Bacterial growth and respiration responses upon rewetting dry forest soils: Impact of drought-legacy

Longer periods of drought and droughts of higher intensity are expected to become increasingly frequent with future climate change. This has implications for the microbially mediated turnover of soil organic matter (SOM), which will feedback to the global C cycle. In this study, we addressed the microbial dynamics underlying the pulse of respiration following rewetting of dry soil, and how the drought-legacy of the soil modulated this response. We studied the microbial dynamics upon rewetting of dry soils from a field-experiment in a temperate forest soil exposed to two seasons of experimental summer-drought, or ambient conditions, by rewetting air-dried soil samples, and monitoring the respiration and bacterial growth responses. The respiratory responses in drought-exposed soils were slower and reached lower rates than control soils, translating to less C mineralised one week after rewetting. While the bacterial growth in drought-exposed soil also was slower, this was only a delayed response, and no differences in cumulative bacterial growth one week after rewetting could be established between drought-exposed and control soils. The pulse in respiration and microbial growth following the rewetting appeared to be due to facilitated microbial C availability caused by physical perturbation of the soil induced by the rewetting event. Reduced C input by trees during drought probably contributed to differences between drought-treated and control soils. Our results indicate that a history of drought increases the microbial C-use efficiency during a rewetting, suggesting a negative feedback to climate warming.