Carbon cycling dynamics in the headwater Radovna stream recharged by Lipnik springs, a carbonate catchment in the Julian Alps, Slovenia, based on stable isotope analysis

Abstract

Carbon cycling was investigated monthly (July, August, September, October, November, March and May) from July 2023 to May 2024 in the Radovna stream originating from the permanent Lipnik spring in the Julian Alps, Slovenia using isotopic composition of carbon in dissolved inorganic carbon (δ13CDIC) and particulate organic carbon (δ13CPOC). The investigated catchment is composed of massive coarse - crystal dolomite and limestone. Total alkalinity ranged from 2.2 to 2.7 mM and is characteristic for carbonate pristine environments. In-situ parameters e.g. dissolved oxygen ranged from 11.0 to 12.0 mg/L, pH from 7.9 to 8.1 and specific electrical conductivity from 275 to 318 μS/cm, respectively. The values of dissolved oxygen reflect that the water system is well oxygenated. CO2 presented a source of carbon to the atmosphere during all investigated months. Oversaturation with CO2 is 1.01 to 4.1 times of atmospheric value. δ13CDIC ranged from -11.8 to -9.7 ‰. This indicates that dissolved inorganic carbon under different discharge conditions mainly originates from the dissolution of carbonates (from 50.3 % to 57.3 %), followed by the degradation of organic matter (from 42.8 % to 49.7 %). Equilibration with atmospheric CO₂ has a negligible impact, ranging from 0.01 % to 0.11 %. δ13CPOC in river water indicate sources such as plant debris, with δ13CPOC of -29.4 ‰, and highly degraded soil organic matter, with δ13CPOC of -24.9 ‰. The δ13CDIC and δ13CPOC values are typical for a forest stream flowing over a limestone substrate. Strong statistical negative significant correlation was obtained between electrical conductivity and water temperature, mass of total suspended solids (mTSS) and pH.