南仁山區低海拔亞熱帶雨林土壤氮礦化作用及硝化作用之探討

Abstract

墾丁國家公園內的南仁山生態保護區保留了臺灣僅存的低海拔原始森林，極富研究價值，卻缺乏土壤氮養分動態變化的研究。本研究擇於受強烈東北季風影響的古湖樣區及有明顯植被壓縮現象的樣帶樣區，在現地以管柱孵育法測定自然環境下之土壤氮轉變，並同時以實驗室孵育法評估潛在氮礦化作用及硝化作用，研究主要目的為了解不同氣候、地形及植生下對土壤氮礦化作用之影響。 2003年10月起在古湖樣區進行擋風處理組與對照組之比較，為期四週的現地孵育共進行三次。擋風處理組與對照組之間的土壤淨氮礦化及淨硝化作用無顯著差異，可能因擋風處理期相對較短且土壤空間變異大，無法顯現東北季風之效果。土壤微生物生質碳 (microbial biomass C, MBC)、微生物生質氮 (microbial biomass N, MBN) 及微生物生質碳氮比在三次孵育期間沒有一致的變化趨勢。2004年2月同時採集古湖樣區及樣帶樣區土壤進行25℃恆溫實驗室孵育，古湖樣區的分析結果與現地孵育得到的趨勢相同，即土壤潛在礦化作用低，且以銨氮 (NH4+–N) 為主要之無機態氮型態。反之，以相同條件孵育之樣帶樣區土壤則明顯不同，總無機態氮濃度隨孵育時間增加而增加，且以硝態氮 (NO3––N) 為主要的無機態氮型態，顯示樣帶樣區是以硝化作用較佔優勢。由古湖樣區研究結果，東北季風影響下的森林土壤有效氮及礦化作用均低，土壤銨氮與MBN濃度雖呈負相關但未達顯著水準，無法確定微生物之競爭是否造成古湖樣區土壤之有效氮濃度低。當土壤中的溫度上升時，微生物生質碳氮之比值下降，顯示土壤溫度可能影響土壤中細菌與真菌所佔的比例。 在2006年1月及8月，以開放管柱法及加蓋管柱法在樣帶樣區之頂坡及麓坡位置進行現地孵育，並採樣帶回實驗室，以二種溫度 (15℃, 30℃) 及二種水分含量 (20%及40%重量百分比) 進行孵育。1月的現地孵育試驗中，硝態氮及總無機態氮濃度均隨孵育時間增加而增加，頂坡與麓坡、加蓋管柱法與開放管柱法之間均無顯著差異。8月現地孵育試驗之硝態氮及總無機態氮濃度則受到降雨及氣溫下降的影響，開放管柱法在孵育第三、四週顯著低於加蓋管柱法，不同地形之間之無機態氮濃度仍無顯著差異。若以單位土壤重量而言，頂坡和麓坡的淨礦化量、淨礦化速率、淨硝化量及淨硝化速率無明顯差異，若比較每單位有機碳濃度之無機態氮，則以麓坡較高，顯示在不同地形位置之間，植群差異造成土壤有機質的質與量不同，因而影響淨氮轉變作用。 頂坡土壤MBC顯著高於麓坡，MBN則因地形與季節而不同，1月孵育之前頂坡高於麓坡，兩地區土壤之MBN經四週孵育後有濃度增加的趨勢；8月孵育之前的MBN顯著高於1月，經孵育後兩地形土壤之MBN均顯著降低。MBC與有機碳之比 (Cmic/Corg) 明顯較高，以麓坡顯著高於頂坡，顯示麓坡土壤之碳有效性較高。MBN與全氮之比 (Nmic/Nt) 則在文獻值範圍內，不同地形及季節之間均有差異，以8月高於1月，頂坡高於麓坡。將樣帶樣區現地孵育之土壤無機態氮、微生物生質量、水分及溫度進行Pearson相關分析，結果顯示微生物性質與不同地形下的水分、溫度呈顯著相關。同一地形不同季節的相關分析結果顯示，麓坡土壤MBN與溫度呈顯著正相關，微生物生質碳氮比則與溫度呈顯著負相關；頂坡土壤銨氮、硝態氮及總無機態氮濃度受溫度影響比麓坡更為明顯，微生物生質氮則較不受溫度影響。 以不同水分及溫度條件進行的實驗室孵育結果發現，改變溫度條件對樣帶樣區土壤淨氮礦化及硝化作用之影響大於改變水分條件。與現地孵育不同的是，實驗室孵育下的淨銨化作用較佔優勢，尤其是頂坡土壤，推測此差異應與孵育前的風乾處理及孵育之水分條件偏低有關。

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.