修枝碎木敷蓋對茂谷柑園土壤有機碳儲量之影響

Abstract

枝條修剪是果園管理的重要措施，但同時也產生大量廢棄枝條處理問題。透過作物殘體回田，可作為具潛力的果園土壤碳匯策略。因此本研究以果園修剪殘枝作為碳源，將茂谷柑修剪枝條粉碎成碎木後，以兩種不同厚度 (5公分、10公分)直接敷蓋於土面，並於敷蓋後每6個月採深度0-10公分和10-30公分土樣進行土壤性質與有機碳含量測定。在敷蓋處理後6個月，兩種碎木敷蓋厚度處理皆顯著提高土壤有機碳與有機碳儲量，深度0-10公分和10-30公分土壤有機碳儲量平均增加6.2和5.6 t·ha⁻¹，同時敷蓋厚度5公分處理可有效降低深度0-10公分的土壤總體密度。在敷蓋18個月後，敷蓋處理總有機碳儲量較未敷蓋土壤高約8.4 t·ha⁻¹。碎木敷蓋可增加樹體磷和氮含量，並促進敷蓋前期的小果膨大及後期果實果皮增厚。綜合土壤性質與樹體生長之結果，以碎木敷蓋厚度5公分處理效果較佳。本研究進一步以孵育試驗評估碎木與有機質肥料共同敷蓋對土壤影響，包含全碎木、碎木與有機質肥料以3:1與1:1 (w/w) 比例混合後的不同敷蓋處理，並持續調查敷蓋物的質量變化。各處理敷蓋物質量在敷蓋3天內即明顯損失至少20%，且在42天的孵育過程中以全碎木敷蓋分解最快 (分解常數k = 0.0057天⁻¹)，碎木：有機質肥料1:1 (w/w) 處理分解最慢 (分解常數k = 0.0022天⁻¹)。不同比例的碎木與有機質肥料共同敷蓋對於土壤有機碳含量的短期提升效益有限，但有機質肥料比例提高會導致pH值降低與電導度上升。本研究再以碎木敷蓋5、10公分及未敷蓋處理進行90天的孵育試驗，以評估果園碎木分解與土壤性質之變化。兩種敷蓋厚度的碎木質量留存率變化趨勢相似，孵育結束時的質量留存率約72%。敷蓋厚度5公分處理預估碎木半衰期為223.5天，10公分處理預估碎木半衰期為210.0天。依單一負指數衰減模式與分解常數推算果園碎木分解18個月後質量留存率為15.94%-17.89%。本研究結果證實以果樹修剪殘枝碎木作為敷蓋材料可在6個月後增加土壤有機碳含量，不影響果園生產，配合敷蓋碎木分解速率及與有機質肥料共同施用的探討，可供發展枝條殘體應用於碳匯果園農耕技術之參考。

Pruning is an important practice in orchard management, but it simultaneously generates a lots of waste branches that require proper disposal. Returning crop residues to the field can serve as a potential strategy for enhancing soil carbon sink in orchards. Therefore, this study used pruned branches from the orchard as a carbon source. After the pruned branches of ‘Murcott’ mandarin were shredded, they were directly mulched on the soil surface at two thicknesses (5 cm and 10 cm). Soil samples from depths of 0-10 cm and 10-30 cm were sampled every 6 months after mulching to determine soil properties and soil organic carbon content. 6 months after mulching, both mulching thicknesses treatment significantly increased soil organic carbon and soil organic carbon stock. The soil organic carbon stocks in the 0-10 cm and 10-30 cm depths increased by an average of 6.2 and 5.6 t·ha⁻¹, respectively. Additionally, the 5 cm thick mulching treatment effectively reduced the bulk density of the 0-10 cm soil layer. After 18 months of mulching, the total soil organic carbon stocks under mulching treatments were approximately 8.4 t·ha⁻¹ higher than that of the non-mulched soil. Shredded pruning residues mulching also enhanced the phosphorus and nitrogen contents in leave, promoted fruit enlargement in the early stage of mulching, and increased fruit peel thickness in the later stage. Based on the soil properties and tree growth results, the 5 cm thick mulching treatment showed the best performance. This study further conducted an incubation experiment to evaluate the effects of mulching with shredded pruning residues and organic fertilizer applied together on soil, including treatments of all-shredded pruning residues and composed of shredded pruning residues and organic fertilizer at ratios of 3:1 and 1:1 (w/w). Changes in mass of mulch were continuously monitored. Mulch mass of all treatment decreased by at least 20% within the first 3 days of mulching, and during the 42-day incubation period, the all-shredded pruning residues treatment decomposed the fastest (decomposition constant k = 0.0057 day⁻¹), while the treatment with shredded pruning residues mixed with organic fertilizer at a 1:1 (w/w) ratio decomposed the slowest (decomposition constant k = 0.0022 day⁻¹). Mulching with shredded pruning residues and organic fertilizer at different ratios had limited short-term effects on increasing soil organic carbon content, but increasing the proportion of organic fertilizer led to lower soil pH and higher electrical conductivity. Another 90-day incubation experiment using the 5 cm and 10 cm thick shredded pruning residues mulching and non-mulching treatments was conducted to assess shredded pruning residues decomposition and changes in soil properties. The change in mass remaining of shredded pruning residues were similar between mulching thicknesses, with final mass remaining of approximately 72% at the end of incubation. The estimated half-life of shredded pruning residues was 223.5 days for the 5 cm thick mulching treatment and 210.0 days for the 10 cm thick mulching treatment. Based on a single-exponential decay model and decomposition constants, the predicted mass remaining of orchard shredded pruning residues after 18 months was 15.94% to 17.89%. The results of this study confirm that mulching with shredded pruning residues can increase soil organic carbon content within 6 months without negatively affecting orchard production. Combined with the analysis of shredded pruning residues decomposition rates and mulching with organic fertilizer, these findings provide valuable references for developing orchard carbon sequestration practices using pruned branch residues.