Original articleThe influence of a shrub-based intercropping system on the soil nematofauna when growing millet in Senegal
Introduction
In Sub-Saharan Africa, food security remains a major concern [1], [2]. West African soils exhibit a poor inherent fertility [3] while agriculture is based on small-scale farming with very low external inputs. Moreover agricultural systems in this semi-arid region are mainly rain fed and thus highly vulnerable to climate variability and drought. Such constraints encouraged the development of alternative cropping systems tailored to social and environmental local conditions.
Native perennial woody shrubs are dominant in the West Africa landscape. Piliostigma reticulatum, one of the most common Sahelian shrubs, provides rural people with fuel, materials for construction, fodder for livestock and traditional medicine [4]. Native shrubs co-exist with staple food crops in farmers' fields. The accumulation of fertile soil particles beneath the shrub canopies resulting from a landscape-scale redistribution of resources generates nutrient-rich soil plots known as “fertility islands” [5], [6]. Shrubs also confer improved microclimatic regimes within the vicinity of their canopy due to their deep rooting systems and associated hydraulic lift [7], [8], [9]. In semi-arid Senegal, traditional management of native perennial woody shrubs involves coppicing and burning aboveground residues in the spring, prior to the planting of row crops, to clear fields. Alternative systems in which annual crops and shrubs are intercropped while shrub residues return to soil as mulch are receiving increasing attention all over Sub-Saharan Africa [10]. Such a shrub management resulted in both nutrient and moisture-related benefits to pearl millet (Pennisetum glaucum) when associated with common shrubs in the Senegalese peanut basin [8], [11]. Better carbon storage and nutrient cycling, and higher soil moisture improved cereal yields [8], [9], [11], [12]. Microbial communities beneath shrubs are more diverse, more active, and different from soil outside the influence of the shrub [11], [13].
Abundant and functionally diverse, soil nematodes are important members of the soil biotic community and play an essential role in ecosystem functions [14], [15], [16]. Different nematode trophic groups are defined according to their feeding habits. Plant feeding nematodes, i.e. plant parasitic and root-hair feeders, cause damage to roots that alter the plant's ability to take up nutrients and water [17]. Bacterial and fungal feeding nematodes affect soil organic matter decomposition and nutrient cycling [14], [18]. Other important trophic groups of free-living nematodes are the predators and omnivores for their role in regulating the populations of other soil organisms [15]. While plant species identity and diversity may affect soil nematode community [19], its analysis provides useful indicators to document soil processes and assess changes in soil conditions of agricultural systems [20].
This study aims at evaluating the response of soil nematode communities, as well as key food web indices, to intercropping pearl millet with P. reticulatum. We attempted to highlight possible impacts on plant feeding nematodes when millet is cultivated with shrub. We also postulated that beneficial nematode communities vary as a result of the acknowledged impacts of shrubs on soil water balance, increased biological activity, soil organic matter build-up, and fertility replenishment.
Section snippets
Experimental field site
The study site was located at Nioro-du Rip in the Southern region of the Senegalese Peanut Basin (13°45 N, 15°47 W, and 18 m above sea level). The climate is semi-arid with mean annual precipitation of 750 mm distributed from July to September and mean air temperatures ranging from 20 °C in December–January to 35.7 °C in April–June (i.e. BSh climate unit according to the Köppen–Geiger’s classification). The soil is a fine-sandy, mixed Haplic Ferric Lixisol [21], locally referred to as a
Soil properties under the different treatments
There is no significant difference in the soil total carbon or nutrient status between control (C) and millet (M) (Table 1). Soil contents of total carbon, nitrogen and phosphorus were significantly higher under shrub canopy (S) than in bare soil (C). However these parameters did not significantly differ between millet cultivated alone (M) or millet in the presence of the shrub (M + S; Table 1). Similar trends were observed for soil ammonium (NH4+–N) while soil nitrate content was significantly
Discussion
Abundance of nematodes in the control soil (C) was very low (400 individuals 100 g−1 soil) while the nematode community was well structured (SI = 88) with a dominance of cp-3 to 5 guild classes. The MDS representation clearly separates treatments according to shrub presence, which affected both taxa composition and relative abundance of nematodes trophic groups. The taxa diversity (Shannon–Weaver index) and richness were higher in the presence of shrubs. The change in composition was related to
Conclusion
The analysis of soil nematofauna, characterized by abundance of the different trophic groups and related maturity and enrichment indexes, allowed discriminating for the presence of P. reticulatum in the studied Senegalese agricultural soils. When millet is cultivated with P. reticulatum, (i) plant parasitic nematodes abundance was decreased, (ii) the nematode community was dominated by enrichment opportunistic and general opportunistic bacterial feeders, and soil food web was characterized by
Acknowledgements
The work was conducted in close collaboration with a project funded by the NSF's PIRE3 program (Grant No. OISE-0968247). We are grateful to Prof. R. Dick (Ohio State University) for support and suggestions. We thank Pape Omar Dieye (ISRA Bambey) and Moussa Diop (ISRA Nioro) for help with field sampling. We also would like to thank the persons that had helped to complete the laboratory analysis at LAMA and LEMSAT (Dakar). Sidy Diakhaté gratefully acknowledges doctoral fellowship of the DPF
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