Soil Respiratory Indicators Including Carbon Isotope Characteristics in Response to Copper

Abstract

The human impact on soil functions with reference to environmental pollution and climatic changes have received great attention during the last decades. A widespread and integral environmental indicator is the soil microbial respiration and can be estimated either on the basis of carbon dioxide evolution or of oxygen uptake. In accordance with many previous studies, we found that soil respiration decreased in the presence of heavy metals such as Cu. The Cu spiking of an acid and unpolluted, organic-rich pine forest soil at approximately the limits for sewage sludge (100 to 200 μ g Cu g ⁻¹ soil) led to high concentrations of up to 0.35 μ g Cu per ml in the soil solution and reduced carbon dioxide evolution by 27% and the oxygen uptake by 18% during basal respiration. Thus, the respiratory quotient (RQ) which is defined as the carbon dioxide evolution related to oxygen uptake was reduced with increasing Cu levels. The Cu effect was also detected after the addition of glucose as a readily available substrate. In the soil investigated, the carbon isotope characteristic of the ¹³ C-CO ₂ evolved did not give any indication of specific carbon use and fractionation processes during basal metabolism as indicated by enrichment or depletion of ¹³ C values. However, carbon isotope characteristic of the ¹³ C-CO ₂ evolved showed that glucose addition stimulated mineralization of the soil organic matter (SOM) which derived from either increased biomass turnover or simulated dead SOM degradation and is defined as priming effect. The results showed that (1) the evaluation of heavy metal effect on soil respiratory indicators and the transformation of active SOM are method-dependent and (2) under aerobic conditions, soil microbial communities consumed relatively more oxygen in the presence of Cu than in its absence in this acid, unpolluted and organic rich soil under pine forest.

Keywords:

• carbon dioxide evolution
• copper
• heavy metals
• humus degradation
• microbial indicator
• oxygen uptake
• priming effect
• respiratory quotient
• soil respiration

Acknowledgments

The authors are grateful for the excellent contribution of Julia Kleindienst and Martin Lange from the Steenbeck Gymnasium in Cottbus and Dr. Yakov Kuzyakov from the Bayreuth Center of Ecology and Environmental Research for helpful discussion. This investigation was in part financially supported by the Deutsche Forschungsgemeinschaft (DFG) project no. 436 RUS 17/41/06.