Organic materials amendments can improve NPK availability and maize growth by reducing heavy metals stress in calcareous soil

The deficiency of nitrogen (N) and phosphorus (P) in calcareous soils of semi-arid climates is well recognized, and various green-based strategies have been employed to enhance their availability. However, the combined effects of heavy metal stress and soil calcareousness on NPK availability remain unclear. Thus, a pot experiment was conducted to assess the impact of biochar (BC), compost (Comp), and poultry manure (PM) at 1% and 2% w/w rates on NPK availability, soil characteristics, and maize performance under cadmium (Cd), lead (Pb), and chromium (Cr) stress. The results demonstrated significant improvements (P = 0.05) in soil properties, NPK availability, and maize growth with each amendment rate increment. Notably, the highest increases in soil pH, from 7.1 to 7.73, soil electrical conductivity (EC), from 0.32 to 0.4 dS m(-1), and soil organic matter (SOM) content, from 0.79 to 1.37%, were observed with 2% BC. The highest increase in soil total nitrogen, from 0.03 to 0.08%, and extractable P, from 2.69 to 6.86 mg kg(-1), occurred with 2% PM, while 2% BC led to an increase in K content, from 43.87 to 92.2 mg kg(-1). Incremental rates of each amendment resulted in greater immobilization of bioavailable Cd, Pb, and Cr in the soil, leading to reduced bioaccumulation in roots and decreased translocation to shoots. The most pronounced effects were observed with 2% BC, which exhibited higher immobilization indexes for Cd (48%), Pb (53%), and Cr (51%). The effect of treatments at 2% was similar for plant height and shoot dry weight. A negative correlation was identified between NPK availability and bioavailable Cd, Pb, and Cr, indicating that organic materials reduced heavy metal stress while increasing NPK availability. BC emerged as the most effective amendment for improving NPK availability and stabilizing Cd, Pb, and Cr in the soil.