Seasonal pattern of nitrogen mineralization and soil moisture beneath Faidherbia albida (synAcacia albida) in central malawi

On fertile alluvial soils on the lakeshore plain of Malawi, maize (Zea mays L.) yields beneath canopies of large Faidherbia albida (synAcacia albida) trees greatly exceed those found beyound tree canopies, yet there is little difference in soil nutrients or organic matter. To investigate the possibility that soil nutrient dynamics contribute to increased maize yields, this study focused on the impact of Faidherbia albida on nitrogen mineralization and soil moisture from the time of crop planting until harvest. Both large and small trees were studied to consider whether tree effects change as trees mature.During the first month of the rainy season, a seven-fold difference in net N mineralization was recorded beneath large tree canopies compared to rates measured in open sites. The initial pulse beneath the trees was 60 g N g^–1 in the top 15 cm of soil. During the rest of the cropping cycle, N availability was 1.5 to 3 times higher beneath tree canopies than in open sites. The total production of N for the 4-month study period was 112 g N g^–1 below tree canopies compared to 42 g N g^–1 beyond the canopies. Soil moisture in the 0–15 cm soil layer was higher under the influence of the tree canopies. The canopy versus open site difference grew from 4% at the beginning of the season to 50% at the end of the cropping season.Both N mineralization and soil moisture were decreased below young trees. Hence, the impact of F. albida on these soil properties changes with tree age and size. While maize yields were not depressed beneath young F. albida, it is important to realize that the full benefits of this traditional agroforestry system may require decades to develop.     

Nitrogen fixation in L. leucocephala L. as affected by rooting volume and competition with E. camaldulensis

Fast growing trees, in particular nitrogen fixing trees, are essential for the establishment of sustainable agroforestry practices by resource-poor and/or environmentally conscious aarmers. An important selection factor for trees is therefore high N₂ fixing capability. This study was conducted to examine biological nitrogen fixation (BNF) in Leucaena leucocephala L., as affected by growth in pots containing 4, 10, 15 and 30 kg soil and when grown together with Eucalyptus camaldulensis. BNF was measured by the ¹⁵N isotope dilution technique. Leucaena and Eucalyptus were grown either sole (SL, SE) or in two mixtures (1:1 - L1:E1 and 1:2 - L1:E2) for a period of 6 months. Planting pattern influenced (P < 0.05) dry matter and N content of shoot and roots of Eucalyptus, as the sole planted trees gave the highest yield. Dry matter, N content and total amount of N fixed by Leucaena significantly decreased with decreasing rooting volume. Also, the smaller the rooting volume (or soil weight), the higher was the proportion (%) of N derived from the atmosphere (%Ndfa) by leucaena. For each rooting volume, the sole or mixed planting pattern did not significantly influence either the proportion (%) or amount of N fixed by leucaena. For these trees, therefore, rooting volume rather than cropping pattern influenced the proportion and amount of nitrogen fixed.     

Agroforestry and soil conservation needs of smallholders in Southern Zambia

An agroforestry and soil conservation needs assessment survey conducted in southern Zambia revealed valuable insight into needs, constraints and development options as perceived by smallholders themselves. Inadequate efforts to conserve soil are due to technical and socio-economic factors, they do not result from a lack of awareness of the widespread erosion threat. Fruit tree planting, windbreak establishment as well as the protection of the natural Faidherbia albida regeneration in cropland are popular agroforestry interventions whose positive effects are widely acknowledged. Forty-two perennial species were found to directly contribute to people's diet. Although local fuelwood and fodder shortages as well as the need for fencing are recognized only few respondents envisage agroforestry solutions such as fuelwood and fodder tree planting or live fencing. Most smallholders are interested in tree planting but have so far only planted few trees. Those planted are usually exotic fruit trees. Drought hardly, termite and browse resistant perennials adapted to smallholder tree planting must be provided by the extension services. The widespread exclusion of women from decision making and the lack of tenure security hampers female participation in agroforestry development and consequently threatens sustainable development altogether.     

