One of the most pronounced effects of compost application are increasing microbial activities in the soils (Giusquiani et al., 1995; Perucci, 1992). Since most of the studies describing microbial activities use enzymatic parameters, the role of the microbial biomass itself as a transitory pool for carbon metabolism remains uncertain. In soil science, the soil respiration, the amount of soil microbial biomass and its ratio to the OC content (Cmic:Corg), and the metabolic quotient (qCO2) are used as indicators to characterise soil conditions (Witter and Kanal, 1998).
When compost is applied to the soil, the basal respiration as a parameter for the actual carbon turnover by the micro-organisms increases as shown by Hadas et al. (1996) for manure compost. Such higher respiration rates in general can be assigned to an additional mineralisation of the soil organic matter (priming effect) (Senesi, 1989; Stevenson, 1994), though Sikora and Yakovchenko (1996) found no priming effect after compost application. Higher respiration rates after recent addition of compost to soils may be due to higher proportions of easily degradable compounds in the composted material as a result of the relative short decay process in the compost facilities. Therefore, the maturity of a compost may significantly affect the carbon turnover in the amended soil. Other factors controlling the carbon turnover are soil specific, like the soil texture or the soil pH. When assessing the effects of compost amendment, these soil characteristics have to be considered.
The amount of soil microbial biomass is closely correlated to the amount and availability of substrate. Therefore, a wide Cmic:Corg ratio indicates a high soil fertility (Fraser et al., 1988). The specific respiration of the microbial biomass qCO2 is used to describe the physiological condition of the microbial biomass in soils (Anderson and Domsch, 1985). The qCO2 reflects the efficiency of substrate use and is found to be higher in young microbial populations (Anderson and Domsch, 1990; Wu et al., 1993). Also stress conditions, such as low pH or high levels of contaminants may result in elevated qCO2 (Anderson and Domsch, 1993; Jörgensen et al., 1995).
Measurement of the soil basal respiration, the microbial biomass as expressed by the Cmic:Corg ratio and the metabolic activity of the micro-organisms may also contribute to a better understanding of the carbon turnover in compost amended soils. From this point of view, we determined the carbon mineralisation, the entire soil microbial biomass and the specific respiration to soils that had been exposed to compost application. In a controlled 18- month laboratory study at 14°C, two biowaste composts (immature and mature compost) were amended to two agricultural soils, differing in texture, pH and management. Carbon mineralisation was monitored every 1 to 2 weeks, microbial biomass was determined every 2 to 6 months during the experiment.