Does a longer drought change how soil bacteria recover and release carbon when water returns?

Details

Rewetting dry soil can produce two patterns of bacterial growth and respiration. In type 1, bacterial growth begins increasing linearly immediately after rewetting, and respiration rates are highest immediately after rewetting. In type 2, bacterial growth begins increasing exponentially after a lag period, with a secondary rise in respiration at the start of that exponential growth increase.

The researchers had previously observed a type 1 response after rewetting soil dried for 4 days and a type 2 response in soil dried for 1 year. This study examined in detail how drought duration related to these two responses of bacterial growth and respiration.

In a controlled drying–rewetting experiment, soil was air-dried for different periods ranging from 4 days to 48 weeks. After rewetting, bacterial growth and respiration were measured repeatedly at 17 °C for one week.

Drought periods of ≤2 weeks produced a type 1 response, whereas periods of ≥4 weeks produced a type 2 response. The lag period increased with drought duration, reaching a maximum of approximately 18 hours.

Bacterial growth responses were also affected by incubation of moist soil before drying and rewetting. The longer moist soil was incubated before a 4-day drying–rewetting event, the longer the lag period became, again reaching a maximum of approximately 18 hours.

The exponential growth increase in the type 2 response coincided with a secondary rise in respiration, whose magnitude increased with drought duration. Cumulative respiration also increased with drought duration and was approximately 4 times higher after 48 weeks of drought than after 4 days.

The researchers concluded that prolonged drought changed the response type of bacterial growth and respiration after rewetting and increased the lag period, the magnitude of the secondary respiration rise, and total carbon release. However, the length of moist-soil incubation before drought modified the drought effect, suggesting that soil conditions before drying and rewetting need to be considered when evaluating microbial responses.