Abstract:
This study investigated the potential of cover crops which have been green manured, to control nematodes and improve soil microbial diversity of vegetable crop fields in Louis Trichardt, Limpopo Province. For the purpose of this study, six different cover crops were initially cultivated in trial plots located in fallow vegetable crop fields and were then green manured by ploughing the biomass under when the plants were mature. These were Lablab purpureus (Dolichos bean), Brassica rapa (Mustard), Eruca sativa (Rocket/Nemat), Avena sativa (Common Oats), Avena strigosa (Black Oats), and Trifolium pratense (Red clover). All trials were conducted on the same crop field which was a fallow crop field where the last crop had been Green Pepper. The control was selected from a portion of this fallow crop field. Not all cultivations were successful as a result of unforeseen circumstances and consequently all were green manured and assessed.
Soil samples were collected from the treatment plots of three of the cover crops which were successfully cultivated soon after green manuring of the cover crops. These were then analysed to determine how green manuring from different cover crops affected nematode community profiles. Similar soil samples were collected from these three cover crops at a much later stage (after allowing for decomposition of green plant material), before analysing it for aspects of soil microbial diversity. These consisted of analyses of soil organic carbon, active carbon, soil microbial and functional diversity and enzymatic activity of the soil.
Nematode taxa were sorted into functional guilds and assigned to a colonizer-persister (cp) scale. Soil microbial species richness and abundance were measured using the Shannon-Weaver and Evenness diversity indices, respectively, while microbial enzymatic activities (ß-glucosidase, phosphatase, urease) were analysed to evaluate ecosystem functioning.
Overall results from this study showed a general domination of bacterial, hyphal and plant feeding nematodes indicative of disturbed soil and less predator and omnivore nematodes which are indicative of undisturbed soil. Concerning microbial soil activity, the control treatment (fallow crop field) displayed the highest soil microbial diversity (p < 0.05) compared to the cover crop treatments which had significantly lower soil microbial diversities. Of all the cover crops, the Dolichos beans (Lablab purpureus) had the highest microbial diversity.
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In conclusion, results indicate that no substantial variances occurred in terms of nematode (parasitic and non-parasitic) community structures and population levels among samples collected from the various treatments and the soil microbial functional diversity and activity were sensitive to and varied between the various treatments.