An evaluation of the Acacia albida-based agroforestry practices in the Hararghe highlands of Eastern Ethiopia

Growing Acacia albida as a permanent tree crop, on farmlands with cereals, vegetables and coffee underneath or in between, is an indigenous agroforestry system in the Hararghe highlands of Eastern Ethiopia. However, there is practically no systematic record or data on the merits and benefits of this practice.The paper presents the results of an investigation into the effects of the presence of A. albida on farmlands on the yield of maize (Zea mays L.) and sorghum (Sorghum bicolor L. Moench). Twenty seven plot pairs each consisting of one plot underneath the A. albida foliage cover and the other in the open, away from the tree-on farmers' fields, in a 40 km radius around the Alemaya College of Agriculture, were sampled and the yield components analyzed. A statistically significant increase in crops yields by 56% on average was found for the crops under the tree canopies compared to those away from the trees. This increase was caused by the improvement in 1000 grain weight and number of grains of plants under the tree, indicating that the trees enhanced the fertility status of the soil and improved its physical conditions in terms of crop growth.Additional benefits from the A. albida trees include supply of fuelwood and fodder. Quantitative estimates of these outputs as well as their monetary values are presented in the paper. However, in order to realize these benefits to a discernible extent, higher stand densities of the tree than at present are required.Based on an enquiry about the farmers attitude towards A. albida, the prospects for an extension of this promising agroforestry technique are discussed against the background of the state and trends of development of agriculture in the area. It is surmised that despite some shortcomings like the relatively slow and highly variable growth of A. albida and a conflict with the spreading cultivation of Ch'at (Catha edulis Forsk.), the prospects of extension of this technique are good. It is recommended that its propagation should be incorporated into the programmes of the extension agencies of the various governmental agencies concerned with land use.     

To prune or not to prune Faidherbia albida: competing needs for water,wheat and tree products in semi-arid Ethiopia

Faidherbia albida is one of the scattered trees commonly intercropped with most cereals in Ethiopia due to its positive impacts. The tree is pruned for various purposes including for fencing and fuelwood. In this study, the impact of pruning on water relations of F. albida and on understorey wheat productivity was investigated. The on-farm study was conducted in Ejerssa Joro, semi-arid Ethiopia. Six mature trees were selected; three were fully pruned and three were left unpruned. Sap flow and leaf water potential were measured on these trees. Crop gas exchange, aboveground biomass and grain yield were measured under and outside tree canopies. The highest and the lowest sap volumes, recorded from unpruned F. albida, during the dry period, were 153 L day^?1 and 20 L day^?1, respectively. The highest and the lowest sap volumes were 13.4 L day^?1 and 0.04 L day^?1 recorded during the wet period. Wheat CO₂ assimilation was highest (7.8 µmolm^?2 s^?1) at 1 m distance and declined away from the tree trunk under unpruned trees. Aboveground biomass and grain yield under unpruned treatments were significantly (P?<?0.05) higher than outside of canopy of same tree and outside canopies of pruned trees. Pruning reduced aboveground biomass and grain yield by 30% and 27%, respectively; despite the higher water uptake by unpruned trees. We recommend that intensive pruning of F. albida be discouraged and propose further studies on optimal pruning for increased food production and provision of tree products to meet farmers’ needs.

     

Acacia albida and other multipurpose trees on the fur farmlands in the Jebel Marra highlands,Western Darfur,Sudan

This paper describes the traditional agroforestry systems based on Acacia albida and other multipurpose trees as practised by the sedentary Fur people on the lower slopes and highlands of the Jebel Marra massif, Sudan. The basic agrosilvopastoral system consists of terraced village fields, where semipermanent rainfed cropping of staple millet and other subsistence crops takes place under stands of multipurpose trees dominated by Acacia albida, Cordia abyssinica and Ziziphus spina-christi. Trees have been retained primarily for food, wood and fodder. Thorn from cut and browsed branches makes a good fencing materal.This system has been able to sustain self-sufficiency of a densely settled population over centuries. However, recent out-migration of people from the montane dry-farming areas has caused gradual return to shifting cultivation. As a consequence, the present-day subsistence farming in the region is characterized by a general level of carelessness and exploitative management and this is reflected in a successive decrease of the tree cover.The evaluation of the AF practices described includes a discussion on their regional importance and extrapolability within the framework of similar situations, especially in Africa, emphasis being given to mountain and highland conditions. The outstanding potential for Acacia albida-based AF systems to be sustained and spread almost all over semiarid to semihumid Africa is highlighted by illustrating its ecologic and economic variability. Extrapolation of such examples, however, is not feasible, without thorough feasibility studies concerning the ecologic, ethnologic and socio- economic conditions in the respective project areas. Basic research needs for improving and extending the system are also indicated.     

