Nitrogen dynamics in multi-source recharge lakes: A comprehensive exploration using multi-isotopic tracing, Bayesian mixing models and structural equation models

Excessive nitrogen levels have become a pressing global environmental issue, especially within lake ecosystems. Despite this, research on nitrogen dynamics within lakes fed by multiple replenishment sources are scarce. To address this gap, our study employs Baiyangdian Lake (BYDL), a quintessential example of a multi-source replenished lake, as a case study. In a novel approach, we integrated multi-isotope analysis, Bayesian mixing models (MixSIAR), and structural equation models (SEMs) to provide a systematic examination of nitrogen sources, pathways, and transformation mechanisms in such lakes. The isotopic compositions indicated that sewage, ammonium fertilizer and soil nitrogen constitute the principal nitrogen sources. The contribution of sewage to lake nitrogen varies across seasons, accounting for 54.4 %, 53.2 %, and 42.5 % during normal, flood, and dry seasons, respectively. Nitrification dominates the nitrogen cycle in BYDL, accompanied by significant assimilation and mineralization, whereas denitrification is minimal. SEM analysis revealed that the primary determinants of nitrate concentration distribution in multi-source lakes are the differentiation of nitrogen forms from replenishing water sources and the differential effects of hydrodynamic mixing, exerting a standardized total effect of 0.614 ( p < 0.01). Additionally, redox conditions and organic matter content exert significant influences on nitrogen distribution. In summary, multi-isotope analysis can effectively identify nitrogen sources and elucidate their migration and transformation pathways. SEMs clarified the key factors governing nitrogen concentration distribution, while MixSIAR precisely quantified the contributions of various nitrogen sources. Together, these methodologies provide a comprehensive approach to decipher the dynamic patterns of nitrogen in multi-source lakes, deepening our understanding of nitrogen cycling in complex aquatic systems. • Integrated multi-isotope tracing, MixSIAR, and SEMs provide a holistic view of nitrogen cycling. • SEMs elucidates the controlling pathways of nitrogen in multi-source recharge lakes. • Primary nitrogen sources were sewage, fertilizer, and soil nitrogen in multi-source Lake. • Nitrification dominates nitrogen cycling in multi-source Lake, with significant assimilation and mineralization. • Nitrate distribution is mainly affected by nitrogen form differentiation from sources and hydrodynamic mixing effects.