Phosphorous (P) is one of the major limiting nutrients in plant production and is a finite resource. An evaluation of standard methods to assess the utilization of P from different fertilizers showed that standard soil analyses and determination of plant P content are insufficient for providing information on P availability during a growth period and does not generate knowledge about P uptake efficiency from different fertilizers or internal P utilization. Isotopes are a powerful tool for tracing nutrients in the soil-plant system. Phosphorus only has one stable isotope, and radioactive phosphorus is hampered by short half-lives and requires licensing. Oxygen has three stable isotopes (16O, 17O, 18O) and oxygen isotopes in phosphate can be used as a proxy tracer for phosphorus in the soil-plant system but remains underexplored in plants. The objective of this PhD project was to apply and investigate the validity of using oxygen isotopes in inorganic phosphate as a tracer of P in the soil-plant system under different fertilizer regimes. A uniform protocol for phosphate extraction from plant material for stable oxygen isotope analysis was developed and applied. It was observed that the original oxygen isotope composition of the P source was lost inside the xylem sap and leaves of barley plants suggesting a rapid turnover of P inside the plant imprinting oxygen from ambient water into the inorganic phosphate. There are limitations for using the approach as a proxy tracer of P in plants during a growth period, and to gain knowledge about P uptake efficiency from different fertilizers. Yet, clear deviations from theoretically calculated equilibrium values were observed and nutrient deficiencies impacted the oxygen isotope composition of inorganic phosphate in plants.

Assessment Committee

Professor Roland Bol, Institute of Bio- and Geosciences, Forschungszentrum Jülich, Germany Professor Jørgen Eriksen, Dept. of Agroecology, Aarhus University, Denmark
Associate Professor Dorette Müller-Stöver (Chair), PLEN, University of Copenhagen, DK

Supervisors

Associate Professor Kristian Holst Laursen, PLEN, University of Copenhagen, Denmark Associate Professor Jakob Magid, PLEN, University of Copenhagen, Denmark