pH與有機質對紅壤吸附-脫附硼的影響

Abstract

硼是植物維持正常生長所必須的微量要素，植物從土壤可攝取硼的多寡，取決於土壤溶液中的硼濃度高低，因此，維持土壤溶液中硼介於足夠植物生長且不至於引起硼毒害的濃度是很重要的。硼吸附-脫附反應主導了硼在土壤中固、液相的分布。硼對植物的有效性除了受土壤性質影響，同時也受施用石灰、肥料或有機質等管理手段的影響，施用石灰導致土壤pH上升而增加土壤對硼的吸附，進而降低了硼的有效性。因此，本研究旨在探討（1）在紅壤中影響硼吸附-脫附的因子；（2）pH對吸附性硼的可逆脫附的影響；以及（3）施用堆肥對土壤吸附-脫附硼的影響。 本試驗選取五種細質地的紅壤，測定在不同pH處理的等溫吸附曲線，在結束硼吸附試驗後，立即以連續稀釋法進行脫附試驗，吸附-脫附數據皆很適合以Freundlich方程式描述。發現紅壤的黏粒、游離氧化鐵、游離氧化鋁、無定形氧化鋁含量較高時，土壤對硼有較高的吸附量。當土壤pH升高時，土壤吸附硼量明顯的增多，尤其以Ca(OH)2取代NaOH調整土壤pH，土壤能夠吸附更多的硼。 土壤的脫附數據分別以傳統表示法與時間依賴表示法呈現土壤等溫脫附曲線，在土壤原有的pH，硼脫附有明顯的遲滯現象，以傳統表示法所得的遲滯係數ndes/nads與λtrad和土壤有機碳含量呈現顯著正相關，而與游離氧化鐵、鋁成顯著負相關。當pH增加時，可觀察到遲滯現象逐漸縮減，吸附性硼變得容易脫附，這個發現或許是因為硼吸附反應與調高pH會增加吸附表面的負電荷，土壤溶液中的B(OH)3為一路易士酸受到負電荷吸引而與表面吸附位置形成外圈錯合物，此種吸附鍵結強度較配位交換鍵結弱，所以被吸附的硼很容易被釋出。 選用龍崗系、老埤系、坡堵系與淡水系土壤添加0、2.5、5、10%的蔗渣堆肥或雞糞堆肥的試驗結果顯示：除了老埤系5%蔗渣堆肥處理土壤對硼的吸附量降低，龍崗系、坡堵系與淡水系土壤，硼的吸附量會隨著蔗渣堆肥施用量增加而有上升的情形。而2.5%雞糞堆肥處理使得四種試驗土壤對硼的吸附量增加，或許是因為添加雞糞堆肥提昇土壤的pH，而有機質或黏土礦物對硼的吸附皆會隨著pH增加而增加，因此，施用有機質所造成土壤pH改變應該是影響硼吸附的重要因子。至於更高的雞糞堆肥施用量所產生的影響則因試驗土壤不同而有所差異。龍崗系土壤硼吸附量會隨著雞糞堆肥施用量增加而略有增加；10%雞糞堆肥處理使得坡堵系與淡水系土壤硼吸附量降低，推測是因有機質的添加導致遮蔽了吸附位置。添加堆肥土壤的硼脫附結果，土壤在低蔗渣堆肥添加量2.5%處理下，土壤脫附硼的遲滯現象有些微的增加，隨著堆肥添加量增加，遲滯係數ndes/nads與λtrad逐漸降低，被土壤吸附的硼能夠較容易被釋出。施用雞糞堆肥的四種土壤，硼脫附的遲滯現象明顯地比未施用雞糞堆肥小。 相對地，以次氯酸鈉去除龍崗系土壤的有機質，發現去除有機質的土壤硼吸附量有大幅度的增加，推論雖然移除有機質會減少土壤中硼的吸附位置，但也因此將原本被有機質遮覆的吸附位置裸露出來，因此增加了硼的吸附。去除有機質土壤對硼的脫附有遲滯現象，但是已去除有機質土壤與未去除有機質土壤有相近的遲滯係數，似乎在同一pH有機質存在與否對硼脫附的難易並沒有很大的影響。 由本論文結果可知，pH的變化是影響土壤供硼狀態最重要的因子，無論是考慮以添加石灰物質改善紅壤pH，或是添加有機質至紅壤改善土壤的理化性質，都必須考慮pH的變化所導致土壤溶液中硼濃度的變動，以適時適量的補充硼肥。

