Labile substrates in root exudates can affect microbial decomposition activity and soil carbon (C) and nitrogen (N) cycles, but the effect can be either positive or negative. We hypothesized that microbes in spring and autumn soils may have distinct responses to labile-substrate addition because microbial metabolisms change with season.
Forest soils sampled in October 2014 and April 2016 were incubated for 30 days. A mixture of glucose, citric acid, and glycine was added to the soils every day with the amount of C equivalent to 0.2, 0.5, 1, 2 and 5 times of the mean flux-rate of root exudates previously measured in this forest. Microbial biomass C (N) and respiration rates were measured on day 1 and day 30. Activity of C- and N-degrading enzymes was measured only on day 30.
  Although soil microbial C and N were not different between the seasons on day 0, the microbial C and N of autumn soils on day 30 increased much more than the spring ones as C doses increased. The respiration fluxes and the microbial metabolic quotient (respiration-to-biomass ratio) of autumn soils were also significantly higher than those of the spring ones. The enzyme activity of autumn soils decreased with C doses, while that of spring soils increased. The microbes of spring soils assimilated C for both biomass growth and enzyme production but the autumn ones only assimilated C to biomass. As a result, the decomposition of soil organic matter may be accelerated in spring but suppressed in late autumn by root exudates. The phenology of soil microbes should be considered in the future research.