Effects of Calcium Salts on the Decomposition Process of Straw and the Characteristics of Three-Dimensional Excitation-Emission Matrices of the Dissolved Organic Matter in Decomposition Products

Straw is an important organic resource. To study the effects of different calcium salts on the straw decomposition process and decomposition products, which can provide a theoretical basis and technical reference for the efficient utilization and high-quality decomposition of organic materials. The straw decomposition experiment was carried out by adding different types of calcium salts (without adding (CK) CaC2O4, Ca(OH)(2), CaCO3, CaCl2, CaSO4 and Ca(H2PO4)(2)) in the laboratory. Then the decomposition rates and chemical properties of the decomposition products were measured at different decomposition stages (30, 60 and 180 d). Three-dimensional excitation-emission matrices (3DEEM) and parallel factor analysis (PARAFAC) were used to explore the chemical composition characteristics of the dissolved organic matter (DOM) of the decomposition products of straw. The results showed that : (1) compared with the control, the carbon conversion rate of straw in CaC2O4, Ca(OH)(2), CaCO3 and CaSO4 treatments increased by 25. 6%, 44. 1%, 33. 6% and 29.7%, respectively, and decreased by 76. 8% and 17. 5% in CaCl2 and Ca(H2PO4)(2) treatments, respectively. CaC2O4 and Ca(OH)(2) treatments significantly increased the pH of the decomposed products. CaCl2 and Ca(H(2)PO(4))2 significantly increased the EC of the decomposed products. The relative humus content of CaC2O4 and Ca(OH)(2) treatments was 3. 4%similar to 20. 9% and 2. 3%similar to 25. 3% higher than that of the control, respectively. (2) The composition of DOM was analyzed by 3DEEM-PARAFAC method and three fluorescent components were identified, including tryptophane-like (C1),fulvic-like (C2) and humic-like (C3). Ca(OH)(2), CaCO3 and CaC2O4 treatments had a higher humic-like/fulvic-like ratio (H/F) than CK treatment, which increased the complexity of the decomposed products. The humification (HIX) index of DOM in CaC2O4, Ca(OH)(2) and CaCO3 treatment was slightly higher than that in CK treatment, and the HIX index of Ca(H2PO4)(2), CaSO4 and CaCl2 treatment was significantly lower than that in CK treatment. (3) Correlation analysis showed that pH and EC were the main factors affecting straw decomposition after adding calcium salt. The humification degree of straw decomposition products was positively correlated with pH and negatively correlated with EC. In conclusion, CaC2O4, Ca(OH)(2) and CaCO3 treatment can promote the process of straw humification and improve the quality of decomposed products, while Ca(H2PO4)(2) and CaCl2 have opposite effects. In addition, pH and EC are the main factors affecting straw humification. This study can provide a scientific reference for selecting suitable calcium salt additives to promote straw decomposition and improve the quality of straw decomposition products.