| More than 85 percent of soil nitrogen present in the organic nitrogen in terrestrial ecosystems. Wherein, the organic nitrogen components present in natural soil solutions referred to dissolve organic nitrogen (DON), and that in air-dried soil extract by pure water or saline solution called soluble organic nitrogen (SON). DON and SON represent the main part of the soil solution dissolved nitrogen, although only a small part of soil organic nitrogen, which is one of the most active components. Thus, the research of soil organic nitrogen in forest system has important environmental and ecological significance.Typical forest soil in profiles (0-100cm) were researched in Dawei Mountain National Forest Park and Dashanchong Forest Park. Based on the study of soil total nitrogen, inorganic nitrogen and organic nitrogen components, focused on the cross-sectional distribution of dissolved organic nitrogen and influencing factors combined with field survey, situ monitoring,and position research method. The main contents showed as blow.1. Researched the variation characteristics of soil organic nitrogen and soluble organic nitrogen in soil profiles of different elevation in Dawei Mountain National Forest Park. The results showed that:Total nitrogen, acid hydrolysable organic nitrogen and soluble organic nitrogen decreased with the increase of depth, content of each component in mountain soils were much higher affected by altitude. The average percentage of soil organic nitrogen to total nitrogen was 97.39%±1.17%, and soil acid hydrolysable organic nitrogen was 64.38% ± 10.68%, each component decreased with the increase of soil depth. Soil soluble organic nitrogen content (9.92-23.45) mg·kg-1, free amino acids (1.62-12.02 mg·kg-1) accounted for about 27.36% ± 9.95% of soluble organic nitrogen. Soil acid hydrolysable organic nitrogen and soluble organic nitrogen were significant positive correlation to total nitrogen, total soluble nitrogen and inorganic nitrogen (P<0.05, n=30), were highly significant correlation with soil bulk density, organic carbon, and total phosphorus (P<0.01, n=30).2. Typical soil profiles in three type’s forest vegetation area (coniferous forest mixed broad-leaf forest, evergreen broad-leaved forest and deciduous forest) were choose to research sectional variation and migration characteristics of soil dissolved organic nitrogen at Dashanchong state-owned forest Park. The results showed that:(1) There were significant differences in soil nitrogen and components content of different forest type. Full cross-sectional of nitrogen component content as follows:coniferous forest (TN:1.85 ± 0.46 g kg-1; available N:61.12 ± 15.50 mg kg-1; MBN:8.38 ± 5.99 mg kg-1)< evergreen broad-leaved forest (TN:1.87 ± 0.90 g kg-1; available N:71.45 ± 44.92 mg kg-1; MBN: 17.80± 19.52 mg kg-1)< deciduous forest (TN:2.10± 0.64 g kg-1; available N:85.48 ± 37.68 mg kg-1; MBN:20.07 ± 15.65 mg kg’1). (2) There were significant differences in soil dissolved organic nitrogen content from different forest types. Result showed that: coniferous forest<evergreen broad-leaved forest<deciduous forest. Dissolved organic nitrogen content in coniferous forest was 0.29～0.70 mg kg-1, and total free amino acid content was 0.05～0.11 mg kg-1. The total free amino acid content in the soil solution accounted for dissolved organic nitrogen content ratio expressed as, coniferous forest (17.61% ± 3.88%)< evergreen broad-leaved forest (21.54%±8.22%)< deciduous forest (34.98% ± 9.52%). (3) Soil dissolved organic nitrogen were highly significant correlation with total orgnic carbon, humic acid, fulvic acid, total nitrogen, total phosphorus, available N, microbial carbon and nitrogen, and pH (P<0.01, n=24), were significant correlation with soil bulk density and moisture content (P<0.05, n=24).3. Researched and established a culture- centrifugal method to obtain soil solutions, the new method to measure soil dissolved nitrogen. Soil dissolved organic nitrogen content were far below than soluble organic nitrogen content, highly significant correlation exist among different methods. Compared the content of soil dissolved organic nitrogen determined by new method and traditional methods, analyzed the differences and correlation between different method. The basic performance of soluble organic nitrogen content extract by different extraction agent was:coniferous forest<evergreen broad-leaved forest<deciduous forest. The performance of extraction efficiency extract by different extraction agent was:ultrapure water< 0.5 mol·L-1 K2SO4 solution<1 mol-L"1 KC1 solution. Extraction efficiency of 1 mol·L-1 KCl solution was 1.29-3.81 times to ultrapure water.4. Typical forest vegetation area (coniferous forest mixed broad-leaf forest, evergreen broad-leaved forest and deciduous forest were choose to research the amount of litters, nitrogen content, and dynamic change characteristics of surface soil dissolved organic nitrogen at Dashanchong forest Park. The results showed that:(1) Dissolved organic nitrogen (DON) content in surface soils reached maximum around May, with an average 0.72 ± 0.48 mg kg-1. Soil dissolved organic nitrogen content in deciduous forest was lower than that in the other two forest types. (2) DON/TN ratio showed gradually increase and then decrease trends, then rapid arise in November. The performance of annual averages of DON/TN ratio was:coniferous forest (0.64±0.23)>evergreen broad-leaved forest (0.53±0.20)>deciduous forest (0.37±0.14). (3) DON were highly significant correlation to DOC, DOC/DON, DON/TN, DOC/TOC, DON/TN-in-litters, DOC/TC-in-litters, and TN/TN-in-litters (P<0.01, n=54), were significant correlation to TN-in-litters and C/N-in-litters (P<0.05, n=54).Organic nitrogen was the main part of soil nitrogen in typical subtropical forests. Affected by altitude, climate change, forest type and soil physic-chemical properties, the components of nitrogen decreased with the increase of soil depth. Due to the high activity, effectiveness, and strong mobility of soil dissolved organic nitrogen, its variation characteristics will profoundly affect the nitrogen cycle in forest ecosystems. |
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