| Nitrogen fertilization improves grain yields, however, this improvement causes some problems, such as the low fertilizer use efficiency and high environmental pollution load because of irrational use. In addition, gaseous losses of nitrogen would decrease the fertilizer use efficiency and threaten terrestrial and aquatic environment. Gaseous losses of nitrogen include ammonia volatilization and nitrification-denitrification. The ecosystem of Three Gorges Reservoir (TGR) is typically fragile. Since the dam impoundment, the water level of TGR seasonally fluctuating between 145 m and 175 m (145 m in summer and 175 m in winter) results in the changes of climate conditions and environmental factors, which might affect gaseous losses of nitrogen in a different way from soil-air interface. However, in the circumstance of water level fluctuated significantly, it was not deeply elaborated that ammonia volatilization and nitrification-denitrification respond to the natural environment and anthropic activities.In order to understand the mechanism of nitrogen balance and transformation in different fertilization systems which would help to reduce the nitrogen loss and to improve the fertilizer use efficiency. The targets of this study were to monitor the ammonia volatilization and loss of N2O from nitrification-denitrification and to analyze influence on different land uses (dry land and paddy field), different elevations (175 m,170 m and 165 m) of watershed in TGR, respectively with ventilation and the static opaque chamber from April to September in 2014, and to estimate the cumulative ammonia volatilization and loss of N2O from nitrification-denitrification of the watershed. Results of this study showed as bellow:(1) Ammonia volatilizationThe differences of ammonia volatilization in different land uses and different elevations were significantly, respectively. The fluxes of ammonia volatilization had obvious characteristics of seasonal variation. The cumulative ammonia volatilization as followed:com (groove) (14.68 kg ha-1)> corn (ridge) (1.80 kg ha-1), paddy field (1.93 kg ha-1)> dry land (1.86 kg ha-1), peanut land (1.82 kg ha-1)> control experiment land (1.80 kg ha-1). However, the cumulative ammonia volatilizations with different elevations were adjacent and were 1.94 kg ha-1,1.92 kg ha-1 and 1.92 kg ha-1 at elevation of 165 m,170 m and 175 m, respectively.The influencing factors of soil ammonia volatilization were different. In the case of different land uses, ammonia volatilization was mainly influenced by air temperature, nitrogen fertilizer and fertilization methods. The variation of ammonia volatilization in three elevations mainly related to the concentrations of NH+-N in the field. Correlation analysis showed that a significant positive correlation between the ammonia volatilization and NH4+-N concentration in the paddy field at the sites of 175 m and 165 m.(2) N2O emissionsLike the variation of ammonia volatilization, the fluxes of N2O emissions in different land uses and elevations were significantly. The fluxes of N2O emissions had obvious characteristics of daily and seasonal variation. The N2O emissions in different land uses were shown:paddy field (4.53 kg ha-1)> dry land (0.13 kg ha-1), maize land (0.80 kg ha-1)> peanut land (-0.25 kg ha-1). The cumulative N2O emissions of different elevations were 10.16 kg ha-1,3.05 kg ha-1 and 0.39 kg ha-1 responding to 165 m,175 m and 170 m, respectively.The N2O emissions was mainly affected by soil temperature, soil moisture, content of NO3--N and NH4+-N in the soil. Among them, the soil temperature was main factor affecting N2O emissions of dry land. The soil moisture was the main factor affecting that of maize land. The content of NH4+-N was the main factor affecting that of peanut land. But the N2O emissions in paddy field had no major impact factor. The main reasons why N2O emissions in three elevations varied greatly were frequent and strong dry-wet alternation in soil because of hydro-fluctuation and seasonal changes of soil water. Meanwhile, available soil carbon from submersed vegetation was sufficient for suitable conditions for N2O emissions.(3) Gaseous losses of nitrogen of the watershedDuring the monitoring days, the cumulative gaseous losses of nitrogen in Yetang watershed get up to 6007.8-17986.2 kg a-1, accounting for 12.02～28.40% of the total chemical fertilizer in the watershed. Ammonia volatilization was the main loss way in the watershed (1674～13652.4 kg a-1), while the cumulative N2O emissions was 4333.8 kg a-1, accounting for 3.35～21.55% and 6.84～8.67% of the total chemical fertilizer in the watershed, respectively,In the case of different land uses, gaseous losses of nitrogen were 935.3 kg a-1 in dry land and 16096.4 kg a-1 in paddy field, accounting for 1.48～1.87% and 25.41～32.20% of the total chemical fertilizer in the watershed, respectively. The ammonia volatilization showed that the amount in dry land (8742 kg a-1) was more than that in paddy field (656.2 kg a-1), accounting for 1.38～1.75% and 1.04～1.13% of the total chemical fertilizer in the watershed, respectively. The N2O emissions showed that the amount in paddy field (1540.2 kg a-1) was more than that in dry land (61.1 kg a-1), accounting for 0.10～0.12% and 2.43～3.08% of the total chemical fertilizer in the watershed, respectively.(4) Influencing factors of gaseous loss of nitrogenGaseous losses of nitrogen of farmland ecological system were affected by many factors, including human factors and natural factors. Human factors included the types of nitrogenous fertilizer, amount and ways of fertilization and farming measurements. Natural factors included soil moisture, soil temperature and NO3--N and NH4+-N content in soil, frequent and strong dry-wet alternation of soil, as well as synthetical impactions of these factors.In general, gaseous nitrogen losses were greatly influenced by human activities and fertilizing, the ratio of organic and inorganic fertilizer, and unreasonable types of nitrogen fertilizer might be the main reason for the large gaseous loss of nitrogen in the small watershed. |
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