While a wealth of nutrient export models exists, a knowledge gap persists regarding how climate and land-use changes specifically drive dissolved inorganic nitrogen (DIN) export in subtropical catchments.

A new study led by a research team from the Nanjing Institute of Geography and Limnology of the Chinese Academy of Sciences has investigated DIN export patterns in a representative subtropical catchment in southeastern China. Leveraging the Hydrological Predictions for the Environment (HYPE) model, the study assessed the primary drivers of fluctuations in DIN export, as well as the potential impacts of projected future climate and land-use shifts.

The findings were recently published in Journal of Hydrology.

Using the HYPE model, the researchers simulated streamflow dynamics from 2000 to 2019, successfully capturing interannual variations in DIN export. The analysis showed that elevated DIN loads observed in spring and summer were primarily fueled by agricultural fertilizer application, coupled with denitrification processes occurring in both soil and river systems.

Model projections indicated that land-use change alone—specifically a 7.0% expansion of arable land and a 7.5% reduction in forest cover between 2015 and 2050—would boost riverine DIN concentrations by 12.8% and DIN loads by 19.1%, driven by increased nitrogen inputs. By contrast, annual evapotranspiration and runoff were projected to remain relatively stable, with only a marginal 0.5% change expected.

Additionally, the team conducted climate change projections for the 2031–2050 period and discovered divergent runoff trends across different climate models and emission scenarios, with estimates ranging from a 24.0% decrease to a 24.8% increase. Over the same timeframe, average riverine DIN concentrations could surge by up to 36.0% compared with the 2000–2019 baseline, depending on the variability of hydroclimatic conditions.

The combined impacts of future land-use and climate change were projected to push riverine DIN loads up by as much as 32.6%.

The study further underscored that climate-driven effects dominate runoff variations, and may amplify the influence of land-use changes on nitrogen pollution in subtropical catchments such as the Yifeng River basin. This amplifying effect stems from reduced precipitation and river discharge, which weaken the water body's capacity to dilute nitrogen inputs.