Bacterial control of fungal growth in soil: Assessing the fungal-bacterial niche overlap using growth measurements in a removal experiment

Background/Question/Methods It is important to know the contributions of bacteria and fungi to decomposition in soil both in connection with the food web structure and the ecosystem functioning. However, the relative importance of the two decomposer groups and the extent of competition between them are insufficiently understood. Fungal and bacterial aspects of decomposer ecology have to this date largely been based on biomass measurements, where the Selective respiratory Inhibition (SI) technique has been one of the predominant methods to separate between fungal and bacterial biomass. Using a growth based technique, we have demonstrated that the inhibitors that SI is based on do not efficiently or specifically inhibit the fungal and bacterial contributions to respiration in the detection period studied in SI. For instance, concentrations of the bacterial inhibitor streptomycin that subdued bacterial growth completely had no influence on the respiration measurement used in SI, and the fungal inhibitors captan and benomyl inhibited the non-target bacterial growth. Conclusions about bacteria and fungi in soil based on the SI technique thus are flawed. There is subsequently a need for a revaluation of the decomposer ecology, with regard to the fungal and bacterial subcomponents, using adequate techniques, which we initiated with a removal experiment.

Results/Conclusions The bacterial influence on fungal growth in soil was studied by applying three different bacterial inhibitors – bronopol, tylosin and oxytetracycline – in a series of concentrations, and comparing the resulting bacterial (using the leucine incorporation technique) and fungal (using acetate-incorporation-into-ergosterol) growth rates after 3-5 days incubation. Direct measurements of growth showed that fungi increased after adding inhibitors; the level of increase in fungal growth corresponding to that of the decrease in bacterial growth, irrespective of the inhibitor used. Fungal growth increased by up to 30 times, when bacterial growth was totally inhibited. Similar antagonistic effects of the bacteria on fungal growth were found after adding the inhibitors in unamended soil and together with additional substrate. The bacterial influence on fungal growth was corroborated both after adding different plant material and pure substrates, where additions that favoured bacterial growth tended to decrease fungal growth. In conclusion, there were strong indications that bacteria decisively control fungal growth in soil.