Soil microbial biomass and organic C in a gradient of zinc concentrations in soils around a mine spoil tip

Soil microbial biomass carbon (biomass C) and its activity were measured in soil from Gabuzkoa (Spain) contaminated with heavy metals from a mine spoil tip. Soil was collected along a natural gradient of heavy metal contamination characterized by different organic C contents, clay contents, cultivation and topography. Biomass C and ninhydrin-N were measured by fumigation–extraction (FE), substrate induced respiration (SIR) and by soil ATP content. Microbial activity was measured by CO2 production and the arginine ammonification rate. The maximum soil concentration of zinc (about 6500 μg Zn g−1 soil) was up to 27 times current European Union (EU) limits for agricultural soils. Microbial biomass, arginine ammonification rate and biomass C as a percentage of soil organic C decreased with increasing soil Zn concentration, while CO2 production and specific respiration rate increased. Soil biomass C was negatively (% variance accounted for=45) and CO2 evolution positively (% variance accounted for=72) significantly correlated with Zn concentration, in exponential relationships. Both biomass specific respiration rate (% variance accounted for=83) and biomass as percentage of soil organic C (% variance accounted for=76) were more significantly correlated to soil Zn concentration than was biomass alone. It was concluded that both biomass and activity measurements could serve as indicators of environmental stress due to metals in non-experimental sites. Additionally, these `linkedʹ measurements generally provided more sensitive indicators of environmental stress by heavy metals than either biomass C or CO2 evolution alone.