Bacterial activity in a forest soil after soil heating and organic amendments measured by the thymidine and leucine incorporation techniques

The effects of soil heating (200°C, 1 h) and organic amendments (straw and poultry manure) on bacterial activity in a forest soil were studied in a laboratory experiment. Measurements were made over a 15-week incubation using the thymidine (TdR) and leucine (Leu) incorporation techniques. Soil heating initially reduced TdR and Leu incorporation rates by 96–98% within 2 weeks of incubation. Thereafter the bacterial activity recovered, but by the end of the incubation the values were still lower than in the corresponding unheated soil. Reinoculation of heated soil with fresh soil improved the reestablishment of bacterial activity after a long lag, since the values were comparable to those observed for the unheated soil only after 13 weeks of incubation. The organic amendments affected bacterial activity in both unheated and heated soils. An enhanced bacterial activity was found within 2 weeks after the addition of straw or poultry manure to the unheated soils, the increase being greater and maintained longer for the latter treatment. In the heated soil no effect of straw addition was detected, while a large increase in bacterial activity following poultry manure treatment was observed after a long lag (up to 8 weeks). The presence of a toxic substance appeared to be the cause of the prolonged reduced bacterial activity in the heated soils, since an inhibitory effect of water extract from heated soil was found on TdR and Leu incorporation rates of bacteria from both unheated and heated soils. Furthermore, the inhibition was less pronounced for bacteria from the heated soil, indicating that a bacterial community tolerant to the toxic substances had developed.