Cassava, maize and tree root development as affected by various agroforestry and cropping systems in Bénin, West Africa

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1 Agriculture, Ecosystems and Environment 100 (2003) Cassava, maize and tree root development as affected by various agroforestry and cropping systems in Bénin, West Africa S.J. Lose, T.H. Hilger, D.E. Leihner, J. Kroschel Institute of Plant Production and Agroecology in the Tropics and Subtropics (380), University of Hohenheim, D Stuttgart, Germany Abstract Soil fertility levels, climate, aboveground interaction for light in plant canopies, and belowground interactions of the root systems for nutrients and water have an important influence on the performance of cropping systems. This study aims at identifying the impact of agroforestry treatments and fertilizer application on the root development of cassava-based cropping systems. At three sites in southern Bénin, the root development of intercropped maize (Zea mays) and cassava (Manihot esculenta) vs. sole-cropped cassava was monitored in an NPK fertilizer treatment and a no-fertilizer and no-mulch treatment. The latter represented local farmers practice. In addition, root development of sole-cropped cassava in three agroforestry systems was recorded at the same sites consisting of: (i) annually planted Cajanus cajan, (ii) perennial alleys, and (iii) a tree block of a Gliricidia sepium, Flemingia macrophylla, Parkia biglobosa, Millettia thonningii mixture. Soil monolith sampling was used to extract roots and generate data on the morphological development of root systems and their interactions. In general, intercropping reduced cassava root length density (RLD). Fertilizer applications significantly increased (P 0.05) the RLD of intercropped maize and intercropped cassava at all sites. Annual alleys of C. cajan developed smaller RLD and root weight densities (RWD) than the perennial tree mixture, leading to less interference with cassava. Block arrangement of the tree mixture had detrimental effects on the cassava growth in the adjacent rows, indicated by high RLD and RWD of the tree mixture. However, the overall effect on cassava yield was positive, when the crop yield is calculated on an entire field basis Elsevier B.V. All rights reserved. Keywords: Root interactions; Cassava; Maize; Sole cropping; Intercropping 1. Introduction Presently, West Africa is facing a situation where the fallow periods of the traditional bush fallow system are becoming too short for restoring soil fertility. This is mainly caused by the increasing food demand of a still rapidly growing population and by the strong contribution of the agricultural sector to the gross national product of the countries in this region. Corresponding author. Tel.: ; fax: address: losesigi@uni-hohenheim.de (S.J. Lose). Shortening fallow periods leads to mining of the soil nutrient stocks, including severe and partly irreversible soil degradation (Buresh et al., 1997; Scherr, 1999). In Bénin, nutrient losses are highly likely to be caused by leaching (Stahr et al., 1999a) and volatilization (Stahr et al., 1999b), although atmospheric inputs to the soil compensate for some of the nutrient losses (Herrmann, 1996). Progressively declining crop yields and loss of soil fertility are the consequences small farmers face while at the same time having to produce their crops on increasingly scarce land. This creates very uncertain socioeconomic conditions for the /$ see front matter 2003 Elsevier B.V. All rights reserved. doi: /s (03)

