Respiratory responses of soil micro-organisms to simple and complex organic substrates

We have measured the rates of respiration over a period of 13 days (304.5 h) at 90-min intervals for two contrasting soils amended with nothing, D-glucose, L-alanine, albumin, starch, D-glucose plus L-alanine, D-glucose plus albumin, L-alanine plus starch or albumin plus starch at 2 mg each individual substrate per gram soil. These treatments represent simple and complex model substrate additions with widely different C:N ratios and enable investigation of the interaction between substrates as they induce microbial respiratory responses. When added singly, the times taken for the maximum respiratory response to occur were less than 15 h for D-glucose, approximately 50 h for L-alanine and approximately 120 h for albumin. Addition of starch alone did not lead to a respiratory response within 304.5 h (13 days). The peak respiratory response from soils amended with L-alanine plus D-glucose was advanced by between 6 and 13 h depending on the soil compared with either substrate added singly. The patterns of respiratory response from soil amended with D-glucose plus albumin were similar to that predicted by the summed data, implying no interaction. The addition of L-alanine plus starch and of albumin plus starch led to increased respiratory responses, implying that the nitrogenous substrate facilitated the utilisation of starch by the soil micro-organisms.