Soil Properties and Phosphorus Isotopic Exchangeability in Cropped Temperate Soils

Abstract

Isotopically exchangeable phosphate (P) is the main source of P for most crops. The amount of P that is located on the solid phase of a soil and that can exchange with P in the soil solution can be calculated knowing the concentration of water-extractable P (C_P ) and two kinetic parameters: (i) the ratio between the total introduced radioactivity added as ³³P in a soil/water system and the radioactivity remaining after 1 min in this solution (R/r_1minute ) and (ii) the rate of disappearance of ³³P from the solution with time (n). The aim of this article was to propose statistical models describing R/r_1minute and n as a function of C_P and other soil properties. By doing so we made the hypothesis that it is possible, knowing C_P and selected soil properties, to calculate the amount of P located on the solid phase of the soil that remains isotopically exchangeable after 1 min and 60 min (P_r1minute , P_r60minutes ). This work was done in two steps. First, isotopic exchange kinetic experiments were carried on nine soil samples (set A) sampled in 1998 that had been incubated in the presence of increasing concentrations of water-soluble P. Results from these experiments allowed development of models to calculate R/r_1minute as a function of C_P , soil pH, and dithionite-extractable Fe (Fed) and n as a function of C_P . In the second step, we compared R/r_1minute , n, P_r1minute , and P_r60minutes values obtained from the isotopic exchange kinetic experiments conducted on an independent set of 27 soil samples (set B) to the values computed from our statistical models. Modeled and experimental values were highly significantly correlated to each other, showing that the three parameters C_P , soil pH, and Fed explained more than 75% of the variability of the experimental results. Experimental and modelled values of R/r_1minute and P_r1minute were not different from each other, whereas experimental values of n and P_r60minutes were statistically significantly different from the modeled values. Finally, the limits and applicability of our approach for the development of a pedotransfer function to predict P_r are discussed.

Keywords:

• Isotopically exchangeable phosphate
• pedotransfer function
• soil properties
• statistical models
• temperate soils

Acknowledgments

The authors thank Else Bünemann, Jiri Cerny, Marion Dumas, Thomas Flura, Astrid Oberson, Werner Stahel, and Grégoire Tombez (ETH Zurich) and the two anonymous reviewers for their help during the preparation of this manuscript. Fritz Birrer (Canton of Lucerne) is thanked for his help in sampling the soils of set B. The financial support of the Swiss National Foundation for Science to Paolo Demaria and the financial support of the Swiss Federal Office for the Environment for the analyses of the soils of set B are gratefully acknowledged.