The Influence of Sewage Sludge and Fly Ash Fertilization on the Total Number of Bacteria (TNB) and Bradyrhizobium Species in Soybean Agroecosystem

Abstract

The aim of this study was to determine the impact of different chemical properties in soil, including changes in magnetic susceptibility and the impact of humic substances from sewage sludge and fly ash on the microbial community in the soybean agroecosystem. A field experiment was carried out using the random plot method on Cambisol with a texture of silt loam. The experiment consisted of 24 plots (six treatments and four replications) using different dozes of sewage sludge and fly ash. The following physicochemical and chemical analyses were performed in the soil samples: pH in KCl, conductivity (λ), total content of heavy metal, magnetic measurements and fractional composition of organic matter. The TNB and the number of the species of Bradyrhizobium in the field cultivation of the soybean variety Lissabon were evaluated using the spread plate method. The total content of heavy metals (Cu, Zn, Cd, Ni, Pb, Cr, Hg, Fe) in the analyzed treatments showed a significant difference between them. The study of the magnetic parameter χ indicated a significant differentiation between treatments from 34.0 to 65.8 × 10−8·m3·kg−1. High correlation coefficients between χ and Fe (r = 0.789), Zn (r = 0.653), Cr (r = 0.617) and TOC (r = 0.949) indicated that the source of these elements was external organic matter. Biological tests (TNB and species of Bradyrhizobium) in different experimental treatments indicated significant relationships between them and showed the resistance of the microbial community in the field cultivation of Lissabon soybean to heavy metal contamination from sewage sludge and fly ash. The study confirmed that external organic matter, such as sewage sludge, can be used as an alternative to natural fertilizers for soybean production.