Nematode communities : bio-indicators of soil quality in conventional and conservation agricultural cropping systems

Abstract

The primary aim of practising conservation agriculture (CA) is to optimise crop production. This is done by using practices and principles that promote soil quality by increasing population levels and/or diversity of beneficial soil organisms, including non-parasitic nematodes. Conservation agriculture is also considered as an alternative means to control plant-parasitic nematode population levels. The main aim of this study was to determine the effect of CA compared to conventional agriculture (CTA) on plant- and non-parasitic nematode communities under maize-based cropping systems in South Africa. Furthermore, the identity of two types of eggs („large‟ and „small‟), present in 50-g root samples of maize (survey study) and maize and other crops (CA study), were clarified and the genera representing them determined. Associations between plant- parasitic nematodes and maize yield were also determined. Additionally, correlations between the different soil chemical parameters and soil nematode assemblages (plant- and non-parasitic nematodes) were investigated. For determining the effect(s) of CA vs. CTA on nematodes, a survey was conducted in the two the main maize-producing areas of South Africa (Free State and North-West provinces). Furthermore, nematode samples (root and soil) from two CA-trials were also obtained. For the survey, root and rhizosphere soil samples (including maize fields under CA and CTA as well as natural veld (NV)) were obtained from 24 ecosystems during the 2012/2013 growing season. For determining the effects of CA vs. CTA on soil nematode population assemblages over a four-year period, two rain-fed field trial sites, were monitored under different maize-based cropping sequences (CRS). These sites were Buffelsvallei (sandy loam soil) and Erfdeel (sandy soil), which is part of a CA programme of the Agricultural Research Council‟s Grain Crops Institute. At these sites, CA and CTA maize monoculture were practised, while maize was also rotated with cowpea, pearl millet and sunflower in different crop-sequence combinations. Rhizosphere soil samples for nematode analyses were obtained before/during planting during each growing season from all plots as well as 60 and 100 days after planting. Additional soil samples were also obtained for soil chemical and physical analyses. Root samples were obtained during the 60- and 100-day sampling intervals. Standard techniques were used to extract nematodes from root and soil samples. Morphological and molecular (only for Meloidogyne spp.) techniques were used to identify nematodes to genus/species (plant-parasitic nematodes) and/or genus/family (non-parasitic nematodes) levels. During both the survey and CA study, 50-g root samples contained two different types of eggs („large‟ and „small‟) when using the NaOCl method. The two egg types were measured (length and width) and were subjected to deoxyribonucleic (DNA) sequencing to identify the nematode genera these two types of eggs belonged to. This activity was conducted first and formed the basis of this study. The two types of eggs were successfully identified with the „large‟ eggs belonging to the genus Meloidogyne and the „small‟ eggs to Rotylenchulus. The measurements furthermore showed that the Meloidogyne eggs were significantly longer than the Rotylenchulus eggs. The same applied for the width of the eggs, with those belonging to Meloidogyne being wider than those of Rotylenchulus. This research is a novel and significant contribution to the Nematology discipline since it will assist in more accurate research related to the presence and effects of Meloidogyne and Rotylenchulus on maize and other food crops. During the survey a total of 13 plant-parasitic genera, 21 species and one subspecies were identified. Of these, 10 genera and 11 species as well as one subspecies were associated with CA maize. Data showed that fields under CA had higher prominence values (PV) for most of the plant-parasitic nematode general/species identified compared to those present in CTA maize as well as NV. This was evident for Rotylenchulus parvus and Pratylenchus spp. populations. However, maize under CTA had higher PV values for Meloidogyne spp. The opposite scenario was observed for non-parasitic nematodes that were present in soil samples. The NV in general had higher PV values and diversity of non-parasitic nematodes, followed by the CA maize for most of the non-parasitic nematode genera/families. Nematode-food web structures showed that most of the soils (including those from NV) sampled represented resource-depleted nematode communities. The non-parasitic nematode genera, Acrobeles and Acrobeloides was the predominant bacterivores present within the soils collected from maize (both CA and CTA) as well as NV. The predominant fungivores present were Aphelenchus and Aphelenchoides while the family Discolaimidae and genera Discolaimoides were identified as the dominant predators. For the CA study, nine plant-parasitic nematode genera and 11 species were identified at Buffelsvallei while at Erfdeel a total of six genera and nine species were observed. Rotylenchulus parvus was the predominant nematode pest present in the 50-g root samples at Buffelsvallei, while Meloidogyne incognita was dominant at Erfdeel. Results also showed that when either cowpea or sunflower were included in the CA cropping sequences (CRSs) a significant decline in R. parvus population densities occurred over the four-year duration of the study. Pratylenchus spp. was the dominant plant-parasitic nematode genus present in the 5-g root samples at both localities. As for R. parvus (50 g roots) when either cowpea or sunflower was included, a decline in the population density of this nematode species was observed. This study ultimately showed that the CA CRSs, in general, had lower plant-parasitic nematode densities than the monoculture CTA. However, both Scutellonema brachyurus (Buffelsvallei) and Tylenchorhynchus goffarti (Erfdeel) had higher population densities in soils under CA than CTA. With regard to non-parasitic nematodes present in soils from the CA study, 31 nematode genera and two families were identified at Buffelsvallei. For Erfdeel a total of 21 non-parasitic nematode genera and four families were observed. Related to the CRSs under CA, higher nematode densities and diversity of non-parasitic nematodes were recorded than from those in CTA maize. This was also demonstrated by the nematode food-web analyses. Bacterivores dominated in soils from both CTA and CA cropping sequences followed by the fungivores. This scenario was evident at both localities. Food-web structures showed that non-parasitic nematode diversity was higher when either cowpea or sunflower was included in the CRSs under CA. However, the majority of the CRSs (both CA and CTA) had soil qualities characterised as stressed and depleted in terms of their inhabiting non-parasitic nematode communities. Identification of non-parasitic nematodes to genera/family level seemed to provide a more accurate reflection of the community structure than when using trophic levels alone. This was further substantiated by the noticeable differences observed between the nematode assemblages and the different CRSs used. Negative correlations existed between maize yield and R. parvus at Erfdeel but not at Buffelsvallei. Correlations between nematodes and soil chemical parameters showed that organic matter (%C) in general had positive, but weak associations with most of the plant- and non-parasitic nematodes. However, %C did had strong, positive correlations with Nanidorus minor and Scutellonema brachyurus at Buffelsvallei. Also, strong, positive correlations between %C and non-parasitic nematode families Panagrolaimidae, Cephalobidae, Dorylaimidae and Tylenchidae existed. Furthermore, at Buffelsvallei sodium (Na) had a positive, strong association with the non-parasitic nematode family Rhabditidae. For this latter locality, phosphate (P) also had strong, positive associations with Cephalobidae and Aphelenchidae (both non-parasitic). Ultimately, results of this study demonstrated that differences in plant- and non-parasitic assemblages existed between maize fields under CA and CTA. Further investigation is, however, needed with regard to nematodes vs. crop yields and soil chemical and physical parameters under local maize-based cropping systems. This study also warrants further investigation with regard to R. parvus and its potential as a pathogenic plant-parasitic nematode in the local maize industry. No information on its reproductive potential and pathogenicity on maize are available worldwide. Further research should also focus on longer term studies related to nematode-CA investigations to obtain an extensive body of information. This will also result in the generation of data on associations between nematode pests and crop yields, as well as chemical soil parameters which are limited and fragmented at present