Biomass and turnover of bacteria in a forest soil and a peat

Abstract

Short-term changes in numbers and biomass of bacteria, determined by direct counts, are descrived for a subarctic mire and for the humus and mineral soil layers of pine forest podsol. For biomass, monthly fluctuations for 15 months are presented. Almost as large fluctuations in bacterial numbers were found during 2 weeks as were found during the whole year. Precipitation resulted in increases in bacterial biomass even when the soil moisture content was non-limiting for bacterial growth, but these increases did not last for more than 1–2 days. Observed rapid declines in bacterial numbers were interpreted as the result of grazing by the microfuana. Changes in cell sizes and shapes after rainfall indicated that even under favourable growth conditions only 15–30% of the bacteria were active. The increases in bacterial biomass were used to calculate a minimum production. For the bacteria in the peat the production over 9 days was 40% of the mean standing crop value per day. The corresponding values for the bacteria of the humus and mineral layer over 13 days were 19 and 15%. The average generation times, estimated from increases in numbers, were 39 h for the peat bacteria and 66 and 55 h for the bacteria of the humus and mineral layers respectively. Based on the number of falls of rain a yearly bacterial production value of 210 g d.w.m −2 was calculated for the forest site. This figure is discussed in relation to the yearly energy input through primary production.