As an important recharge source for lakes, groundwater has become one of the dominant routes of nitrogen (N) input into lakes. Understanding the source and behaviour of N around plateau lakes is essential for improving groundwater and lake water quality. This study used NO3- stable isotopes (delta N-15-NO3- and delta O-18-NO3-) combined with hydrochemical data, environmental parameters and a Bayesian stable isotope mixing model (SIAR) to elaborate the NO3- sources and contributions and N concentrations and cycling processes in groundwater (n = 476) from 8 plateau lake basins in regard to land uses and seasonal changes. We found that multiple N form concentrations in the rainy season (RS) and cropland (CA) were higher than those in the dry season (DS) and residential area (RA); NO3--N was the most dominant N form, and 45% of groundwater samples exceeded the WHO NO3- drinking water threshold. The NO3- isotopes and SIAR model revealed that soil N (SN), manure and sewage (M&S) and N fertilizer (NF) were the main NO3- sources in groundwater, contributing 47.5%, 30.5% and 19.5% based on land uses and 39.0%, 37.5% and 20.0% based on seasonal changes, respectively. Moreover, denitrification was considered the primary N transformation process in the groundwater in the RS, CA and RA, but the N transformation process in the DS was dominated by nitrification. These results indicated that N concentration and source varied with land use and seasonal changes, and the cross distribution of CA and RA and difference in seasonal rainfall resulted in the difference in multiple N sources and source contributions in groundwater. Despite changes in land uses and seasons, SN and M&S remain the largest NO3- contributors to groundwater, which poses new challenges to control groundwater NO3- pollution in the area around plateau lakes with a cross distribution of CA and RA. Therefore, improving septic tanks and sewage networks and scientific soil N management are essential for controlling groundwater pollution.