Phosphate saturation on organic carbon in agricultural soils

Promotors: Prof. dr. ir. Steven Sleutel
Researchers: Ir. Marie Tuerlinckx

This research project investigates how long-term phosphorus (P) fertilisation affects soil organic carbon (SOC) sequestration in light-textured arable soils. While nitrogen fertilisation is well known to influence SOC stocks, the role of P has received far less attention. Research suggests that SOC stock may respond to increasing soil P availability in a bell-shaped pattern: SOC initially increases in P-deficient soils due to higher plant productivity, reaches a plateau and may decline when soil P levels become excessive. The mechanisms behind this potential decline remain poorly understood. 

The project aims to document and explain SOC losses at high soil P levels by combining large-scale field surveys, controlled experiments and modelling. 

First, historically P-loaded agricultural soils will be analysed to assess how elevated phosphate saturation degree affects SOC stocks in both topsoil and subsoil. Next, biological mechanisms will be examined, focusing on how excess P may alter root growth, rhizodeposition, microbial communities and the efficiency with which plant-derived carbon is converted into stable SOC. Stable carbon isotope techniques will be used to trace plant carbon inputs into soil.

In parallel, the project will investigate chemical mechanisms related to soil P geochemistry. Phosphate competes with organic matter for binding sites on soil minerals, potentially weakening organo-mineral associations that protect SOC from decomposition. Long-term P enrichment may therefore reduce SOC stability and enhance mineralisation.

Finally, insights from field observations and mechanistic studies will be integrated into soil carbon models to improve predictions of SOC dynamics under long-term P fertilisation.