『Abstract
Inorganic P is the least mobile major nutrient in most soils
and is frequently the prime limiting factor for plant growth in
terrestrial ecosystems. In this study, the extraction of soil
inorganic P with CaCl2 (P-CaCl2)
and geochemical modelling were combined in order to unravel the
processes controlling the environmentally available P (EAP) of
a soil over a range of pH values (pH〜4-10). Mechanistic descriptions
of the adsorption of cations and anions by the soil constituents
were used (1-pK Triple Plane, ion-exchange and NICA-Donnan models).
These models are implemented into the geochemical code Visual
MINTEQ. An additive approach was used for their application to
the surface horizon of a Cambisol. The geochemical code accurately
reproduced the concentration of extracted P at the different soil
pH values (R2=0.9, RMSE=0.03 mg kg-1). Model
parameters were either directly found in the literature or estimated
by fitting published experimental results in single mineral systems.
The strong agreement between measurements and modelling results
demonstrated that adsorption processes exerted a major control
on the EAP of the soil over a large range of pH values. An influence
of the precipitation of P-containing mineral is discounted based
on thermodynamic calculations. Modelling results indicated that
the variations in P-CaCl2 with soil pH were
controlled by the deprotonation/protonation of the surface hydroxyl
groups, the distribution of P surface complexes, and the adsorption
of Ca and Cl from the electrolyte background. Iron-oxides and
gibbsite were found to be the major P-adsorbing soil constituents
at acidic and alkaline pHs, whereas P was mainly adsorbed by clay
minerals at intermediate pH values. This study demonstrates the
efficacy of geochemical modelling to understand soil processes,
and the applicability of mechanistic adsorption models to a ‘real’
soil, with its mineralogical complexity and the additional contribution
of soil organic matter.』
1. Introduction
2. Material and methods
2.1. Soil properties and experimental data
2.2. Geochemical modelling
2.3. Model parameters
2.4. Undetermined binding properties
2.5. Statistics
3. Results
3.1. Estimation of missing binding properties
3.2. Geochemical processes
3.3. Phosphorus and electrolyte adsorption mechanisms
3.4. Relationship between the adsorption of cations and anions
4. Discussion
4.1. Controlling geochemical process
4.2. Validity of estimated binding properties
4.3. Distribution of adsorbed P: mineral composition and electrolyte
influences
4.4. Influence of phosphorus surface complexes
5. Conclusions
Acknowledgements
References