『Abstract
Phosphorus (P) gradually accumulates in surface soils if there
is a continuous input of inorganic P fertilizers, manures and
composts. In turn, P is lost from soil through runoff and leaching,
which lead to increased P concentrations in surface and ground
waters. Previous work showed that, in Northern European agricultural
soils, below a certain Olsen P concentration (P extractable in
0.5 M NaHCO3, pH 8.5), there was little or
no P leaching or 0.01 M extractable-CaCl2 soil P (a surrogate
for P leaching). however, at above this soil P concentration (termed
the ‘Change Point’) there was a linear relationship between P
loss and soil Olsen P concentration. The aim of the work described
here was to attempt to characterize the ‘Change Point’ in a range
of Chinese soils, to determine the maximum soil P concentrations
that should be permitted before P leaching would occur. Accordingly,
23 soils were sampled from arable land in 13 Chinese provinces.
Organic carbon, pH, carbon exchange capacity (CEC),＜0.01 mm and
＜0.002 mm clay particle size fractions, exchangeable Ca and Mg,
and oxalate-extractable Al and Fe were measured in air-dried and
sieved (＜2 mm) soils. The maximum P adsorption capacity (Qm) and
P adsorption affinity (K) were calculated from the Langmiur equation.
Following the addition of increasing P concentrations (ranging
from 0 to 400 mg P kg-1 soil as KH2PO4), the soils were adjusted to 50％ water holding
capacity (WHC) and then incubated at 25℃ for 4 days. After three
cycles of air-drying, incubation and rewetting, the ‘Change Points’
ranged from about 30 to 160 mg P kg-1 soil, corresponding
to 0.02-0.75 mg CaCl2-P l-1. Below
pH 6.0, the ‘Change Points’ increased with soil pH (minimum around
30 mg P kg-1 soil), but decreased above pH 6.0. The
highest ‘Change Points’ were found in soils of about pH 6.0 (maximum
about 96, with one outlier of 156 mg P kg-1 soil).
The ‘Change Points’ were positively correlated with soil organic
carbon concentration (R2=0.50) and amorphous Fe (R2=0.46).
Correlations were closer in the soils below pH 6.0 than in the
soils above pH 6.0.
Keywords: Olsen P; CaCl2-P; ‘Change Point’』
1. Introduction
2. Materials and methods
2.1. Soils
2.2. Soil analyses
2.3. Preparing gradients of soil P
2.4. soil P adsorption isotherm
3. Results
3.1. Relationship between soil Olsen P and CaCl2-P
3.2. Relationship of ‘Change Points’ to soil properties
3.3. Relationship of soil P adsorption to soil properties
4. Discussion
4.1. The ‘Change Point’
4.2. Factors affecting the ‘Change Points’
4.3. Soil P adsorption saturation
5. Conclusion
Acknowledgements
References