Zhao,X., Zhong,X., Bao,H., Li,H., Li,G., Tuo,D., Lin,Q. and Brookes,P.C.(2007): Relating soil P concentrations at which P movement occurs to soil properties in Chinese agricultural soils. Geoderma, 142, 237-244.

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wAbstract
@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 eChange Pointf) 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 eChange Pointf 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 eChange Pointsf 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 eChange Pointsf increased with soil pH (minimum around 30 mg P kg-1 soil), but decreased above pH 6.0. The highest eChange Pointsf were found in soils of about pH 6.0 (maximum about 96, with one outlier of 156 mg P kg-1 soil). The eChange Pointsf 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; eChange Pointfx

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 eChange Pointsf to soil properties
@3.3. Relationship of soil P adsorption to soil properties
4. Discussion
@4.1. The eChange Pointf
@4.2. Factors affecting the eChange Pointsf
@4.3. Soil P adsorption saturation
5. Conclusion
Acknowledgements
References


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