The relationship between agricultural practices and selected heavy metals in vineyards of the Cape Winelands, Western Cape

Abstract

Heavy metal toxicity is a major threat to the health of both humans and ecosystems. Toxic levels of heavy metals in food crops, such as grapes, can have devastating effects on plant health and the market value of the produce. Two important factors that may influence the prevalence of heavy metals in grapevines are seasonal change and farming practices. The objectives of this study were (i) to conduct a detailed pioneer screening of heavy metal levels in soils and grapevine leaf tissues in selected wine farms and (ii) to study the influence of season and farming on heavy metal levels in soils and grapevine leaf tissues. Soil and grapevine leaf samples were collected from de-marcated areas in selected vineyards in the Cape Winelands region of South Africa. The sampling was conducted in winter and summer from the same sites. The soil and the leaf samples were an-alysed using inductively coupled plasma mass spectrometry (ICP-MS) techniques. The pooled data from the farms practicing conventional or organic farming showed that seasonal variation had no significant effect (DF =1, 22; P > 0.05) on the heavy metal contents in the soil. When soil data from winter and summer months were compared separately or pooled, the influence of agricultural practice was well-pronounced in As (DF =1, 22 or 46; P < 0.05) and Cu (DF = 1, 22 or 46; P <0.05). The agricultural practice greatly influenced (DF = 1, 22; P< 0.05) Cu, As, Cr, and Hg up-take, with little effect on Ni, Co, Cd, and Hg leaf contents. Generally, the heavy metals studied (Cr, Co, Ni, Zn, As, Cd, Hg, and Pb) were substantially below the maximum permitted levels in plant and soil samples, per the recommendations of the WHO and Er indices, respectively. However, moderate contamination of the soils was recorded for Cr, Ni, Zn and Pb. Remarkably, Cu levels in organic vineyard soils were significantly higher than in conventional vineyards. Furthermore, based on the Igeo index, Cu occurred at moderate to heavy contamination levels.