不同土地利用之土壤磷含量與型態

Abstract

磷是維持地球生命的重要元素，磷在土壤中會以多種化學形態存在。不同的土地利用方式，特別是在不同農業管理下，造成土壤磷型含量和型態有所不同。台灣過去積極開發山林，因環保與國土保安意識提高加上林業政策推動，對不當使用農地進行廢耕，而導致廢耕造林的出現。當農地轉為廢耕地後，對土壤磷含量和型態影響，並不是十分清楚。本研究選擇四個採樣樣區，依海拔從低至高，分別為瑞穗、名間、清境與梅峰等樣區，並於各採樣樣區選取三種不同土地利用型態，包括廢耕林地、耕犁農地與無耕犁農地等型態，探討不同土地利用型態對土壤磷的含量和型態影響。本研究結合Hedley Fractionation、K-edge X光吸收光譜分析 (XANES)、固態與液態核磁共振光譜 (31P-NMR) 等不同分析技術，綜合評估土壤磷型態與含量的改變。 實驗結果顯示除梅峰樣區外，全磷含量農地高於廢耕林地。無機磷方面，大部分樣區的Resin-P、NaHCO3-Pi和NaOH-Pi含量，農地顯著高於廢耕林地。HCl-P含量，無耕犁農地含量高於其他兩種土地利用或呈無顯著差異。而有機磷含量，則是在耕犁農地含量最低。從XANES的分析結果得知，無耕犁農地皆有碳酸鈣的訊號，而耕犁農地只有清境樣區有明顯的shoulder與峰值訊號，其餘無訊號或不明顯，廢耕造林則是中海拔有訊號。液態31P核磁共振光譜分析得出，正磷酸鹽為最主要的磷型態 (65- 96%)，而農地土壤的磷酸二酯比例與含量，均小於廢耕林地。固態31P核磁共振光譜分析結果顯示相較於瑞穗樣區，Ca-P為梅峰樣區主要的土壤磷型態。上述三種實驗分析結果與磷的序列萃取在全磷、Ca-P的結果相似。並非所有不同植物可利用有效性的磷分級皆會隨海拔上升而含量增加，而年均溫除了無耕犁農地的HCl-P外，其餘磷分級皆會隨年均溫下降而含量增加。總而言之，不同土地利用會改變土壤磷的含量和型態，年均溫或許比海拔更能解釋磷分級的變化。

Phosphorus (P) is an essential element for life and may exist in various chemical speciation in the soil. Different land uses, such as agricultural management, is proposed to affect the content and speciation of soil P. Similar with other countries in the world, Taiwan used to conduct forest reclamation actively. However, some improper using arable fields have been abandoned recently due to the awareness of environmental protection and reservation. Little is known about the changes in the content and speciation of soil P after the conversion from arable fields to abandoned fields. In this study, I selected four study sites, including Ruisui, Minjian, Wuling, and Meifung with the altitude ranged from 300 m to 2200 m. In each study site, three different land uses of abandoned fields, tillage, and non-tillage fields were compared. I applied Hedley fractionation, X-ray absorption near edge structure (XANES), and solid- and liquid-state 31P nuclear magnetic resonance (NMR) spectroscopies to examine how different land uses affected the content and speciation of soil P. The results indicated a significantly lower total P in the abandoned fields, except the Meifung site that had a lower total P in the tillage field. The resin-P, NaHCO3-Pi, and NaOH-Pi were higher in the abandoned fields. HCl-P in the non-tillage fields was higher or no significant difference compared to other land uses. Except for the Meifung site, significantly lower organic P was found in the tillage fields. The XANES spectra of the residues after NaOH extraction showed the Ca-P signal in the non-tillage fields. The spectra in the Qingjing site and Meifung site had the clearest Ca-P signal, suggesting the highest Ca-P signal in the middle elevation in abandoned fields. Phosphorus was mainly composted in the orthophosphate form in the NMR extract (65- 96%). The proportion and concentration of diesters in both tillage and non-tillage fields were less than those in the abandoned fields. Solid 31P-NMR showed that the primary soil P speciation in the Meifung site was in the Ca-P formation. The measurements from both chemical and spectroscopic methods showed similar results in P content and speciation, particularly in total P and Ca-P. Except for HCl-P in the non-tillage fields, P content increased with the elevation or the decreasing mean annual temperature. The results indicated that the different land uses had different phosphorus content and speciation.