Chemical structure and nanomechanics relevant electrochemistry of solid-phase humic acid along a typical forest-river-paddy landscape section in eastern China and its environmental implications

Solid-phase humic acid (HAₛₒₗᵢd) plays a critical role in global carbon cycle and redox biogeochemistry of topsoil, because of its unique physicochemical properties, including electrochemical property. In this study, topsoil HAₛₒₗᵢd along a typical forest-river-paddy landscape section in eastern China was investigated in the aspects of electrochemical property, chemical structure and nanomechanics, and their relationship. Nano-size HAₛₒₗᵢd particles were extracted from topsoil of paddy soil (PS-HAₛₒₗᵢd), forest soil (FS-HAₛₒₗᵢd) and riverside sediment (RS-HAₛₒₗᵢd). Results showed that all the HAₛₒₗᵢd were conductive and played an important role in conductivity of topsoil suspension. HAₛₒₗᵢd contained both reversible and irreversible redox peaks, with redox activity of PS-HAₛₒₗᵢd > RS-HAₛₒₗᵢd > FS-HAₛₒₗᵢd. Owing to limited humification, electron exchange capacity (EEC) values of topsoil HAₛₒₗᵢd suspensions were modest (3.18–4.45 μmol e⁻g⁻¹ HAₛₒₗᵢd). Compared with RS-HAₛₒₗᵢd and FS-HAₛₒₗᵢd, PS-HAₛₒₗᵢd showed higher aromaticity and higher degree of humification with simple and even nanomechanical property. Long-term cultivation (human activities) as well as high content of polyvalent metals in paddy soil were supposed to facilitate formation of aromatic carbon and improve humification of HAₛₒₗᵢd. The results suggested that aromatic carbon and high humification degree of PS-HAₛₒₗᵢd contributed to simple and even nanomechanics, which further optimized its electrochemical property. This study not only provides novel insight into the mechanism of HAₛₒₗᵢd mediating electron transfer, but also inspires ideas for soil and environmental management with different purposes based on regulation of HAₛₒₗᵢd.