The impact of organic and inorganic fertilizers on availability and speciation of phosphorus and heavy metals in calcareous soils

Application of organic and inorganic fertilizers to agricultural soils could affect the status of phosphorus (P) and heavy metals (HMs). To examine the availability of P and HMs in soils, ten agricultural soils were treated with monopotassium phosphate (KH2PO4) or poultry manure at a rate of 200 mg P kg−1. Different extractants were used to extract various forms of P and HMs from different treatments. The various P and HMs species and saturation indices were also studied using the Visual MINTEQ software. The highest percentage of P extractability was observed for KH2PO4-treated soils with 54.6, 7.6, and 2.4% for Olsen, water-extractable P, and calcium chloride-extractable P extractants, respectively. The application of fertilizers increased the percentage of degree of P saturation (DPS) in treated soils. The mean DPS of poultry manure-treated soils was 70.7%, which is 46.7% higher than the threshold (24%), and the mean DPS of KH2PO4-treated soils was 109.3%, which is 85.3% higher than the threshold. The results from correlation analysis revealed that soil texture, pH, and organic matter are generally important factors affecting HMs availability in soils. The Cd2+ was the highest Cd species in all three treatments. The application of poultry manure or KH2PO4 resulted in a decrease of Cd2+ species and an increase of CdHPO4 species. The findings indicated that increasing the P extractability by applying poultry manure or KH2PO4 at the rate of 200 mg P kg−1 increased the risk of P leaching; however, the application of poultry manure or KH2PO4 did not significantly affect the availability of Cd, Cu, Fe, Mn, Ni, and Zn in soils, with the exception of available Zn in poultry manure-treated soils, extracted by DTPA.