Spectral characteristics of dissolved organic mattwe in rice-shrimp co-cropping soils and its relationship with exchangeable cadium in soils

In order to explore whether the relationship between the spectral characteristics of heavy metal cadmium (Cd) and soil dissolved organic matter (DOM) changes with the cultivation process of rice and shrimp, traditional rice cultivation (CK) was taken as the control, and rice and shrimp cultivation mode (RS) was taken as the research object. UV spectrum (UV-VIS) and 3D fluorescence spectrum (3D-EEMS) were used to analyze the changes of soil DOM spectral characteristics and composition of RS during rice growth stage (early stage T1, tillering stage T2, booting stage T3, yellow ripening stage T4). The relationship between DOM and exchangeable cadmium (Cdex) content in RS and CK soil was established by using correlation and step-up regression model. The results are as follows: UV spectrum parameters show that the DOM of aromatics and hydrophobicity is the most in T4. Compared with CK, the molecular weight of DOM in RS soil decreased with rice growth. UV254 (ultraviolet absorbance at wavelength 254 nm), SUVA254 (ratio of UV254 to dissolved organic carbon concentration), SUVA260 (ratio of UV260 to dissolved organic carbon concentration) and other parameters were more stable in T3~T4 than CK, and DOM tended to be biogenic in the yellow ripe stage. The fluorescence spectra showed that soil DOM of RS and CK were composed of 4 components, namely tryptophan C1, humus C2 and C3, and fulvic acid C4. Among the fluorescence parameters, the fluorescence parameters (FI) of CK and RS are greater than 1.9 in different periods, the humification index (HIX) is less than 4, the autogenetic index (BIX) of CK is about 0.6~0.7 from T1 to T4, and the BIX value of RS is between 0.7 and 0.8, indicating that the DOM of both modes has typical endogenous local characteristics. There was no significant change in the proportion of DOM components during the growth period. Correlation analysis showed that Cdex content was positively correlated with spectral slope ratio SR and component C4 (P < 0.05), indicating that Cdex was closely related to the humification degree, molecular weight, source and component of DOM. Stepwise regression analysis showed that the factors related to molecular weight were important for predicting the content of Cdex. The results can provide scientific reference for the prevention and control of Cd pollution in the breeding process of rice and shrimp.