Farmers estimated the labor requirements for mucuna and lablab to be less than for crotalaria. Farmers independently experimented on how these species can be integrated into banana (Musa spp.), coffee (Coffea robusta), sweet potato (Ipomoea batatas), and cassava (Manihot esculenta) production systems. Farmers reported that the beneficial effects of the green manures included higher food-crop yields; weed suppression; improved soil fertility, soil moisture, and soil tilth; and erosion control. Mucuna and lablab were preferred because of reduced labor requirements and increased net benefits compared with continuous cropping. Farmer participation in the green manure research resulted in efficient generation and adaptation of green manure technology now being promoted in eastern and central Uganda.This revised version was published online in November 2005 with corrections to the Cover Date.     

Biomass production of tree fallows and their residual effect on maize in Zimbabwe

The rotation of maize (Zea mays) with fast-growing, N₂-fixing trees (improved fallows) can increase soil fertility and crop yields on N-deficient soils. There is little predictive understanding on the magnitude and duration of residual effects of improved fallows on maize yield. Our objectives were to determine the effect of fallow species and duration on biomass production and to relate biomass produced during the fallow to residual effects on maize. The study was conducted on an N-deficient, sandy loam (Alfisol) under unimodal rainfall conditions in Zimbabwe. Three fallow species — Acacia angustissima, pigeonpea (Cajanus cajan), and Sesbania sesban — of one-, two-, and three-year duration were followed by three seasons of maize. Pigeonpea and acacia produced more fallow biomass than sesbania. The regrowth of acacia during post-fallow maize cropping provided an annual input of biomass to maize. Grain yields for the first unfertilized maize crop after the fallows were higher following sesbania (mean = 4.2 Mg ha^–1) than acacia (mean = 2.6 Mg ha^–1). The increased yield of the first maize crop following sesbania was directly related to leaf biomass of sesbania at the end of the fallow. Nitrogen fertilizer did not increase yield of the first maize crop following one- and two-year sesbania fallows, but it increased yield following acacia fallows. Nitrogen fertilizer supplementation was not required for the first maize crop after sesbania, which produced high-quality biomass. For acacia, which produced low-quality biomass and regrew after cutting, N fertilizer increased yield of the first post-fallow maize crop, but it had little benefit on yield of the third post-fallow maize crop.This revised version was published online in November 2005 with corrections to the Cover Date.     

Nitrogen contribution of five leguminous trees and shrubs to alley cropped maize in Ibadan, Nigeria

There are abundant local legume trees and shrubs potentially suitable for alley cropping systems in the sub-Saharan Africa, which are yet to be studied. The nitrogen contribution of two years old Albizia lebbeck and S. corymbosato yield of maize grown in alley cropping was compared to that of Senna siamea, Gliricidia sepium and Leucaena leucocephala in four seasons at Ibadan. Maize shoot biomass and maize grain yield in A. lebbeck alley compared favourably with that in G. sepium and L. leucocephala. Maize biomass and grain yield in S. corymbosa alleys were the lowest. Within A. lebbeck, L. leucocpehala, and G. sepium alleys there were no significant differences in the maize yield in the alleys that received 0, 40 or 80 kg N/ha. Application of more than 40 kg N/ha in S. corymbosa alleys was not necessary as there was no significant increase in maize yield at the higher level of nitrogen. Maize yield and N uptake in A. lebbeck alleys were not significantly different from yield and N uptake in G. sepium, and L. leucocephala at the same fertilizer level. There was a significant correlation between hedgerow tree biomass and maize grain yield. At the end of twelve weeks after pruning application, the organic residues of the pruning applied in the alleys ranged from 5% in G. sepium and 44% in A. lebbeck in the first year compared with the original pruning applied which showed that the slow rate of A. lebbeck decomposition could have a beneficial effect on the soil. The maize N recovery from applied N fertilizer was low (10–22%). Percentage N recovery from the prunings was low in the non-N fixing trees (12–22%), while the recovery was high (49–59%) in A. lebbeck as well as in the other nitrogen fixing tree prunings. Thus A. lebbeck, apart from enhancing maize growth and grain yield like in L. leucocephala and G. sepium, had an added advantage because it remained longer as mulching material on the soil because of its slow rate of decomposition. It was able to survive pruning frequencies with no die-back. This indicates that A. lebbeck is a good potential candidate for alley cropping system in West Africa. S. corymbosa performed poorly compared with the other legume trees. Though it responded to N fertilizer showing a positive interaction between the hedgerow and fertilizer application, it had a high die back rate following pruning periods and termite attack.This revised version was published online in November 2005 with corrections to the Cover Date.