Bacterial and fungal growth on different plant litter in Mediterranean soils: Effects of C/N ratio and soil pH

Plant litter represents an important source of nutrients and energy for soil microorganisms, but will also selectively affect which organism group, fungi or bacteria, that will be favoured during decomposition. The balance of fungal to bacterial growth will furthermore be affected by soil chemistry like pH. A laboratory experiment was carried out using two different Mediterranean forest soils differing in pH, adding five types of litter varying in C/N ratio from 15 to 75, including the major litter type from the two soils. Growth of bacteria (using the leucine incorporation technique) and fungi (using the acetate into ergosterol incorporation technique) was then followed during 6 weeks. The balance of fungal to bacterial growth was positively affected by litter with increasing C/N ratio, while the C availability, as judged by evolved CO2, did not have any influence. Furthermore, low pH in the soil further favoured fungal growth, irrespective of the litter type. Despite differences in fungal to bacterial growth this appeared to have little influence on respiration rates from the added litter, suggesting functional redundancy. Our results highlight how both initial soil conditions (pH) and litter composition (C/N ratio) independently affects fungal and bacterial growth during decomposition.