2 138 S.J. Lose et al. / Agriculture, Ecosystems and Environment 100 (2003) existing local farm family household system (Floquet, 1994; Schlauderer, 1997). Against this background, a Special Research Program on Adapted Farming in West Africa, focusing on various low to medium input alternatives for improving traditional cropping practice, was carried out by the University of Hohenheim from 1986 to During the first phase, agronomic studies concentrated on on-station trials and were replaced by on-farm trials in various edaphoclimatic zones of Bénin in 1991 (Böhringer and Leihner, 1997). On-station research results collected from alley cropping trials with Leucena leucocephala (Lam.), and Cajanus cajan (L.) Millsp. established at the IITA Bénin-Station showed the importance of aboveground interactions, such as competition for light, on the long-term productivity of intercropped maize (Zea mays L.) and cassava (Manihot esculenta Crantz) (Leihner et al., 1996). Furthermore, these experiments indicated the limitations of belowground effects, especially for nutrients and water (Akondé et al., 1996), and resulted in a detailed analysis of the nutrient budget for maize, cassava and trees (Akondé et al., 1997). In many experiments, the evidence of root system interactions in cropping systems is mentioned and their importance for growth and yield formation processes is underlined (Singh et al., 1989; Rao et al., 1990, 1991; Ruhigwa et al., 1992; Salazar et al., 1993; Schroth, 1995; Sanchez, 1995; Gregory, 1996). In the on-station experiments mentioned above, these effects were mainly studied by indirectly measuring changes in soil, water and plant parameters (Kühne, 1993; Ernst-Schaeben, 1994; Akondé et al., 1996). However, detailed root system observations are necessary to generate information on the morphological development of roots and interactions among the various components of multispecies cropping systems. This information is particularly important for understanding the complexity of the positive aspects of complementarity as well as the negative aspects of competition in agroforestry systems (Sanchez, 1995; Schroth, 1995; Van Noordwijk and Purnomosidhi, 1995; Ong and Huxley, 1996; Vandermeer, 1998). The general objective of this study was to broaden the knowledge on rooting systems and patterns in cropping systems. The specific objectives were to: (i) clarify the effect of mineral fertilizer on the root growth of cassava in sole cropping and cassava maize mixed cropping stands, (ii) monitor cassava root growth in three agroforestry systems, and (iii) to quantify the influence of edaphoclimatic conditions on the root development of crops and agroforestry species at representative sites in Bénin. 2. Materials and methods 2.1. Site description In 1991, three on-farm trials were established between longitude 2 E and 3 E, close to the villages of Houeto (6 27 N, 20 m above mean sea level), Attotinga (6 41 N, 82 m a.m.s.l.) and Agouagon (7 58 N, 190 m a.m.s.l.) in the Départements Atlantique and Zou in the southern part of the Republic of Benin (Böhringer and Leihner, 1997). Houeto and Attotinga are located in the Guinea-Congo savanna zone, whereas Agouagon represents the southern-guinea savanna zone (Bohlinger, 1998). Houeto and Attotinga have a bimodal rainfall distribution with a long rainy season from March to July and a short rainy season from September to November, whereas a unimodal rainfall distribution, lasting from March to October, characterized Agouagon (Fig. 1). Houeto and Attotinga are located on the old Triassic sandstone plateau of Eocene clay, called continental terminal. The soils are locally known as terre de barre (Raunet, 1973). Agouagon is situated on the Precambrian basement of the Precambrian crystalline base rock (granite and gneiss). Chemical properties and texture of the soil are very similar and differ in the morphology as iron concretions increase hydraulic conductivity (Stahr et al., 1996). Soil classification, and major chemical and physical soil properties of the sites are summarized in Table 1. The lower soil organic matter content at Houeto indicates a higher degree of degradation with no further differences in texture and chemical soil properties (Stahr et al., 1996) Experimental design At each site, 20 plots ranging from 400 to 600 m 2 were established in a randomized complete block design with five cropping systems as treatments and

