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The terrestrial export of dissolved organic nitrogen (DON) is associated with climate, geography and land use, and thus is influenced by geo-climatic variability, human interference, the farmland and woodland in soil, and hydrological connection levels to rivers. A data-set was presented including two catchments covering the major land use types and different hydrological connection levels to rivers within Eastern China: Middle Yangtze (river-isolated lakes) and Huai River (non-river-connected lakes). Total dissolved nitrogen [TDN, including DON and dissolved inorganic nitrogen (DIN)], total organic carbon (TOC) and physical variables (T, DO, pH, OPR, conductivity) were measured in seventeen lakes over summer and winter. In all study lakes, both DON and NH₄ ⁺-N were the major fractions of TDN, with much lower proportions of NO₃ ⁻-N and NO₂-N. A variation pattern with higher DON and lower NH₄ ⁺-N concentrations was observed in non-river-connected lakes, and yet reverse pattern in river-isolated lakes. Higher DON concentrations were recorded with a high extent of farmland in land use, reflecting the influence of human impact on DON loads. These relationships and correlation analysis indicated that DON concentrations were controlled by several factors including temperature, latitude, precipitation, hydrological process and farmland cover in land use. Seasonal variations showed that higher mean surface DON concentration (0.004–0.666 mg/L) was measured in winter, which might be due to resuspension of the organic matter from bottom sediments. The uptake of DON by phytoplankton and bacteria was likely to be the most significant reason for low DON concentrations (0.006–0.335 mg/L) during the summer, supported by increasing DON concentrations accompanied with disappearing Chla after phytoplankton died. Much higher NO₃ ⁻-N concentrations in summer were attributed to nitrogenous fertilizer, high rainfall, and inorganic nitrogen suspending brought by frequent wind wave.
