Observation of the nitrogen deposition in the lower Liaohe River Plain, Northeast China and assessing its ecological risk

Little information about the nitrogen (N) deposition in the lower Liaohe River Plain (LRP) of Northeast China was available. A continuous 5-year observation (from 2004 to 2008) was conducted to determine the nitrogen deposition in precipitation collected by a standard rain gage, and then we assessed its ecological consequences. The mean inorganic nitrogen concentrations were 1.82 mg N L−1 for NH4+–N and 0.89 mg N L−1 for NO3−–N. On an annual basis, the averaged amounts of N deposition were 14.5 kg N ha−1 year−1, which were much lower than that of hotspots in China. However, the amount of deposited N should still be taken account into the N fertilizer management of the agroecosystems. Compared with the critical loads (CL) of N deposition, the deposited N would likely threaten the natural ecosystems in LRP. The N concentrations in rainwater and the amount of N deposition showed a clear seasonal pattern, and precipitation played an important role in regulating the N concentration. Furthermore, the seasonal fluctuation of NH4+/NO3− ratio reflected that the deposited N originated from both fertilizer application and fossil fuel combustion, and environmental factor (soil temperature, lightning and sunshine) might also contribute to the seasonal cycles of NH4+/NO3− ratio. The high annual NH4+/NO3− ratio (2.05) compared with the more developed region suggested that N deposition in this region was mostly affected by agricultural activities rather than industrial activities.