Nematodes as bioindicators of irrigated soil health in the Crocodile (West) and Marico catchments

PhD (Environmental Science), North-West University, Potchefstroom CampusGlobally, irrigated crop production accounts for 40% of produce. However, crop yield and quality is threatened by the deterioration of freshwater resources as a result of anthropogenically induced pollution. The threat of irrigating with low quality water furthermore extends to soil health/quality, which plays an important role in sustainable crop production. In South Africa, the Hartbeespoort and Crocodile (West) irrigation schemes (Crocodile [West] Catchment), representing the experimental sites for this study, are supplied with water from the Crocodile (West) River system. This river system has historically been subjected to pollution (e.g. metals, nutrients, and salts) that originates from urban, industry, and agricultural landscapes. Conversely, water utilized by the Marico-Bosveld Irrigation Scheme (Marico Catchment; reference system) is regarded as minimally impacted. Although the threat posed to crop production can be evaluated using region-specific irrigation water quality guidelines (e.g. South African Water Quality Guidelines for Agricultural Use: Irrigation), such guidelines only consider soil health from an abiotic (physico-chemical properties) perspective and disregards biotic attributes. This even though soil fauna play a fundamental role in fulfilling important soil ecosystem functions (e.g. nutrient cycling and pest control). Assessing and monitoring soil health thus requires a holistic approach. Therefore, the soil quality TRIAD approach, which integrates the chemistry, ecology, and ecotoxicology lines of evidence (LOEs) into an ecological risk assessment (ERA) framework, can be applied to assess the health of irrigated soils. A need also exists to expand the toolset for evaluating the toxicity of environmental samples. Subsequently, the aims of this thesis were to: 1) evaluate the quality of irrigation water utilised in selected irrigation schemes associated with the Crocodile (West) and Marico (reference system) catchments, 2) develop a high-throughput assessment method for evaluating the toxicity of spiked and environmental (aqueous) samples, and 3) assess the subsequent threat to the health of irrigated soils following the soil quality TRIAD approach, as part of a site-specific ERA, with nematodes as bioindicators. Results generated for the first aim confirmed that the Crocodile (West) Catchment has historically been subjected to anthropogenic pollution that posed a risk to crop production. Historical water quality data from 2005 − 2015 showed that the Hartbeespoort and Crocodile (West) irrigation schemes were exposed to calcium sulfate enrichment, while significant differences in water quality parameters occurred between these irrigation schemes and the reference system. Also, specific salt ions and nutrients concentrations exceeded threshold values provided by irrigation water quality guidelines. The Marico Catchment, in turn, was subjected to minimal anthropogenic disturbance. The second aim was also completed successfully, showing that the oxygen consumption rate (OCR) of the bacterivore nematode Caenorhabditis elegans can be used as an endpoint of toxicity in high-throughput assessments. The design of this high-throughput protocol facilitated assessments of the toxic effect of specific toxicants or mixtures (aqueous environmental samples) by measuring the OCR inhibition of C. elegans after 48 h of exposure. Results produced significant concentration-response relationships following benzylcetyldimethylammonium chloride monohydrate (BAC-C16) and cadmium (Cd) exposure, respectively, allowing the calculation of effective concentration values. Furthermore, a strong, positive correlation was evidenced between C. elegans OCR and growth inhibition, validating OCR as a sublethal endpoint of toxicity. The third aim was represented by the soil quality TRIAD for which soil samples were collected from selected farmlands associated with the Hartbeespoort, Crocodile (West), and Marico-Bosveld irrigation schemes and analysed in line with each LOE. The ecology LOE, represented by terrestrial, non-parasitic (beneficial) nematodes as bioindicators of soil quality, showed that all the studied farmlands presented either disturbed or disrupted ecosystems. Together with data from the chemistry LOE, it was shown that inorganic nitrogen (N) content, likely influenced by the application of fertilizers, presented a strong, positive correlation to the abundance and diversity of beneficial nematodes, which are indicative of enriched soils. For the ecotoxicology LOE, testing the toxicity of selected soil water (capillary water that occupies soil pores) samples was achieved using C. elegans reproduction and growth inhibition (ISO 10872), as well as C. elegans OCR inhibition using the newly developed high-throughput protocol. While C. elegans growth presented the lowest percentage inhibition/stimulation, a broad range of reproduction and OCR inhibition/stimulation was evidenced for both the study and reference farmlands. Integration of results from the three LOEs into the ERA framework concluded that irrigation water quality posed only a low risk at some of the studied farmlands. This is largely attributed to agricultural activities resulting in soil ecosystem disturbance, enrichment of inorganic N, and soils presenting toxicity at the reference system, which was used for background correction in the calculation of risk numbers. Outcomes of this study ultimately highlighted the impact of anthropogenic activities on irrigation water quality in the Crocodile (West) Catchment. Nonetheless, it remained difficult to elucidate the subsequent effects on irrigated soil health, likely as a result of agricultural activities (e.g. tillage and fertilizer application) causing an even greater disruption. This study concludes that there is a need to address the paucity of information relating to the health of irrigated soils.Doctora