Degradation of vinasse in soil under different humidity levels: CO2 liberation, microbial biomass formation and immobilization of added nitrogen

Degradation of vinasse added to a sandy Red-Yellow Latosol at the rate of 200m3/ha and kept at 40,60 and 80% of the holding capacity (w.h.c.), was studied and compared for liberation of CO2, formation of microbial biomass and immobilization of nitrogen added. The CO2 liberated was evaluated by NaOH retention followed by titration with HCl. The microbial biomass was determined by using gamma radiation as biocide. Nitrogen immobilization was determined using the Kjeldahl method and 15N enrichment according to Rittemberg's method. Soil moisture, which affected the oxygen level of the soil, had a significant influence in CO2 liberation, formation of microbial biomassa and nitrogen immobilization. Samples kept under drier conditions (40% w.h.c.) showed initially greater Co2 liberation. However, at the end of 3 month incubation period, total carbon evolved was similar at all misture levels used, with an average of 3805μg C/g soil. The microbial biomass showed greater formation for the drier samples (40% w.h.c.), reaching a maximum of 519μg C/g soil. Immobilization of the N added showed an increasing initial rate, which was greater with dryness of the soil, followed by stabilization. Nevertheless, at the end of 3 month incubation period, the percentages of immobilization were similar and about 40% of total 15N irrespective of the soil moisture content. Therefore, the increasing rate of carbon assimilation was not totally acompanied by an increasing immobilization for the N added. The greatest intensity was reached by CO2 liberation in residue degradation, 2/3 of the carbon having evolved to CO2 and than 1/3 having been immobilized by the microbial biomass. (author)