Dry Matter Production, Nutrient Cycled and Removed, and Soil Fertility Changes in Yam-Based Cropping Systems with Herbaceous Legumes in the Guinea-Sudan Zone of Benin

2.1. Study Sites

The study was carried out in the Guinea-Sudan transition zone of Benin (centre of Benin) in four sites: Miniffi (District of Dassa-Zoumè), Gomè (Glazoué), Akpéro, and Gbanlin (Ouessè) with latitudes 7°45′ and 8°40′ north and longitudes 2°20′ and 2°35′ east (Figure 1).

The climate is tropical transitional Guinea-Sudan with a rainfall distribution gradient from bimodal (Southern Benin) to monomodal (Northern Benin). The average annual rainfall during the study period was 1052 mm (2002), 1386 mm (2003), 983 mm (2004), and 797 mm (2005). The rainfall regime in the study area is variable and unequal distribution (i.e., number of rainy days per month) varies from one site to another. The 2002 and 2003 cropping seasons were wet and had better rainfall distribution with an average annual precipitation of 1200 mm, whereas 2004 and 2005 were dry (890 mm) with relatively low rainfall distribution.

Most of the soils are tropical ferruginous soils [19], originally from Precambrian crystalline rocks (granite and gneiss), and classified as plinthosols (Gbanlin and Akpéro) and luvisols (Miniffi and Gomè) [20] (Table 1). Miniffi, Akpéro, and Gbanlin are located on a plateau while Gomè is on lowland. Akpéro is close to forest while Gbanlin, Miniffi, and Gomè are far. There is a rising gradient of fertility from the continuous cropping system on degraded soils towards the forests. This degradation is related to soil organic matter decrease, which leads to nutrient depletion (nutrients removed in the crop harvest, leaching, and erosion). Vegetation is a degraded woody savannah type. Maize, yam, cassava, and groundnut are annual cropping systems and the cash crops are cotton and soybean. Mineral fertilizer application appears to be essential. Smallholder farmers use fertilizers on maize on depleted soils depending on cash and inputs availability. Cotton is not mixed cropping, but pure crop in rotation with other crops (maize or sorghum).

2.2. On-Farm Experiment

The concept of the experiment was to produce residue biomass followed by planting yam in rotation cropping systems. A previous cover crop (fallows or intercropped maize/legume) was designed to provide organic matter for the following main crop (yam) (Table 1).

2.3. Data Collection

Composite soil samples were collected in each field before the beginning of the experiment along plot transects at soil depths of 0–10 cm and 10–20 cm (32 farm fields × 2 depths = 64 samples) in order to determine soil characteristics. At the end of 2005 before yam harvesting, composite soil samples were collected at the same depths in the moulds along plot transects (32 farm fields × 4 treatments × 2 depths = 256 samples).

Prior to ridging, in four 1 m² quadrats within each plot the aboveground biomass of herbaceous legumes and fallow was collected in October 2002 and 2004. The biomass samples were dried at 60°C until constant weight and then dry weight was determined. At maturity, maize grain and stover were harvested per row on each plot and dry matter (DM) determined. DM of yam tubers and shoots was estimated on each plot in December 2003 and 2005 (Tables 2 and 3).

2.4. Soil and Plant Nutrients Content

The nutrients contents of the soil samples were performed in the Laboratory of Soil Sciences, Water and Environment (LSSEE) of INRAB (Benin National Research Institute). The plant nutrient content was estimated according to the biomass amount.

Soil and plant macronutrients content (N, P, and K) were analyzed. Nitrogen (N) content was analyzed using the Kjeldahl method [22], available phosphorus with Bray 1 method [23], potassium with the FAO method [24, 25], organic carbon with the Walkley and Black method [26], and soil fractionation with Robinson method [27] and pH (H₂O) (using a glass electrode in 1 : 2.5 v/v soil solution). Only yam tuber and maize grain were removed, and all other plants parts were recycled (A. gayanus, maize stover, yam shoot, A. histrix, and M. pruriens). Yam or M. pruriens shoot included leaves. Nutrient removed or recycled was calculated as a summation of nutrient concentration time dry matter of the respective plant parts. Dry matter removed or recycled was calculated as a summation of dry matter of the respective plant parts.

2.5. Analyses of Variance to Test the Effect of Site, Year, and Treatment on Yam Yield

Analysis of variance (ANOVA) using the general linear model (GLM) procedure [28] was applied to the DM production (tubers and shoots), nutrient contribution to the systems, and soil properties at depths 0–10 and 10–20 cm. The experiment was conducted with 32 farmers, eight in each site. For each of them, a randomized complete block design with four treatments and four replicates was carried out using a partial nested model with five factors: Year, Replicate, Farmer, Site, and Treatment. The random factors were “Year” and “Replicate” and “Farmer.” Farmer was considered as nested within “Site” and “Replicate” as nested within “Farmer.” The fixed factors were “Treatment” and “Site.” Sites were considered as fixed based on certain criteria such as landscape (lowland and plateau), soil type, and initial soil fertility. Yield values were logarithmically transformed to normalize the data and to stabilize population variance. The GLM was computed to assess the interactions between the factors involved. Least square means and standard error were also computed for factor levels, and the Newman and Keuls test was applied for differences between treatments. Significance was regarded at P ≤ 0.05.

