Mineralização de carbono e nitrogênio e recuperação de nutrientes em solos com distintas texturas adubados com lodo de esgoto

Abstract

The regulation of the agricultural layout of the sewage sludge is made by Conama Resolution 375/2006, establishing that the recommendation of the dose of the residue is made from the availability of mineral nitrogen and its mineralizable organic fraction. The objective of the study was to evaluate the mineralization and nutrient recovery rates of dry and composted sewage sludge in soils with different textures. Two studies were carried out, one in the field and the other in a greenhouse. The study in field was assembled in the randomized block design (RBD), with four replications, in a subdivided plot. The plots were composed by the 3 x 3 factorial scheme. The factors consisted of soils with textures (clayey, medium and sandy) and three types of management (application of composted sewage sludge (LC), sludge stabilized at 350 ºC (LE) and without the application of the sludge (SR). And the subplots were composed by the collection times of 0, 5, 15, 35, 50, 80 and 120 days. The experimental units consisted of pits with 20 cm deep and 50 cm in diameter. Analyzes of the content of organic C (CO), mineral N, total and organic (NO), P, Ca, Mg and K were performed. In the study in laboratory, it was adopted a completely randomized design (CRD) with four replications in factorial 3 x 3 x 6, adopting the same factors of the field study. The experimental units consisted of plastic containers with a capacity of 500 cm³, containing 100 g of soil, hermetically closed. Analyzes of CO, mineral and total N, evolution of C-CO2, fractionation of humic substances and analysis of 15N e13C were performed. The obtained data were submitted to analysis of variance (p <0.05). The averages obtained for the qualitative factor were compared using the Tukey test and, for the quantitative factor, were compared through the use of regression models. The fractions of NO mineralization (FM) and CO degradation (FD) were calculated based on the adjusted first order kinetic model. In field, the FM of NO was influenced by the soil textures, being observed FM higher than 50, 60 and 75% in clayey, medium and sandy soils, respectively, for both sludge. The mineralization of the LE is more intense, being observed by the greater release of mineral N and greater degradation of the C. The FD of the CO were superior to 75 and 45% in the soils with application of the LE and LC, respectively. The application of the sludges increased P, Ca and Mg contents in soils. The availability of Ca and Mg occurred steadily throughout the monitoring, with lower levels of sandy soils. The contribution of the sludges to the supply of K in soils is insignificant. Higher availability of P was observed with addition of LC and in soils of sandy and medium texture. In laboratory, for both sludges, the FM of the NO was higher than 40%, regardless of soil texture. Higher accumulated respiration was observed in soils that received LE, with a C-CO2 evolution higher than 230 mg kg-1 in clayey and sandy texture soils and of 196 mg kg-1 in medium texture soil. Already on the soils with LC addition, respiration was below 100 mg kg-1. The FD of the CO for clayey, medium and sandy texture soils with LC addition were 12.5; 5.4 and 13.3%, respectively, being lower than the soils that received the LE of 26, 31 and 17%, respectively. The humina fraction (HU) was responsible for the majority stocking of C and N. More than 65, 68 and 50% of the total CO of the soils of clay, medium and sandy texture were found in this fraction. And more than 60% of the total N of the soil, in all treatments, were found in the HU. By the isotopic analysis of 15N and 13C, it was identified that more than 40% of the N and C from the sludge are stored in the soils at 120 days after application.