Soil dissolved organic matter (DOM) is a critical component of the global carbon cycle, mediating energy and nutrient transfer between terrestrial, aquatic, and atmospheric environments. Its transformation and mobilization influence nutrient availability, microbial activity, ecosystem carbon balance, and ambient environmental conditions. Terrestrial DOM input into water systems significantly impacts water quality and aquatic ecosystems, affecting drinking water treatment and human health. The complex molecular properties of DOM vary greatly with ecosystem type, making it crucial to understand its behavior in diverse environments. Grasslands, covering a significant portion of the Earth's land surface, store substantial amounts of soil organic carbon, including DOM. In China, grasslands represent a vast and climatically diverse area, making them ideal for studying the spatial variability of soil DOM and its environmental impacts. This study addresses two key questions: (1) What are the compositional features and potential environmental functions of soil DOM in large-scale grassland ecosystems? (2) How do environmental factors, particularly climate variables, drive the spatial patterns of DOM at the continental scale? The study's findings are crucial for understanding the role of soil DOM in ecosystem services and human health, particularly given the ongoing climate change.

Previous research has established the importance of soil DOM in various ecosystem processes and its impact on water quality. Studies have highlighted the role of DOM in regulating nutrient availability, microbial activity, and carbon cycling. The impact of terrestrial DOM on water quality and the subsequent challenges in drinking water treatment have also been well-documented. However, large-scale systematic studies examining the spatial patterns and controlling factors of DOM geochemistry in grasslands, particularly in China, are limited. Existing regional-scale studies provide valuable insights but lack the continental perspective necessary to understand the broad environmental implications of DOM variability. A major gap in knowledge exists concerning the potential link between terrestrial DOM and human health, particularly the potential association with endemic diseases.

This nationwide survey characterized DOM in 89 grassland soil samples collected across 30 provinces of China during the dry season. Soil samples (0-20 cm depth) were collected from diverse grassland sites, accounting for variations in climate, vegetation, and soil type. DOM was extracted from air-dried soils using Milli-Q water. The following analyses were performed: pH, electrical conductivity (EC), redox potential (Eh), dissolved organic carbon (DOC), total dissolved nitrogen (TDN), and inorganic nitrogen. Disinfection byproduct formation potential (DBP-FP) and chlorine reactivity (SDBP) were assessed through chlorination experiments. DOM characterization included UV-visible absorbance spectroscopy, three-dimensional excitation-emission matrix (3D-EEM) fluorescence spectroscopy, high-performance size exclusion chromatography (HPSEC) for molecular weight distribution, and Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR MS) for molecular composition in selected samples. A DOM incubation experiment was conducted to determine biodegradability. Spatial patterns were analyzed in relation to environmental variables (temperature, precipitation, solar radiation, soil properties, vegetation indices, net ecosystem productivity, and soil respiration) and cancer incidence and mortality data obtained from the 2019 China Cancer Registry Annual Report. Statistical analyses included Wilcoxon tests, Spearman correlations, regression analysis, variation partitioning analysis (VPA), principal component analysis (PCA) with PERMANOVA, random forest regression (RF), and partial least squares path modeling (PLS-PM).

The study identified four fluorescent components (C1-C4) in the grassland soil DOM. Humic-like (C1-C3) and high molecular weight (3.4–25 kDa) fractions dominated the DOM composition, with concentrations of DOC and DON consistent with previous regional records. The northern regions of China exhibited higher abundances of humic-like and HMW compounds, greater humification (HIX), and higher DOC, DON, and DBP precursor concentrations. In contrast, southern China showed higher levels of LMW, MMW, C4, BIX, and FI, indicating a greater proportion of autochthonous, proteinaceous compounds. The study revealed a significant South-North divide in DOM geochemistry, with the Yangtze River basin showing a remarkable transition. DOM from the northern regions displayed significantly lower biodegradability (21-35%) than that from the southern regions. HMW and humic-like fractions were negatively correlated with soil respiration (Rs) and net ecosystem productivity (NEP), while LMW and protein-like compounds showed positive correlations. Significant correlations were observed between DOM composition and cancer incidence and mortality rates. Higher LMW and protein-like fractions were associated with nasopharyngeal cancer, while HMW and humic-like fractions were linked to pancreatic cancer. Random forest analysis indicated that edaphic factors were key predictors for DOM quantity, while climate parameters were more influential in determining DOM composition. PLS-PM analysis showed that climate and soil factors negatively impacted DOM quantity and quality. The study found that grassland soil DOM explained a significant portion of the variance in annual ecosystem carbon exchange and cancer rates.

The findings demonstrate a clear spatial heterogeneity in grassland soil DOM geochemistry across China, primarily driven by climatic and edaphic factors. The contrasting roles of HMW humic-like and LMW proteinaceous fractions in ecosystem carbon cycling highlight the complexity of DOM's influence on soil respiration and productivity. The observed associations between DOM composition and cancer incidence warrant further investigation into the potential mechanisms, including direct toxicity of specific DOM compounds and interactions with other environmental pollutants. These findings emphasize the importance of considering local DOM characteristics when developing strategies for ecosystem management and water resource protection. The study's limitations regarding seasonal variability and the need for further research to establish causal relationships should be carefully considered.

This continental-scale study reveals significant spatial variability in the composition and functions of grassland soil DOM in China. The South-North divide in DOM geochemistry underscores the influence of climate and edaphic factors on DOM properties. Different DOM fractions play distinct roles in ecosystem carbon exchange, and certain fractions show potential links to cancer incidence. Future research should focus on elucidating the mechanisms behind these associations and investigating the seasonal dynamics of soil DOM. This knowledge is crucial for developing sustainable management practices that optimize ecosystem services and minimize potential risks to human health.

The study's primary limitation is the focus on the dry season, which may not fully represent the annual dynamics of soil DOM. The observed correlations between DOM and cancer incidence do not establish causality; further research is needed to clarify the underlying mechanisms. The study relies on existing datasets for environmental variables and cancer rates, which may contain uncertainties or limitations. The sample size, although large for a continental-scale study, could be increased for enhanced statistical power and a refined analysis of spatial heterogeneity.