Variations of dissolved organic matter vertically in the scenario of sludge composting acting as nitrogen fertilizers: perspectives of spectral implications.

Soil dissolved organic matter (DOM) plays a vital and indispensable role in terrestrial ecosystems due to its high reactivity and remarkable mobility. The present study was conducted to determine the vertical characteristics of DOM derived from soil incorporated with sludge composting after 210-day of winter wheat cultivation, using UV-visible and excitation-emission matrices (EEMs) coupled with parallel factor (PARAFAC) modeling. Our findings showed that the amounts of DOM released from the soils amended by sludge composting underwent a significant change from 0.31 g kg to 0.82 g kg (p < 0.05). Five EEMs-PARAFAC fluorescent components (components 1-5), including UVA + UVC humic acid-like, and fulvic acid-like materials, were obtained. Furthermore, the proportion of C1-C5 observed in 72 soil DOM samples varied significantly with the following descending order: C2 (31.2 %) > C1 (28.8 %) > C3 (21.1 %) > C4 (18.4 %) > C5 (0.46 %) (p < 0.05). The fluorescence intensity (FI) of DOM derived from sludge compost-amended soils suggested terrestrial and allochthonous origin in the topsoil, while indicating a predominantly microbial source in the subsoil. Partial Least Squares Structural Equation Modeling (PLS-SEM) analysis indicated that the EEMs-PARAFAC components were positively regulated by soil depths (effect coefficient: 0.001), and fluorescence factors (effect coefficient: 0.775). Conversely, they were negatively mediated by soil DOM contents, presenting an effect coefficient of -0.295, and UV-visible factors, with an effect coefficient of -0.227. The findings reveal the vertically distributions of DOM spectral components and strengthen the understanding of soil DOM decomposition and transformation processes under the agricultural application of sludge composting.