Assessing Silicate Catchment Dynamics of the Oplotnica River (Slovenia) Through Stable Carbon and Nitrogen Isotopes

This study investigated the sources of carbon and nitrogen in a small, silicate-dominated catchment (Oplotnica River, Pohorje, Slovenia) with two creeks, Lukanjski and Javorski, from 2012 to 2014. Additionally, carbon and nitrogen cycling in soil profiles was studied at Javorski Creek. Isotopic analyses of river water, particulates, and sediments provided a more holistic view of the sources of carbon and nitrogen, weathering contributions, and the evasion or sequestration of CO₂ in the atmosphere within the catchments. The weathering of rocks, such as granodiorite and quartz diorite, influences water geochemistry. The Oplotnica River and its tributaries were characterized by the ion composition: Na⁺  > Ca²⁺  > Mg²⁺  > K⁺ and HCO₃⁻ > SO₄²⁻ > Cl⁻ > NO₃⁻. Partial pressure of CO₂ concentrations in river and creeks ranged from 1.1 to 13.4 times that of atmospheric pressure, representing a source of CO₂ to the atmosphere. The carbon isotope value of dissolved inorganic carbon (δ¹³C_DIC) ranged from − 9.8 to − 1.4‰ in river, while in the creeks, it ranged from − 26.1 to − 4.7‰, reflecting the degradation of organic matter and exchange with the atmosphere. The intensity of bicarbonate weathering for the Oplotnica River at its gauging station was 10.4 mmol/(l⋅km²⋅s), characteristic of silicate watersheds. The isotopic composition of carbon (δ¹³C) and nitrogen (δ¹⁵N) in river sediments reflects values typical of soil and temperate (C3) plants. This study is significant on both local and global levels, as it addresses the contribution of weathering rates and the release of CO₂ to the atmosphere from small silicate watersheds.