Handbook for Irrigation of Wine Grapes in South Africa

Chapter 5

Figure 5.28 and references therein). Soil microbial enzyme activity was most sensitive to changes triggered in the topsoil layers. It was highest in the 0 to 10 cm layer and gradually decreased with increasing depth. Since this gradient in enzyme activity was observed before and after irrigations, it implies that irrigation with winery wastewater had no negative consequences on organic matter in soil. In fact, it suggests that easily decomposable organic matter added to the soil via irrigation promoted soil enzyme activity. Enzyme activity also seemed to have been stimulated over time as more irrigation was applied (Fig. 5.28). When assessed over the entire trial period, microbial population sizes also decreased with depth, but the impact of irrigation with winery wastewater on general microbial counts and shifts in soil microbial communities were inconclusive (data not shown). Glomalin content also decreased with an increase in soil depth, but did not respond to level of COD in the diluted wastewater (data not shown). Given that both glomalin and soil microbial enzyme activity are considered good indicators of soil microbial health, irrigation with winery wastewater should be of little to no consequence to general soil health. Furthermore, soil fertility may even be improved, given the positive effects of winery wastewater on soil microbial enzyme activity under the prevailing conditions. However, the foregoing should be received with caution, as some of the findings need to be substantiated by further research.

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ȕ - glucosidase activity (µg/g soil/h)

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FIGURE 5.28. Effect of irrigation with diluted winery wastewater on mean β -glucosidase activity. Data are annual means, irrespective of irrigation cycle or soil depth. Columns designated by the same letter do not differ significantly (p ≤ 0.05). Irrigation of grapevines using winery wastewater diluted up to a maximum COD level of 3 000 mg/  did not affect the grapevine water status, vegetative growth or any of the yield components, compared to the river water control (Howell et al. , 2016). The mean mid-day stem water potential was -0.6 PMa, which indicated that Figure 5.29

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IRRIGATION OF WINE GRAPES 141