3 S.J. Lose et al. / Agriculture, Ecosystems and Environment 100 (2003) Fig. 1. Rainfall distribution at the three on-farm sites at Attotinga, Houeto and Agouagon, Bénin, during the experimentation period (bars: actual values; lines: monthly average of the last 10 years). four replications. The plot size represented the average field size of small farmers in the research area. In 1996, each plot was further subdivided in one half with sole-cropped cassava and the second half with maize cassava intercropping. Three agroforestry treatments, (i) alley cropping of annually planted C. cajan (L.) Millsp., (=C) (ii) alley cropping of a Gliricidia sepium (Jacq.) Walp., Flemingia macrophylla (Willd.) Blume ex Miq., Parkia biglobosa (Jacq.) Benth. and Millettia thonningii (Schum. & Thonn.) Bak. mixture (=A), (iii) tree block planting of the same tree mixture, planted on one side of the plot (=B), were compared with two treatments without trees, (iv) NPK fertilizer application of 90 kg N ha 1, 39 kg P ha 1 and 75 kg K ha 1 (=+NPK), and (v) local farmers practice without use of fertilizer or mulch as control (= NPK). For feasibility reasons, only agroforestry treatments with sole-cropped cassava were included in the root study whereas sole- and intercropped cassava were considered in the no-tree treatments. The monitoring of the root system was carried out from March 1996 to April For detailed information on planting and pruning dates of the field trial, refer to Table 2. The trials were managed according to local farmers practice and supervised by researchers, e.g., the practice of ridging before planting at Agouagon and the weeding frequency at all sites. Fertilizer was split-applied in two doses, one at planting and one 4 weeks later Data collection and analysis Root sampling was coordinated with the destructive sampling of the aboveground biomass at 30, 90, 120 and 240 days after planting (DAP). Starting with a cassava plant, the sampling scheme was designed to cover the entire interface between two rows in the cassava maize intercropping of the control and the fertilized as well as in the sole-cropped plots of the agroforestry treatments (Fig. 2a). In the cassava sole cropping of the control and the fertilized plots, only half the distance between two rows was covered by sampling of soil monoliths, assuming a symmetrical root distribution for the second half of the distance between two cassava rows (Fig. 2b). This sampling procedure was completed in each of the four replications and repeated twice per plot. Thus, eight individual values were obtained for each position and depth. The sampling was carried out with a set of rectangular iron frames, which were driven into the soil with a hammer, cutting the soil into monoliths. All frames had the same basal surface (0.2m 0.16 m) but differed in height. These heights were 0.1, 0.2, 0.4, and 0.6 m, representing the depth of the soil monoliths, which were taken successively by placing each of the frames at exactly the same location. The maximum depth of 0.6 m was chosen as the soil was impenetrable to a greater depth during the dry season. At Attotinga and Agouagon, soil monolith samples were taken in all treatments. At Houeto, the sampling

4 Table 1 Chemical and physical soil properties of the central trails at the start of experiment (Source: Agbo, 1999) Site Soil type a Depth (cm) Horizon ph (H 2 O) OM (%) Ct (%) Nt (%) C/N P bray (mg kg 1 ) K (cmol kg 1 ) Mg (cmol kg 1 ) Ca (cmol kg 1 ) Na (cmol kg 1 ) CEC (cmol kg 1 ) Houeto Ferrali-haplic 0 7 A h acrisol 0 23 E B t Attotinga Ferrali-haplic 0 8 A h acrisol 0 30 E B t Agouagon Ferric lixisol 0 15 A h Ae Be Btcs a According to FAO (1990). BS (%) Sand (%) Silt (%) Clay (%) Stones (Vol. (%)) Bulk density (g cm 3 ) 140 S.J. Lose et al. / Agriculture, Ecosystems and Environment 100 (2003)

5 S.J. Lose et al. / Agriculture, Ecosystems and Environment 100 (2003) Table 2 Characteristics and management dates of the on-farm sites in Bénin (Source: Agbo, 1999) Site Fallow period prior to cultivation (years) Planting date, 1996 Pruning dates of Harvest date of Tree mixture (DAP) C. cajan (DAP) Maize, 1996 Cassava, 1997 Houeto 6 26 April 0, 35, 80, , 130, August 17 March Attotinga 6 29 March 0, 37, 85, , 135, July 17 February Agouagon 5 23 May 0, 35, 69, , 150, September 25 March was done in all treatments, except the control plots, which were too poorly established to provide the necessary number of plants to cover the entire growing season. The soil monolith samples were washed, sieved (Ø 0.2 mm) and roots of maize, cassava, trees, and other plants were separated phenologically by their different colors and rooting patterns. The separated root fractions of each sample were weighed and root lengths were measured by using desktop scanners (HP Scan Jet 3p and 4c) and the Delta-T Image Analysis Software package (Delta-T-Devices Ltd., Cambridge, UK) to measure the root length (Delta-T, 1993). Since the volume V of the soil monoliths was known, root weight density (RWD) and root length density (RLD) could be calculated as RLD i;a = RL i;a (1) V a and RWD i;a = RW i;a (2) V a where RLD is root length density, RWD is root weight density, RL the root length, RW the root weight, i the root fraction of a distinct species, i.e. cassava, maize, tree or weeds, a the volume of a distinct soil monolith position. The RLD and RWD of the weeds were recorded but not shown in this paper to keep the figures more comprehensible. The statistical analysis of variance (ANOVA) of the randomized complete block design was carried out with SAS for PC V , by using the mixed procedure. Means were separated in case of significant F-test, with Fisher s protected least significant difference test (LSD) at a 5% probability level for comparisons between two treatments. For more than three treatments, the LSD was replaced by Tukey s honest significant difference test (HSD) at a 5% probability level (SAS, 1996). 3. Results 3.1. Root length density (RLD) At Houeto, the analysis of variance of the factor cropping system showed no significant effects except for the soil depth to 0.2 m at 240 DAP (Table 3). However, NPK fertilization led to increased root growth of sole-cropped cassava at soil depths of and m after application of the second fertilizer dose as indicated by the higher RLD of cassava up to 90 DAP. At 120 DAP, the RLD of sole-cropped and intercropped fertilized cassava reached the same magnitude as the RLD of cassava in the tree mixture and the block planting agroforestry treatments, which were significantly higher than in the C. cajan treatment (Fig. 3). At this sample date, the analysis of variance of the treatment effects had become significant for the upper soil layers (Table 3). Data collected at Attotinga and Agouagon showed that maize root growth was positively affected by fertilizer applications as indicated by the higher RLD of maize. This is also emphasized by significant treatment effects on maize RLD in the ANOVA until 90 DAP. At the beginning of the cropping season, RLD distribution of maize and cassava was significantly different (Figs. 4a and b and 5a and b). The analysis of variance for the two factors, cropping system and treatment of maize and cassava, was significant during almost the entire cropping period within all examined depths (Table 3). The faster root development of higher maize RLD, accompanied by a slower initial development

