| Summary: | 博士 === 國立臺灣大學 === 農業化學研究所 === 98 === The Nanjenshan Ecological Reserve Region of Kenting National Park preserves the lowland native forests in southern Taiwan. Many researches about vegetation has been performed, however, our understanding of soil nitrogen nutrient dynamics conditions were still insufficient. The main objective of this study is to evaluate the soil nitrogen transformations under different climate regions, landscape positions and vegetation types in Nanjenshan. In-situ incubations and laboratory incubations were conducted in the Lake Site and the Transect Site to estimate the soil net nitrogen mineralization and net nitrification.
In the Lake Site, wind-block treatment plots and control plots were compared to show the effect of northeastern monsoon on soil nitrogen. Three in-situ incubations have been conducted since Oct 2003. Net N mineralization and nitrification were not significantly different between wind-block treatment and control treatment, probably due to the relatively short time of treatment and high spatial variability of soils. Soil microbial biomass carbon (MBC), microbial biomass nitrogen (MBN) and the MBC/MBN ratios had no consistent trend for three in-situ incubations. In Feb 2004, soils of the Lake site and the Transect Site were both collected for incubation in the room temperature (25℃). Consistent with in-situ incubation results, the Lake Site soils showed a low rate of net N mineralization, and ammonium nitrogen (NH4+–N) was the dominant inorganic N form in soil. The Transect Site soils were apparently different from the Lake Site soils. Concentrations of total inorganic nitrogen increased with extended incubation time, and nitrate (NO3––N) was the dominant inorganic form in the Transect soils. This indicated that nitrification is the predominant soil N process in the Transect Site. According to the studies in the Lake Site, available N and nitrification of forest soil are low under strong northeastern monsoon conditions. Concentrations between NH4+–N and MBN have the trend of negative correlation but not statistical significant, thus we can not confirm that low available N is due to the competition of soil microbes. MBC/MBN ratios were negatively correlated with temperature regime; it suggested that temperature may have influence on proportions of bacteria and fungi in the soil.
In January and August 2006, in-situ incubations were conducted in the summit and footslope position of the Transect Site with open core and closed-top core methods. Soils were also collected and incubated in the laboratory under two temperature (15℃, 30℃) and two moisture (20%, 40%, w/w) regimes. In January 2006, the amount (mg N m−2 28d−1) of mineralized and nitrified N and the N transformation rate (mg N m−2 d−1) of the footslope were similar to those of the summit. The net rates and amounts of mineralization and nitrification in August were 1.3- and 2.1-fold higher than those in January on the footslope and 2.29- and 2.46-fold higher on the summit, respectively, both due to significantly higher concentrations in August. In addition to the seasonal variation at a given landscape position, the differences in the N transformation rates (mg N m−2 d−1) between the footslope and summit positions were greater in August, and the net N mineralization rates and net nitrification rates at the summit position were 1.8- and 1.3-fold higher than those at the footslope position, respectively. On a per gram organic carbon basis (mg N g OC−1 d−1) and per gram nitrogen basis (mg N g N−1 d−1), the net N mineralization and net nitrification rates of the footslope were both higher than those of the summit in January and August. It revealed that the differences in the N transformation rates between landscape positions were influenced by the substrate quality, which is directly linked to the vegetation type or tree species.
Soil MBC of summit was significantly higher than of that footslope. In Jan 2006, MBN concentrations of summit and footslope after incubation were both higher than those of before incubation. In Aug 2006, MBN concentrations were significantly higher than that in Jan before incubation; after incubation the concentrations decreased significantly. Ratios of MBC to soil organic carbon (Cmic/Corg) were relatively high as compared to literatures, but ratios of footslope soils were consistent and significantly higher than those of summit soils, suggesting a higher C availability was found in footslope position. Ratios of MBN to total nitrogen (Nmic/Nt) varied between landscape positions and incubation time. Besides, soil microbial properties were significantly correlated with soil moisture and temperature regimes, which reflected the differences between summit and footslope positions. MBN of footslope position were positively correlated with soil temperature, and the MBC/MBN ratios were negatively correlated with soil temperature. Concentrations of soil ammonium, nitrate and total inorganic N in summit were more significantly correlated with soil temperature than in footlope, however, it seemed that summit MBN was less affected by soil temperature regime.
According to the results of laboratory incubations under different soil moisture and temperature conditions, changing temperature condition had greater effects on soil net N mineralization and nitrification than those of changing moisture condition. As contrasted with field incubations, net ammonification was the predominant process under laboratory conditions, especially for summit soils. It was possibly due to the air-dry process before incubation and the low moisture condition in experiments.
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