Effect of organic-complexed superphosphates on microbial biomass and microbial activity of soil

Organic complexed super-phosphates (CSPs) are formed by the complexation of humic acid (HA) with calcium monophosphate. The aim of this study was to determine whether two CSPs, characterized by different HA concentrations, added to a calcareous soil at an agronomic dose, were able to maintain the phosphorus (P) in a soluble form longer than the superphosphate fertilizer. Another important goal was to verify if CSP could positively influence soil microbial biomass and soil microbiological activities. Organic complexed super-phosphates were capable of keeping a large portion of P in a soluble form under different soil water conditions. In particular, the CSP with the highest organic C content was the most effective product, capable of maintaining, in an available form, the 73 % of the initially added P at the end of the experiment. In addition, it was the most effective in increasing C-CO2 soil emission, microbial biomass carbon (C) and nitrogen (N), fluoresceine diacetate hydrolysis and activities of alkaline phosphomonoesterase, beta-glucosidase and urease. The addition of CSPs to soil probably produced a priming effect, increasing several times C-CO2 release by the treated soil. The significant correlation (p < 0.05) between C-CO2 emission and the amount of C added to soil by CSP suggests that the added HA acted as trigger molecules.