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

Organic Complexed Super-Phosphates (CSP), is formed by a Humic Acid-calcium monophosphate complex. The aim of this study was to determine whether two CSPs, characterized by different Humic Acid 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 some soil microbiological activities. CSPs 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 a 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 phosphatase, -glucosidase and urease. The addition of CSPs to the 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 humic acid acted as trigger molecules.