Nitrogen deposition in typical cropland in the Leizhou Peninsula, China

Abstract

Reactive nitrogen (Nr) creation has increased sharply over the past 150 years because, as the world population has increased, food and energy consumption have also increased continuously. Consequently, Nr emissions and nitrogen deposition have increased rapidly since the Industrial Revolution. Nitrogen deposition causes a series of environmental problems, including soil acidification, water eutrophication, loss of plant diversity in sensitive ecosystems and indirect N2O emissions. The Leizhou Peninsula is famous for cash crop cultivation. Because of the specific meteorological characteristics of this area, for example, the high air temperature, high annual rainfall and strong winds, quantifying nitrogen deposition may be very important for evaluating nitrogen cycling in cropland and other related environmental impacts. Previous studies undertaken in this area have considered wet nitrogen deposition. In our study, we investigated total wet and dry nitrogen deposition in a typical cropland in Zhanjiang. The atmospheric concentrations of NH3, HNO3, NO2, pNH4+ and pNO3-were 5.62, 0.88, 3.16, 3.30 and 2.02 μg N /m3, respectively, over the duration of the sampling period. The peak NH3 concentration was observed in summer, and was attributed to NH3 emission simulation from different NH3 emission sources (especially N fertilization), induced by high temperatures. pNH4 and pNO3 concentrations were higher in winter due to the low winter rainfall, as were inorganic nitrogen concentrations in precipitation. Atmospheric Nr concentrations were muchhigher than those reported by the European and American Nr monitoring networks, but much lower than those reported in the North China Plain. Dry nitrogen deposition is difficult to estimate directly because of the complicated meteorological and surface conditions. In this study, we tried to choose reasonable deposition velocities from those summarized in the literature from other studies. The American Clean Air Status and Trend’ s Network only has data for HNO3, pNO3, and pNH4 deposition velocities, and so could not be used in this present study. However, we were able to use the deposition velocities adopted by the European monitoring network. These were considered appropriate because we used the same equipment as was used to monitor nitrogen deposition in the European NitroEurope network. The NH3, HNO3, NO2, pNO3 and pNH4 deposition velocities from this network were 0.53cm/s, 0. 8 cm/s, 0.12 cm/s, 0.25 cm/s and 0.25 cm/s, respectively, while the dry nitrogen deposition rate was 17.6 kg hm-2 a-1. The average NO3--N concentration in precipitation samples was (0.86 ± 0. 36) mg N/ L, while the average NH4+-N concentration was (1. 11 ± 0. 68) mg N/ L. Inorganic nitrogen concentrations in precipitation samples were higher in winter and lower in summer because of the variation in rainfall between the different seasons. The total wet nitrogen deposition was 25.3 kg N hm-2 a-1. Wet NH4+-N and NO3--N, and dry NH3, HNO3, NO2, pNH4 and pNO3 contributed 30. 8%, 28. 0%, 23. 7%, 5. 4%, 2. 8%, 3. 9% and 5. 4% to the total nitrogen deposition, respectively. The large contributions from wet NH4+-N deposition and dry NH3--N deposition in this study indicated that fertilization played a large role in airborne NH3 and nitrogen deposition. Comparison of the inorganic nitrogen concentrations in rainfall in other regions of China shows that the NH4+-N concentrations and NH3--N concentrations in the Leizhou Peninsula were much lower than those for the North China plain, but that they were consistent with the inorganic nitrogen concentrations reported for southern and eastern China. The total nitrogen deposition recorded during this study was 42.9 kg N hm-2 a-1. Wet nitrogen deposition rates showed a close relationship with rainfall events, showing that wet nitrogen deposition changes with the rate of rainfall in different years. Dry deposition velocities of Nr species should be measured directly, or by using inferential methods, to decrease uncertainty in dry nitrogen deposition studies. Further, there should be some concern about cropland nutrient management and the related impacts on water, forest and grassland because of the high rates of background nitrogen deposition in this area.