Microbial biomass and enzyme activities: Production and persistence

The production and persistence of microbial biomass and also urease, phosphate and casein-hydrolysing activities were investigated when either glucose or ryegrass were added, as energy sources, with ¹⁵NO₃^? to a clay-loam soil. Both direct count and fumigation methods were used to determine soil microbial biomass. Microbial biomass and enzyme activities increased after the addition of energy sources. Increases in phosphatase and urease activities coincided mainly with increases in bacterial biomass and with the rapid immobilization of labeled N. Conversely, the increase in casein-hydrolysing activities preceded the phase of net mineralization that occurred during the later period of incubation.Although microbial biomass and the biochemical activities tested increased in the soils treated with energy supplies, they eventually decreased to the level of the control soil. Even the increases in biomass and enzyme activities present at zero time, as a result of the addition to the soil of exogenous microorganisms and enzymes with the ryegrass, were not maintained after extensive incubation. The influence of homeostatie mechanisms which tend to maintain a stable biological composition in the soil microbial population is discussed. A possible relationship between the available “active” or “biological” space, total microbial biomass and total enzyme activity in soil is suggested.