Biodegradation kinetics of monosaccharides and their contribution to basal respiration in tropical forest soils

Abstract

Low molecular weight (LMW) organic compounds in soil solution are easily biodegradable and could fuel respiration by soil microorganisms. Our main aim was to study the mineralization kinetics of monosaccharides using 14 C-radiolabelled glucose. Based on these data and the soil solution concentrations of monosaccharides, we evaluated the contribution of monosaccharides to basal respiration for a variety of tropical forest soils. Further, the factors controlling the mineralization kinetics of monosaccharides were examined by comparing tropical and temperate forest soils. Monosaccharides comprised on average 5.2 to 47.7% of dissolved organic carbon in soil solution. Their kinetic parameters (V max and K M ), which were described by a single MichaelisMenten equation, varied widely from 11 to 152 nmol g À1 h À1 and 198 to 1294 mmol L À1 for tropical soils, and from 182 to 400 nmol g À1 h À1 and 1277 to 3150 mmol L À1 for temperate soils, respectively. The values of V max increased with increasing microbial biomass-C in tropical and temperate soils, while the K M values had no correlations with soil biological or physicochemical properties. The positive correlation between V max values and microbial biomass-C indicates that microbial biomass-C is an essential factor to regulate the V max values in tropical and temperate forest soils. The biodegradation kinetics of monosaccharides indicate that the microbial capacity of monosaccharide mineralization far exceeds its rate at soil solution concentration. Monosaccharides in soil solution are rapidly mineralized, and their mean residence times in this study were very short (0.4-1.9 h) in tropical forests. The rates of monosaccharide mineralization at actual soil solution concentrations made up 22-118% of basal respiration. Probably because of the rapid and continuous production and consumption of monosaccharides, monosaccharide mineralization is shown to be a dominant fraction of basal respiration in tropical forest soils, as well as in temperate and boreal forest soils.