Relating soil P concentrations at which P movement occurs to soil properties in Chinese agricultural soils

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, cation 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 °C 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.