Bacteria constrain the fungal growth response to drying-rewetting

Bacteria and fungi are the two principal decomposer groups in soils, determining rates of biogeochemical cycling. Rewetting of dry soils induces enormous dynamics in biogeochemistry. Bacteria have been shown to exhibit large variation in growth over time upon drying-rewetting (D/RW), however, in studies to date, fungal growth has shown limited responsiveness. Here we investigated whether fungal growth responses to D/RW are constrained by competition with bacteria by using the bactericide bronopol to suppress bacterial growth during D/RW. We examined responses for two different soils, previously shown to exhibit different bacterial growth responses to D/RW. Experimental elimination of bacterial growth lead to increased fungal growth in both soils upon D/RW, indicating a competitive release of fungal growth when bacteria were suppressed. We also observed a pronounced fungal growth response to D/RW for one of the soils, which has not been previously reported. In this case, even when rewetting with water (i.e. without bacterial suppression), fungal growth increased to reach rates 10-times greater than in the moist control soil. The peak in fungal growth coincided with a secondary peak of respiration, revealing a functional importance of fungi for C-cycling during D/RW. The decline in fungal growth following this peak also coincided with the onset of exponential bacterial growth, further strengthening evidence for a negative correlation between bacteria and fungi, suggesting that competition with bacteria can constrain the fungal growth response to D/RW.