6 142 S.J. Lose et al. / Agriculture, Ecosystems and Environment 100 (2003) Fig. 2. Scheme of soil monolith sampling in: (a) intercropping systems (identical to alley cropping and block planting) and (b) sole-cropping systems. of cassava RLD, indicated a temporal sharing of the same soil volume and resources such as water and nutrients. Furthermore, the higher RLD of intercropped cassava in deeper horizons compared to the more profuse distribution of roots in the sole-cropped system in the upper horizons emphasizes a stratification of the root distribution or a spatial sharing of soil resources. However, the impact of fertilizer on the RLD

7 S.J. Lose et al. / Agriculture, Ecosystems and Environment 100 (2003) Table 3 Analysis of variance for cassava, maize and tree root length density (RLD) at Attotinga, Agouagon and Houeto, Bénin, for the cropping period Site Plant Source a Soil depth (m) RLD (DAP) Attotinga and A gouagon Tree T b 0.1 NS c NS NS 0.6 NS Maize T 0.1 NS d NS NS NS Cassava C e NS NS NS 0.6 NS NS T 0.1 NS NS NS 0.2 NS NS NS C T 0.1 NS + NS 0.2 NS NS NS 0.4 NS NS NS NS Houeto Tree T NS 0.2 NS NS NS 0.6 NS NS + NS Cassava C 0.1 NS NS NS NS 0.2 NS NS NS NS NS NS NS 0.6 NS NS NS NS T 0.1 NS NS NS NS NS 0.4 NS NS NS NS NS NS C T 0.1 NS + NS NS 0.2 NS NS 0.4 NS + NS NS 0.6 NS NS N NS a Source of variance. b Treatment effects calculated by comparing all treatments. c F-test not significant at the 0.1 probability level. d No maize roots detected. e Cropping system effects calculated by comparing the two cropping systems. + P 0.1. P P P

