SUBSTRATE-INDUCED RESPIRATION IN FOREST AND ARABLE SOILS MEASURED BY O-2-MICROCOMPENSATION - MOISTURE CONDITIONS AND RESPIRATORY QUOTIENT

We investigated if the substrate-induced respiration (SIR) measured by an O-2-microcompensation apparatus differs when glucose is added as s olution or as powder. Results from 15 different soils (5 forest soils and 10 arable soils) indicate that SIR measurements of soils amended w ith glucose as solution are equivalent to those of soils amended with glucose as powder. In a second experiment the respiratory quotient (RQ ) of microorganisms of the 15 soils, unamended and amended with glucos e, was measured with and without pre-incubation with alkali. Pre-incub ation significantly reduced the RQ of unamended and amended soils, sug gesting that CO2 from carbonates contributed to the amount of CO2 evol ved in samples without pre-incubation. In glucose amended forest soils the RQ was 1.00, indicating that aerobic microorganisms metabolized t he added glucose. In contrast, in glucose amended arable soils the RQ was 1.25, suggesting that the glucose added was also used by anaerobic microorganisms. In arable soils without glucose addition the RQ was 0 .72, indicating microbial use of materials low in oxygen (fats, protei ns). In contrast, in forest soils without glucose addition the RQ was 1.19, indicating some activity of anaerobic microorganisms in these so ils. It is concluded that caution is needed when comparing data on bas al respiration and substrate-induced respiration based on measurement of oxygen consumption or carbon dioxide production.