Decoupling factors influencing spatial and temporal trends of total nitrogen and ammonia nitrogen levels in lakes across China

Abstract. The heterogeneity of lake nitrogen pollution in its spatial distribution and long-term evolution influences ecosystem functioning and the effectiveness of water environment management. However, whether the drivers shaping the spatial patterns of different nitrogen forms differ from those governing their temporal dynamics remains poorly understood at large spatial scales. Here, we constructed monthly time series (2010–2024) of total nitrogen (TN) and ammonia nitrogen (NH₃-N) concentrations for 3,020 lakes across China’s five limnological regions using an ensemble machine learning framework and to reveal the difference in factors that influence spatial patterns and long-term changes of the two nitrogen forms at the large scale. The results showed that the factors determining where lakes are nitrogen-enriched are not necessarily those controlling whether nitrogen conditions improve or deteriorate over time and that different nitrogen forms respond to management and environmental change through distinct pathways. For TN, the spatial patterns are jointly shaped by external nitrogen loading, land cover, and hydro-climatic conditions, highlighting strong landscape-scale controls. However, its long-term evolution is governed more strongly by temporal variations in external nitrogen inputs than by static spatial characteristics. In contrast, NH₃-N exhibits a more direct and rapid response to external emission reduction measures, with both its spatial and temporal dynamics demonstrating high sensitivity to changes in anthropogenic nitrogen inputs. By highlighting the decoupled controls on spatial patterns and temporal trends, this study underscores the necessity of shifting lake management from static, location-based regulation toward an integrated 'state-rate' adaptive strategy.