The neglected role of forest eco-hydrological process representation in regulating watershed nitrogen loss

Forest eco-hydrological functions are relevant to multiple water and ecosystem services essential for watershed management. However, these have received limited attention compared to pollution sources such as farming and fossil fuel power plants. In this study, we systematically analyzed the characteristics, pathways, and sources of nitrogen (N) loss in a forested watershed using a combination of field sampling, remote sensing, and a modified Soil and Water Assessment Tool (SWAT) model with enhanced forest eco-hydrological representation. The modified SWAT has improved the performance in simulating forest leaf area index (LAI), evapotranspiration (ET), and water and sediment yield from a mechanistic perspective (the coefficient of determination increased by 0.64 and 0.11 for monthly LAI and ET, respectively). The modified SWAT and in situ sampling were then utilized to identify the N loss pattern and physical mechanism of various sources. The results show that organic N (ORGN) loss dominated the forest N loss and contributed up to 44.37 % of the total ORGN load in the upstream area. High nitrate leaching was observed in both upstream forest (0.11 ± 0.3 kg/ha) and farmland (16.21 ± 36.13 kg/ha) due to the high gravel content and soil saturated hydraulic conductivity. A comparison with forests or forested watersheds in similar northern temperate regions worldwide revealed that less-developed soil with high permeability usually has a higher nitrate leaching load than other soil types, ranging from 0.5 to 39 kg/ha/yr. Most notably, comparative analysis shows the original SWAT substantially overestimated the ORGN, nitrate loss, and nitrate leaching load from forestland by 9.69 %, 26.98 %, and 24.03 %, respectively. Such a large difference in N source calculation can potentially mislead mitigation strategies that aim to reduce N load from diffuse sources. Advanced watershed management for sustainability and water quality should adopt an integrated approach that acknowledges the forest eco-hydrological functions in the soil-aquatic continuum.