Microbial functional diversity can be influenced by the addition of simple organic substrates to soil

The influence of the composition of added organic C on microbial functional diversity was investigated in an arable soil. Soil was treated with single large additions of organic C compounds (asparagine, gluconic acid, glutamic acid, mannose, methyl–glucamine and urocanic acid) or repeated additions of compounds (asparagine, glucosamine, α-ketovaleric acid or urocanic acid) over 4 d. The effects on microbial functional diversity were assessed by measurement of patterns of in situ catabolic potential (ISCP) in each experiment at 1, 7 and 20 weeks, and 1, 4 and 7 d, respectively. Microbial biomass C was generally increased by the substrate additions in both experiments and remained greater than the control over 7 d in the repeated-addition experiment. After 20 weeks, however, only the microbial biomass C in the asparagine and mannose treatments remained increased in the single-addition experiment. The direction of changes in the patterns of ISCP were distinctly different between most treatments in both the single- and repeated-substrate addition experiments. Over 20 weeks, factor analysis showed that the patterns of ISCP in the glutamic acid, mannose, methyl–glucamine and urocanic acid treatments tended to converge with that in the non-amended treatment. The changes in patterns of ISCP were also different after repeated additions of different substrates. However, respiration responses to the added substrate in each of the treatments were not specifically increased relative to the respiration responses to other organic C compounds. These results support the hypothesis that addition of different types of organic C can influence patterns of ISCP. However, it is difficult to distinguish the direct effects of the added compounds on the respiration response to individual substrates.