The effects of glucose loading rates on bacterial and fungal growth in soil

Microbial activity in soil is usually limited by the availability of carbon (C). Adding an easily available C source, like glucose, has therefore been a common approach to study alleviation of resource limitations. Most such studies have relied on respiration to study microbial dynamics, with few following the explicit growth response. We determined the response in bacterial and fungal growth, as well as respiration, to additions of glucose (0.5–32 mg C g−1 soil) during up to 6 days, using leucine incorporation for bacterial growth and acetate-in-ergosterol incorporation for fungal growth. A concentration of 2 mg glucose-C g−1 soil, where the fungal contribution appeared to be small, was also studied with a high time resolution. Adding glucose resulted in an initial lag phase of stable respiration and bacterial growth. Bacterial growth was similar to the unamended control, while respiration was 8 fold higher during this period. The 14-h lag phase was followed by an exponential increase for both respiration and bacterial growth, with a similar intrinsic growth rate (μ) of around 0.25 h−1. After the exponential phase, bacterial growth decreased exponentially. The respiration initially decreased even more rapidly than bacterial growth. At concentrations exceeding 4 mg glucose-C g−1 the relative stimulation of fungal growth surpassed that of bacteria, with the highest amendment rates, 32 mg C g−1, resulting in mainly fungal growth. Lower loading rates than 4 mg glucose-C g−1 appeared to stimulate mainly bacterial growth.