Mechanisms of the Priming Effect in a Savannah Soil Amended with Cellulose

Abstract

Two mechanisms have been hypothesized to explain the priming effect (PE), that is, the acceleration of soil C decomposition by fresh C input to soil. First, extracellular enzymes that are produced to decompose fresh C by fresh C specialized microbes may be partly efficient in degrading soil C. Second, depending on the competition with fresh C specialized microbes, part of the fresh C may be absorbed by soil C decomposing microbes. This absorption increases the populations of soil C decomposing microbe and hence the decomposition rate of soil C. The PE was quantified in a savannah soil amended with 13C-labeled cellulose at a rate of 495 mg C kg−1 Cellulase was applied to the soil at a rate of 30 000 units kg−1 soil to quantify the contribution of cellulase to the PE of cellulose. The rate of soil C decomposition increased by 55% with cellulose addition leading to PE of 234 mg C kg−1 Cellulase released 32 mg C-glucose from soil cellulose, representing only 14% of the PE. This indicated that the decomposition of soil C required the production of specific enzymes, and that the PE resulted from the stimulation of microbes able to provide soil C decomposing enzymes. Our results also showed that cellulose stimulated at least two types of microbes: soil C decomposing microbes that may also use cellulose and cellulose specialized microbes that exclusively decompose cellulose. These results indicate that the PE depends on microbial competition. We estimated that, following cellulose addition, soil humus stock was depleted by 174 mg C kg−1