Nutrient availability and changes in microbial biomass of organic amendments during field incubation

Field evaluation of release and availability of nutrients and potentially t oxic elements from composts is necessary to estimate their nutrient contrib ution to crops, potential effect on soil and environmental quality. A bioso lids (BSD), a yard waste (YW), and a West Palm Beach cocompost (WPCC) were incubated under field conditions in a citrus grove on an Oldsmar fine sandy soil (sandy, siliceous, hyperthermic Alfic Arenic Haplaquods). The incubat ion columns and the soil underneath each column were sampled on 0, 240, and 360 days after incubation and analyzed for KCl extractable NH4-N and NO3-N , 0.5 M NaHCO3 extractable P, and Mehlich 3 extractable K, Fe, Mn, Zn, Cu, and microbial biomass. The total concentration and extractable proportion o f each element greatly varied among the three organic amendments. Approxima tely 34-73% of K, 1-14% of Fe, 7-68% of Zn, 7-47% of Mn, and 2-34% of Cu in the three organic amendments were extractable by the Mehlich 3 reagent at the beginning of incubation. Incubation of these amendments under field con ditions for a period of 1 yr increased the availability of N, P, K, and sev eral micronutrients including Fe, Cu, Zn, and Mn. Microbial biomass-C and - P were markedly increased during the field incubation. However, the BSD, co ntaining high total C and other nutrients, produced less microbial biomass- C than the two composts. The rapid increase in concentrations of available metals including Cu, Zn, and Mn in the BSD during the incubation may have a dverse effects on microbial biomass growth and proliferation in this compos t. A combination of BSD and YW improved conditions for the microbial biomas s growth as evidenced by the increase in microbial biomass C and P of this combination during the course of incubation.