关键词: 天山北坡经济带, 地表水, 营养盐, 土地利用, 空间分布

Abstract:

The northern Tianshan Mountain economic zone serves as a core growth pole for China’s Western Development Strategy and a key node of the “Silk Road Economic Belt,” while also functioning as a vital ecological security barrier for western China. In recent years, the rapid advancement of national strategies has accelerated regional socio-economic development and urbanization, leading to significant shifts in land use patterns. These anthropogenic changes have exerted significant pressure on the surface water environment, causing nutrient exceedances in certain water bodies and threatening regional sustainable development and ecological security. Although previous studies have evaluated water quality in specific small catchments within the region, there is a lack of systematic research covering the entire northern Tianshan Mountain economic zone that integrates spatial distribution characteristics, land use response mechanisms, and pollution source apportionment. To support differentiated pollution control and water environment protection, this study conducted comprehensive field investigations from June to September in 2023 and 2024. A total of 185 surface water samples were collected from rivers, lakes, and reservoirs across the region. The study employed the single-factor evaluation method to assess nutrient pollution status and utilized Spearman correlation analysis, Redundancy Analysis (RDA), and multi-ring buffer analysis (at 100 m, 500 m, and 1000 m scales) to reveal the response characteristics of nutrient indicators to land use patterns. Furthermore, nitrate pollution sources were apportioned using a combination of hydrochemical ion tracing and dual stable isotope tracing. The main research findings are as follows: 1) Overall water quality status and pollution characteristics: Nutrient indicators in the surface water of the northern Tianshan Mountain economic zone were generally at low levels, yet specific pollution risks persist. Assessment results indicate that 42.16% of sampling points met the Class III standards of the Environmental Quality Standards for Surface Water (GB 3838—2002), suggesting acceptable overall quality but prominent localized pollution. Specifically, the exceedance of nitrate nitrogen NO₃⁻-N) and Total Nitrogen (TN) in rivers was significant. In contrast, lakes and reservoirs exhibited higher non-compliance rates for organic pollution indicators (COD_Cr and BOD₅), with lakes showing significant organic pollution risks. 2) Spatial heterogeneity of nutrients: The study reveals distinct spatial differentiation in nutrient distribution across the four sub-regions (West, Northwest, Central, and East). The Northwest and East regions primarily face nitrogen pollution risks. In particular, the Northwest region exhibited the highest average TN concentration (1.62 mg·L⁻¹), followed closely by the East region (1.61 mg·L⁻¹), both exceeding Class III limits. The Western region also showed elevated TN levels but relatively lower ammonia nitrogen concentrations. Conversely, the Central region, having the highest degree of industrialization and urbanization, exhibited a distinct pollution profile characterized by higher Total Phosphorus (TP) and organic pollution (COD_Cr and BOD₅). Watershed-scale analysis further identified the Emin River and Bortala River basins as high-risk areas for nitrogen enrichment, while the Urumqi River basin faces composite pollution challenges involving nitrogen, phosphorus, and organic matter. 3) Response of water quality to land use patterns: Land use structure was confirmed as a critical factor driving surface water nutrient variations, with its impact varying by water body type and spatial scale. River water quality demonstrated a sensitive and significant response to land use changes, whereas lakes and reservoirs showed no significant correlation with land use in adjacent buffer zones due to hydrological inertia and internal sediment release. For rivers, construction land was the dominant factor influencing nutrient levels across all buffer scales (100 m, 500 m, and 1000 m). Redundancy Analysis indicated a positive correlation between construction land and multiple nutrient indicators, suggesting that impervious surfaces accelerate runoff and pollutant transport from urban point and non-point sources. Cultivated land acted as a significant “source” landscape for nitrate and TN; its influence diminished with increasing buffer distance but remained a key factor in the 500 m and 1000 m buffers. Conversely, grassland and woodland function as “sink” landscapes, exhibiting negative correlations with nutrient concentrations, thereby highlighting their ecological interception and purification functions. 4) Nitrate source apportionment: The integration of hydrochemical and isotopic tracing achieved precise identification of nitrate sources, revealing significant regional differences driven by distinct dominant land use types. Western region: Nitrate pollution is primarily driven by cropland utilization. The main sources of nitrate are agricultural chemical fertilizers and soil nitrogen mineralization, reflecting the intensive cotton and grain production activities in the Bortala and Jinghe watersheds. Northwest and Eastern regions: the main sources of nitrate are domestic sewage and manure. This suggests that rural settlements and livestock farming associated with construction land and scattered residential areas are the primary contributors to nitrate loading. Central Region: Nitrate sources are complex and driven by the synergistic effects of cropland and construction land. Isotopic evidence points to a mixed contribution from soil nitrogen mineralization, domestic sewage, and manure, corresponding to the dense population and overlapping industrial-agricultural activities in this region. Conclusion and policy implications: In summary, although the overall water quality in the northern Tianshan Mountain economic zone remains manageable, structural and regional pollution risks related to nitrogen and organic matter are prominent. The study confirms that land use patterns—specifically the expansion of construction land and agricultural activities—are the primary drivers of riverine water quality deterioration. Based on these findings, differentiated control strategies are proposed: The Western region should focus on upgrading agricultural practices, including precision fertilization and establishing ecological buffer zones to intercept agricultural runoff. The Central region requires a dual focus on upgrading urban sewage treatment facilities and controlling industrial discharge to mitigate organic and phosphorus pollution. For the Northwest and Eastern regions, priority should be given to managing rural domestic waste and livestock manure to reduce nitrogen inputs from human settlements. Furthermore, the protection of riparian grasslands and woodlands should be strengthened across all regions to enhance the natural purification capacity of river corridors. This study provides a theoretical basis and data support for the implementation of precision pollution control and the optimization of territorial spatial planning in the northern Tianshan Mountain economic zone.

Key words: the northern Tianshan Mountain economic zone, surface water, nutrients, land use, spatial distribution patterns