In this study the authors determined the nitrogen isotopes of nitrate and the carbon and nitrogen contents of water samples from the Hongfeng Lake and inflowing streams, and, on this basis, discriminated the extraneous nitrogen input and the interior nitrogen release (organic matter decomposition) in the lake. The results showed that the amount of extraneous nitrogen input into the Hongfeng Lake increases and the interior nitrogen release from the lake is intensified in winter and spring. Nitrogen mixing is relatively obvious at the catchments between the lake inlet and inflowing streams, while organic matter decomposition (nitrification) is most intense in the middle and lower reaches of the lake. In winter and spring the organic matter decomposition takes place mainly at the middle level of the water column in the middle and lower reaches of the lake, which has a bearing on the abundance of degraded organic matter at this water level. This part of degraded organic matter contains a considerable proportion of hydrogenous nitrogen in the lake.