Adaptive responses of soil microbial communities under experimental acid stress in controlled laboratory studies

Soil samples from nine different beech forest sites (A(h) horizon) with a mean initial soil pH close to neutral (6.4, SD 0.6) were treated with different amounts of H+-ions using acid water of pH 0.5 (H2SO4). The H+-input needed to lower the pH by one or more than three units was in the range between 0.006 and 0.6 mg H+ kg(-1), designated as mild, strong or extreme acid stress. The soil samples were incubated for a maximum of 200 days at 20 degrees C and their microbial biomass-C (C-mic), qCO(2) and pH was measured at intervals. In addition, the ratio of fungal:bacterial contributions to total respiration was determined at the beginning and end of the experimental period. The extent of microbial biomass-C loss (32-87%) and the increase in the qCO(2) (1.8->7 times) in comparison to acid-untreated samples followed the amount of initial H+-ion input. Differences between treatments based on one-way ANOVA were significant for C-mic depression at day 8 (p<0.01) and day 80 (p<0.05), and for qCO(2) at the beginning (p<0.001) and at day 80 only between mild and extreme acid stress (p<0.027). Over time some recovery of the microbial biomass was observed with a concomitant decrease in the qCO(2), an indication of adaptation to acidic conditions by the surviving and newly formed biomass. After 80 days of incubation microbial biomass values expressed as percent microbial-C in total soil carbon (C-mic:C-org) resembled those recorded for natural sites at comparable soil pH. Then was a strong reduction in bacterial respiration following mild, strong or extreme acid treatment. A recovery here over time was only noted for mild or strong acid treatments. The results confirm that soil pH is a significant controlling parameter for microbial biomass build-up and the fungal:bacterial ratio as found previously with natural site studies. (C) 1999 Elsevier Science B.V.