Isolating the mechanisms that govern the contrasting relationships of the major decomposer groups in soil, fungi and bacteria, to pH

Background/Question/Methods The influence of pH on the relative importance of the two principal decomposer groups in soil, fungi and bacteria, was investigated along a uniform pH gradient, ranging from pH 8.3 to 4.0, within 180 m in a silty-loam soil where barley has been continuously grown for >100 years (Hoosfield acid strip, Rothamsted Research, UK). We estimated fungal growth by measuring acetate incorporation into ergosterol and bacterial growth using leucine/thymidine incorporation. The growth-based measurements revealed a five-fold decrease in bacterial growth, and a five-fold increase in fungal growth, with lower pH. Thus, at pH 4.5 fungi appeared to be more dominating, while at pH 8 the relationship reversed with bacteria dominating. Using different plant material amendments combined with the suppression of bacterial and fungal growth with specific growth inhibitors we attempted to identify the causes for the observed difference between decomposer groups in relation with pH. Results/Conclusions When barley straw or alfalfa were added in a five-day incubation experiment in laboratory microcosms, the pattern for bacterial and fungal growth originally observed was largely repeated, only at higher rates. There was a tendency for barley straw to induce more fungal growth at the low pH end than alfalfa, and, reciprocally, for alfalfa to induce more bacterial growth at the high pH end compared with barley straw. However, the added plant materials did not induce significant higher bacterial growth at the low pH end, nor fungal growth at the high pH-end. The bacterial growth was efficiently inhibited along the whole pH gradient after adding a bacterial inhibitor. As a response fungal growth was stimulated also in the high pH-end of the gradient, suggesting that the originally low fungal growth at high pH was an effect of competitive exclusion. Contrastingly, there was no indication of stimulated bacterial growth at the low pH-end when fungal growth was selectively inhibited. Consequently, our results suggest that one mechanism for the dramatic shift in the decomposer community along the pH-gradient was bacteria outcompeting fungi at the high pH-end. At the low pH-end the bacterial growth appeared to be physiologically limited by edaphic factors allowing for extensive fungal growth.