Rice, grown in all Brazilian regions, is particularly important in the southern region, where 60 % of the total production of this cereal is raised. The rice parboiling process is a hydrothermal treatment of partially cooking the grains in the husk, improving the nutritional value. This process results in an effluent rich in organic matter and nutrients, which can be used as organic fertilizer in agriculture. In this sense, this study aimed to evaluate the quality of a soil based on changes of some microbial properties arising from the addition of anaerobic sludge from an effluent treatment plant of parboiled rice. Two experiments were conducted. The first in a greenhouse, with corn grown on an Ultisol enriched with anaerobic sludge of parboiling rice at doses of: 2.15; 4.31; 8.62; 12.93; and 17.24 g kg-1. Soil organic carbon and nitrogen and the microbial biomass were evaluated. The second experiment was developed in the laboratory using respirometric vessels with the same soil and treatments as in the previous experiment; the microbial activity was determined by basal respiration. Soil without fertilization and NPK fertilized soil constituted control treatments. Treatments were arranged in a completely randomized design with four replications. The sludge increased soil microbial activity, and the carbon and nitrogen contents of the microbial biomass, and decreased the soil metabolic quotient. Sludge application did not affect the ratios COT/NT, CM/NM, CM/COT and NM/NT in comparison to the NPK treatment. Sludge increased the contents of soil organic carbon and caused a decrease in the levels of mineral N (NH4+ and NO3-), compared to the NPK treatment. The sludge originated from rice parboiling can be applied to soil at the recommended doses, with benefits for the microbial activity and biomass.
organic carbon; soil nitrogen; basal respiration; microbial biomass
