Feng et al. (2026) Subsurface stormflow concentration-discharge relationships reveal DOC and nitrate transport mechanisms across land uses in karst hillslopes

Identification

• Journal: Journal of Hydrology
• Year: 2026
• Date: 2026-02-03
• Authors: Na Feng, Jun Zhang, Fa Wang, Zhiyong Fu, Kelin Wang
• DOI: 10.1016/j.jhydrol.2026.135070

Research Groups

• Institute of Subtropical Agriculture, Chinese Academy of Sciences, Changsha 410125, China
• Guangxi Key Laboratory of Karst Ecological Processes and Services, Huanjiang Observation and Research Station for Karst Ecosystems, Chinese Academy of Sciences, Huanjiang 547100, China
• University of Chinese Academy of Sciences, Beijing 100049, China

Short Summary

This study investigated dissolved organic carbon (DOC) and nitrate transport dynamics and concentration-discharge relationships in subsurface and epikarst flows across four land-use types in karst hillslopes. It revealed epikarst flow as the dominant nutrient export pathway and highlighted the impact of land use and rainfall patterns on carbon and nitrogen fluxes.

Objective

• To quantify the transport dynamics of dissolved organic carbon (DOC) and nitrate, and their concentration-discharge relationships, in subsurface and epikarst flows across different land-use types in karst hillslopes.

Study Configuration

• Spatial Scale: Karst hillslopes with four land-use types (cropland, forage grassland, planted forestland, shrubland).
• Temporal Scale: High-frequency sampling, focusing on the early rainy season (e.g., April-May), heavy rainfall (25–49.9 mm d⁻¹), and large rainstorms (>100 mm d⁻¹).

Methodology and Data

• Models used: Not explicitly mentioned; empirical study based on high-frequency sampling and analysis of concentration-discharge relationships.
• Data sources: Manual high-frequency sampling of dissolved organic carbon (DOC) and nitrate concentrations, and discharge from subsurface and epikarst flows.

Main Results

• DOC and nitrate concentrations were highest during heavy rainfall (25–49.9 mm d⁻¹) in the early rainy season (April-May) but lower during large rainstorms (>100 mm d⁻¹).
• Export DOC and nitrate yields reached 1.71–15.19 kg km⁻² h⁻¹ and 0.60–8.88 kg km⁻² h⁻¹ in large rainstorms, respectively, which were 1.11–13.09 times and 1.30–6.53 times those in other rainfall events (25–99.9 mm d⁻¹).
• Epikarst flow exported 10.27 times more DOC and 17.33 times more nitrate yields per hour than subsurface flow, establishing it as the dominant pathway for nutrient export due to its greater runoff depth.
• Planted forestland generally had the highest DOC yields (2.78 kg km⁻² h⁻¹) in subsurface flow, while cropland showed peak nitrate exports (1.77 kg km⁻² h⁻¹).
• Forage grassland reduced DOC yield by 80.5% relative to planted forestland and nitrate by 97.4% relative to cropland.
• Mechanistically, DOC export was source-limited with near-synchronous concentration-discharge coupling in subsurface flow, contrasting with delayed release in epikarst flow.
• Nitrate export was transport-limited, exhibiting clockwise hysteresis in cropland and shrubland, anticlockwise in planted forestland, and no significant hysteresis in forage grassland.
• Land use indirectly influenced DOC and nitrate concentrations by altering soil water content, while precipitation affected DOC and nitrate via discharge and geogenic ions (e.g., Mg²⁺), respectively.

Contributions

• Provides novel insights into the transport dynamics and concentration-discharge relationships of DOC and nitrate in complex karst subsurface and epikarst flows.
• Quantifies the relative importance of epikarst flow as the dominant pathway for nutrient export compared to subsurface flow.
• Demonstrates the significant impact of different land-use types on DOC and nitrate yields and transport mechanisms.
• Identifies forage grassland as an effective land-use strategy for reducing carbon and nitrogen export.
• Elucidates the mechanistic differences in DOC and nitrate export across different flow paths and land uses.
• Highlights the critical role of epikarst systems and rainfall patterns, particularly large rainstorms, in regulating nutrient exports, informing groundwater quality protection strategies.

Funding

• Not explicitly mentioned in the provided text.

Citation

@article{Feng2026Subsurface,
  author = {Feng, Na and Zhang, Jun and Wang, Fa and Fu, Zhiyong and Wang, Kelin},
  title = {Subsurface stormflow concentration-discharge relationships reveal DOC and nitrate transport mechanisms across land uses in karst hillslopes},
  journal = {Journal of Hydrology},
  year = {2026},
  doi = {10.1016/j.jhydrol.2026.135070},
  url = {https://doi.org/10.1016/j.jhydrol.2026.135070}
}