Identifying organic matter fractions and revealing their dynamic spatial variation within riparian soil profiles along an urbanized river <i>via</i> solid-phase SFS with moving window 2-dimensional correlation spectroscopy

Solid-phase fluorescence with synchronous fluorescence spectroscopy (SFS) is a relatively innovative method for characterizing soil organic matter (SOM). Herein, dynamic spatial variation and source of SOM fractions from riparian soils were elucidated along an urbanized river. Composite soil samples of four depths were collected from six land use types, <i>i.e.</i>, eco-conservation area (ECA), industrial area (INA), urban area (URA), town area (TOA), rural area (RUA) and agricultural area (AGA). Based on synchronous fluorescence spectra coupled with Gaussian band fitting, SOM was found to predominantly contain tyrosine-like (TYLF), tryptophan-like (TRLF), microbial humic-like (MHLF), fulvic-like (FLF₁ and FLF₂) and humic-like (HLF) substances within each soil profile. Among these, TYLF and TRLF (39.69-57.07%) were the most representative components. The decreasing order of the content of fluorescence materials within the six soil profiles was TOA > RUA > AGA > INA > URA > ECA. According to the moving window 2-dimensional correlation spectroscopy, SOM within the ECA soil profile was mainly derived from the degradation of terrestrial plant metabolites and residuals, while SOM within the INA soil profile was associated with the treated/untreated industrial wastewater, which penetrated the river and partially flowed into the riparian zones. SOM in both the URA and TOA soils could be correlated with scattered domestic sewage and livestock wastewater, whereas the SOM in the RUA and AGA soils could be correlated with crop metabolites and residuals. Noticeably, variations in SOM fractions within the ECA, RUA and AGA soil profiles virtually occurred in the range of 20-80 cm and within the INA, URA and TOA soil profiles in the range of 20-40 cm. Based on SEM, the humification degree of SOM within the former three soil profiles was found to be indirectly affected by MHLF on HLF, while the latter soil profiles were found to be directly affected by MHLF. This study can provide technical support for ecological restoration of riparian zones.