Response of soil health index to untreated wastewater irrigation in selected farms under different vegetable types

Crop irrigation with wastewater has drawn global attention in water-scarce areas as a strategy to offset water scarcity. However, the response of soil ecosystem functioning and soil health to long-term and continuous wastewater (WW) irrigation has not been well-understood. A total of 12 vegetable farms irrigated with WW and fresh water (FW) were analyzed to assess WW irrigation impacts on soil health using the minimum data set (MDS) and soil health index (SHI) framework. We determined the combination of soil physicochemical, nutritional, and biological indicators (22 soil indicators) along with linear SHI (L- SHI) and nonlinear SHI (NL- SHI) quantification approaches in the WW-irrigated fields versus the FW-irrigated fields. Significant changes were observed in most soil attributes in the WW-irrigated farms versus the FW-irrigated farms. These changes could mostly improve soil health (e.g., soil stability index, exchangeable cations, organic matter (OM), total N, and available P), but some of them have a degrading effect on soil health (e.g., electrical conductivity (EC), sodium absorption ratio (SAR), and heavy metals). Soil health indicators identified through the MDS approach were clay, EC, OM, exchangeable K, Pb, and soil microbial respiration, contributing to the SHI value by 40.6%, 40.6%, 23.8%, 23.8%, 20.8%, and 14.9%, respectively. The SHI values computed through the non-linear scoring technique were more sensitive to WW irrigation than those computed through the linear scoring technique, suggesting that NL- SHI would outperform L- SHI in representing variations in soil functions. The L- SHI and NL- SHI values were 17–31% and 21–32% higher in the WW-irrigated farms than in the FW-irrigated farms, respectively, suggesting a good improvement in soil capacity and functions after WW irrigation. To conclude, our results revealed that an integrated SHI, particularly NL- SHI, would be more instrumental for evaluating post-WW irrigation soil functions and soil health in the WW-affected croplands than single soil indicators.