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The objective of this project is to study the nitrogen budget and cycle of Nanjenshan forest ecosystem. At Kuhu experimental area, we explored soil properties, soil water contents, amounts and compositions of litterfall, mineralization of organic matter, dry and wet deposition and lake water quality.
The results of soil property analysis showed that soils to be extremely acidic（pH4.3∼4.8）, fine-textured（clay and clay loam）, low in organic matter（1.0﹪∼5.34﹪）, total（0.06﹪∼0.17﹪）and available nitrogen, but high in "free" Fe and Al. Analyses of soil water samples found their pH values were between 4.6∼6.2, decreasing with increasing depth. The NH4+－N content was higher in surface layers than in bottom layers. Lake water and rain water had higher NO3- content than soil water. Nitrite wa not detectable. The amount of litterfall at the experimental site was 6.56 ton/ha*year. Leaves were the main litterfall component（66.47﹪）. There were two pronounced peaks of higher monthly litterfall, occurring during the growing season（from March to May）and during the typhoon period（from July to September）. The average nitrogen content of the litterfall was 1.04﹪and the total amounts accumulated by litterfall were 68.58 kg/ha*year.
The results of incubation experiments indicated that the amounts of soil organic matter, inorganic nitrogen, NH4+－N and NO3-－N increased about 0.18﹪, 123.1 mg/kg, 76.3 mg/kg and 37.7 mg/kg per month, respectively. Based on these results, the annual inputs of NH4+－N and NO3-－N through mineralization were 2783.4 and 1357.3 kg/ha*year, respectively. The annual inputs of NO2-, NO3-, NH4+ and HNO3 from dry deposition were 0.03, 0.10, 0.012 and 1.53 ton/km2, respectively. While NO3- and NH4+ from wet desition were 4.13 and 3.87 ton/km2, respectively. According to this data, the amounts of nitrogen originating from dry and wet deposition were, respectively, 16.64 and 80.08 kg/ha*year. Lake water pH values ranged from 4.39 to 8.69. The respective concentrations of NO3-, NO2- and NH4+ were 0∼2.7, 0.03, and 0∼0.4 mg/L.
Within the studied area, the main sources of nitrogen were dry deposition（16.64 kg/ha*year）, wet deposition（80.08 kg/ha*year）and decomposition of litterfall（73.54 kg/ha*year）. On the other hand, the nitrogen outputs were soil and sediment absorption（103.35 kg/ha*year）, plant uptake（68.58 kg/ha*year）, denitrification and release by soil water（1.33 kg/ha*year）. Since the soil were extremely acidic and low in available nitrogen, it is assumed that denitrification is negligible.
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