The vertical distribution of fine roots of five tree species and maize in Morogoro,Tanzania

In order to assess the possibility of root competition in agroforestry, the vertical distribution of fine roots (< 2 mm in diameter) of five tree species in pure two-year-old stands was compared to that of mature maize.Cassia siamea, Eucalyptus tereticornis, Leucaena leucocephala andProsopis chilensis had a rooting pattern similar to that of maize, i.e. a slow decline in fine root mass from 0–100 cm soil depth.Eucalyptus camaldulensis had its roots evenly distributed down to 100 cm. On an average, the fine root biomass of the tree species was roughly twice as that of the maize. We conclude that the studied tree species are likely to compete with maize and other crops with a similar rooting pattern for nutrients and water.     

Alchornea cordifolia,a promising indigenous browse species adapted to acid soils in southeastern Nigeria for integrated crop-livestock agroforestry production systems

Dry matter (DM) production, crude protein, phosphorus fibre contents and goat preference for eight indigenous browse species,Alchornea cordifolia, Diallum guineense, Ficus capensis, Baphia nitida, Manniophytum fulvum, Homalium aylmeri, Glyphaea brevis andRauwolfia vomitoria, and for two exotics,Leucaena leucocephala andGliricidia sepium, in cultivated plots were compared on acid soil in southeastern Nigeria. Total DM production was higher (P<0.05) forAlchornea cordifolia than for the other browse species.Glyphaea brevis andL. leucocephala were the most preferred species, whileA. cordifolia, G. sepium andR. vomitoria were the least. Mean crude protein content of browse species in this study was higher, whileP and neutral detergent fibre were lower than reported for other browse species in Nigeria. It is argued that the ultimate goal of a crop-livestock agroforestry system such as alley farming, could be better achieved through the complementary use of browse species.     

Faidherbia albida and its effects on Ethiopian highland Vertisols

On a Vertisol under 850 mm rainfall, at an altitude of 1850 m, Faidherbia albida trees that occurred at 6.52 trees ha^–1, with a canopy cover that range from 97 m² ha^–1 to 229 m² ha^–1 were recorded. Studies on the lateral and vertical influence of the tree canopy on some physical and chemical properties of the soil, show apparent higher organic matter on the West side of the tree than the East due to accumulated wind blown litter by prevalent wind direction, and organic matter, N, P and K levels were higher under the tree canopy than outside for all directions and depths studied. Soil pH, exchangeable Na, Ca and Mg under the canopy and outside were similar. Available water capacity was 1.5 to 2 times more under than outside the tree. Noticeable increases in the silt fraction under the tree was recorded and discussed in relation to soil condition improvement and plant growth.     

The vertical distribution of fine roots of five tree species and maize in Morogoro,Tanzania

In order to assess the possibility of root competition in agroforestry, the vertical distribution of fine roots (< 2 mm in diameter) of five tree species in pure two-year-old stands was compared to that of mature maize.Cassia siamea, Eucalyptus tereticornis, Leucaena leucocephala andProsopis chilensis had a rooting pattern similar to that of maize, i.e. a slow decline in fine root mass from 0–100 cm soil depth.Eucalyptus camaldulensis had its roots evenly distributed down to 100 cm. On an average, the fine root biomass of the tree species was roughly twice as that of the maize. We conclude that the studied tree species are likely to compete with maize and other crops with a similar rooting pattern for nutrients and water.     

