Monitoring early stages of P release from fertilizer pellets to bulk soil using non-invasive sampling techniques

Phosphorus (P) is a development-limiting nutrient for crops, hence global food production relies on P fertilizer application. However, P mobility in soil depends on many abiotic and biological processes, most notably its chemical interactions with the soil particles. Optimizing the timing and amount of fertilization could lead to higher production efficiencies and also reduce P runoff and subsequent contamination of water bodies. Plants have developed strategies to improve P uptake by optimizing the root system architecture and exuding organic acids for enhancing P mining locally to the root tips. However, these adaptations are mainly a response to low P availability or to already immobilized P patches in soil, and little is known about the fate of P in the early stages of fertilization.

In this framework, we developed an experimental assay for investigating P release from the fertilizer pellets, and its movement through soil using non-invasive microdialysis sampling techniques and inductively coupled plasma - mass spectrometry (ICP-MS) analytical techniques. Microdialysis allowed for time resolved in-situ samplings and the small size of the probes also allows for a fine spatial resolution.

Results showed a very rapid release of the P from the fertilizer pellet (triple super phosphate, TSP), producing a high concentration pulse that lasts a few hours. P concentrations then decrease over time until reaching steady low concentrations after 6-8 days and P replenishment from the pellet was not observed after the first pulse. The experiments showed that the speed of P movement in soil is greatly influenced by soil particle size distribution, and that gravity plays an important role in promoting quick P movement in the downward direction, while diffusion can account for P observed in the upward direction. Modelling was also applied to data fitting for quantifying trends and deriving an effective P diffusion coefficient in saturated soil.