Microbial growth responses upon rewetting soil dried for four days or one year

A pulse of respiration is induced by rewetting dry soil. Here we study the microbial responses underlying this pulse of respiration when rewetting soil dried for 4-days or 1-year. In the 4-days dried soil, respiration increased to a maximum rate immediately upon rewetting after which it decreased exponentially. In the 1-year dried soil, respiration also increased immediately, but then remained high for 16 h, after which it increased further, exponentially, with a peak rate after 20 h. The level of bacterial growth was initially lower in rewetted than in constantly moist soil, but started to increase linearly immediately upon rewetting 4-days dried soil. In 1-year dried soil, bacterial growth started only after a 16 h lag period of zero growth, and then increased exponentially to a peak after 30 h, at rates superseding those in continually moist soil. Fungal growth started to increase immediately upon rewetting, and reached the rate of the control soil after 2 days for the 4-days dried soil, and after a week for the 1-year dried soil. Thus, prolonged drying altered the pattern of bacterial and fungal growth after rewetting. Our results suggest that both fungal and bacterial growth are uncoupled from the initial respiration pulse and that growth responses and microbial C-use efficiency can be affected by prolonged drying.