Influence of GM crops, aromatic crops, allelopathy and litter decomposition on species assemblages of meso-arthropods in cultivated soils of the Free State Province, South Africa

[摘要] English: Integrated methods in land use and land management are needed, in addition to traditional agricultural practices, to provide an increasing human population with the necessary food security. By conserving soil organisms in crop agro-ecosystems, farmers can in essence be practicing sustainable conservation agriculture, where soil biodiversity is responsible for soil health. Potential toxic plants, whether natural (allelopathic) or anthropogenic (GMOs), cause a concern regarding this biodiversity in agro-ecosystems. Maize that has been genetically engineered using the soil bacterium Bacillus thuringiensis (Bt), known as Bt maize, expresses the synthetically modified Cry1Ab, Cry1F, Cry1A.105 or Cry2Ab2 proteins that are toxic to some insects. The impact of Bt-maize on non-target soil organisms is an important aspect in soil health and agricultural sustainability. The same goes for allelopathic crops, which can influence other crops in their immediate vicinity or in succeeding seasons.The aims of this study were to determine the possible effects of GMOs (Bt-maize), allelopathic crops (alfalfa and sunflower) and aromatic crops (onion) on soil meso-arthropod assemblages. A trial on humus decomposition rates and the potential occurrence of a Home field advantage (HFA) of decomposing litter was also conducted, the relevance being that decomposition is the driver of soil organic matter (SOM) production which enriches soil and, in turn, benefits soil organisms.Soil samples were taken at the roots of the plants in the porosphere where the plant interacts directly with its environment. To extract soil mesofauna, the Tullgren extraction method was used. Samples were collected from the following localities in the Free State: Bainsvlei area (maize, onion, and decomposition samples on the farm Geluk), Bainsvlei area (alfalfa and decomposition samples on the farm Maranatha), Bloemdal area (maize �?on the farms Karee Laagte and Feather Stone) and Petrusburg area (sunflower and onion �?on the farm Thornberry). To analyse data statistically, the Shannon diversity index, Sørensen similarity index and Home field advantage index (HFAI) was used.No immediate negative effects of Bt maize on soil faunal diversity were observed. However, in a 2012 study, a higher diversity of soil mesofauna was observed in the Bt fields, indicating that plants with the insect resistant gene may very well benefit soil faunal groups due to increased plant health and production of a larger root mass (podosphere). The influence of allelopathic crops on soil meso-arthropods showed that stressed allelopathic plants had an overall lower diversity than non-stressed plants. However, there is some uncertainty here, since lower diversity can also be attributed to low soil humidity and exposure to external post-harvest factors during the trial. Overall diversity in onion fields was lower than in the control fields, whilst some species of soil organisms only occurred in the natural fields and not in the onion field. There was no indication that the toxins produced from these plants actually kill the soil fauna, but the assumption could be made that onion plants were at least repellent. Certain mesofaunal species specifically occurred only in the onion fields, indicating opportunism and resistance towards onion repellent odours.The different sampling methods used in the decomposition trial showed some filtering effect in terms of the organisms allowed into the traps. The HFAI patterns for the four successive sampling dates (16, 24, 30 April and 07 May 2014) temporally correlate with the abundance of soil arthropods within the litter traps and litterbags at the given sampling date. Noteworthy during this trial is that certain trophic groups, such as microbes and predators, fulfil a vital role in decomposition and that this process is not only dependant on the litter producing plants as such. Furthermore, allelopathic alfalfa litter was seemingly also preferred by certain introduced, opportunistic collembolan species, indicating the important role alien species can play in the soil environment. In spite of all this and albeit that the sampling methods used in this trial created an unnatural scenario (to a certain degree) for litter decomposition agents by excluding certain size groups of soil arthropods, the overall conclusion is that a HFA (to a certain extent) was confirmed and demonstrated across all the sampling methods used for this short-term decomposition study.All of these aspects in crop agriculture can play a significant role in determining soil fertility and productivity. A better understanding of these processes can provide farmers with the necessary expertise and knowledge to manage sustainable crop farming systems.

[发布机构] University of the Free State