桃園中壢臺地不同水分境況下含鐵網紋極育土氧化還原形態特徵之鑑定與鐵錳結核生成機制

Identification of the Redoximorphic Features and Formation Mechanism of Fe-Mn Nodules in Ultisols with Plinthite under Different Moisture Regime in Chungli Terrace

簡士濠(Shih-Hao Jien)

指導教授 ： 陳尊賢

國立臺灣大學/生物資源暨農學院/農業化學研究所/博士(2006年)

https://doi.org/10.6342/NTU.2006.00453

摘要

鐵錳聚積物 (例如鐵錳軟團塊、鐵錳結核)於含鐵網紋 (plinthite)極育土中為一明顯的形態特徵。鐵錳聚積物的生成與水文狀況關係密切，諸如季節性地下水位的變動或表面灌溉水的灌排都將影響土壤中元素的重新分佈或氧化還原形態特徵上的變化。本研究在桃園中壢臺地上，針對不同水分境況且不同形態特徵之土壤，探討其氧化還原形態特徵之鑑定方法及鐵錳結核生成與當地水文狀況之關係並且推論鐵錳結核生成機制。
本研究選定桃園中壢臺地上之三種不同水分境況土壤，分別為湖口土系 (Plithic Paleaquults)，竹圍土系 (Typic Plinthaquults)和蘆竹土系 (Typic Plinthaquults)。分別於三種土壤之表面下25、50、100及200公分處於2004年1月至2005年12月每兩星期土壤水分基質勢能、土壤氧化還原電位，及另設置一200公分監測井以觀察地下水位變化。將10×10公分之鋁製方盒於每土層中收集土樣，攜回實驗室後以濕篩方式收集不同粒徑大小之鐵錳結核，收集之粒徑分別為2-5, 5-10, 10-20和>20 mm。
研究結果發現處於地勢較低的湖口土系，常年季節性地下水位為三土系中最高，長年於Bt1層(20-40 cm)處變動。此深度附近由於乾濕交替頻繁，導致整個剖面中之鐵錳結核含量於Bt1和Bt2 (20-60 cm)處最多 (9-33 kg/55cm/m2) (20公分至75公分間)。湖口土系為三土系中還原時間最久之土系，表土50公分以下之平均還原時間為一年之74%，總結核含量為約9 kg/150cm/m2 (50-200公分)，為三剖面中之最少量。原因為土壤長期處於還原狀況下，導致鐵錳結核不易生成。竹圍土系為三種土壤中總鐵錳結核含量最多之剖面 (表土50至200公分總量約740 kg/150cm/m2)。由於此剖面地下水位變動頻繁，長年於50-180公分間變動，且表土50公分下之平均還原時間為一年之40-50%，總鐵錳結核含量為740 kg/150cm/m2 ((50-200公分))。由此結果推測中等還原狀況和頻繁地下水位變動乃是造成鐵錳結核形成的主要因子。蘆竹土系排水等級較其餘兩土系較佳，但受暫棲水和地下水位變動的雙重影響，在Btv1 (20-50 cm)層內，氧化還原作用盛行，鐵錳結核含量稍多 (22-53 g/kg)且粒徑較大，以10-20mm的結核為主。剖面愈往下層部分，雖長期浸水飽和但卻未達還原狀況，應該為地下水含氧的關係，使得還原狀態不盛行，導致鐵錳分凝作用不佳，鐵錳結核不易生成。此土系表土50公分下鐵錳結核總量只達 220 kg/150cm/m2 (50-200公分)。
由無定形、游離態和全量鐵和錳分析可判定鐵錳結核之結晶化程度及鐵活性指標。結晶程度可由結晶化指數 ((Fed-Feo)/Fed)比較得知，研究結果發現，三剖面中各不同粒徑下之鐵錳結核結晶化程度與土壤一年中還原時間具有顯著之負相關 (r=-0.30*, p<0.05)，顯示當土層處於還原狀況愈久時，現地所形成之結核結晶程度越差。另一方面，鐵活度指標 (Feo/Fed)與土壤還原時間具顯著正相關 (r=0.43**, p<0.01)，結果表示，當鐵錳結核粒徑越大時，Feo/Fed有較大的趨勢。此現象可解釋鐵錳結核為現地生成，由小粒徑之鐵錳結核為核心並逐漸而增大。由微形態方面之觀察，結果可證實大部分結核確實為現地生成 (邊界漸往外擴散)，且大部分之結核皆為以錳為核心。由此現象，可推測當土層處還原狀態一段時間後，錳還原而往下淋洗至孔洞或石英等不易風化之礦物上，待氧化而後沉澱，接續還原的鐵或錳再逐漸洗入原先沉澱的錳核裂隙中或沉澱並包覆於表面上逐漸增大。
本研究推論在含鐵網紋之極育土中的鐵錳結核生成機制有三個步驟：(1) 受還原的錳先移動至土壤微孔隙中或不易風化之礦物表面上沉澱形成結晶性不佳之錳核；(2) 接續由於地下水位變動或表面灌溉水之淋洗，導致還原的鐵隨黏粒或獨自洗入移動至一開始之錳核表面或裂隙中再氧化而沉澱；(3) 長久乾溼交替的水文變動，鐵錳結核逐漸氧化累積增大。