8 144 S.J. Lose et al. / Agriculture, Ecosystems and Environment 100 (2003) Fig. 3. Development of cassava root length density (RLD) down to 0.6 m soil depth at Houeto as affected by: (a) NPK application (( ) cassava; (- - -) intercropping; ( ) sole-cropping) and (b) tree root length density of the agroforestry treatments ( A : alley cropping with tree mixture; B : block planting of tree mixture; C : alley cropping with C. cajan), Error bars refer to Fisher s least significance difference test between cropping systems (a) or treatments (b) within depth and date at P development was obviously stronger at Attotinga (Fig. 4a and b). At Agouagon, the traditional management practice of ridging the topsoil before planting affected root growth until 120 DAP when much lower RLD was obtained during this period (Fig. 5a and b). At the beginning of the cropping season, the tree root system of the alley cropping with tree mixture was much better developed at Attotinga (Fig. 4c), whereas both other sites presented a much lower RLD at this date (Figs. 3b and 5c). This is possibly associated with a better restoration of the root system during the period from harvest of the previous cropping season to the start of the next at Attotinga. The tree root concentration, however, shifted to upper horizons after the first pruning, but decreased further with subsequent prunings (Fig. 4c). At Houeto, the ANOVA showed significant treatment effects on tree RLD (Table 3). The root system of alley cropping with tree mixture was also well recovered at the beginning of the growing season. The tree RLD of the topsoil horizons started to decrease after the first prunings and remained low until the end of the season. After the end of the pruning period, the RLD of trees started to recover immediately (Fig. 3c). In the presence of high tree RLD, the RLD of the associated cassava was low at both sites Houeto and Attotinga (Figs. 3b and 4c). At Attotinga and Agouagon, the ANOVA showed significant treatment effects on the tree RLD, except for the topsoil layer between 90 and 120 DAP and at 240 DAP for the three lower compartments (Table 3). The local crop management practice of ridging also strongly influenced tree RLD development in the agroforestry treatments.

9 S.J. Lose et al. / Agriculture, Ecosystems and Environment 100 (2003) Fig. 4. Development of cassava and maize root length density (RLD) down to 0.6 m soil depth at Attotinga as affected by: (a) cropping systems (( ) cassava; ( ) maize; (- - -) intercropping; ( ) sole-cropping), (b) NPK application and (c) tree root length density of the agroforestry treatments ( A : alley cropping with tree mixture; B : block planting of tree mixture; C : alley cropping with C. cajan), Error bars refer to Fisher s least significance difference test between cropping systems (a) and (b) or treatments (c) within depth and date at P At Agouagon, tree roots of all agroforestry treatments were forced to explore deeper soil horizons until 120 DAP when the ridges started to flatten out (Fig. 5c). Subsequently, the root systems of the tree mixture in the alley cropping and the block planting treatment also started to grow towards the topsoil and explore the upper soil compartments. Therefore, the cassava root system benefited from ridging until the period when the trees started to re-explore the topsoil. At Attotinga, the root system of C. cajan developed continuously into deeper soil horizons until 120 DAP and interfered with the simultaneous growth of the cassava root system only marginally (Fig. 4c). Towards the end of the growing season, C. cajan showed

10 146 S.J. Lose et al. / Agriculture, Ecosystems and Environment 100 (2003) Fig. 5. Development of cassava and maize root length density (RLD) down to 0.6 m soil depth at Agouagon as affected by: (a) cropping systems (( ) cassava; ( ) maize; (- - -) intercropping; ( ) sole-cropping), (b) NPK application and (c) tree root length density of the agroforestry treatments ( A : alley cropping with tree mixture; C : alley cropping with C. cajan; B : block planting of tree mixture), Error bars refer to Fisher s least significance difference test between cropping systems (a) and (b) or treatments (c) within depth and date at P a severe curtailing of roots in the alleys due to pruning and fungus infestation. Nevertheless, the low interference of both species favored the development of an intense cassava RLD in all horizons, which was reduced only during the period of low rainfall in summer. At Houeto, alley cropping with C. cajan led to low RLD of cassava (Fig. 3b), whereas C. cajan did not show any tendency for root growth into deeper horizons at Agouagon (Fig. 5c). But the cassava root system exploited the upper horizons evenly Root weight density (RWD) At Attotinga and Agouagon, the ANOVA of the treatment effects on cassava and tree RWD were significant for the entire cropping period (Table 4).

11 S.J. Lose et al. / Agriculture, Ecosystems and Environment 100 (2003) Fig. 6. Development of average cassava and tree root weight density (RWD) down to 0.6 m soil depth as affected by NPK application and agroforestry treatments at Houeto, Attotinga and Agouagon, Bénin, Error bars for cassava RWD refer to Tukey s honest significant difference test between treatments within date at P Error bars for tree RWD refer to Fisher s least significance difference test between treatments within date at P 0.05.

