Soil Nematode Communities under Organic and Conventional Farming Systems in Chuka, Tharaka Nithi County, Kenya

Plant parasitic nematodes (PPN) are biotic constraints to crop production worldwide resulting in significant yield losses. Management of these nematodes has primarily depended on chemical nematicides; but due to the increased pressure for more economic and environmentally friendly strategies, alternative methods like organic and conventional amendments, have been considered. To test the efficacy of conventional and farmer practice systems against organic farming in the suppression of PPN, on abundance and diversity of free living nematodes (FLN), field trials were conducted in Chuka in January 2015 at two sites (farmers’ fields and demonstration site) over three seasons: season 1 (cowpea sole crop), season 2 (maize-bean intercrop) and season 3 (bean sole crop). Organic systems received neem cake + compost + Tithonia + ash; conventional received Marshall EC + Calcium ammonium nitrate + Di-ammonium phosphate (DAP) and farmers’ practice received DAP + Manure. A non-amended system was included in the study as control. Soil sampling was done every six weeks where ~500 g soil and 50 g cowpea, beans and maize roots were collected per plot from five sampling points. Nematodes were extracted from 100 ml of soil and 5 g of roots then identified to genus level. Data on abundance and diversity were subjected to analysis of variance in R version 3.2.5 with differences at p≤0.05 considered statistically significant. Twenty nine genera belonging to bacterivores, fungivores, omnivores, predators and PPN were found in the study area. Bacterivores dominated the organic systems while PPN prevailed in the non-amended control system at both sites. High population of bacterivores in the organic system could be attributed to compost which is known to provide soil organisms with a new energy source that increases their diversity and activity. Soils under maize-bean intercrop showed varying population of nematodes as organic system recorded significantly higher population when compared to other systems at farmer fields (2,182 ± 89.78) and demonstration site (2,014.5 ± 98.00). Renyi diversity profile showed no significant differences among the farming systems indicating nematodes were evenly distributed across the systems. Principle response curves were used to establish the effect of farming systems on individual nematode genera over time and only the organic model was significant. Tylenchus, Meloidogyne and Helicotylenchus spp. were effectively reduced up to the second month. Diversity and ecological indices during the bean sole crop differed significantly (p≤0.05) where organic systems had significantly higher values of plant parasite index (4.16 ± 0.88), enrichment index (85.52 ± 10.61) and structural index (87.42 ± 8.06) at farmers’ fields with a similar trend at the demonstration site. This indicated the soils were high in nutrients and very stable. In general, significantly lower numbers of PPN were recorded in the organic system across all seasons at both sites. In conclusion, organic farming appears to suppress populations of PPN whilst promoting that of FLN. Organic farming is therefore recommended for small holder farmers and extension officers to use in awareness creation. Further research should be done to identify nematodes to species level using molecular methods under controlled environment.