Remediation Effect and Mechanism of Biochar in Combination with Nitrogen Fertilizer on Cd-contaminated Paddy Soil

Abstract

Cadmium (Cd) heavy metal pollution has posed serious threats to soil health and the safe production utilization of agricultural products. A pot experiment was conducted to study the effects of biochar (BC) and nitrogen fertilizer with three levels, namely 2.6 g·pot-1 (N1), 3.5 g·pot-1 (N2), 4.4 g·pot-1 (N3) biochar combined with nitrogen fertilizer (BCN1, BCN2, and BCN3), on soil Cd fractions, Cd enrichment, the transport of rice, and soil enzyme activity, as well as the changes in microbial community composition and complex interactions between microorganisms through high-throughput sequencing. The results showed that biochar combined with nitrogen fertilizer led to the transformation of Cd from the exchangeable state to the residue state, and the proportion of the exchangeable state was significantly reduced by 6.2%-14.7%; by contrast, the proportion of the residue state increased by 18.6%-26.4% relative to that in CK. In addition, singular treatments of nitrogen fertilizer enhanced the accumulation capacities of Cd in roots, which increased by 22%-33.5% compared with that in CK. By contrast, the BC and BCN treatments reduced Cd accumulation in roots and the transfer capacity from stems to rice husks and husk to rice. Furthermore, the BCN treatments promoted soil enzyme activities (urease, acid phosphatase, invertase, and catalase). MiSeq sequencing showed that BCN treatments increased the abundance of the main species of soil bacterial microbes (such as Acidobacteriales, Solibacterales, Pedosphaerales, and Nitrospirales). Moreover, co-occurrence network analysis showed that the complexity of the soil bacterial network was enhanced under the N, BC, and BCN treatments. Overall, biochar combined with nitrogen fertilizer reduced soil Cd availability, inhibited the capacity of Cd accumulation and the transport of rice, and improved the soil eco-environmental quality. Thus, using BCN could be a feasible practice for the remediation of Cd-polluted agricultural soil.