Organic fraction of biosolids and effect of the carbon stock and quality of organic material of a latosol cultivated with Eucalyptus

Abstract

The agricultural use of biosolid has been recommended as a practice for the maintenance or increase in the levels of edaphic organic matter (OM) and to obtain all the benefits associated with OM. However, little is known about the quality of the organic fraction of the biosolids, the dynamic of carbon (C) in soils treated with the residue and real contribution of the organic fraction of the biosolids in the sequestration of C and capacity for cationic exchange (CCE) in treated areas. The present research plan is proposed with the general objective of studying the degradation of biosolids after application in the soil, correlating it with the initial chemical composition of the organic fraction of the residues and using the carbon 13C isotope, to determine the effect of the biosolids in the degradation of the original OM in a latosol cultivated with eucalyptus and treated with doses of a biosolid. To this end, it is necessary to reconcile experiments under controlled laboratory conditions with field evaluation (case study). In controlled conditions we will determine the rates of degradation of five biosolids after mixture with samples of a red oxisol of clayish texture. The incubation time will be 70 days. The results of accumulated CO2 released, discounted from the control, will be used in adjustments to mono, bi and triphasic equations of first-order kinetics, so as to obtain values of K (speed of degradation constant) and degradation half-life. The organic fraction of biosolids will be characterized by means of the determination of the total levels of OM, MO, C, N, P, C soluble in water, carbohydrates, proteins, lipids, cellulose, lignin, tannins and phenols; levels of C, N and P in organic compounds; and C, N and P in inorganic compounds. These results will be correlated with the rates of biosolids after 70 days of incubation and, if possible, with the rates in each phase of degradation (bi and tri-phasic equations). Using the difference between the values of natural abundance of the stable isotope of 13C in the oxisol and in three of the biosolids, we will evaluate the real degradation of the biosolids in the soil, measured periodically during the 70 days of incubation, the relative percentage of C deriving from the residues and original C of the soil, in the incubated soil samples and in the respirated CO2. This approach will permit the evaluation of the conventional respirometric method (capture and determination of CO2) with regard to the possibility of over or under estimation of the rates of degradation of biosolids, as well as making it possible to determine the effect of biosolids in the degradation of the original C in the soil (priming effect). In the field, we will determine the levels and stocks of C and N, up to a depth of 60 cm, in a red-yellow oxisol of low fertility, cultivated with Eucalyptus grandis and treated with doses of an alkaline biosolid. We will also evaluate possible alterations in the quality of the soil’s OM as a result of the application of the residue and, in this case, the indicator variables will be: 1) total levels of carbohydrates, proteins, lipids, cellulose, hemicellulose and lignin; 2) concentrations of carbon in three pools, determined by reason of different degrees of oxidation; and 3) potential values of CCE at pH 7.0 and CCE effective on the natural pH. It should be noted that the study of the field case is complementary to a multidisciplinary project which began in 1998, the object of which involves the evaluation of silvicultural, agronomical, environmental and economic aspects, related to the use of biosolids in fastgrowth forest plantations. (AU)