Beneficial nematodes as bioindicators of ecosystem health in irrigated soils

Irrigated crop production, which accounts for 40% of global produce, is threatened by the deterioration of freshwater resources as a result of pollution originating from anthropogenic activities. While the effects of irrigating with low quality water on crop yield and quality is well appreciated, little remains known about the threat posed to soil ecosystems. The present study represents, with nematodes as bioindicators, the ecological line of evidence (of the TRIAD approach) and is aimed at evaluating the soil health status of farmlands associated with the Hartbeespoort, Crocodile (West), and Marico-Bosveld (reference system) irrigation schemes (South Africa). Irrigation water and soil samples were collected during the winter and summer growing seasons of 2016 and analysed for physico-chemical properties [pH, electrical conductivity, organic carbon content, particle size distribution, and metal, nutrient (inorganic nitrogen and phosphorus), and salt (chloride, fluoride, sulfate, calcium, magnesium, potassium, and sodium) concentrations]. Results indicated that the Hartbeespoort and Crocodile (West) irrigation schemes utilized water with an increased salinity (as indicated by the electrical conductivity) and inorganic nitrogen and phosphorus concentrations. The associated soil ecosystems were classified, using nematode-specific indices, as either degraded or disrupted. However, the reference system also presented degraded or disrupted soil ecosystems, which suggests that irrigation water quality was not the main factor influencing soil ecosystem health. Instead, it is likely that conventional farming practices (e.g. tillage) were the main drivers behind the observed disruptive effects. A redundancy analysis triplot evidenced a strong correlation between inorganic nitrogen, crop production, and nematode r-strategists, indicating that the nematode assemblages responded rapidly to agricultural activities such as the addition of fertilizers. No significant influence of irrigation water quality on soil ecosystem health was thus evidenced.