Comparative Analysis of Four Methods for Accurate Estimation of Soil Phosphorus Storage Capacity: a Case Study in a Typical Red Soil

Efficient utilization of agricultural soil phosphorus (P) and mitigation of loss risks necessitate a precise evaluation of soil P storage capacity (SPSC). This study compared the effectiveness of four soil test P methods (Oxalate, Bray, Olsen, and Mehlich-1) to accurately estimate SPSC and simplify P loss risk assessment of soils located in a typical red soil in the Sunjia Watershed, Yingtan, Jiangxi Province, China. The extraction efficiencies of these methods for Fe, Al, P, and P saturation ratio (PSR) were compared, and conversion equations between SPSC_Ox (extracted using Oxalate) and soil test P (Bray, Olsen, and Mehlich-1) were derived through fitting analysis. The results underscored Oxalate as the optimal extractant for gauging P loss risk in red soils. Structural equation modeling (SEM) unveiled the substantial impact of amorphous iron-aluminum oxides (Fe_o, Al_o) on SPSC, with Fe_o exerting a more pronounced influence than Al_o. Among soil physicochemical properties, total carbon emerged as the most influential, and a strong interaction was noted between the physicochemical properties and Fe_o and Al_o. The study delineated three crucial P concentration ranges for practical P management in red soils. When Bray-P < 48.2 mg kg^–1, the soil acted as a P sink with no P loss risk, allowing for continued P application to augment crop yield. Conversely, within 48.2 mg kg^–1 < Bray-P ≤ 55.2 mg kg^–1, the soil attained its maximum secure P capacity; further P application significantly escalated the peril of P loss. Subsequently, when Bray-P > 55.2 mg kg^–1, the soil turns into a source of P release. This signifies an escalated risk of P loss, demanding the immediate implementation of environmental protective measures.