12 148 S.J. Lose et al. / Agriculture, Ecosystems and Environment 100 (2003) Table 4 Analysis of variance for the average of soil depths of cassava and tree root weight density (RWD) at Attotinga, Agouagon and Houeto, Bénin, for the cropping period a Site Plant Source a RWD (DAP) Attotinga and Agouagon Tree Tb Cassava T + Houeto Tree T NS c NS NS Cassava T NS NS NS a Source of variance. b Treatment effects calculated by comparing all treatments. c F-test not significant at the 0.1 probability level. + P 0.1. P P P Whereas at Houeto, significant effects were observed only for tree RWD at 240 DAP and for cassava RWD at 90 DAP. The two less fertile sites, Houeto and Agouagon, showed some similarities in the development of RWD (Fig. 6a and c). This is, however, the result of different factors. At Houeto, the already highly degraded soil with its low nutrient status was the main reason for the decrease of cassava and trees RWD obtained at the end of the growing season. To some extent, fertilizer applications increased RWD of cassava during the short rainy season. At Agouagon, however, water shortage due to low rainfall depressed the cassava RWD during the second half of the growing period (Figs. 1 and 6c). The lower tree RWD at this site supported this observation. The very low C. cajan RWD at Agouagon indicated a more complementary root distribution, which did not interfere significantly with the cassava row. The local cropping practice of ridging at the beginning of the season contributed to a distinctly separate root growth of the annually cultivated species, cassava and C. cajan. Nevertheless, at this site dieback of C. cajan towards the end of the growing season attributed to the low RWD values. At Attotinga, none of these effects were observed (Fig. 6b). Contrasting RWD results were found at this location as soil conditions and rainfall distribution were much more favorable. In all treatments, the highest RWD was observed at the end of the growing period. Among the sole-cropped treatments, highest cassava RWD was obtained when fertilizer was applied. Intercropping of maize and cassava, however, produced a slightly higher cassava RWD. The two alley cropping systems favored cassava development only at the beginning of the season, where high cassava RWD were observed at 90 DAP. However, the annually re-planted C. cajan seems to be more suited for an alley planting design, simultaneously leading to high RWD of both cassava and C. cajan (Fig. 6c). The most obvious competitive effect on the RWD of the adjacently growing cassava was produced by block planting treatment with tree mixture. This planting pattern strongly suppressed cassava RWD as indicated by high tree RWD in the adjacent row. 4. Discussion The results of the root studies presented here support the presumptions of the earlier studies of the above mentioned Special Research Program on the degree of competition in agroforestry systems, caused by root system interactions and which were assessed only indirectly in the previous periods (Akondé et al., 1996; Böhringer and Leihner, 1997) Site and management effects on RLD Site effects had a strong impact on the RLD and led to different values. At Agouagon and Houeto, a generally lower soil fertility level or an already ongoing process of soil degradation may be responsible for the lower RLD at both sites when compared to Attotinga with its higher RLD. In addition, data derived from Attotinga and Agouagon underline the potential of root studies to understand the impacts of intercropping on the biomass production of the associated crops. In this case, data from maize cassava intercropping treatments clearly indicate that fertilizer application mainly favored maize root growth, which finally led to higher maize grain yields at Attotinga (Agbo, 1999). The root yield of the intercropped cassava did not increase with fertilizer use. Fertilizer application possibly improved the potential of maize in competing for nutrients, which was reflected in the simultaneous reduction of the cassava RLD during the entire growing season at

