Insights into bioavailable heavy metal impact driven by sludge application on soil nitrification: Toxicity thresholds and influential factors

Strict regulations on heavy metal (HM) limits impede the sludge land utilization for carbon emission reduction. This study aimed to evaluate the impact of bioavailable HMs (Cd, Cu, and Zn) on soil nitrification and determine toxicity thresholds via two cycles of sludge land application tests over 185 days. HMs inhibited gene abundance in their labile fractions, with the most affected being nitrite-oxidizing bacteria (NOB)-nxrB, followed by ammonia-oxidizing bacteria (AOB)-amoA, NOB-nxrA, and ammonia oxidizing archaea (AOA)-amoA. Toxicity thresholds for incremental labile fractions of HMs (in mg/kg) were determined as 0.35 for Cd, 21.73 for Cu, and 84.04 for Zn. Additionally, AOB, as the core nitrifiers, significantly correlated (P < 0.05) with ammonia nitrogen, soil organic matter, total phosphorus, and total potassium, playing a pivotal role in maintaining intricate interactions within HMs-spiked sludge-treated soil systems. The acute toxicity effects of HMs on potential ammonia oxidation (PAO), measured by inhibition rates, were 77.04 %, 73.63 %, and 67.06 % for Cd, Cu, and Zn, with labile fractions contributing 33.79 %, 40.19 %, and 28.37 %, respectively. Long-term sludge land application revealed chronic toxicity of HMs to PAO through the reshaping of ammonia-oxidizing microorganisms, particularly Cu and Zn. These findings provide insights into HM toxicity thresholds and their impact on nitrification, supporting sustainable sludge land management.