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 degrees C, 1 h) and organic amendment s (straw and poultry manure) on bacterial activity in a forest soil we re studied in a laboratory experiment. Measurements were made over a 1 5-week incubation using the thymidine TdR) and leucine (Leu) incorpora tion techniques. Soil heating initially reduced TdR and Leu incorporat ion rates by 96-98% within 2 weeks of incubation. Thereafter the bacte rial activity recovered, but by the end of the incubation the values w ere still lower than in the corresponding unheated soil. Reinoculation of heated soil with fresh soil improved the reestablishment of bacter ial activity after a long lag, since the values were comparable to tho se observed for the unheated soil only after 13 weeks of incubation. T he 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 treatm ent. In the heated soil no effect of straw addition was detected, whil e a large increase in bacterial activity following poultry manure trea tment 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 bac terial activity in the heated soils, since an inhibitory effect of wat er extract from heated soil was found on TdR and Leu incorporation rat es of bacteria from both unheated and heated soils. Furthermore, the i nhibition was less pronounced for bacteria from the heated soil, indic ating that a bacterial community tolerant to the toxic substances had developed.