Project properties
Title Long-term chemical losses of phosphorus fertilizer
Group Soil Chemistry and Chemical Soil Quality
Project type thesis
Credits 36
Supervisor(s) Hendrik Holwerda (PhD student) and Gerwin Koopmans
Examiner(s) Prof. Rob Comans
Contact info Hendrik Holwerda (hendrik.holwerda@wur.nl)
Begin date 2024/09/01
End date 2025/07/01
Description Goal: Improve our understanding of long-term chemical losses of P fertilizer

Background and why: Phosphate is one of the most important nutrients for crop growth. However, phosphate reacts strongly with various soil particles which reduces its availability for crop uptake. Presumably oxides are the most important P sorbent. The reaction of phosphate with oxides is generally considered to consist of a fast sorption reaction to the external surface of oxide particles resulting in labile sorbed P, followed by a slow sorption reaction to the internal surface of the oxide particles resulting in stable sorbed P. In case phosphate is surplus applied, the slow adsorption reaction may lag resulting in a relatively large labile poor. For situation when phosphate fertilization is withheld, the slow desorption from the stable pool may lag resulting in a relatively large stabile pool. To what degree this occurs under field conditions where P input and output is small compared to the total sorbed pool is not clear. Under field condition, equilibrium may set between P in solution <> external sorbed P <> internal sorbed P. However, this requires more study. A good understanding of the slow reaction is very important to both assess whether availability of freshly applied P fertilizer is relatively high compared to the total sorbed P pool and to assess whether availability of the total sorbed P pool is relatively low when P fertilization is withheld.

How: To study this, you have access to a unique soil sample dataset which originates from a long-term phosphate field fertilization experiment. In this field experiment two different sections are present. For one part phosphate has been yearly surplus applied for 30 to 50 years in varying rates. For the other part, phosphate has been surplus applied for 25 years but fertilization is ceased for the last 25 years. This provides the unique opportunity to compare soil samples with similar soil properties and a similar total P loading for which one received P fertilizer very recently and the other no P fertilizer for a long time. With these samples provide a unique change to study whether the ratio of external sorbed P versus internal sorbed P differs between soils with recent P application and soils with no recent P application while the total sorbed P content is similar.
To determine the amount of external P versus the internal P, you will make use of a non-destructive P desorption technique by desorbing P from soil with dialysis bags filled with a strong P sorbent. With this technique it is possible to distinguish between phosphate which is quickly desorbed to the soil solution and phosphate that desorbs only slowly to the soil solution since its release is limited by diffusion out the oxide particles. This technique is relatively complex and therefore different more commonly used soil P extractions may be tested to find out if specific wet chemical extractions can also provide a good estimation of the labile pool as measured by the desorption technique.

During your thesis you will be supervised by Hendrik Holwerda (PhD student) and Gerwin Koopmans.

Interested in this topic? Please contact Hendrik Holwerda (hendrik.holwerda@wur.nl).
Used skills Literature research, lab skills, data processing and analysis, scientific writing and reporting, presentation skills
Requirements As a requirement, we ask for 12 credits of SOC/SBL-courses in the form of at least one course from the following list: SBL-21806 Soil Quality, SOC-21306 Soil Pollution and Soil Protection, SOC-40806 Field Training Soil-Vegetation-Atmosphere Interactions, and SOC-33806 Environmental Analytical Techniques. Furthermore, you have completed at least one course from this list: SBL-35306 The Soil Carbon Dilemma, SOC-36306 Biogeochemical Cycles and Climate Change Mitigation, and SOC-34806 Applications in Soil and Water Chemistry.