Long-term soil organic phosphorus dynamics: evidence from 14C time series

Soil organic matter (SOM) dynamics involve interactions between carbon (C) and phosphorus (P) cycling, yet the long-term dynamics of organic phosphorus (OP) in soils remains poorly understood. While radiocarbon ( 14 C) has been widely used to study soil organic carbon (OC) dynamics, the persistence of OP is commonly assumed to mirror that of OC, despite indications that phosphorylated compounds interact more strongly with minerals and may turn over more slowly. Here, we use the excess atmospheric 14 C resulting from nuclear weapons tests in the 1950s and 1960s (so called bomb-derived 14 C) to investigate multi-decadal OP dynamics in agricultural soils using a recently developed extraction–precipitation method that isolates total precipitated organic P (TPOP) for radiocarbon analysis. We studied archived topsoil samples (0–20 cm) collected at regular intervals between 1957 and 2019 at three long-term experimental sites in southern Sweden. Total organic carbon (TOC) declined strongly over 60 years, while total organic phosphorus remained stable across all sites. Across all sites, Δ 14 C values of TOC rose in the 1960s following the 14 C in the atmosphere and decreased slowly during the last decades. In contrast, the Δ 14 C of TPOP did not change significantly over time. These results provide the first field-based radiocarbon evidence that soil OP turns over more slowly than TOC, likely due to stronger associations with minerals. Our findings indicate a partial decoupling of C and P cycling and highlight OP as an intermediately stable SOM component.