Functional Diversity and Ecological Role Of Nematodes in Pigeon Pea And Maize-Pigeon Pea Agro-Ecosystems

Abstract

Kenya relies heavily on pigeon pea (Cajanus cajan L.) and maize (Zea mays L.) as essential food crops, widely grown by small-holder farmers for food, feed, and income. These crops face significant production challenges due to plant-parasitic nematodes (PPN), which cause enormous yield losses. The four main species of PPN that negatively impact pigeon pea and maize are Meloidogyne spp., Rotylenchulus spp., Heterodera spp., and Pratylenchus spp. Plant-parasitic nematodes cause 50% and 8-35% yield reductions in maize and pigeon pea, respectively. Besides the presence of PPN, Kenyan agro-ecosystems have a high density of free-living nematodes (FLN) that perform vital functions, including breaking down organic matter, cycling nutrients, pest regulation and serving as bio-indicators of ecosystem quality. The objective of this study was to assess the variety of nematodes present in monocrop pigeon peas (PP) as well as the nematode community structure and their role as soil health indicators in maize-pigeon pea (MPP) agro-ecosystems. The study also determined the nematode functional guild diversity and their ecological role in PP and MPP agro-ecosystems, and the distribution and prevalence of PPN feeding groups in PP and MPP agro-ecosystems. For all objectives, baseline field surveys were carried out in respective PP and MPP farmers’ fields in Embu County using probability frame sampling design coupled with three separate W shaped “sample walk” pattern. For PP across the eight regions in Mbeere North, Embu County, 46 nematode genera were identified, of which Meloidogyne, Rotylenchulus, Acrobeloides and Cervidellus differed significantly among the regions. No variations were observed in Simpson diversity, Shannon, Pielou's evenness, and genus richness measures between regions at P > 0.05. Canonical correspondence analysis (CCA) showed marked connections between soil characteristics and specific nematode genera, with the first two axes explaining 56.65% of the total variation. For MPP agro-ecosystems across eight regions in Mbeere South, Embu County, 41 nematode genera were identified with Longidorus spp., Heterocephalobus spp. and Mononchus spp. varying significantly among the regions. Following structure and enrichment indices analysis, Irabari soil food web was degraded, while the other seven regions were well-structured. In all the studied regions, the energy pathways for decomposing organic matter were predominated by fungivores. Among the assessed nematode metabolic footprints, only bacterivore footprint showed marked variations. Regarding PP and MPP cropping systems in Mbeere South and Mbeere North sub-Counties, Embu County, nematode assemblages consisted of 45 and 44 genera in PP and MPP, respectively, of which Pratylenchus, Rotylenchulus, and Aphelenchoides varied markedly between the two cropping systems. Non-metric multidimensional scaling analysis indicated a deep overlap of nematode genera in the two systems at the community level. The MPP fields favored herbivores population assigned to PP2 and PP3 over the PP fields. Pearson correlation coefficients and CCA indicated sand, clay, Cu and pH as the most relevant soil properties for accounting the pattern of nematode community assemblages. In addition, the migratory endoparasites and epidermal/root hair feeders were significantly higher in MPP than in PP, while sedentary parasites and ectoparasites were more in PP than in MPP system. These results reveal a significant presence of economically important PPN in PP and MPP systems, which highlights the need for effective nematode management programs. The study also provides beneficial understanding of the structure and functioning of the nematode soil food web in PP and MPP, potentially contributing to the improvement of their yields and soil health.