Soil water competition in a temperate hedgerow agroforestry system in South Africa

An on-farm trial was conducted to determine dry matter production of four fodder tree species and their effect on soil water and maize production. The trees were planted in rows intercropped with maize. The four tree species selected were Acacia karroo Hayne (indigenous fodder tree), Leucaena leucocephala (Lam.) De Wit (nitrogen fixing), Morus alba L. (fodder and fruit), and Gleditsia triacanthos L. (fodder and fuel). Volumetric soil water was measured in the upper 0.3 m of soil in each row of the trial using the time domain reflectometry technique. The neutron probe technique was used for monitoring the water content deeper in the soil. Geostatistical methods were used to analyse treatment differences in the upper 0.3 m of soil. The soil water content did not differ significantly between the maize and tree rows indicating that competition for water in the upper horizon was not the reason for lower maize yields. However, at greater soil depths (75–125 cm) trees in the wide spacing used less water than those in the narrow spacing. Light interception was an important factor in reducing maize yields in the row nearest to the trees. High soil water values recorded during summer indicated that in the current cycle of good rainfall the plants in the agroforestry trial were not stressed. Thus the trees do not compete with the crops for soil moisture in good rainfall seasons. However, this study would need further evaluation for the competition for water for the low rainfall years. Since the trees have access to water at greater depths, they are likely to be more productive into the dry season than shallow rooted crops.     

Characterization of <Emphasis Type="Italic">Faidherbia albida</Emphasis> (Del.) A. Chev. population in agroforestry parklands in the highlands of Northern Ethiopia: impact of conservation,environmental factors and human disturbances

Concerns about sustainable management and conservation of multipurpose trees in their habitat have led to increased number of studies on the ecological characterization of their population. Such knowledge on Faidherbia albida, the most used tree in agroforestry parklands in Ethiopia, is limited. F. albida population was characterized in and compared between two agroforests having different conservation status in Northern Ethiopia. Population structural parameters along with environmental factors and human activities were assessed in 42 randomly installed plots using a transect method. Size class distribution was used to describe and analyze the species long-term population dynamics. Adult density was almost three times higher in Zongi agroforest (19.9 ± 2.9 trees ha^?1) where the species has been conserved and managed for longer time compared to Abraha-atsbeha agroforest (7.9 ± 2.5 trees ha^?1). The same trend was observed for tree morphological parameters which were significantly higher at Zongi than Abraha-atsbeha. However, size class distributions coefficient of skewness and the median diameter indicated a declining and vulnerable population at Zongi and an increasing population at Abraha-atsbeha. Species population characteristics were influenced by environmental factors such as altitude, stone cover, erosion severity, slope, and human-related disturbances including land use, fodder harvesting, distance away from the center of the village and proximity of household to the plots. The study confirms the impact of conservation, environmental factors and human disturbances on shaping F. albida population and recommends the consideration of the trade-offs between them to design effective conservation and management strategies to sustain F. albida agroforests.     

Millet production under pruned tree crowns in a parkland system in Burkina Faso

As a tree management tool, three treatments of crown pruning (total-pruning, half-pruning and no-pruning) were applied to Vitellaria paradoxa (karité) and Parkia biglobosa (néré) in agroforestry parkland systems in Burkina Faso. The area under each tree was divided into four concentric tree influence zones (Zones A: up to 2 m from the tree trunk, B: up to half of the radius of the tree crown, C: up to the edge of the tree crown and D: up to 2 m away from the edge of the tree crown). Millet production under these zones and outside was assessed during two cropping seasons over the study period of three years and the results showed that tree crown pruning had significant effect on millet production and the highest millet grain yield and total dry matter were produced under total-pruned trees (507 ± 49 and 2033 ± 236 kg ha⁻¹ year⁻¹, respectively). Light transmission, transpiration and soil nutrient status under the trees were also analysed in relation to millet production. The results of the analysis showed that total-pruned trees gave the highest millet production due to the reduction by crown pruning of the effects of large tree crowns on PAR transmission below crowns and rates of transpiration by trees. Soil was more fertile closer to the tree trunks than outside tree crowns. This may also be one of the reasons why millet overall performed better under Zone B than outside tree crowns. The higher production of millet under Zone B than under Zone A, the zone closer to the tree trunk, may be due to lower light intensity and more intense competition for water between trees and crops under Zone A. It was concluded that at least in the short term millet production could be improved by crown pruning of both karité and néré, but long term effects may depend on the ability of the trees to maintain the amelioration of soil fertility and on how quickly the trees recover from pruning. This revised version was published online in June 2006 with corrections to the Cover Date.     

