Respiration and priming effects after fructose and alanine additions in two copper- and zinc-contaminated Australian soils

The objective of the present study was to determine whether substrate-induced priming effects in soils are sensitive to increasing levels of Cu and Zn. Soils were collected from ten plots of two Australian field experiments (Spalding and Avon) where increasing amounts of Cu or Zn had been added 2 years prior to sampling, reaching maximum values of 5,880 mg kg(-1) for Cu and 7,400 mg kg(-1) for Zn. In a 21-day incubation experiment, the effect of uniformly C-14-labeled fructose and alanine on the mineralization of the soil organic carbon (SOC) was investigated. With increasing heavy metal content, the initial peak of soil respiration after substrate addition was retarded, indicating that the microorganisms utilizing these substrates were inhibited in soils highly contaminated with heavy metals. Both substrates strongly changed the mineralization of the soil organic matter (SOM), i.e., priming effects were induced. In the soil samples with high Cu concentrations from Spalding, fructose induced a stronger additional mineralization of the SOC than in the lower contaminated samples. In the samples with the highest Zn contamination level, negative priming effects, i.e., a reduced mineralization of SOM, were observed. In contrast, heavy metal effects in the Avon soil (pH 7.6) were less pronounced since substrate mineralization and priming effects were not directly related to the increasing heavy metal content. Apart from direct toxic heavy metal effects, the tested microbial activity parameters were also indirectly affected through the toxic heavy metal effects on plant growth. At the highest heavy metal contaminations, no fresh biomass inputs occurred during the past 2 years so that microorganisms in these soils were highly substrate-limited. As a consequence, complex interactions between different levels of heavy metal contamination, the microbial activity, and the input of SOC via plant biomass have to be considered.