Chie Hayakawa

Biodegradation of low molecular weight organic compounds and their contribution to heterotrophic soil respiration in three Japanese forest soils

Low molecular weight (LMW) organic compounds in soil solution could be important substrates for heterotrophic soil respiration. The importance of LMW organic compound mineralization in heterotrophic soil respiration needs to be confirmed for different types of soils. The concentrations of LMW organic compounds in soil solution and mineralization kinetics of 14C-radiolabelled glucose, acetate, oxalate and citrate were studied in three Japanese forest soils (Andisol, Spodosol and Inceptisol) with varying adsorption capacities. Based on those results, the fluxes of LMW organic compound mineralization and their magnitude relative to heterotrophic soil respiration were quantified. Monosaccharides and organic acids comprised on average 5.9–11.2% and 0.9–1.4% of dissolved organic carbon in soil solution, respectively. Monosaccharide mineralization make up 49–74% of heterotrophic (basal) soil respiration at the soil-profile scale, while organic acid mineralization accounts for between 5% (Andisol) and 47–58% (Spodosol and Inceptisol) of heterotrophic soil respiration. The mineralization of LMW organic compounds is a substantial fraction of heterotrophic soil respiration regardless of soil type, owing to their rapid and continuous production and consumption. The specific contribution of organic acid mineralization to heterotrophic soil respiration varies depending on soil adsorption capacities, namely iron and aluminum oxides.

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

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 14C-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 KM ), which were described by a single Michaelis-Menten equation, varied widely from 11 to 152 nmol g−1 h−1 and 198 to 1294 µmol L−1 for tropical soils, and from 182 to 400 nmol g−1 h−1 and 1277 to 3150 µmol L−1 for temperate soils, respectively. The values of V max increased with increasing microbial biomass-C in tropical and temperate soils, while the KM 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.

Fluxes of dissolved organic carbon and nitrogen throughout Andisol, Spodosol and Inceptisol profiles under forest in Japan

Leaching of dissolved organic matter (DOM) is an important process in the translocation and stabilization of organic carbon (C) and in influencing nitrogen (N) availability in forest soils. The roles of DOM in soil carbon and nitrogen cycles were evaluated by quantifying the fluxes of dissolved organic carbon (DOC) and nitrogen (DON) entering and leaving the organic (O), A and B horizons. In Spodosol and Inceptisol soils, DOC fluxes were highest in the O horizon (149 to 344 kg C ha−1 yr−1), decreasing in the A and B horizons. In Andisol soils, DOC fluxes were low throughout the profile because of low DOC production in the O horizon (53 kg C ha−1 yr−1) and the high adsorption capacity of amorphous aluminum (Al) and iron (Fe) (hydr)oxides in the mineral horizons. In Spodosol soil, DOC from the O horizon represented a large proportion of C input into the mineral soil, whereas this contribution appeared to be small in Andisol soil. The DOM was enriched in nitrogen during decomposition and humification of the foliar litter, but DON was a small proportion (5–31%) of total dissolved nitrogen (TDN) in surface soil solutions. The narrow DOC/DON and DON/TDN ratios were attributable to the low C/N ratios of the foliar litter (33–40). It was quantitatively shown that the importance of DOM in C and N cycles in forest soils varied depending on soil types and litter C/N ratio.