Nine years of grazing and fertilization shape dynamics of soil phosphorus fractions in Karst pasture ecosystems

Understanding soil phosphorus (P) cycling is critical for sustaining grassland productivity and soil health, especially in P-limited karst ecosystems. This study evaluated the long-term effects of three pasture management strategies — grazed-abandoned pasture (three years grazing followed by six years abandonment), continuous grazing without fertilization, and grazing with NPK fertilization—compared to unused land, in a subtropical karst region of Guizhou, China. Using a randomized complete block design established in 2012 (n = 4), soil samples were collected across five depths (0–45 cm) in 2012, 2015, and 2021 to assess changes in P fractions and related soil properties. Grazed-abandoned pasture had higher C/P and N/P ratios but lower soil organic matter than other treatments in the 0–10 cm soil. The effect of fertilized-grazed pasture on bioavailable P, active Po, and secondary mineral P shows an interannual cumulative effect; these effects shifted down the soil profile during the experiment. Grazed-abandoned pasture reduced total soil P and shifted P composition by increasing resin-extractable P at the expense of mineral and active organic P, maintaining available P. High acid and alkaline phosphatase activities in grazed-abandoned pasture supported organic P conversion and microbial biomass P. Grazed pasture did not alter the size or composition of P fractions but had lower MBP due to slower microbial P turnover. Fertilized-grazed pasture increased mineral and occluded P fractions (10,7–21 % of TP) and primarily supplied plants through NaHCO3-Pi and NaOH-Pi, with 1 M HCl-Pi. Thus, fertilization combined with grazing proved most effective in enhancing soil P availability and supporting long-term soil fertility, while abandonment may reduce P sustainability in karst grasslands. These findings provide important insights for optimizing pasture management in fragile subtropical ecosystems.