Microbiological Mechanism of Nitrogen Forms and Concentrations in Regulating Cadmium Accumulation in Crops following Stabilization in Contaminated Agricultural Soil

Regulating the secretion of protons and acidic compounds by crops and microbes, to minimize the re-mobilization of stabilized cadmium (Cd) in soil, is an essential strategy in agricultural management to ensure food safety. In this study, we managed the re-mobilization of stabilized Cd in soil and its subsequent accumulation in crops using nitrogen fertilization strategies implemented with varying forms and concentrations of nitrogen - specifically, ammonium nitrogen (NH4+-N) and nitrate nitrogen (NO3--N); at 60 and 120 mg kg-1). The findings revealed that, in contrast to the NO3--N treatment, the NH4+-N treatment significantly enhanced the conversion of stabilized Cd in the soil to NH4NO3-extractable Cd, and also amplified the accumulation of Cd in edible amaranth, with concentrations in roots and shoots rising from 1.7-6.0 mg kg-1 to 4.3-9.8 mg kg-1. The introduction of NH4+-N can result in a decrease in the pH value of the rhizosphere soil and stimulate the production and secretion organic and amino acid metabolites, such as oxalic acid, lactic acid, stearic acid, succinic acid, and L-serine, from the crop roots. In comparison to NO3--N, the combined interaction of root exudates with NH4+-N has a more pronounced impact on the abundance of microbial genes associated with glycolysis pathway and tricarboxylic acid (TCA) cycle, such as pkfA, pfkB, sucB, sucC, and sucD. The effects of NH4+-N on crops and microorganisms ultimately result in a significant increase in the re-mobilization of stabilized Cd. However, based on simulated experiments, the contribution rate of microorganisms to the re-mobilization of stabilized Cd in soil is only 3.8-6.6%. Therefore, regulating the secretion of protons and organic acids in crop metabolism, rather than focusing on microbial activities, is the key measure to inhibit the re-mobilization of stabilized Cd during agricultural activities.