3.1. Initial Soil Characteristics

The relevant general soil physical and chemical characteristics before are presented in Table 4.

Site physical characteristics such as soil texture (sand) were relatively high (74.778%–88.79%) followed by silt (5.55%–17.36%) and clay (5.66%–7.861%) with the lowest content. The soils had a neutral reaction, with pH (H2O) ranging from 6.3 to 6.8.

The initial soil fertility status of different sites was low. Soil organic matter (SOM) contents were low in all fields, ranging from 0.93% to 2.258%, and the C : N ratio ranged from 8.69 to 11.70. Available P levels were very low and varied from 3.012 to 20.125 mg/kg-soil. Soil N concentration ranged from 0.056% to 0.112%. N, P, and SOM contents were significantly higher in 0–10 cm than in 10–20 cm depth, except at Gbanlin site for N and SOM. Gomè site showed, for both soil depths, the lowest values of carbon (C%), N%, P (mg/kg-soil), and organic matter (%), whereas Akpéro had the highest values.

3.2. Dry Matter Production and Nutrient Contribution to the Systems

In the 2002 and 2004 cropping seasons, the highest biomass dry matter (DM) amount recycled was recorded on TMM (Table 5).

The ANOVA partial nested model shows that yam yield DM differed significantly depending on the factor Treatment (P < 0.001). The factors Site and Year were not significant for yam yields DM. But Replicate (P < 0.001), Treatment × Farmer (P < 0.01), and Year × Farmer interactions (P < 0.001) were significant (Table 6).

Dry matter (t ha⁻¹) of yam tubers removed and yam shoots recycled, N, P, and K content (kg ha⁻¹) dry matter of plant parts removed in the crop harvest, and those returned to the soil in yam-based cropping systems were significantly higher in TMA and TMM than in T0 and TM during both cropping seasons (Tables 7 and 8).

Therefore, total plant N, P, and K (kg ha⁻¹) dry matter removed in the crop harvest and those returned to the soil in yam-based cropping systems were significantly higher in TMA and TMM than in T0 and TM during both cropping seasons (Table 9).

3.3. Effects of Treatments on Soil Characteristics

Afterwards soil characteristics at the end of the experiment globally showed relatively low clay, silt, and relatively high sand concentration on different sites under different treatments (T0, TM, TMA, and TMM) in comparison with initial soil characteristics at the beginning of the experiment. Soil organic matter concentration was improved at 10–20 cm depth particularly in Miniffi (1.247%, 1.176%, 1.326%, and 1.409%) on T0, TM, TMA, and TMM, respectively, and Gomè (1.010%, 0.959%, 1.046%, and 1.126%). Globally, soil N and P concentrations were improved on different sites on treatments TMA and TMM in 0–10 cm or 10–20 cm depth (Tables 10(a)–10(d)).

The end of study soil analysis showed soil chemical properties (SOM%, N%, P (mg/kg-soil), K⁺ cmol kg⁻¹, and pH water) significantly higher in TMA and TMM than in traditional systems T0 and TM (P < 0.001). Soil clay contents were significantly higher in TMA, TMM, and T0 than in TM (P < 0.001). No significant difference was observed for silt and sand concentrations for different treatments (Table 10(e)).

4.1. Dry Matter and Nutrients Recycled in Yam-Based Cropping Systems

The highest biomass dry matter (DM) amount recycled was recorded on Mucuna/maize intercropping (TMM). Mucuna grows rapidly and DM production can reach 10 t ha⁻¹ [2, 17, 29]. In fact, Mucuna creeps and climbs maize straw in pattern crop allowing the lianas staking. Therefore, Mucuna large leaves profit from solar radiations improving the photosynthetic activity and the plant productivity. Mucuna reaches the physiological maturity (flowering time) between 180 and 240 days after grains planting in the study area in comparison with Aeschynomene (200–306 days) [30, 31].

DM of yam shoots recycled on TMA and TMM were significantly higher in 2005 (dry year) than in 2003 (humid year). The chemical fertilizers applied and the above biomass DM of intercropping maize and herbaceous legume recycled and accumulated in 2002, 2003, and 2004 could have resulted in a combined beneficial effect of water, nutrient use, and plant growth in 2005. DM amounts of M. pruriens, A. histrix, and maize stover recycled were higher in 2002-2003 (humid year) than in 2004-2005 (dry year). In fact, plant yields and agronomic productivity were constrained by recurring drought stress exacerbated by highly variable and unpredictable rains. M. pruriens stover showed the highest DM amount followed by A. histrix whatever the year and this could reach 10 t ha⁻¹ [18] because M. Pruriens, compared with A. histrix, grows more rapidly and close.