關鍵字

鐵網紋 ； 極育土 ； 氧化還原形態特徵 ； 鐵錳結核 ； 鐵活度指標 ； 鐵結晶化指標

並列摘要

Iron and manganese nodules are dominant redoximorphic features in most of rice growing soils of Taiwan that are characterized by the effects of variability in anthraquic and seasonally fluctuating of shallow ground water tables. A field experiment was conducted in Chungli terrace from January of 2004 to December of 2005 to characterize the chemical and physical properties of Fe-Mn nodules and to examine the possible mechanisms for nodule formation under different moisture regimes. It was found that the quantities and characteristics of these nodules were to a greater extent influenced by fluctuating soil moisture regime. The objectives of this study are(1) to characterize the chemical and physical properties of Fe-Mn nodules, and (2) to propose possible mechanisms for nodule formation under different regimes of soil water conditions
Three Ultisols with Fe-Mn rich nodules and different moisture regime were selected in Chungli rice growing terrace located in the northern Taiwan. The selected soils were Typic Plinthaquult (Luchu soil), Typic Plinthaquult (Chuwei soil) and Plinthic Paleaquult (Houko soil) within an elevation of 20 to 30m above sea level. Routine soil sampling and analyses included variables such as the measurements of ground water table, soil matric potential and soil redox potential (Eh) which were than combined with the measurements of physical and chemical characteristics of whole soils and Fe-Mn nodules with different sizes to understand the formation mechanism of the redoximorphic features in these anthraquic soils with plinthite.
It was observed that the soil pedon (below 50 cm depth in the profile) in Plinthic Paleaquult (Houko soil) located at the bottom of the hydrosequence was most reduced (70% of the year) compared with other soils in the hydrosequence. The highest reduction at 50 cm depth in Houko soil was found to be associated with the slightly development of Fe-Mn nodules (9 kg/150 cm/m2). The Chuwei soil had moderately reduced pedon (40% of the year) but was found to be associated with the highest development of Fe-Mn nodules (740 kg/150 cm/m2). Based on these observations, it was assumed that moderately reduced duration was the most suitable condition for the formation of Fe-Mn nodules along this toposequence.
Iron and manganese nodules of four sizes (2-5, 5-10, 10-20, and >20mm radius) were also determined to study their formation under different water regime conditions. These nodule samples were analyzed for amorphous material (Feo, Mno, and Alo), crystalline material (Fed, Mnd, and Ald), and total content (Fet and Mnt) of Fe, Mn and Al. The results indicated that Fed and Fet decreased with increasing sizes of the nodules. No trend was found in any fraction of Mn of three pedons for Mno, Mnd and Mnt. The crystalline index ((Fed-Feo)/Fed) of different size nodules ranged from 0.5 to 1. A significantly negative correlation was observed between the crystalline index and the duration of reduced time in a year (r=0.30*, p<0.05). It indicated that the degree of crystalline of nodules decreased with length of reduced duration.
Nodules of different sizes had similar Fe activity index (Feo/Fed). It showed a trend such that Feo/Fed ratio slightly increased with the nodule sizes, and the largest nodule (> 20 mm) had significantly lower bulk density compared with other nodule sizes. These evidences implied that the poor crystalline iron nodules are still accruing which could be explained by the occurrences of nodules of varying sizes as a nucleus.
The micromorphological features of Fe-Mn nodules with different sizes and water regimes were also studied. These features indicated that the most nodules are formed in-situ with diffuse boundary. The Ap horizon of Luchu soil, as well as soils in Ap and AB horizons of Chuwei soils that were mostly in reduced condition had maximum amount of Mn nodules of different sizes . In these horizons, it appears that reduced Mn would have been moved and precipitated on the surface of the coarse minerals such as quartz, and then reduced Fe and Mn continuously precipitated on their surface or over time infilled into the minerals and grew to the larger ones. On the other hand, concentric nodules (always smaller than 0.5 mm) presenting only in the Luchu soil could be attributed to the dominant role of the finer soil texture (i.e., clay loam). Relatively fewer, smaller and poorer structures of nodules observed in Houko pedon might be because of long-term reduced condition and higher content of organic matter.
Based on the results of this study as above, it appears that Fe and Mn nodules formation in these Ultisols with plinthite of Chungli of northern Taiwan have undergone changes through three stages of development: (1) in the initial stage of nodule formation, reduced Mn moved to mineral surfaces and then re-oxidized to poor crystalline Mn oxide form; (2) Fe or Mn continued depositing on or infilling into the Mn nodules and these deposits or infillings of Fe were developed to a recognizable crystalline Fe forms; and (3) depending on the redox condition of the profile over time, these particles continued growing, more of the small nodules were added and poorly crystalline Fe formed into the bigger volumes of Fe-Mn nodules.

並列關鍵字

plinthite ； Ultisols ； redoximorphic features ； iron and manganese nodules ； iron activity index ； iron crystalline index

參考文獻

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