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 m ean initial soil pH close to neutral (6.4, SD 0.6) were treated with differ ent amounts of H+-ions using acid water of pH 0.5 (H2SO4). The H+-input nee ded to lower the pH by one or more than three units was in the range betwee n 0.006 and 0.6 mg H+ kg(-1), designated as mild, strong or extreme acid st ress. The soil samples were incubated for a maximum of 200 days at 20 degre es C and their microbial biomass-C (C-mic), qCO(2) and pH was measured at i ntervals. In addition, the ratio of fungal:bacterial contributions to total respiration was determined at the beginning and end of the experimental pe riod. The extent of microbial biomass-C loss (32-87%) and the increase in t he qCO(2) (1.8->7 times) in comparison to acid-untreated samples followed t he 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 d ay 80 (p<0.05), and for qCO(2) at the beginning (p<0.001) and at day 80 onl y between mild and extreme acid stress (p<0.027). Over time some recovery o f 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) resembl ed those recorded for natural sites at comparable soil pH. Then was a stron g reduction in bacterial respiration following mild, strong or extreme acid treatment. A recovery here over time was only noted for mild or strong aci d 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.