長期有機和慣行農法試驗對土壤中碳和磷的垂直分布的影響

Abstract

磷是植物生長所必需的關鍵營養元素，其有效管理對於糧食安全與環境永續至關重要。然而，全球磷資源的枯竭以及過度施肥造成的環境污染風險，對現代農業提出了嚴峻挑戰。為探討不同施肥策略對土壤磷動態的長期影響，本研究以長期施肥試驗田加以比較慣行、折衷及有機施肥管理策略，對旱田和水田土壤中磷與碳的垂直分布及其移動性的影響，並評估不同施肥處理對土壤磷有效性和環境風險的差異。 旱田和水田的有機處理有最高濃度的總碳和總磷，有機處理碳和磷的累積，主因是施肥量不同所致。因此，透過磷活性因子(phosphorus activation coefficient, PAC)分析不同處理的磷轉換效率，有機處理仍有最高的PAC值，且隨著深度逐漸上升，顯示有機處理磷垂直移動的風險高於其他處理。同時，序列萃取結果顯示磷物種的垂直分布，在旱田表土層(0-30公分)有機處理土壤中可溶性磷濃度最高，代表其生物有效性較高；慣行處理則以鐵磷為主，其生物有效性較低。而水田區，因為浸水還原作用的影響，使可溶性磷的濃度上升。水田土壤之XANES LCF磷物種擬合結果顯示，缺乏有機質的慣行處理，土壤鐵磷物種以吸附在水鐵礦磷為主，而有機處裡則是以磷酸鐵為主。土壤膠體萃取可以分析磷在土壤中的垂直移動機制:在0-30公分分的土層，慣行處理膠體碳和磷釋出比例最高，顯示慣行處理下，磷主要以膠體的型式垂直移動。有機處理因為具有豐富的土壤有機質，增強土壤團粒結構，抑制膠體磷的釋出。 透過分析碳和磷總量、有效性、磷物種和膠體萃取，研究結果顯示長期施用有機肥料可增加土壤總碳含量並提升土壤有效磷轉換效率。其通過增加土壤有機質，並提升磷的可溶性含量與生物有效性，實現了更高的磷轉化效率。然而，有機處理中磷的累積，特別是可溶性磷濃度的提升，可能帶來磷垂直移動的環境風險，進一步影響地下水質的安全性。相比之下，慣行處理中，磷物種主要以鐵磷的形式存在，但低碳環境下土壤團粒結構的不穩定，導致膠體磷的釋出和移動性增強，加劇了表土磷流失的風險。

Phosphorus (P) is an essential nutrient for plant growth, and its effective management is critical for both food security and environmental sustainability. However, the depletion of global phosphorus resources and the environmental risks associated with excessive fertilization pose significant challenges to modern agriculture. This study investigates the long-term effects of different fertilization strategies on soil phosphorus dynamics. Using a long-term fertilization experiment, we compared conventional, integrated, and organic fertilization practices to evaluate their impact on the vertical distribution and mobility of phosphorus and carbon in upland and paddy soils, as well as their effects on phosphorus availability and environmental risks. Both upland and paddy soils under organic treatment exhibited the highest concentrations of total carbon and total phosphorus, primarily due to differences in fertilizer inputs. The phosphorus activation coefficient (PAC) analysis revealed that organic treatment had the highest PAC values, which increased with depth, indicating a higher risk of phosphorus vertical transport compared to other treatments. Sequential extraction results further showed variations in phosphorus speciation across soil depths. In upland soils, the organic treatment had the highest concentration of soluble phosphorus in the 0–30 cm surface layer, suggesting higher bioavailability, whereas the conventional treatment was dominated by iron-bound phosphorus, which is less bioavailable. In paddy soils, the reductive conditions induced by waterlogging increased soluble phosphorus concentrations. X-ray absorption near-edge structure (XANES) linear combination fitting (LCF) analysis indicated that in the absence of organic matter, iron-bound phosphorus in conventionally managed paddy soils was primarily associated with ferrihydrite, while in organically managed soils, phosphorus was predominantly in the form of iron phosphate. Colloidal extraction further elucidated the mechanisms of phosphorus vertical transport. In the 0–30 cm soil layer, conventional treatment resulted in the highest release of colloidal carbon and phosphorus, suggesting that phosphorus in this system primarily moves vertically in colloidal form. In contrast, the organic treatment, with its higher soil organic matter content, enhanced soil aggregate stability and suppressed colloidal phosphorus release. By analyzing total carbon and phosphorus, phosphorus bioavailability, phosphorus speciation, and colloidal extraction, this study demonstrates that long-term organic fertilization increases total soil carbon content and enhances phosphorus transformation efficiency. Organic management improves phosphorus solubility and bioavailability through increased soil organic matter content, leading to more efficient phosphorus utilization. However, the accumulation of soluble phosphorus under organic treatment raises concerns about potential phosphorus leaching and groundwater contamination. In contrast, under conventional treatment, phosphorus was predominantly present in iron-bound forms, but the instability of soil aggregates in low-carbon conditions led to increased colloidal phosphorus release and surface phosphorus loss.