The effect of contrasting biosolids application strategies on soil quality

Abstract

Purpose: Incorporating biosolids into the soil improves plant yield compared with surface application, but it can result in the increased uptake of trace elements. However, there is a lack of knowledge about how different types of biosolids applications affect soil quality. We aimed to determine the effect of the type and rate of biosolids application on soil quality and the mobility of contaminants. Methods: Soil quality was determined by soil fertility (inorganic N, exchangeable P, Mg, Ca, K), exchangeable trace and non-essential elements (Al, Mn, Zn, Cu and Cd) and biological activity (dehydrogenase activity). We measured the properties of soil pore water, bulk soil and rhizosphere in a pot and a rhizobox experiment, with increasing concentration of biosolids (equiv. 16 t ha¯¹, 48 t ha¯¹ and 145 t ha¯¹ dry weight), applied on the surface, incorporated to 25 cm, or incorporated into a patch. Results and discussion: The incorporation of biosolids into the soil increased the exchangeable Zn, Cu, Cr, Ni and Cd, compared with surface application. The surface application of biosolids increased the inorganic N in the soil compared with biosolids incorporation (680 mg kg¯¹ vs. 380 mg kg¯¹), and decreased soil pH by 1.1 units. This aligned with solubilisation of Al (43 mg kg¯¹ vs. 6 mg kg¯¹) and Mn (43 mg kg¯¹ vs. 33 mg kg¯¹) and explains the decreased microbial activity in the soil compared with the unamended soil. Incorporating biosolids in the soil increased the biological activity, likely due to biosolids-borne microbes. The root systems significantly increased microbial activity, pH, and the concentration of NH₄⁺, NO₃⁻, and exchangeable P, S, Mg, Na, Zn, Cu and Ni, and significantly decreased exchangeable concentration of Mn and Fe.