Abstract
Tree planted fallow is an agroforestry system that may restore degraded soils and protect them from erosion. In this study, sandy soils properties of Acacia senegal planted fallows (AF) were assessed and compared to those from the continuous cropped system (CC) in 3 sites from Northern Cameroon in order to determine its suitability to restore soil fertility and sustain crop productivity. Soil samples were collected from the topsoil (0 to 20 cm) and the subsoil (20 to 40 cm) and subjected to physicochemical analyses. The trials were established for 2 consecutive years, respectively with sorghum (Sorghum bicolor) and cowpea (Vigna unguiculata). Results confirmed the sandy (more than 80% of sand) and acidic (4.42 ≤ pH ≤ 6.59) soil characters. In every site, topsoil from AF was relatively more fertile than from CC. Globally, nutrients content were influenced by tree density and fallow duration. The more improved elements were organic matter, nitrogen and pH. Sorghum and cowpea yields were quite variable depending on fallow duration, tree density and conversion form. The highest crop yields (3.4 tha-1 for sorghum and 2.4 tha-1 for cowpea) were obtained in 19 years old AF converted by partial clear-felling. The intercropping process by partial clear-felling of trees was the best conversion form. Overall findings indicated that fallowing with A. senegal can reduce soil acidity, restore nutrients and therefore it constitutes a suitable agroforestry system that may sustain annual crops productivity. However, researches have to determine the best tree density for intercropping and the tools for their sustainable management. Key words: Acacia senegal fallow, agroforestry, sandy soils, continuous cropping, North Cameroon.  
Highlights
Soils in sub-Saharan Africa are characterized by nutrients depletion along with time
The high difference in yields observed between crops in fallowed plots (AsEc and AsCb) and in continuous cropped (CC) could be due in one hand to soil fertility improvement through trees litter fall and on the other hand to the microclimate created which were favorable to crops growth
This study was focused on the conversion of old A. senegal plantation in order to determine their suitability, to restore degraded sandy soils and sustain crop productivity
Summary
Soils in sub-Saharan Africa are characterized by nutrients depletion along with time. An integration of this tree in agroforestry system as a means of restoring the soil fertility and promoting gum arabic production was observed and has been widely published (Fadl, 2010; Palou Madi et al, 2010; Kissi, 2011) It was recognized and considered as one of the most successful forms of natural forests management in tropical dry lands. In semiarid sub-Saharan, planting A. senegal seemed to be more adapted because of its high adaptation potential to drought and fodder generation for animal as well as wood and gum arabic (Nasreldin, 2004; Palou Madi et al, 2010; Abib et al, 2013) It can stabilize sandy soils and restore their fertility, protecting them from erosion (Mallet et al, 2002; Harmand et al, 2012). This paper (1) determines the influence of A. senegal fallow duration on soil properties; (2) determines the effect of conversion form on sorghum and cowpea production and (3) assesses and compares sorghum and cowpea production of fallowed land to continuous cropping without A. senegal
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