13 S.J. Lose et al. / Agriculture, Ecosystems and Environment 100 (2003) Attotinga and the reduced cassava RLD during the maize growing period at Agouagon. The two climatically different environments of Attotinga and Agouagon favored cassava root growth in the alley cropping system with C. cajan compared to the alley cropping system with a perennial tree mixture. This may be explained by the delayed and less interfering root development of C. cajan in the topsoil horizons during critical growth periods of cassava at around 3 months after planting and by the generally deeper rooting habit of C. cajan, compared to that of cassava. Generally, G. sepium is considered a relatively non-competitive tree species with pronounced response of its root system to shoot pruning (Schroth and Lehmann, 1995). Even where competition occurs, this is not necessarily negative for the system as a whole, as total nutrient and water use may be increased. Defining and obtaining the right amount of competition within the tree crop association is a very difficult but necessary task for agroforestry design and management. The choice of the appropriate woody species for the use in an agroforestry system requires a thorough knowledge of their fine and structural root architecture (Akinnifesi et al., 1999). At Agouagon, ridging prevented tree root exploitation of the upper horizons in the cassava row effectively excluded direct root competition. Therefore, ridging may be considered as a cropping practice for improving the performance of the agroforestry system itself. The importance of management implications on the performance of agroforestry systems is outlined by Rao et al. (1998). Deeper rooting species that extent beyond the crop-rooting zone, appear to be better suited to systems on acidic soils. Spatial complementarity in water use between tree and crop components is also desirable as a large percentage of fine roots of many hedgerow species were in the top 0.5 m soil layer; where crop roots are also concentrated, so competition may result. Alley cropping with a tree mixture showed strongest root growth in the upper soil horizons, diminishing cassava RLD. According to Van Noordwijk et al. (1996), this is undesirable as a high horizontal spread of tree roots in the topsoil may lead to competition with crops for nutrients and water. The root architecture requirements of a simultaneous agroforestry system include low occurrence of large woody roots and low extensive root systems of the tree in the crop-rooting zone (Van Noordwijk and Purnomosidhi, 1995). At Attotinga, the block design suppressed the root system of adjacent cassava rows in a soil depth of m more strongly than the alley cropping system of the same tree mixture. Severe competition, however, occurred only in the cassava rows adjacent to the block. Thus, with respect to the whole field, the block diminished competition by reducing the length of the effective tree crop interface Further factors influencing root system development In the same ecoregion, Tossah et al. (1999) emphasized the importance of the factors soil and climate for the efficiency and sustainability of alley cropping systems. As our studies show, these factors are particularly important for the development of different root systems. However, even more important than these is the selection of the best-suited hedgerow species (Aihou et al., 1999) in the most adapted cropping system. Furthermore, major effects on the root activity pattern in space and time can be attributed to management practices such as pruning. Although this has been discussed by Rao et al. (1998) and Rowe et al. (1999), it is still difficult to draw general conclusions. Facilitative, complementary, and competitive effects of root systems in agroforestry cause an inherent conflict in defining the desirable root characteristics (Schroth, 1999). Not only are root systems genetically determined but they are also the product of environmental factors, which may be manipulated by the land-user, i.e., associations of plants, fertilizer distribution, and shoot pruning. Fine root length density is not directly proportional to root function. Competition between neighboring roots occurs when the nutrient depletion zones around the roots overlap, which is caused by the uptake of nutrients into the roots (Ong et al., 1999) Implication of RWD The RWD, although obtained only from the root fraction greater than 0.2 mm diameter, represents approximately 80 90% of entire root biomass of the studied species (Böhm, 1979; Toky and Bisht, 1992)

