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Studying the chemical composition and characteristic differences of soil dissolved organic matter (DOM) is significant for understanding the mechanism of Cd immobilization by soil amendments. Soil amendments have been widely applied to contaminated farmlands to reduce the accumulation of heavy metals in crops, but the spectral characteristics of DOM in soils under amendment regulation have rarely been studied. Typical Cd-contaminated paddy soil from South China was collected, three categories of amendments (organic-based, inorganic-based, and lime-based, a total of 11 types) were applied, rice planting pot trials were done, and the effects of different amendments on soil DOM were investigated. The spectral characteristics of rhizosphere soil DOM under the regulation of different amendments were comparatively analyzed using UV-Vis spectroscopy, 3D fluorescence spectroscopy, and parallel factor analysis (PARAFAC). The results showed that the lime-based amendments increased the soil pH and promoted the dissolution of organic matter from the soil solid phase, resulting in a significant increase in soil DOM content. Compared with that of the control, organic-based amendments increased the relative molecular weight and recent autochthonous origin contribution of soil DOM, inorganic-based amendments increased the aromatic content and hydrophobic components of soil DOM, and lime-based amendments increased the chromogenic components and the degree of humification of soil DOM. Four fluorescence components, C1 (255/465), C2 (325/400), C3 (275/390), and C4 (240/460), were identified using PARAFAC and verified with the OpenFluor database, all of which were humic-like. Two types of spectra corroborated with each other, indicating that soil DOM dominated by humus-like matter originated from terrestrial source input. The correlation analysis showed that the fluorescent component C4 of soil DOM could be used to predict Cd accumulation in brown rice in the red limestone soil-rice system. These results will provide a reference for the selection of the appropriate soil amendments.

Based on the total and available concentrations of heavy metal elements in agricultural soil, and the concentrations of heavy metals in rice collected from Daye city, Hubei province, and the thresholds of available concentrations of heavy metals were derived by species sensitivity distribution (SSD). The over-standard rates of Cd and Cu in the soil were 90.7% and 42.6%, respectively, which indicates that agricultural soil in Daye city exhibits signs of Cd and Cu pollution. The rates of Cd and Pb were 50.9% and 89.8% in brown rice samples, respectively, which exceeds the safety standards of agricultural products. The SSD curves for Cd and Pb were fitted with BurrⅢ distribution based on the ratio of the concentration in brown rice and the available concentration in the soil. The hazardous concentrations (HC 5 ) were derived inversely from the agriculture product safety standards. The thresholds of available Cd and Pb were 0.02 mg·kg -1 and 0.005 mg·kg -1 , respectively. Compared with those of available Cd in other countries and studies, the threshold of available Cd is scientifically shown to be reasonable. Soil is not necessarily the main source of Pb in rice; thus, the rice species does not represent the derivation of the threshold of available Pb. Moreover, a lack of the accumulation data of Pb at low accumulation levels and multiple soil pollution levels leads to unreliable derivation. The results indicate that the threshold of available Pb does not have practical significance in soil pollution control.