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Study on the Nitrogen Fixation and Nutrient Dynamics of Casuarina spp. Windbreak at Tai-Chung Harbor
本區木麻黃的全年枯落物量達16.25 t/ha，在月份間有顯著差異，以10月4.44 t/ha為最高，造成大量凋落的原因可能與颱風及東北季風有關。枯落物組成成分中，以接合小枝所佔比例最高（59.6-87.1%），其次為枝條2.49-23%、繁殖體2.66-30.33%、其他成分0.11-12.06%。全年養分輸入量碳為8.15 t/ha，氮為288.99 kg/ha，磷為5.75 kg/ha，鉀為51.78 kg/ha，鈉為75.67 kg/ha，鈣為119.31 kg/ha，鎂為19.86 kg/ha。
土壤養分含量調查顯示枝葉層養分含量分別為氮462.02 kg/ha，磷5.62 kg/ha，鉀43.47 kg/ha，鈉27.59 kg/ha，鈣196.56 kg/ha，鎂28.15 kg/ha；土壤層養分含量集中在0-5 cm土層。有機碳與全氮量分別為2.6%及0.13%；有效磷、NO3--N及NH4+-N分別為5.43、54.2及10.7 mg/kg；交換性鉀、鈉、鈣、鎂與CEC依序為0.62、1.17、1.78、0.57、7.62 cmol/kg。除了氮無季節性變化，其餘養分以秋冬季節較高。
木麻黃根瘤生物量隨月份有明顯差異。活根瘤以7月16.06±25.49 g/m2為最大量，顯示根瘤生長主要發生於夏季。利用乙炔還原法測定固氮速率，以3月10.81±6.14 μmol/g/hr最高，11月5.94±2.05 μmol/g/hr最低，證實固氮速率以春季較旺盛。根瘤年平均生物量為6.06 g/m2，平均固氮速率為7.8 μmol/g/hr，推估25年生木麻黃林每年約可固定氮素38.65 kg/ha。
The purposes of this study were to explicate how did Casuarina adapt to a mineral deficient site. Litterfall, decomposition, soil nutrient, nodule biomass and N2 fixation in the 25-year-old Casuarina stand at Taichung Harbor were studied during October 2000 to April 2002.
The results showed that the amount, one yearly basis, of litterfall in the Casuarina stand was 16.25 t/ha and indicated that litterfall varied significantly with month. Maximum litterfall occurred in October (4.44 t/ha); the main reason may cause by typhoons and northeast monsoon in winter. The litterfall of which 59.6-87.1% was composed of branchlet, 2.49-23% was branch, 2.66-30.33% was reproduction, and the remainder (0.11-12.06%) was comprised of other materials. Annual amounts of these nutrients included 8.15 t/ha C, 288.99 kg/ha N, 5.75 kg/ha P, 51.78 kg/ha K, 75.67 kg/ha Na, 119.31 kg/ha Ca and 19.86 kg/ha Mg, respectively.
With respect to the decomposition by litterbag showed the highest rate during the initial stage (0-2 months). However the decomposition became slow down in 4 months later, which remaining mass ranged from 64.34% to 61.54%. During the period, the C concentration was decreased until the study end, the N, K and Na concentration were decreased initially but than increased, and the Ca, Mg and P concentration were increased until the study end.
The results showed that the litter layer nutrient contents were N 462.02 kg/ha, P 5.62 kg/ha, K 43.47 kg/ha, Na 27.59 kg/ha, Ca 196.56 kg/ha and Mg 28.15 kg/ha, respectively. It indicated the major nutrient pool in the soil was in the 0-5 cm depth. The soil layer nutrient concentration showed that total C and N were 2.6% and 0.13%; available P, NO3--N and NH4+-N were 5.43, 54.2 and 10.7 mg/kg; extractable K, Na, Ca, Mg and CEC were 0.62, 1.17, 1.78, 0.57 and 7.62 cmol/kg. Soil N concentrations didn't fluctuate seasonally, but the others were higher in October or January.
Active nodule biomass of Casuarina varied significantly with month. The nodule biomass was highest in July (16.06±25.49 g/m2) and nodule formation occurred in summer. Nitrogen fixation was monitored using the acetylence reduction assay. Maximum rate of N2 fixation occurred in March (10.81±6.14 μmol/g/hr) and the minimum occurred in November (5.94±2.05 μmol/g/hr). It explicated that nitrogen fixation activation was vigorous in spring. Annual average of nodule biomass was 6.06 g/m2, the rates of N2 fixation was 7.8 μmol/g/hr. From this estimations, it can be calculated that 25-year-old Casuarina stand would fix about 38.65 kg N2 per ha per year.
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