Decoupling of Cellulose Decomposition and Glucose Mineralization in Volcanic Soils

Abstract

Volcanic soils (ando soils) have paradoxical relationship between high microbial activities and high stocks of soil organic matter (SOM). The high amounts of SOM are hypothesized to fuel microbial biomass and increase decomposition by supplying energy-rich substrates, which challenge the high stability of the SOM. To analyze the effects of microbial biomass on the decomposition of energy-rich substrates, we measured the decomposition rates of cellulose filter papers and the mineralization rates of 14 C-labelled glucose in five volcanic soils (four forest soils and one cropland soil) in Japan. The responses differed among both soils and substrates (glucose/cellulose). Cellulose decomposition rates increased with increasing soil temperatures but tended to decline with decreasing pH. Glucose mineralization increased with increasing microbial biomass, whereas cellulose decomposition decreased as fungal dominance increased in the acidic forest soils. The decoupling of cellulose decomposition and glucose mineralization in the acidic forest soils suggests that the depolymerization of energy-rich polymers is a rate-limiting step in the production of soluble, energy-rich substrates that drive the decomposition of native SOM. High microbial biomass, when dominated by slow-growing fungi, does not necessarily lead to high SOM decomposition activities in volcanic forest soils.

Keywords: Basal respiration, Cellulase, Exoenzyme, Fungi, soil organic matter