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Keywords:

  • biomass C to organic C ratio;
  • fumigation–extraction method;
  • lag period;
  • qCO2;
  • substrate-induced respiration

Abstract

Soil microbial biomass C, the metabolic quotient ([qCO2] respiration rate to biomass ratio) and growth characteristics, such as lag period and specific respiration increment after the addition of glucose to the soil, were determined for 19 surface soils varying widely in pH and land use under different climatic conditions in Asia. The soil samples included natural undisturbed forest and grassland soils, and disturbed soils affected by cultivation or slash and burn agricultural practices. Although chloroform labile C was significantly correlated with the substrate-induced respiration (SIR) rate, the ratio of chloroform labile C to SIR was negatively correlated with soil pH. The SIR-biomass C to organic C ratio was significantly correlated with soil pH, but this correlation was not observed between pH and the biomass C to organic C ratio using the fumigation–extraction (FE) method. Similarly, the qCO2 measured using SIR-biomass C (SIR-qCO2) was negatively correlated with soil pH, whereas the qCO2 measured using FE-biomass C (FE-qCO2) was not correlated with pH. These results were in contrast to several reports on the significant correlations between soil pH and FE-biomass C to organic C ratio and FE-qCO2 using soil samples collected from a relatively narrow range of climatic conditions. Therefore, it could be concluded that soil pH can indirectly affect the FE-biomass C to organic C ratio and FE-qCO2 by affecting the quality and decomposition of litter and soil organic matter, but has a more direct effect on the SIR-biomass C to organic C ratio and SIR-qCO2 by inhibiting the mineralization of glucose in acid soils. Although the lag period and the specific respiration increment were not well correlated with any measured variables, the lag period was significantly lower in the disturbed soils than in the natural undisturbed soils. This suggests that the lag period after glucose addition could be used as a good indicator of disturbance.