Effects of maize straw application on water-soluble organic carbon and fluorescence characteristics of albic soil.

Albic soil is a typical low-yield soil in China, characterized by low soil organic matter content. Exploring the impacts of maize straw return on the water-soluble organic carbon (WSOC) content of albic soils is of great significance for improving carbon sequestration and stabilizing soil fertility. In an experiment, we added maize straw (1% of soil mass) to two subtypes of albic soil (typical and meadow albic soils) at depths of 10 cm, 20 cm, and 30 cm, with no straw addition as the control, to analyze changes in WSOC content and fluorescence characteristics at 40, 100, and 290 days after straw addition. Results showed that maize straw addition significantly increased WSOC content at all depths in both subtypes, with a tendency to increase over time. After 290 days, WSOC content in typical albic soil increased by 6.4%, 15.7%, and 131.7% at 10 cm, 20 cm, and 30 cm depths, respectively, while meadow albic soil showed increases of 8.6%, 25.1%, and 54.0% at the same depths. Maize straw addition significantly decreased the water-soluble organic carbon/soil organic carbon (WSOC/SOC) ratio, indicating higher stability of SOC pool. The fluorescence index (1.15-1.79) and biogenic index (0.91-2.63) increased by 5.8% and 2.7%, respectively, while the humification index (0.25-0.62) decreased by 15.0%, suggesting that WSOC was influenced by both exogenous and endogenous substances and that straw addition led to a reduction in the humification of albic soils. Analysis of the WSOC fluorescence spectra identified four components belonging to three types of substances: soluble microbial products, humic acid like substances, and protein-like substances. After the addition of straw, the contents of humic acid like substances in typical albic soil decreased, while that of soluble microbial products and protein like substance increased. In meadow albic soil, the contents of humic acid like substances increased at depths of 10 cm and 30 cm but decreased at 20 cm; soluble microbial products increased at depths of 10 cm and 20 cm but decreased at 30 cm, while that of protein-like substances showed the opposite trend. Straw retuning increased the WSOC content in albic soil, promoted the formation of low-molecular-weight fluorescent components in WSOC solution, simplified the structure and enhanced the nutrient-supplying capacity.