苗栗後龍海岸木麻黃與水黃皮根瘤固氮率及游離固氮率之季節性變化

Abstract

本研究之目的，在測定苗栗後龍海岸生育地木賊葉木麻黃(Casuarina equisetifolia)與水黃皮(Pongamia pinnata)，根瘤固氮活性與游離固氮潛能的季節變化，以瞭解生育地土壤中，氮的供給狀況與植體氮、磷動態。 於研究樣區選取造林後2年生與6年生之優勢木和劣勢木木麻黃與水黃皮，以乙炔還原法測定根瘤固氮活性與游離固氮潛能，並調查其土壤與植體的氮、磷養分。 結果顯示，兩樹種都在2009年6月時有最大的根瘤固氮活性(8.38~15.05 ; 9.37~29.40 C2H4 hr-1 g dwt-1)，於2009年10月至2010年1月時有最小根瘤固氮活性(0.22~0.71; 0.38~1.18 C2H4 hr-1 g dwt-1)。在植體全氮濃度和根瘤固氮活性之間，木麻黃與水黃皮皆呈顯著正相關(p<0.01)，且水黃皮優勢木的根瘤固氮活性較劣勢木佳，推測根瘤固氮能幫助植物的生長。本實驗亦發現，土壤游離固氮潛能與根瘤固氮活性的季節性變化呈負相關，並且游離固氮潛能與生長勢和植體全氮濃度之間呈負相關，而根瘤固氮活性則剛好與游離固氮潛能相反，呈正相關，推測可能是游離固氮能量來源之根系分泌物，會與植物體內的碳/氮比相關。 本研究證實，木麻黃與水黃皮根瘤固氮活性與游離固氮潛能，皆有季節性變化，但根瘤固氮活性與游離固氮潛能彼此間有負相關，而林木的根瘤固氮活性越高，對林木的生長越有幫助。

The purposes of this study were to investigate seasonal dynamics of nodule and free-living nitrogen fixation rate of Casuarina equisetifolia and Pongamia pinnata in the coastal windbreak forest at Houlong, Miaoli, to understand the habitat's nitrogen supply conditions and plants nutrient dynamics in the soil. 2 years and 6 years old of the dominant and suppressed trees were selected in the study plots, using the acetylene reduction method to determine the activity of nodule and free-living nitrogen fixation. The nitrogen and phosphorus nutrients in the soils and plants were also investigated. The results showed that the two species owned the largest nitrogen fixation activity in June, 2009 (8.38~15.05 ; 9.37~29.40 C2H4 hr-1 g dwt-1); and the minimum nitrogen fixation activity from October, 2009 to January, 2010 (0.22~0.71; 0.38~1.18 C2H4 hr-1 g dwt-1). On both Casuarina and Pongamia, there is significant positive correlation between plant total nitrogen concentration and nodule nitrogenase activity (p<0.01). Moreover, in Pongamia, dominant trees have higher nodule nitrogenase activity than those of suppressed. It clearly showed that nodules nitrogen fixation could help the plant growth. This study also found that the free-living nitrogen fixation potential of the soil and the nodule nitrogenase activity in seasonal changes were negatively correlated with growth potential and plant total nitrogen concentration. It was speculated that the root exudate is the free-living nitrogen bacteria's energy source related to the carbon / nitrogen ratio in plants. This study demonstrated that there are seasonal changes in nodules and free-living nitrogen fixation of Casuarina and Pongamia. However, there is negative correlation between nodule nitrogenase activity and free-living nitrogen-fixing has a negative correlation. The higher the nodule nitrogenase activity is, the more it helps the growth of trees.