Boron is an essential micronutrient for plant growth. The uptake of B by plants depends mainly on B concentration in soil solutions. It is important to maintain B levels in soil solutions that are sufficient for plant uptake but nontoxic. Adsorption-desorption process governs B distribution between adsorbed and liquid phase. Boron availability to plants depends on soil properties as well as management practice like liming, fertilization and use of organic matters. The increase in soil pH associated with liming could also result in an increase in B retention by the soils and consequently a lower B availability. Therefore, the objectives of the study are on: (1) the factors affecting B adsorption-desorption in five red soils; (2) the effect of pH on the reversibility of adsorbed B; (3) the effect of compost application on B adsorption and the reversibility of adsorbed B. In this study, boron adsorption and desorption as a function of B concentration and pH were measured in five fine-textured red soils. Immediately after adsorption of B, four consecutive desorption steps were carried out by successive dilution. The sorption results were described well by the Freundlich equation (P<0.01). Boron adsorption capacity increased as soil contains higher amounts of clay, free Fe2O3, free Al2O3, and amorphous Al2O3。Boron adsorption increased markedly as pH increased. This increase was more pronounced when Ca(OH)2, as opposed to NaOH, was used for pH adjustment. The presentation of desorption data was based on the traditional isotherm approach and on the time-dependent isotherms. Apparent hysteresis was observed that both derivational families of desorption isotherms deviated from the adsorption isotherm for the five soils at their original pH. Moreover, hysteresis coefficients, ndes/nads and λtrad for the traditional approach, had a positive correlation with organic carbon content, but they had a negative correlation with free Fe2O3 and free Al2O3 contents. As pH increased, the hysteresis diminished, and adsorbed B became more reversible. This finding may be due to the fact that B adsorption and raising pH increase the negative charge on the surface, the outer-sphere boric acid complexes formed based on the Lewis acidity of the B metal center. Unlike the strong binding by ligand exchange reaction, the physically-bound boric acid may more readily return to the soil solution when solution concentrations decrease. Adsorption-desorption of B was investigated in Lunkang, Laopi, Potu, and Tansui soils receiving varying doses of sugarcane compost (SC) or chicken manure compost (CM) (0, 2.5, 5, 10%). Application of sugarcane compost increased B adsorption in Lungkang, Potu, and Tansui soils except for the decreasing B adsorption in Laopi soil with 5% SC amendment. Application of 2.5% chicken manure compost considerably increased pH and B adsorption in the four soils. Because B adsorption on humus or clay minerals is pH-dependent, higher pH might be an important factor improving B adsorption. The effects of higher rate of chicken manure compost on B adsorption are different for the soils. Boron adsorption by Lungkang soil slightly increased as the rate of chicken manure compost increased. Boron adsorption by Potu and Tansui soils decreased with 10% CM amendment. This decrease might be due to the adsorption sites were occluded by organic matter. The degree of hysteresis slightly increased with 2.5% SC amendment. Hysteresis coefficients, ndes/nads and λtrad, decreased with a higher application rate of sugarcane compost, indicated adsorbed B became more reversible. The degree of hysteresis apparently decreased with the amendment with chicken manure compost. The amount of B adsorbed was considerably greater after the organic matter had been removed from Lungkang soil by NaOCl. It suggested that a portion of adsorption sites are generally coated or occluded by organic matter and become available for B adsorption after removal of the organic matter. Boron desorption showed a hysteretic trend in the treated soil. But little effect of the presence of organic matter in hysteretic desorption was observed compared to the soil without organic matter removal at similar pH. In conclusion, pH is the most important factor in determining B availability in soil solution. The changes of pH must be taken into consideration before liming or application of organic matter, and consequently the B status in soil solution must be satisfied by B fertilization in adequate concentration for plant growth.