Vegetation restorations are crucial strategies for combating land degradation, yet their mechanisms on improving soil quality, especially from perspective of soil fertility, remain far from clear. Sparse trees in semi-arid savanna grasslands (i.e., climate communities) offer a provoking reference for vegetation restoration. Here, taking advantage of Ulmus macrocarpa Hance fertile islands of the savanna ecosystem in the Daqinggou National Nature Reserve, this study aimed to investigate the vertical and horizontal distribution patterns of soil physicochemical properties and DOC fluorescence spectral characteristics. Results showed that soil organic carbon (SOC) and DOC were significantly decreased with both the increasing distance from tree and increasing soil depth. Horizontal and vertical treatments significantly enhanced fluorescence intensities of DOC. Additionally, the soil under canopy exhibited slightly richer concentrations of NH4+–N, NO3––N, TN, and TP at topsoil compared with deep soils. The SOC, TN, TP, NH4+–N, and NO3––N showed significantly positive relationships with the DOC. The study provides evidence that trees can form fertile island effects and enhance soil nutrients and DOC. These results are vital for guiding vegetation restoration degraded ecosystem in semi-arid area.