Intercropping Acacia albida with maize (Zea mays) and green gram (Phaseolus aureus) at Mtwapa,Coast Province,Kenya

Long-term agroforestry demonstrations/trials using Acacia albida and other nitrogen fixing multipurpose trees/shrubs were initiated in mid-1982 to assess soil and crop productivity at a coastal lowland site characterized by low soil fertility, weed problems and consequent poor crop yields. Growth performance (height and diameter at breast height, dbh) of Acacia albida under eight densities rotationally intercropped with maize (Zea mays) and green gram (Phaseolus aureus), crop grain yields, soil fertility changes and weed control were assessed for a 5-year period (May 1982 to March 1987). A parallel-row systematic spacing field layout was used. Intercropped Acacia albida mean hight and dbh were 140 and 24% respectively higher than tree-only controls by the fifth year. Growth rate was low during the first year but increased in subsequent years to mean height and dbh of 9 m and 10 cm respectively by March 1987. While differences in dbh were significant, those between stand heights were not. Crop yields, especially under higher tree densities, declined considerably due to unexpected shade which also caused significant reductions in weed biomass. Soil fertility levels remained unchanged during the experimental period relative to the initial status, and differences between the intercropped Acacia albida plots and the tree — or crop — only control appeared not to be significant. We conclude that an understanding of the mechanism regulating leaf fall/retention phenomena of Acacia albida is crucial towards determining the intercropping potentials of the species.     

Separating the effects of trees on crops: the case of Faidherbia albida and millet in Niger

Faidherbia albida (Del.) A. Chev. is an important tree species of the scattered tree or parkland systems in the Sahel. The improved crop growth under its canopy is well known, and has been attributed to various components notably: higher soil fertility, improved microclimate and better soil physical properties. The relative contributions of each of these components are not known, but knowledge about this is essential for making proper decisions concerning management options. The overall tree effect on crop production (expressed as fraction of the sole crop production) is analysed here as a weighted sum of (positive and negative) relative net tree effects on the resources for crop growth. In this sum, the weights reflect the degree of limitation of the resources in the environment at the tree-crop interface. The paper shows how the relative net tree effects and the accompanying weights can be estimated from field experiments. In an on-farm field experiment in Niger, millet production under the F. albida canopy was about 36% higher than in the open field. The nitrogen availability under trees was estimated to be more than 200% higher than in the open causing a 26% production increase. The phosphorus availability was estimated to be almost 30% higher and because of its high limitation causing a production increase of 13%. The net effect via other resources (notably light and water) was negligible (3% production reduction) and not significant.This revised version was published online in November 2005 with corrections to the Cover Date.     

Restoration of rooting competence in mature Faidherbia albida,a Sahelian leguminous tree,through serial root sucker micrografting

Faidherbia albida is an essential leguminous tree of arid areas of Africa which is widely used in agroforestry. This species naturally produces root suckers. Their morphogenetic ability has been used to initiate a rejuvenation/reinvigoration process in mature subjects. The proposed method contains two stages. The first stage generates the first ramets : our results have shown that suckers picked from root cuttings are more vigorous than explants taken directly from the crown of adult subjects. The second stage furthers the rejuvenation process through cycles of repeated micrografting onto juvenile rootstock, which have been grown in vitro. As early as the third cycle, a significant increase in reactivity is gained. This is determined by growth of the scion and the rooting competence of microcuttings severed from the scions after each micrografting cycle. At that point, the rooting competence is completely restored. So, this sequential method, (i) suckers from root cuttings, (ii) repeated micrografting, seems valid as a technique for the restoration of rooting competence in mature F. albida with a view to its cloning.     