14 150 S.J. Lose et al. / Agriculture, Ecosystems and Environment 100 (2003) and can be used for estimating the nutrient contents in the belowground part of different plants. Lehmann et al. (1995), Risasi et al. (1998), and Agbo (1999) provide analyses of nutrient concentrations in roots of different diameter classes in these ecozones, which can be used for future estimations. 5. Conclusions Evaluating the RLD distribution in a soil volume is considered a valuable method to explain and understand both the temporal and spatial sharing of water and nutrient pools by plant roots and nutrient uptake by roots. The assessment of the RWD, on the other hand, is well suited for explaining the nutrient distribution in the soil. Furthermore, this study showed the potential of traditional cropping practices as means for improving crop performance in agroforestry systems. However, further tests to confirm this observation are required before introducing such technologies in present agroforestry systems. Acknowledgements The authors gratefully acknowledge the Deutsche Forschungsgemeinschaft (DFG) and the Ministry of Science and Research (MWF) of Baden Württemberg who funded this research within the Special Research Program 308 at the University of Hohenheim. The authors would also like to thank the reviewers for their very helpful corrections and suggestions. References Agbo, B.P., Restoring Crop Productivity in West Africa: The Potential of Agroforestry. Markgraf Verlag, Weikersheim, Germany, p Aihou, K., Sanginga, N., Vanlauwe, B., Lyasse, O., Diels, J., Merckx, R., Alley cropping in the moist savanna of West Africa. I. Restoration and maintenance of soil fertility on terre de barre soils in Bénin Republic. Agrofor. Syst. 42, Akinnifesi, F.K., Kang, B.T., Ladipo, D.O., Structural root form and fine root distribution of some woody species evaluated for agroforestry systems. Agrofor. Syst. 42, Akondé, T.P., Leihner, D.E., Steinmüller, N., Alley cropping on an Ultisol in subhumid Benin. I. Long-term effect on maize, cassava and tree productivity. Agrofor. Syst. 32, Akondé, T.P., Leihner, D.E., Kühne, R.F., Steinmüller, N., Alley cropping on an Ultisol in subhumid Benin. III. Nutrient budget of maize, cassava and trees. Agrofor. Syst. 37, Bohlinger, B., Die spontane Vegetation in traditionellen Anbausystemen Benins ihre Bedeutung und Möglichkeiten des Managements. PLITS 16 (2), 175. Böhm, W., Methods of studying root systems. Ecological Studies, vol. 33. Springer, Berlin, Germany, 188 pp. Böhringer, A., Leihner, D.E., A comparison of alley cropping and block planting systems in sub-humid Bénin. Agrofor. Syst. 35, Buresh, R.J., Sanchez, P.A., Calhoun, F. (Eds.), Replenishing Soil Fertility in Africa. Soil Science Society of America (SSSA), Wisconsin, USA, 250 pp. Delta-T, Delta-T Scan User Manual. Delta-T Devices Ltd., Cambridge, UK, p Ernst-Schaeben, R., Potential of alley cropping maize and cassava in South Benin, West Africa: evaluation of the establishing-phase. Ph.D. Thesis. Department of Plant Production and Agroecology in the Tropics and Subtropics, University of Hohenheim, Stuttgart, Germany, p FAO, FAO UNESCO Soil Map of the World. Revised Legend. Soil Bulletin No. 60. FAO, Rome, Italy, 119 pp. Floquet, A., Dynamique de l intensification des exploitation au sud du Bénin et innovation endogènes: Un défi pour la recherche agronomique. University of Hohenheim, Stuttgart, Germany, 411 pp. Gregory, P.J., Approaches to modeling root growth and the uptake of water and nutrients in agroforestry systems. Agrofor. Syst. 34, Herrmann, L., Staubdeposition auf Böden West-Afrikas. Hohenheimer Bodenkundliche Hefte no. 36. University of Hohenheim, Stuttgart, Germany, p Kühne, R.F., Wasser- und Nährstoffhaushalt in Mais-Maniok-Anbausystemen mit und ohne Integration von Alleekulturen, Alley cropping in Süd-Benin. Hohenheimer Bodenkundliche Hefte no. 13. University of Hohenheim, Stuttgart, Germany, p Lehmann, J., Schroth, G., Zech, W., Decomposition and nutrient release from leaves, twigs and roots of three alleycropped tree legumes in central Togo. Agrofor. Syst. 29, Leihner, D.E., Ernst-Schaeben, R., Akondé, T.P., Steinmüller, N., Alley cropping on an Ultisol in subhumid Benin. II. Changes in crop physiology and tree crop competition. Agrofor. Syst. 34, Ong, C.K., Huxley, P., Tree Crop Interactions A Physiological Approach. CAB International, Wallingford, Oxon, UK, 390 pp. Ong, C.K., Deans, J.D., Wilson, J., Mutua, J., Khan, A.A.H., Lawson, E.M., Exploring below ground complementarity in agroforestry using sap flow and root fractal techniques. Agrofor. Syst. 44, Rao, M.R., Sharma, M.M., Ong, C.K., A study of the potential of hedgerow intercropping in semi-arid India using a two-way systematic design. Agrofor. Syst. 11, Rao, M.R., Sharma, M.M., Ong, C.K., A tree crop interface design and its use for evaluating the potential of hedgerow intercropping. Agrofor. Syst. 13,

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