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; while organic phosphorus (OP) constitutes only a small fraction of total SOM, its long-term turnover remains poorly constrained and may strongly influence nutrient availability and long-term biogeochemical cycling. While organic carbon (OC) turnover has been extensively studied using radiocarbon (¹⁴C), OP dynamics are commonly assumed to mirror those of OC, despite evidence that phosphorylated compounds interact more strongly with soil minerals and may persist longer than non-phosphorylated compounds. Here, we use bomb-derived ¹⁴C as a tracer to investigate multi-decadal OP turnover in agricultural soils based on a technique that allows us to isolate soil OP to measure its isotopic signature [1].

We analysed archived topsoil samples (0–20 cm) collected between 1957 and 2019 from a long-term field experiment, replicated at three sites in southern Sweden. This time series spans the period of atmospheric bomb ¹⁴C enrichment caused by thermonuclear weapons testing in the late 1950s, and subsequent decline, enabling thus direct comparison of C incorporation into OC and OP pools over more than five decades. Using a recently developed extraction–precipitation approach [1], we isolated soil organic phosphorus (TPOP) and measured its Δ¹⁴C signature alongside the Δ¹⁴C signature of soil total OC.

At the beginning of the observation period, the Δ¹⁴C values of bulk soil organic carbon (TOC) were consistently lower than those of the total precipitated organic phosphorus (TPOP) fraction across all sites. Over time, Δ¹⁴C of bulk TOC increased, reflecting incorporation of bomb-derived radiocarbon, and subsequently declined following the decrease in atmospheric Δ¹⁴C. In contrast, Δ¹⁴C values of TPOP showed a slower, attenuated response compared to bulk TOC across the study period. This pattern indicates a slower incorporation of recently fixed carbon into the OP-associated pool relative to bulk soil organic carbon.

The attenuated Δ¹⁴C response of TPOP therefore suggests that OP-associated organic matter is preferentially stabilized within mineral-associated pools [2,3], leading to longer persistance compared to bulk soil organic carbon. Although TPOP accounted for only a small proportion of soil TOC (≤ 14%), its older radiocarbon signature indicates a distinct contribution to long-term SOM persistence.

Our results provide the first long-term, radiocarbon-based evidence that soil OP turns over more slowly than TOC, likely due to stronger mineral associations and reduced microbial accessibility. These findings support the view that carbon and phosphorus cycling in soils are partially decoupled at multi-decadal timescales, with OP turnover constrained not by pool size but by stabilization mechanisms.

References:

[1] Tian, Y., & Spohn, M. (2025). A method to isolate soil organic phosphorus from other soil organic matter to determine its carbon isotope ratio. Soil Biology and Biochemistry, 210, 109911.

[2] Kögel‐Knabner, I., Guggenberger, G., Kleber, M., Kandeler, E., Kalbitz, K., Scheu, S., Eusterhues, K., & Leinweber, P. (2008). Organo‐mineral associations in temperate soils: Integrating biology, mineralogy, and organic matter chemistry. Journal of Plant Nutrition and Soil Science, 171(1), 61-82.

[3] Spohn, M. (2020). Phosphorus and carbon in soil particle size fractions: A synthesis. Biogeochemistry, 147(3), 225-242.

Acknowledgement:

This research was funded by the European Research Council (ERC) (grant number 101043387).