Land use-driven regulation of phosphorus fractions in soil aggregates

An excessively high proportion of stable phosphorus (SP) in soil can exacerbate the risk of phosphorus deficiency. Therefore, optimising land use and management practices to promote phosphorus transformation and cycling is essential for increasing soil phosphorus availability and ensuring the sustainable use of phosphorus resources. In this study, the dynamics of phosphorus fractions in soil aggregates under four land use types, sloping farmland (SF), economic fruit forestland (EFFL), woodland (WO), and natural grassland (NG), were quantitatively investigated using long-term monitoring plots and a modified Hedley sequential fractionation method. Aggregate stability was evaluated using mean weight diameter, geometric mean diameter, the proportion of > 0.25 mm aggregates, and aggregate disintegration rate. The proportion of mechanically stable > 2 mm aggregates followed the order EFFL > SF > WO > NG, accounting for 66.2%, 58.7%, 47.4%, and 45.7%, respectively. Relative to dry sieving, wet sieving reduced the proportion of > 2 mm aggregates by 42.6%, 33.4%, 15.9%, and 29.9% in SF, EFFL, WO and NG, respectively. EFFL and SF were characterised by greater total phosphorus (TP) contents and greater accumulation of multiple phosphorus fractions in large aggregates. EFFL had the highest total nitrogen (1.96 g·kg⁻¹), TP (1.01 g·kg⁻¹), and free dithionite-extractable Fe oxide content (35.14 g·kg⁻¹), thereby increasing the proportion of large aggregates and enhancing their mechanical stability. Moreover, the reserves of each phosphorus fraction increased with increasing aggregate particle size. Across all land use types, SP was the dominant phosphorus fraction in soil aggregates, following the order of SP > moderately active phosphorus > active phosphorus, and its accumulation was significantly and positively correlated with Fed, nutrient status, pH, and aggregate stability. Structural equation modelling indicated that the various land use types enhanced TP retention by increasing soil nutrient availability and aggregate stability while simultaneously promoting SP accumulation through enhanced nutrient supply. However, increased phosphatase activity suppressed SP accumulation. Furthermore, the soil water content influenced SP accumulation and transformation by directly regulating metal oxide dynamics and indirectly affecting aggregate stability. Overall, this study provides new insights into enhancing soil phosphorus retention and use efficiency and optimising soil management in karst regions. The findings indicate that rational land use allocation, enhanced aggregate stability, and coordinated regulation of nutrient and water conditions are critical for achieving sustainable phosphorus resource management.