[Dynamics of respiration and the microbial biomass in soil supplemented with straw from corn and soybeans]

Soil organic matter level has an important influence on land productivity. This level depends upon the efficiency in partitioning carbon from plant debris into CO2 and humified substances by microorganisms. The C/N ratio of the residues is one of the factors that affects this process. To study its influence on the growth and activity of the microflora, a soil from the corn region of Buenos Aires was supplemented with different amounts of straw from corn and soybean, and respiration and microbial biomass evolution were followed during four months. Biological activity of supplemented soils overcame control respiration during the first days of incubation, and intense cellular reproduction took place. Later, respiration decreased but biomass remained higher than in unamended soil until the end of the trial. When more carbonaceus material was added the microbial reproduction was faster, while the growth yield and the apparent residual carbon were lower; the C/N ratio of the biomass remained constant with all the treatments. The different nitrogen enrichment of the straw had no effect on the biological properties determined. However, soil mineral nitrogen content decreased in supplemented soils. The decrease of assimilatable nitrogen was stronger when more substrate was incorporated and when the C/N ratio was wider. These results suggest that soil supplies the necessary nitrogen for microbial growth and the straw quality has no effect on the activity and growth of microorganisms.