Changes in microbial biomass and P fractions in biogenic household waste compost amended with inorganic P fertilizers

The present study was conducted to evaluate the changes in microbial biomass indices (C, N, and especially P) and in P fractions in compost amended with inorganic P fertilizers. In the non-amended control, the average contents of microbial biomass C, N, and P were 1744, 193, and 63 μg g−1 compost, respectively. On average, 1.3% of total P was stored as microbial biomass P. The addition of KH2PO4 and TSP (triple super phosphate) led to immediate significant increases in microbial biomass C, N, and P. Approximately, 4.6% of the added TSP and 5.8% of the added KH2PO4 were incorporated on average into the microbial biomass throughout the incubation. Approximately, 4.7% of the 1 mg and 5.8% of the 2 mg addition rate were incorporated on average into the microbial biomass. In the amendment treatments, the average contents of microbial biomass C, N, and P declined by 44%, 64%, and 49%, respectively. Initially, the average size of the P fractions in the non-amended compost increased in the order (% of total P in brackets) resin P (0.7%) < NaOH-extractable Pi (inorganic P, 3.0%) < NaOH–Po (organic P, 6.9%) < NaHCO3–Pi (11.9%) < NaHCO3–Po (17.0%) < residual P (24.6%) < HCl–P (35.7%). Initially, the relative contributions of the P fractions in the amended compost treatments increased in the order: NaOH–Pi (1.0%) < resin P (2.5%) < NaOH–Po (4.5%) < NaHCO3–Po (4.9%) < residual P (14.8%) < HCl–P (15.2%) < NaHCO3–Pi (57.1%). At the end of the 56-day incubation, the largest and highly soluble fraction of NaHCO3-extractable Pi had decreased in place of the less soluble fractions NaOH-extractable Pi and Po, but especially HCl–P, but not in place of the insoluble fraction of residual P. The microbial biomass is able to rapidly store significant amounts of easily soluble P and to prevent it from adsorption or other fixation processes.