Spatial Variation and Influencing Factors of Optical Characteristic of Water Extractable Organic Matter in Soils of Urban Grassland Across Climatic Zones in China

Dissolved organic matter (DOM) is the most active organic component in terrestrial ecosystems. In this study, 165 soil samples which were dominated by moss were collected from grasslands with similar vegetation in urban residential areas from 55 cities across different climatic zones of China. The water extractable organic matter (WEOM) of samples was analyzed. The results showed that WEOM content ranged from 23.5 to 517.6 mg kg⁻¹ in the soil samples. Four fluorescent components including two tryptophan-like components (C1 and C4) and two humic-like components (C2 and C3) were identified by excitation–emission matrices combined with parallel factor analysis (EEM-PARAFAC). Proportions of humic-like components (C2 + C3) were 54–66%, which is 1.2–2.0 times of the tryptophan-like component proportion (C1 + C4) for all samples. The proportions of C2 and C3 were highest in the tropical monsoon climate (TroM) zone compared to that in the other climatic zones. Structural equation model and decomposition of variation showed that soil nutrient content was the main contributing factor of soil WEOM. Content Total nitrogen content and mean annual temperature (MAT) were the main factors influencing soil HIX but they had the opposite effects. Our results suggested that increasing soil nutrient content contributed the increase in soil WEOM content, but increasing MAT reduced the humification degree of WEOM.