Chemical weathering intensity controls the accumulation of nickel in rice (Oryza sativa L.) cultivated in basalt-derived paddy fields

Abstract

Globally, a high concentration of nickel (Ni) in agricultural soil is closely related to basalt weathering. Rice grains readily accumulate Ni, however, the evolution of Ni distribution and mobility in paddy fields with basalt weathering remain unclear. This study investigated the distribution, geochemical fraction, and mobilization of Ni in basalt bedrock-paddy soil-rice plant continuums under contrasting climates through a combination of field surveys, chemical extractions, X-ray diffraction, and scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS) analyses. Results show that the soils developed in tropical climates were more enriched in Ni than those in temperate climates. The concentration of Ni in rice grains from the studied temperate regions (3.71 ± 2.01 mg/kg) was higher than that in the tropical regions (0.57 ± 0.37 mg/kg). The concentration of Ni bound to Fe (hydro)oxides in paddy soils from temperate regions was five times higher than that in tropical regions. In contrast, the concentration of Ni found in phyllosilicate clays (mainly smectite/illite) in paddy soils from the temperate regions was only ∼1/4 of that of clay (mainly kaolinite) in the tropical paddy soils, and the ratio of Ni concentration in phyllosilicate clays to highly reactive Fe (hydro)oxides in paddy topsoil in temperate regions was one order of magnitude less than that in tropical regions, which contributes to the low bioavailability of Ni in tropical paddy soils. Furthermore, the accumulation of Ni in rice grains was insensitive to soil pH in tropical regions but showed significant negative correlations with the degree of chemical weathering (chemical index of alteration and index of laterisation) in temperate and tropical regions. These observations support the hypothesis that chemical weathering and the dissolution–recrystallization of Fe (hydro)oxides induced by frequent redox fluctuations in paddy soils cause Ni repartition between Fe (hydro)oxides and phyllosilicate minerals, which influence Ni geochemical cycling and bioavailability, and fundamentally regulate Ni accumulation in rice. This study elucidates the mechanism and controlling factors underlying Ni partition in the basalt-paddy soil-rice plant continuum under distinct climates, providing new insights into the understanding of geochemical behavior and environmental fate of Ni.