Contribution of LGD source carbon to the carbon emission of eutrophic steppe lakes in northern cold and arid regions

We present a conceptual model for the biogeochemical cycle of carbon in lakes of eutrophic grasslands in northern cold and arid regions containing LGD-derived carbon. The main source of groundwater carbon in steppe lakes is cow and sheep feces, which infiltrates into aquifers, undergoes decomposition, transformation (such as CH 4 oxidation, denitrification, etc.) and accumulation, flows along groundwater, and finally discharge into lakes. LGD is therefore an important source of various forms of carbon (DOC, DIC, CO 2 , CH 4 ) in lakes. We estimate the contribution of LGD to lake GHG (CH 4 : 12.3%, CO 2 : 14.7%) through greenhouse gas flux quantifcation model and 222 Rn isotope mass balance model. • Lake groundwater discharge (LGD) brings a large amount of carbon-containing substances into the lake. • Groundwater and lake water have homology. • A binding model of 222 Rn isotope and greenhouse gas flux was established to estimate the contribution of LGD. • LGD contributes significantly to lake greenhouse gas emissions. Lake groundwater discharge (LGD) represents a critical but underappreciated source of greenhouse gases in lacustrine systems, though its governing mechanisms and quantitative significance are not yet clearly defined. This investigation focuses on Hulun Lake, a representative eutrophic lake in China’s cold-arid northern regions, through integrated analysis of stable isotope signatures (δ 13 C-CO 2 , δ 13 C-CH 4 , δ 13 C-DIC). We employed 222 Rn mass balance modeling and greenhouse gas flux quantification to assess LGD’s contribution to CO 2 and CH 4 emissions. Results demonstrate elevated carbon component concentrations in groundwater relative to surface waters (DOC: groundwater 1755.50 μmol/L vs. lake water 1480.00 μmol/L vs. river water 933.25 μmol/L; DIC: groundwater 5089.00 μmol/L vs. lake water 7311.00 μmol/L vs. river water 3345.75 μmol/L). Groundwater exhibits higher mean dissolved greenhouse gas concentrations compared to lake water (CO 2 : 2.300 vs. 0.458 μmol/L; CH 4 : 0.029 vs. 0.027 μmol/L). Isotopic evidence confirms hydrological connectivity between groundwater and lake systems. Quantitative analysis reveals LGD accounts for 14.7 % of lacustrine CO 2 emissions and 12.3 % of CH 4 emissions. These findings establish LGD as a critical regulator of carbon biogeochemical cycling in lake ecosystems, necessitating its systematic integration into regional carbon budget assessments. The demonstrated significance of LGD in carbon dynamics underscores its essential consideration in regional and global carbon cycle models.