稻稈添加對浸水土壤中鐵物種變化的作用

Abstract

稻稈返田做為去化農業廢棄物的手段已經行之有年且已被證實對土壤營養卓有成效，然而因為水稻產地大多一年多穫，使未分解完全的稻稈在再度進水後因微生物大量產生而出現溫室氣體、植物病蟲害等問題影響生產，即便使用基因改造提升稻稈可分解的程度，也會造成其他副作用如更大量的碳排放等；部分前人研究則指出鐵的物種可固定土壤中的有機質避免流失，且可大幅影響營養元素的有效性，其作用機制來自於微生物造成的氧化還原反應，因此也有將土壤中出現的鐵物種納入評估施肥成效的呼聲。 本研究透過為期84日，使用兩種質地不同但 pH 相近的酸性土壤，以及三種不同脆性稻稈，以模擬水田環境下稻稈返田的孵育實驗，企圖審視稻稈脆性對於稻稈分解的影響，以及添加稻稈時對於土壤中鐵物種變化的作用為何。從實驗結果中得知，在浸水孵育的過程中無論土系，土壤溶液中的鐵濃度與有機碳濃度都是以與中洗纖維含量最低的SA1064稻稈(Brittleness = 3)孵育組為最高，且都在孵育三週後濃度開始降低，然而其代表芳香環物質濃度的254 nm吸光值則未有下降跡象，同時代表有機物分子量的SR值則顯示土壤溶液的有機物分子量隨孵育時間上升；鐵序列萃取結果顯示平鎮系土壤內氧化鐵物種的結晶性會被破壞且在有添加稻稈時現象更為明顯，不過與稻稈脆性無關，而XANES圖譜顯示無水的氧化鐵在孵育過程中變成氫氧化鐵，而有添加稻稈時鐵的配位結構則較容易停留於氫氧化鐵；同樣的序列萃取則顯示台南系土壤氧化鐵物種的結晶性雖未顯著被破壞，但隨時間增加，還原性的二價鐵礦物結晶含量則緩慢上升，XANES圖譜則顯示台南系土壤的二價鐵與三價鐵之間在孵育過程出現消長，而中洗纖維含量最少的Tn B=3處理的消長週期則為最長，顯示處理中較多的溶液有機碳可能具有還原性以穩定鐵物種。 透過土壤溶液中鐵與有機物的基本分析、鐵序列萃取以及同步輻射數據，本研究歸納出土壤中可能的鐵物種變化途徑，並指出在浸水土壤進行稻稈返田的過程中，稻稈中洗纖維含量相對於脆性，更可能是影響土壤溶液性質與土壤鐵物種最主要的因子。

Returning rice straw to the field has been proved to be an effective strategy for improving soil fertility. However, because rice is harvested more than once a year in most of the rice-producing areas, the rice straw which cannot be completely decomposed in the intermittent period could result in the production of greenhouse gas or the occurrences of disease and pests that can damage the subsequent rice production. Although gene modification could enhance the degradability of rice straw there would be several adverse side effects, such as far more carbon emissions to the environment. Previous studies suggested that Fe species relate to fixation of organic matter in the soil and strongly affect the availabilities of nutrients. Thus, it is essential to determine Fe speciation to assess fertilization effectiveness of rice straw returned to paddy field. In this study, three types of rice straw (1 wild type and 2 mutants mutated by sodium azide) with different brittlenesses were mixed with two different paddy soils and then incubated under submerging conditions for 84 days to investigate how straw brittleness affect its degradation and its impact on the transformation of Fe species as a function of soil submergence time. The result showed that, regardless of soil type, the treatments with rice straw SA1064, brittleness = 3, had the most dissolved Fe and dissolved organic carbon (DOC) concentration. Dissolved Fe and DOC concentrations both increased to maximum at 21 days and then decreased until the end. However, soil solution’s concentration of aromatic compounds did not decrease, while the molecular weight of DOC increased during the incubation process. The results of Fe sequential extraction showed that the crystallinity of Fe species in Pc soil decreased during the incubation regardless of brittleness. XANES-LCF showed that iron hydroxide species were transformed from anhydrous iron oxide, and it was more likely for Fe to stay in hydroxide forms with the presence of rice straw. However, the crystallinity of Fe species in Tn soil did not significantly decrease during the incubation, but well-crystalline Fe(II) minerals were formed over time. XANES-LCF showed that Fe(II) and Fe(III) species fluctuated by time, and the treatment with the least neutral detergent fiber content (Tn B=3) had the longest fluctuation period among all the treatments, which indicated that the a larger amount of DOC in such treatment stabilize the reduced Fe species. Through soil solution analysis, Fe sequential extraction, and XANES analysis, the results revealed the transformation pathways of Fe species in soils.