The nutrient (N, P, and K) levels removed or recycled fit the DM production (tubers and shoots) and then varied according to treatment and cropping season.

4.2. Impact of Yam-Based Cropping Systems with Herbaceous Legumes on Soil Properties

Most of the soils as mentioned above are tropical ferruginous soils, originally from Precambrian crystalline rocks (granite and gneiss) and classified as plinthosols (Gbanlin and Akpéro) and luvisols (Miniffi and Gomè). Miniffi and Akpéro are located on a plateau (well-drained soils) while Gomè is on lowland (more poorly drained soils). Gbanlin is located on an undulating plateau with concretions. Soil chemical analysis showed that the soil was deficient in N, P, and K and soil organic matter (SOM). This could be due to the mining agriculture and also a consequence of the mechanical destruction of the soil structure during the ridging for yam crop. In fact yam is a demanding crop in terms of organic matter and nutrients. Research [32] reported that yam yielding about 30 t of fresh tuber ha⁻¹ removes 120 N kg ha⁻¹, 5.1 P kg ha⁻¹, and 111 K kg t⁻¹. When land is used too intensively, the SOM is rapidly reduced in the unstable fraction. In the short and medium term, this reduction leads to a decrease in soil biological activity and, then, contributes to soil degradation and depletion [33]. Many studies report that soil organic matter (SOM) decreases in cultivated soils [33]. This decrease is linked to the depth of the cultivated soil layer and is probably exacerbated in yam-based cropping systems.

Nitrogen is the most deficient component of these soils grown with low organic matter content. Total nitrogen deficiency of these soils lies in the fact that nitrogen is the only major nutrient that does not exist in the bedrock. Further, the transfer of atmospheric nitrogen to the soil by biological and chemical process is slow. Losses of nitrogen in these soils are common because of the high volatility and solubility of this nutrient. Nitrogen is generated by the breakdown of inherent organic matter and needs to be supplemented with other sources of organic materials or mineral fertilizer. Many studies focusing on these elements conclude that there is an indisputable need to correct the lack of N and P in the soil in Africa [2, 6].

It is possible to reduce or stop ongoing soil degradation and the decrease in yield with such rotations including improved short fallows or intercropping with herbaceous legumes. The use of legumes improves levels of concentration of the soil parameters. The improvement of the clay concentration at the end of the perennial experiment could be due to the process of the composite soil samples collected on the ridges resulting from the brewing of the soil deep layer relatively rich in clay and the soil horizon surface after ridging. Indeed, ridging allows increasing the volume of the soil deep layer and contributes to the incorporation of organic residues into the soil.

Significant differences in total SOM and nutrients increase with treatments TMA and TMM in comparison with T0 and TM could be due to the faster decomposition of fermentable green manure (herbaceous legumes) with low humification coefficient (5%) added to the moderate decomposition of lignified maize stover on relatively degraded soils [34]. Our observations are in agreement with those of [35] who reported that cropping systems and organic manures have the most influence on the SOM. Rotations with M. pruriens and A. histrix represented a source of easily available N, P, and K for the yam crop which could be related to their faster decomposition and nutrient release, compared with the slower release of nutrients by poorer quality materials such as maize stover and A. gayanus grass. In Ghana, studying the effect of cropping sequences with cassava and legume crops, [36] indicated that only 30% of M. pruriens litter remained six weeks after incorporation of the biomass. References [37] and [38] that studied the traditional M. pruriens-maize rotation in Honduras estimated that 83% of nitrogen produced by a mulch of M. pruriens was available for the following maize crop. They also observed that available P remained practically constant, with 15 to 20 mg/kg-soil in the surface horizon in spite of P exports by maize. Reference [38] concluded that the practice of continued rotation with M. pruriens and maize prevented soil N depletion for at least 15 years.

Our results showed that legumes improved soil P. Legumes fallows with M. pruriens are known especially for improving the quantity of available P fractions in the soil for subsequent crops [39]. Nevertheless, they depend on the inherent P levels in the soils. M. pruriens root exudates could solubilize P increasing its availability. In the study of [40], organic materials have also been found to reduce P sorption capacity of soils and increase crop yields in P limiting soils.

The soil K concentrations were improved in our study (Table 4). Reference [3] showed soil K concentration of 0.82 cmol kg⁻¹ in the 0–20 cm soil layer and decreasing significantly with cultivation. The rate of decline was about 0.023–0.054 cmol kg⁻¹ year⁻¹ in the 0–20 cm soil layer [3].