Evaluation of newly reclaimed areas in Saudi Arabia for cultivation of the leguminous crop Phaseolus vulgaris under sewage sludge amendment

Herein, the cultivation of Phaseolus vulgaris was evaluated in newly reclaimed areas in Abha City, Saudi Arabia, following amendment with different rates of sewage sludge (SS) collected from a nearby municipal wastewater treatment plant. A pot experiment was designed to assess the soil heavy metal (HM) pools, growth, yield and HM uptake by P. vulgaris plants cultivated under different SS amendment rates (0, 10, 20, 30, 40 and 50 g kg−1). SS application was found to significantly change all of the chemical properties of the reclaimed soil (except Fe and Mn). The application of SS caused a gradual significant elevation in the soil organic matter content. The application of SS also led to a meaningful increase in the soil electrical conductivity and the Cd, Co, Cr, Cu, Mo, Ni, Pb and Zn contents. However, the soil pH decreased in reaction to the treatments. However, the measured HMs (except Fe) in the soil mixtures in all treatments after harvesting P. vulgaris plants were below the maximum permissible limits in agricultural soil. Generally, SS application up to 20 g kg−1 caused a significant increase in most of the morphological parameters and biomass of P. vulgaris compared to the plants grown in the control soil, followed by a decreasing trend at SS application rates of 30, 40 and 50 g kg−1. P. vulgaris flower production decreased with an increase in the SS application rate, and fruiting was only successful at rates of 10 and 20 g kg−1. All the HM contents in different tissues of P. vulgaris were meaningfully higher for plants treated with different SS levels matched to those grown in control soil. The highest contents of all of the HMs were documented at 50 g kg−1 SS in all P. vulgaris tissues. However, all HM contents (excluding Fe and Mn in the roots) were within safe limits, with most of the absorbed HMs persisting in roots. The regression analysis showed that P. vulgaris plants have an increasing ability to absorb more HMs with an increase in the SS amendment rate. Calculations of bioaccumulation factor (BAF) showed that the BAF for most investigated HM was < 1. This means that P. vulgaris is an excluder for these HMs. In addition, calculations of the translocation factor (TF) indicated that the capacity of P. vulgaris to translocate HMs from roots to shoots or pods is < 1 for most of the investigated HMs, indicating that the amounts of HMs that persisted in roots were always higher than those that travelled to the stems, leaves or pods. Taken together, our findings suggest that soil application of SS up to 20 g kg−1 can provide a sustainable safe practice for SS disposal and improve P. vulgaris growth and yield, while exerting no environmental threats provided there is no accumulation of HMs to toxic levels in shoots of the grown P. vulgaris plants or in the amended soils.