Enlightening the dark side of the phosphorus cycle in terrestrial ecosystems: Turnover of organic phosphorus in soils

Project Information

PHOSCYCLE

Grant agreement ID: 101043387

DOI

10.3030/101043387

EC signature date 18 July 2022

Start date 1 October 2022

End date 30 September 2027

Funded under

• European Research Council (ERC)

Total cost

€ 1 930 839,00

EU contribution

€ 1 930 839,00

Coordinated by

SVERIGES LANTBRUKSUNIVERSITET
Sweden

Periodic Reporting for period 1 - PHOSCYCLE (Enlightening the dark side of the phosphorus cycle in terrestrial ecosystems: Turnover of organic phosphorus in soils)

Reporting period: 2022-10-01 to 2025-03-31

Summary of the context and overall objectives of the project

Phosphorus (P) is a macronutrient that limits plant growth in many ecosystems. Soils contain large amounts of organic P (OP) derived from plant detritus, which becomes available to plants through OP decomposition (see conceptual figure). Despite the fact that this process is crucial for plant nutrition, since plants can only take up inorganic P, the rate at which OP is decomposed and the time OP persists in soils before being decomposed remains poorly understood. Currently, many biogeochemical models are based on the assumption that the pools of soil OP and organic carbon turn over at the same rate. However, recent findings suggest that OP persists longer in soil than organic carbon that is not phosphorylated, because OP compounds sorb more rigidly to minerals than compounds without phosphate group, which likely decreases their decomposition rate.
At present, there are no reliable methods for determining the decomposition and turnover of OP in soils, which strongly hampers our ability to implement an accurate mechanistic representation of the P cycle in Earth system models. In contrast to carbon and nitrogen, P has only one stable isotope. In addition, the non-stable phosphorus isotopes have very short half-lives, which makes them unsuitable for exploring the long-term (i.e. >1 year) dynamics of phosphorus in terrestrial ecosystems. Thus, much less is known about the P cycle than about the carbon and nitrogen cycle in terrestrial ecosystems.
This project develops a new approach to study OP decomposition and turnover in soils. The project develops isotope methods to analyze the isotope signature of carbon in OP compounds, allowing us to determine their decomposition and turnover in soils. The results of this project open up new possibilities for studying P dynamics and make a fundamental advance in our understanding of the P cycle in terrestrial ecosystems. The goal of the project is to quantify the turnover time of different OP compounds and the total soil OP pool, understand the factors that determine the turnover, and reveal how soil OP turnover affects P cycling in terrestrial ecosystems.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

The PHOSCYCLE project comprises four work packages (WPs). In WP1, we develop methods for analyzing the isotope signature of carbon in soil OP compounds. We apply these methods in WP2 to explore the persistence and turnover of OP compounds in comparison to the total organic carbon pool and non-phosphorylated compounds in a large range of different soils. In WP3, we study how OP quality and soil minerals affect the sorption and persistence of OP in soils. Finally, in WP4 we model the turnover of OP in soils and the cycling of P in terrestrial ecosystems.

In the first two years, the project has made major advances towards the goal of the project. Some of the results have already been published, for example, a review paper that summarizes the current state of the art of the scientific field of the PHOSCYLE project. The review paper is about preferential adsorption of OP and organic nitrogen compounds to minerals in soils. It synthesizes the current state of knowledge and shows that OP compounds and many organic nitrogen compounds adsorb preferentially to minerals (compared to phosphorus- and nitrogen-free organic compounds), which likely stabilizes them against microbial decomposition in soils. The review paper also formulates hypotheses for future research about how the preferential adsorption of these compounds affects their decomposition in soils. In addition, one paper about nitrogen and phosphorus interactions at ten of the long-term experimental sites whose soil OP dynamics are currently being analysed was recently published. Furthermore, one article is currently being published that explains some challenges in the determination of P fluxes based on radioisotope labeling experiments along with strategies to avoid potential pitfalls.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

In the first two years of this five-year project, we developed techniques that make it possible to determine the isotopic signature of carbon of OP in soils. The new methods solve the problem that arises from the lack of multiple stable phosphorus isotopes, by allowing us to measure the isotopic signature of carbon of OP. The isotope signature of carbon of OP can be used to estimate the turnover of OP in ecosystems. Specifically, it enables us to determine the decomposition and turnover of phytate in soils at sites that experienced shifts in vegetation types (e.g. from C3 to C4 or vice versa). Furthermore, this technique opens up new opportunities to test hypotheses about element cycling in soils. For example, the hypothesis that carbon covalently bound to phosphorus has a longer transit time in soil organic matter than carbon in phosphorus-free organic compounds. The methods developed here can also aid in identifying the sources of OP in aquatic systems through stable isotope fingerprinting.

Logo of the project PHOSCYCLE

Conceptual image: The phosphorus cycle