Influence of Soil Physico-chemical Parameters on Nematode Communities in Banana-grevillea Robusta Agroforestry Systems in Kirinyaga, Central Kenya

Abstract

Banana remains an essential crop mainly cultivated in the tropical and subtropical regions. In Africa, its production is carried out by small scale farmers under different cropping systems for both home consumption and market. However, banana production is hampered by low soil fertility, inadequate banana management practices, pests and diseases resulting in low yields. To overcome these challenges, farmers resort to emerging technologies such as improved fallow, cover crops, integrated pest and soil fertility management, agroforestry with fast growing tree species. Using soil nematodes as an indicator of soil health, this study was carried out to determine the influence of soil physico-chemical properties on soil nematode community in grevillea-banana agroforestry systems in Kirinyaga County, Central Kenya. Soil samples were collected in banana sole stands, Grevillea robusta sole stands and grevillea-banana intercrops in three agro-ecological zones during the dry and rainy seasons. These soil samples were analysed for their physico-chemical properties, nematode extraction, identification to genus level and calculation of nematode community and ecological indices. Soil physio-chemical properties, namely soil moisture content, soil bulk density, soil organic matter, total nitrogen, phosphorus and exchangeable bases, were significantly different among agro-ecological zones, cropping systems and between seasons. The overall nematode density, comprising bacterivore, fungivore, omnivore, predator and herbivore nematode trophic groups increased significantly during the wet season compared to the dry season. The highest population density of bacterivore nematodes was recovered in banana sole stands in the highland zone at 170.2 individuals/ 200 cc sol whereas the least was found in grevillea-banana intercrops in the midland zone at 35.3 individuals/ 200 cc soil. The population density of fungivore nematodes was significantly affected by interactive effects of agro-ecological zones and cropping systems, being higher only in banana sole stands and grevillea-banana intercrops in the lowland zone at 28.5 and 16.4 individuals/ 200 cc soil, respectively; in xiii grevillea sole stands in the midland zone at 16.3 individuals/ 200 cc soil; and in grevillea sole stands and grevillea-banana intercrops in the highland zone at 18.8 and 17.7 individuals/ 200 cc soil, respectively. The density of predator, omnivore and herbivore nematodes were significantly higher in banana sole stands in the lowland zone at 20.66, 56.66 and 371.67 individuals/200 cc soil, respectively. The population density of herbivore nematodes was significantly higher in banana sole stands compared to grevillea sole stands and the intercrops in all the agro-ecological zones. The Shannon’s diversity index and taxonomic richness were significantly higher in the highland zone at 1.99 and 11.8 compared to the midland (1.72 and 8.83) and the lowland zones (1.78 and 9.57). Nematode population was more diverse and evenly distributed in grevillea-banana intercrops and grevillea sole stands than in banana sole stands. Enrichment and Channel indices revealed highland and lowland zones to be nutrient-rich and dominated by bacterial decomposition pathway while the midland zone is nutrient-poor and dominated by a fungal decomposition pathway. The Structure Index and Maturity Index were significantly low in banana sole stands compared to grevillea sole stands and grevillea-banana intercrops. These results suggest that intercropping banana with grevillea trees is more sustainable than growing banana in monoculture since intercrops maintained a healthy soil whereas the banana monoculture favoured the build-up of the population of parasitic nematodes. Key words: Nematode, banana, Grevillea robusta, agroforestry system