Regulation of the respiratory quotient of soil microbiota by availability of nutrients.

Abstract The effects of glucose and mineral nitrogen amendment on the respiratory quotient (RQ), defined as the ratio of moles CO(2) evolved per moles of O(2) consumed, were tested on an agricultural and a forest soil (A horizon) of the Bornhöved Lake District, northern Germany. Both substrate-induced respiration rate and RQ value increased with increasing glucose concentration. Glucose plus nitrate addition gave the highest RQ value in the agricultural soil whereas addition of glucose alone gave the highest RQ value in the forest soil. Generally, glucose plus ammonium addition resulted in lower RQ values than glucose addition alone. When glucose was added to soil at field rates of C input, the RQ value showed fluctuations between 0.44 and 1.35. Low C supply caused rapid return of the initially enhanced RQ value towards those of basal respiration, that was typically <1. The experiment showed that both respiratory activity and respiratory quotient increased with increasing amount of available C. The RQ values >1 refer to both anabolic C uptake and 'unbalanced' C degradation. The microbial communities in the agricultural soil were less efficient in C use than those in the forest soil.