Impact of manure application on forms and quantities of phosphorus in a Chinese Cambisol under different land use

It is critical to understand the effect of manure application on the availability of phosphorus (P) and the potential environmental contamination by runoff and leaching. However, previous studies generally focused on cultivated soil layer in single cropping systems. The aim of this study was to evaluate the effect of manure application on soil P forms and quantities to the 200 cm depth in a Chinese alkaline Cambisol in different cropping systems and the potential environmental implications. The sampling site, Shunyi District, is located in the peri-urban area of Beijing in the North China Plain, where large quantities of manure generated from intensive animal operations have been applied to agricultural fields. A field survey was carried out before sampling to identify soil sampling sites with long-term manure application and an adjacent area receiving no manure used for the same crop production. Soil samples from three cropping systems (vegetables, cereals, and trees) were vertically collected to a depth of 200 cm with the following depth increments: 0-20, 20-60, 60-90, 90-120, 120-160 and 160-200 cm. Soil samples were analyzed for plant-available P (Olsen P) and various P fractions by sequential P fractionation. Degree of P saturation (DPS) was also determined. Soil calcium bound P was the most abundant P fraction, followed by the residual P. Organic P only accounted for less than 5 % of total P in most of the soils. Manure application increased the levels of inorganic P (Pi), with higher proportions of Pi in labile forms than stable forms. After manure application for 8-15 year, available P (Olsen P) and DPS values of the 0-20 cm layer in all sites exceeded the threshold for Olsen P (60 mg kg(-1)) and DPS (30 %) and the risk of P loss by runoff is expected to significantly increase. The DPS values were generally lower than 30 % below 20 cm, indicating a minimal risk of P loss via leaching from deeper soil. The results indicated that in typical peri-urban areas of the North China Plain, the on-going practice of manure application not only increased the size of each of the labile and non-labile P pools, but also caused a shift in the relative sizes of the different pools, regardless of the cropping systems. However, contrary to what was expected, soil P loss through surface runoff would be a greater concern than leaching following long-term manure amendment.