Discriminating surface soil inorganic nitrogen cycling under various land uses in a watershed with simulations of energy balanced temperature and slope introduced moisture

Abstract

Surface soil under various land uses/covers plays an important pool of N discharge in a watershed. Soil temperature and moisture drive soil N cycling, especially important for the watersheds with anthropogenic manipulations. Not all of the ongoing process-based models consider topographic factors of known importance when simulating soil moisture but most of them simulate soil temperature typically derived from empirical relations with air temperature. These assumptions might lead to underestimate the N discharge in the watersheds. This study employed an energy balanced soil temperature module and a slope introduced soil moisture module in coupling with a conceptual model of hydro-bio-geochemical process-based watershed life cycle analysis, i.e. the Watershed Inorganic Nitrogen Dynamic (WIND), to simulate soil N cycling across forest land, farmland, and urban bare land in an urbanizing watershed in southern China. The model was calibrated and validated by two individual survey events in 2013 and 2016–17 respectively using moisture and inorganic N in surface soil (0–10 cm) of the watershed with determination coefficients (R2) above 0.7. Significantly higher soil temperature arose when using the energy balanced module in comparison to the air temperature module. Meanwhile, soil moisture declining with land slope became significant above 15° steepness. Estimates of surface soil N cycling across all land uses were significantly altered. Importantly, land uses determined the extents of their alterations. The WIND provides insights into drivers of hot - spot/hot - moment hydro-bio-geochemical processes of soil inorganic N for land uses in the watershed. More generally, as the findings inform a better constraint of parameters in other physics-based watershed-scale water quality models they also potentially directly contribute to improved strategies for sustainable management of inorganic N discharges under the predicted climate changes.