Atmospheric nitrogen deposition and its environmental implications at a headwater catchment of Taihu Lake Basin, China

Increased atmospheric nitrogen (N) deposition as a result of anthropogenic emissions has induced large negative impacts on ecosystem health. Such influence can be magnified at the fragile headwater watershed with intensified human activities. Here, we use nearly eight years of continuously monitored N deposition to examine the temporal variations in atmospheric N deposition rates and composition and evaluate the potential environmental implications at a headwater catchment of Taihu Lake Basin. Our results showed that there was a significant declining in atmospheric N deposition, which could be divided into three stages at the headwater site. The average annual atmospheric N depositions were 31.1, 27.30 and 18.90 kg N ha−1 yr−1 in stage 1 (from 2011 to 2013), stage 2 (from 2014 to 2016) and stage 3 (2017 and 2018), respectively. The multiyear averaged atmospheric N deposition was 27.69 kg N ha−1 yr−1, of which 54.2% was derived from wet N deposition. The ratio of ammoniacal N and nitrate N significantly decreased from 1.03 in stage 1 to 0.50 in stage 3, which was mainly due to the sharp decline in ammoniacal N deposition (decreasing by 55.6%) and relatively stable nitrate N deposition with the implementation of environmental policies. The overall decline in N deposition would mitigate nitrate N leaching for natural forest ecosystems. Nitrate N concentrations in the outlets of the forest catchment had a declining with decreasing N deposition. Meanwhile, the contribution of N deposition to the total N loading in the surface water body decreased from 17.7% in stage 1 to 10.7% in stage 3. However, atmospheric N deposition remained high in the headwater region. Long-term measurements of atmospheric N deposition are necessary to help develop emission control measures and protect ecosystem health at a regional scale. •Significant declining trend in atmospheric nitrogen (N) deposition was observed in a headwater catchment of Taihu Lake Basin.•Ammoniacal N deposition decreased by 55.6% from 2011 to 2018.•Nitrate N deposition was relatively stable and had a slight increasing trend with intensive tourism activities.•Current environmental policies had positive effects, reducing N deposition to the headwater catchment.•The implications of atmospheric N deposition to freshwater ecosystems cannot be ignored, although the N deposition declined.