Effects of waste water irrigation on soil properties and soil fauna of spinach fields in a West African urban vegetable production system

Highlights

• Gypsum application reduced soil pH but increased soil EC.

• Soil-dwelling arthropods tolerated soils of high pH, but avoided soils with an increased EC.

• Turnover rate of soil pH and EC was positively related to arthropod abundance.

• Waste water irrigation and affected soil pH determined arthropods composition.

Abstract

The usage of inadequately processed industrial waste water (WW) can lead to strong soil alkalinity and soil salinization of agricultural fields with negative consequences on soil properties and biota. Gypsum as a soil amendment to saline-sodic soils is widely used in agricultural fields to improve their soil physical, chemical and hence biological properties. This study aimed at analysing the effects of intensive WW irrigation on the structure and composition of soil-dwelling arthropods on spinach fields (Spinacia oleracea L.) in a West African urban vegetable production system. We used gypsum as a soil amendment with the potential to alleviate soil chemical stress resulting in a potentially positive impact on soil arthropods. A total of 32 plots were established that showed a gradient in soil pH ranging from slight to strong soil alkalinity and that were irrigated with WW (n = 12) or clean water (CW; n = 20), including eight plots into which gypsum was incorporated. Our study revealed a high tolerance of soil-dwelling arthropods for alkaline soils, but spinach fields with increased soil electrical conductivity (EC) showed a reduced abundance of Hymenoptera, Diptera and Auchenorrhyncha. Arthropod abundance was positively related to a dense spinach cover that in turn was not affected by WW irrigation or soil properties. Gypsum application reduced soil pH but increased soil EC. WW irrigation and related soil pH affected arthropod composition in the investigated spinach fields which may lead to negative effects on agronomical important arthropod groups such as pollinators and predators.

Introduction

In many cities of Sub-Saharan Africa, urban agriculture heavily relies on the use of industrial waste water (WW) for crop irrigation. But even treated WW frequently does not meet the international standards for agricultural use developed by WHO, FAO and UNEP with potential consequences for ecosystem health and functions (Corcoran, 2010). Particularly of concern in industrial WW used for agriculture is its often high level of sodium and potassium. High concentrations of both salts may cause, due to their accumulation in the soil, strong soil alkalinity leading to a pH above 8.5 (Soil Survey Division Staff, 1993). This may be accompanied by adverse effects on soil structural properties resulting in a compaction layer with low infiltration capacity (Dastorani et al., 2008, Hussain et al., 2002, Sou et al., 2013). In conjunction with high amounts of nitrates, potassium and chloride, sodium is a major driver for salinization processes in soils. This may add to the often reported problems of West African soils that suffer from poor native soil fertility, low organic carbon, low water-holding and cation exchange capacity, surface crusting and high soil surface temperatures (Bationo and Buerkert, 2001). Gypsum, or calcium-sulfate-dihydrate (Ca[SO₄]·2H₂O), has been often used as an amendment in saline-sodic agricultural fields to improve their soil physical and chemical properties. As gypsum dissolves, it releases calcium ions that displace sodium on the clay colloids in alkaline soils thereby improving soil structure (Shainberg et al., 1982). Given these bio-physical constraints to plant growth the preservation of soil biological properties is likewise important to secure nutrient turnover, help to suppress soil borne diseases and pests and to maintain bioturbation (Postma-Blaauw et al., 2010, Whitford, 1996). Many studies showed the important role of ground dwelling arthropods for nutrient cycling, pedoturbation and sediment transport (Butler, 1995, Garcia and Niell, 1991, Nichols et al., 2008). Particularly ants and termites are known to affect physical and chemical soil properties. The presence of ants can alter soil pH whereby the pH of alkaline soils can be decreased and that of acid soils increased (Frouz and Jilková, 2008). It is well recognized that extreme soil properties such as strong soil alkalinity and high soil salinity influence not only plant performance but also below-ground and above-ground communities of soil biota that are interlinked. However, our understanding about the mechanisms behind the effects of aboveground-belowground feedbacks is still very limited (De Deyn and Van der Putten, 2005, Scherber et al., 2010). Most studies examined invertebrate responses on effects of increased soil pH and soil salinity within the framework of urban ecology (Dunxiao et al., 1999, McIntyre et al., 2001, Santorufo et al., 2012) whereas our knowledge about industrial waste water as driving forces for arthropod distribution within agricultural soils is scarce.

Furthermore, the West African region had received so far little attention regarding the effects of agricultural practises on soil biodiversity, although in Burkina Faso up to 90% of the population depends on agriculture as their main source of income generation.

Hence, this study aimed to analyse the effects of intensive waste water irrigation on the structure and composition of soil-dwelling arthropods found on spinach fields (Spinacia oleracea L.) in a West African urban vegetable production system, using gypsum as a soil amendment to alleviate alkalinity-induced stress with potentially positive effects on soil arthropods.