Soil phosphorus continues to re-equilibrate over 60 years of forest development in the Calhoun Critical Zone Observatory in the southeastern US Piedmont

Abstract

Land use changes and reforestation impact soil phosphorus (P) distribution over extended periods. This study examines P distribution in forest development from 1957 to 2017 at the Calhoun Critical Zone Observatory in South Carolina. Tracking changes in 0–60 cm mineral soil P fractions through six samplings in eight continuously uncut plots and eight plots cut in 2007 (replanted in 2009), we hypothesized an increase in soil organic P (P_o) over time in uncut plots, at the expense of HCl-extractable P (i.e., Ca-P). This hypothesis is based on the expectation of a transfer of P from easily accessible forms, possibly from historic fertilization to organic reservoirs within the soil. Meanwhile, we expect to lose P_o in cut plots due to disturbance and residue decomposition, and increase bioavailable Mehlich-III extractable P (Mehlich-III P), resin inorganic phosphorus (P_i), and HCO₃ P_i. Results revealed a continued decrease in Mehlich-III P in uncut (38.7–15.7 µg g⁻¹) and cut plots (25.2–17.7 µg g⁻¹) within the 0–7.5 cm range. NaOH-P_o in uncut plots remained stable, while in cut plots, it increased by ∼4 µg g⁻¹ in the 0–7.5 cm soil layer. Slowly cycling P_o (NaOH-P_o) increased during forest regrowth in the cut plots, aligning with rising soil organic carbon and decreasing soil pH. Soil P changes indicate minor declines in available fractions during the initial three decades, with the HCl extractable (Ca-P) fraction experiencing the highest depletion. In the later three decades, P associated with NaOH extraction (i.e., iron [Fe] and aluminum [Al] bound P) declined, although estimated mineral soil P removals during this period were less than accumulated P in tree biomass and forest floor. The total soil P decline from 1990 to 2017 was 34 kg ha⁻¹, suggesting a growing role of P recycling from debris decomposition in supplying soil P to plants.