Uptake characterization of soil arsenic species and its effects on nitrogen cycle in soybean (Glycine max (L.) Merrill) cultivation: A comparison with cadmium

This study aimed to elucidate the effects of arsenic species [As(III)/As(V)] and cadmium [Cd(II)] on nitrification and nitrogen fixation in soybean (Glycine max (L.) Merrill) cultivation, and to identify nitrogen cycle disruption mechanisms in realistic soil environments with a focus on soil-metal-plant-microbe interactions. We examined heavy metal(loid)s uptake in plant tissues, changes in nitrogen species in porewater, nitrogenase activity, the contents of essential trace metals (Mo and Fe) in nitrogenase, and nitrogen-related microbial communities. The results revealed that As(III) was oxidized to As(V) by manganese oxides and As(III)-oxidizing bacteria, and subsequently accumulated in roots and nodules. This oxidation promoted NO₃⁻ reduction by As(III)-oxidizing bacteria, increasing toxic NO₂⁻ concentrations in porewater by up to 1.4-fold compared with the control. As-tolerant ammonia-oxidizing bacteria (AOB) maintained nitrification under As exposure, whereas Cd(II) markedly impaired AOB activity, reducing nitrification by 68 % at 10 mg·kg⁻¹ . In nitrogen fixation, As(V) mimicked MoO₄²⁻ and inhibited Fe uptake, blocking Mo and Fe accumulation in nodules and suppressing nitrogenase activity by up to 79 %. Increasing As concentrations progressively inhibited nodule formation, with no nodules observed at 60 mg·kg⁻¹ As pot. Cd(II) exerted minimal effects on Mo and Fe contents; however, it impaired microbial enzymes, resulting in nitrogen fixation inhibition only at high concentrations (48 % reduction). This study highlights differences between heavy metal(loid)s and the role of As speciation in nitrogen cycle disruption, providing comprehensive insights essential for developing strategies to restore biological nitrogen fixation and improve crop productivity in contaminated soils.