Identification of rainy season nitrogen export controls in a semi-arid mountainous watershed, North China

A comprehensive understanding of the nutrient export process and export controls is demanded effective pollution mitigation in fragile riverine ecosystems. In this study, behaviors of the full range of nitrogen (N) under stormflow (5-events) and baseflow (2-events; before and after the rainy season, multiple sites) were assessed to explore N export controlling mechanisms according to the identified main components causing the changes in N exports, N transport pathways, seasonal trends, and nutrient supply watershed regions through the 2020 rainy season in a semi-arid mountainous watershed, northern China. Results showed increments in riverine dissolved organic-N (DON) and particulate N (PN) loadings as the leading cause of N flux and composition changes through the rainy season, although nitrate-N (NO3-N) contributed 69.6% of total-N (TN). Storm runoff generated 3-fold and 4-fold average increments in DON and PN fluxes. DON and PN shared 1-66% (18.1%) and 1-44% (9.7%) of TN through storms, registered consistency in behavior, mainly originated from near-stream soil, and were primarily transported by shallower subsurface flow. Our results broaden the understanding of PN delivery in catchment wetting-up periods by highlighting the decoupling of primary origins/transport pathways of PN from sediments. Results suggested hydrological functioning parallel to the catchment wetting-up as the principal governor of storm N evolution; soil moisture levels build up in the early rainy season, soil water runoff dominance during peak discharge fluctuations, groundwater runoff dominance at the end of the rainy season. Cumulative rainfall and antecedent soil moisture exerted more significant control over storm N exports than individual rainfall features. The assessment of N behaviors through river network disclosed watershed regions responsible for excessive N delivery and influences of unsustainable agriculture, sewage treatment work, and damming on natural riverine N fluxes. These findings could be useful references for the formulation of water pollution control strategies in the future.

Automated summary

This study assessed the behaviors of nitrogen (N) in a semi-arid mountainous watershed in northern China during the 2020 rainy season to explore the mechanisms controlling N export. The results showed that increased loadings of dissolved organic-N (DON) and particulate N (PN) in rivers were the main cause of changes in N flux and composition during the rainy season. Storm runoff contributed the most to DON and PN, which mainly originated from near-stream soil and were transported by shallower subsurface flow. Additionally, hydrological processes played a significant role in regulating N loss during storm events. The study also identified watershed regions responsible for excessive N delivery and examined the impacts of unsustainable agriculture, sewage treatment work, and damming on natural riverine N fluxes.