Influence of inorganic and organic fertilization on soil microbial biomass, metabolic quotient and heavy metal bioavailability

We studied the long-term effects (12 years) of municipal refuse compost: ad dition on the total organic carbon (TOC), the amount and activity of the mi crobial biomass (soil microbial biomass C, B-C and metabolic quotient qCO(2 )) and heavy metal bioavaiability in soils as compared to manuring with min eral fertilizers (NPK) and farmyard manure (FYM). In addition, we studied t he relationships between among the available fraction [Diethylenetriaminope ntacetic acid (DTPA) extractable] of heavy metals and their total content, TOC and B-C. After 12 years of repeated treatments, the TOC and B-C of cont rol and mineral fertilized plots did not differ. Soils treated with FYM and composts showed a significant increase in TOC and B-C in response to the i ncreasing amounts of organic C added. Values of the B-C/TOC ratio ranged fr om 1.4 to 2, without any significative differences among soil treatments. T he qCO(2) increased in the organic-amended soil and may have indicated micr obial stress. The total amounts of metals in treated soils were lower than the levels permitted by the European Union in agricultural soils. DTPA-extr actable metals increased in amended soils in response to organic C. A multi ple regression analysis with stepwise selection of variables was carried ou t in order to discriminate between the influence exerted on DTPA-extractabl e metals by their total content, TOC and B-C. Results showed that each meta l behaved quite differently, suggesting that different mechanisms might be involved in metal bioavailability.