Effects of amendments on phosphorous status in soils with different phosphorous levels

Phosphate fertilizers are used in horticultural production to increase yields, but overuse can result in inefficient crop uptake and large loss of phosphorus (P). Soil amendments could potentially reduce the risk of P loss while maintaining high crop yield. The objective of this study was to investigate the effect of organic and inorganic amendments on P fractions in soils with different P levels (low, medium and high), and the underlying mechanisms. Five treatments were included: no amendment (CK), chemical fertilizers (CF), rice straw (FS), chicken manure compost (CM), and vermicompost (VM). Counterintuitively, the proportion of organic phosphorus (OP) tended to increase in the CF treatment, especially in soil with low P level. Results indicated that the change in the proportion of OP was mainly controlled by soil acid phosphomonoesterase, alkaline phosphomonoesterase and phosphate-solubilizing microorganisms. The proportion of available P increased in soil with low P level, but decreased in soil with high P level in the VM treatment. Non-labile P was preferentially formed in the VM treatment because of the high calcium content in vermicompost. Phosphates associated with aluminum (Al-P) were significantly higher in the CM treatment than in other treatments. This treatment also had a large presence of Pseudomonas and Streptomyces bacteria, which are capable of promoting the formation of Al-P. Our results indicated that mechanisms causing changes in soil P fractions were distinctly different among various amendments, and that vermicompost had the greatest potential to increase P supply in soil with low P level, and to decrease the risk of P loss in soil with high P level.