Screening agroforestry tree mulches for corn growth: a combined soil test,pot trial and plant analysis approach

The response of corn (Zea mays) to incorporated leaf and twig mulches ofLeucaena leucophala, Gliricidia sepium andCassia siamea, andGrevillea robusta as a non-legume comparison was investigated in a 10-week pot trial and a concurrent soil incubation study to evaluate the suitability of various agroforestry trees as mulch sources.Leucaena contributed to the highest N uptake and biomass production of these corn plants, reflecting the benefits of organic mulching.Cassia-treated plants also performed better than the unmulched controls, butGrevillea incorporation suppressed corn growth probably because of the relatively high Mn status of this mulch. Manganese toxicity was confirmed by comparative vector diagnosis of plant nutrient status.Cliricidia mulching resulted in seedling mortality after germination possibly from high soil pH and ammonium toxicity. Mineral N production in mulched soils during the laboratory incubation was well correlated with N uptake in corn shoots. The results suggest that the nutritional effects of agroforestry tree mulches on growth of companion crops may be effectively screened by a combined soil test, pot trial, and plant analysis approach.     

Multipurpose tree prunings as a source of nitrogen to maize under semiarid conditions in Zimbabwe

When multipurpose-tree (MPT) prunings are used as a source of nitrogen to annual corps in agroforestry systems, it is important that high levels of N recovery are attained. In order to test the effect of pruning quality and method of pruning application on N-recovery rates, a field experiment was conducted using prunings of five MPT species and two methods of application (surface versus incorporation). There was an interaction of pruning quality and method of pruning application on N recovery and maize grain yield. For most MPT species used, incorporated prunings gave high nitrogen recovery compared to surface applied prunings. In order to achieve synchrony between N supply from prunings and N demand by a maize crop, relatively large amounts of N should be released into the soil before peak N demand by the maize crop. This could be achieved by incorporating adequate quantities of high quality prunings such as those ofCajanus cajan andLeucaena leucocephala. Florida Agricultural Experiment Station Journal Series Number R-05489.     

Allometric models for predicting above- and belowground biomass of <Emphasis Type="Italic">Leucaena</Emphasis>-KX2 in a shaded coffee agroecosystem in Hawaii

We developed site-specific allometric models for Leucaena leucocephala × pallida var. KX2 trees in a shaded coffee agroecosystem in Hawaii to predict above- and belowground biomass and the regrowth potential of pollarded trees. Models were used to compare tree growth rates in an experimental agroforestry system with different pollarding frequencies and additions of tree pruning residues as mulch. For all allometric equations, a simple power model (Y = aX^b) provided the optimal prediction of biomass or regrowth after pollarding. For aboveground biomass components (stem, branches, leaves, and seed and pods), stem diameter alone was the best predictor variable. Stump diameter provided the best prediction of coarse root biomass and aboveground regrowth after pollarding. Predictions of biomass from generalized allometric models often fell outside the 95% confidence intervals of our site-specific models, especially as biomass increased. The combination of pollarding trees once per year plus the addition of tree mulch resulted in the greatest aboveground regrowth rates as well as accumulation of biomass and C in the stump plus coarse roots. Although optimal prediction required the development of site-specific allometric relationships, a simple power model using stem or stump diameter alone can provide an accurate assessment of above- and belowground tree biomass, as well as regrowth potential under specific management scenarios.     

Influence of Agroforestry and Traditional Management of Maize (Zea mays L.) on Soil Fertility

The aim of the work was to evaluate soil nutrient concentration at 0–5, 5–10, and 10–20 cm in maize (Zea mays L.) grown in sequence with black oats (Avena strigosa Schreb.) under Leucaena diversifolia alley cropping agroforestry system (AFS) and traditional management system/sole crop (without trees–TS), following a randomized block design. The experiment was carried out at the Brazilian Association of Biodynamic Agriculture, in Botucatu, São Paulo, Brazil. The treatments were: control (C), chemical fertilizer (F), biomass of L. diversifolia alley cropping (B), and biomass of L. diversifolia alley cropping + chemical fertilizer (B+F).

After 2 yr, it was observed that pH, organic matter, and nutrient content had a tendency to show higher values in the treatments biomass+fertilizer, biomass, and fertilizer application, in both systems. Higher values in pH, organic matter, phosphorus, potassium, calcium, magnesium, sum of bases, cation exchange capacity, percentage base saturation, boron, copper, and manganese tended to occur in the agroforestry system.