FIELD AND LABORATORY STUDIES ON THE RELATIONSHIPS BETWEEN RESPIRATIONAND MOISTURE FOR 2 SOILS OF CONTRASTING FERTILITY STATUS

The respiration rates of two silt loam soils, both under grazed pastur e but of widely different fertility status were compared both in the f ield over 21 months and in a 56 day laboratory experiment. The tempora l variation in soil respiration [basal and glucose amended (1 h and ov ernight) rates] was measured on samples from the field. Although the b asal respiration rates measured in the low fertility soil were as high , and often exceeded the rate measured at the high fertility sites, th e glucose-amended respiration rates (1 h and overnight) revealed furth er differences between the two soils. This may be explained in terms o f qualitative and quantitative differences in the microbial population s, using the authochthonous - zymogenous concept of microbial ecology. The low fertility soil is rich in filamentous organisms and may suppo rt a predominantly autochthonous flora. The high fertility soil, in co ntrast, seems to support a flora with a substantial zymogenous compone nt. A laboratory experiment simulated soil dry during summer months an d changes in soil respiration rates were monitored in soils during slo w drying conditions from three initially different water contents. Lin ear relationships were found for both soils between soil respiration a nd gravimetric water content. Log-linear relationships were found betw een water potential and soil respiration rate. The laboratory experime nt also quantified the rate of substrate depletion during the experime nt, at a particular water content (-0.05 MPa). The results demonstrate d a clear difference between the two soils. Soil microorganisms from t he low fertility site appeared not to run out of nutrients, despite th ere being no fresh inputs apart from mineralisation within the closed system, whereas the respiration rate of the fertile soil did decline. The significance of the laboratory and field observations on these two soils is discussed in terms of nutrient cycling in soils from under p asture and the organisms involved in these processes.