Factors controlling soil respiration ( RS) are of great interest because RS plays a critical role in determining global atmospheric carbon dioxide (CO 2) concentrations. Substrate availability is one of the most important factors controlling RS. Soil microorganisms consume various substrates ranging from simple sugars supplied by aboveground photosynthesis to complex humic acids in soil organic matter; however, substrate decomposition rates depend on substrate availability. Thus, RS is partly determined by the amount and quality of available substrate. However, accurate quantification of the amount of available substrate is difficult because soil microorganisms utilize carbon (C) substrates of varying quality for RS. Water-soluble C (WSC), hot-water-soluble C (HWSC), and microbial biomass C are known as indicators of the amount of available soil C substrate. We continuously measured RS in two contrasting soils, Andosol and Fluvisol, during the cultivation of soybean and brassica crops with a 6-month fallow period between them. The total annual RS in Andosol and Fluvisol were 37623 and 40849 g CO 2-C m -2, respectively, with no significant difference between them. WSC and HWSC were measured every month during RS measurement. During the soybean growth period, RS and WSC were correlated, and soil type did not affect RS. During the fallow period, RS, HWSC and microbial biomass C in Fluvisol were higher than those in Andosol, despite the total soil C in Andosol being higher than that in Fluvisol. RS during brassica crop growth was not correlated with any of the measured substrate indicators. We therefore concluded that the relationships among the measures of available substrates and RS at the field level could provide vital information on seasonal changes in the interaction between the effects of soil type and plants on RS, thereby leading to a better understanding of belowground C dynamics.
