Fracionamento físico e caracterização da matéria orgânica do solo sob diferentes coberturas vegetais

Abstract

The soil organic matter determines the main attributes that define soil quality, and it is directly affected by the soil management practices. The objective of this study was to amplify the understanding of the influence of soil tillage systems in the stock and quality of soil organic matter (SOM). It is believed that different soil tillage systems and soil coverage act differently on the distribution of SOM pools. The study reported on Chapter I was conducted in a long-term experiment (13 years) in LAGRISA sugar-cane processing plant, in Linhares municipality (ES), to evaluate the effect of sugarcane harvesting systems on SOM s stock and quality. The treatments evaluated were: a) with pre-harvest burning (burnt cane), and b) without pre-harvest burning, with the maintenance of the residue as soil coverage (green cane). The management of the crop as green cane, with residue conservation, contributed with higher C and N stocks and SOM fractions; thus it being more suitable for the soil and climate conditions studied. Light fractions infrared spectrums were very similar with others found in the literature, suggesting that, the quality of the material was very similar, for different environments, but the quant ity of the organic compounds in the light fraction of SOM would, probably, be the difference. The soil management systems influenced more in the quality of SOM than in SOM stocks. The objective of Chapter 2 study was to standardize the methodology used in the densimetric fractionating of SOM, related to the NaI density best suited for extracting the light fraction of SOM. It was tested by using carbon isotopes and analyzing the chemical composition of the fractions extracted. Soil samples used in the densimetric fractionating were from an experimental site located in CEPLAC, Itabela municipality (BA). The experiment was installed in 1988, in a long term system of pasture management study. Soil samples were collected at the 0-5 and 5-10 cm depth, in a Red and Yellow Ultisol, under brachiaria (Brachiaria brizantha) and elephant grass (Pennisetum purpureum) pastures. For this study, it was also used soil samples taken previously to the pasture experiment implantation, when the area had coverage of secondary Atlantic Forest. Besides that, soil samples from pastures with 6 and 8 years after implantation were used. The experimental design of the pasture plots was of randomized block with 4 repetitions in each treatment. Before the NaI density testing, it was necessary to calibrate the ultrasound equipment potency, as well as to identify the energy level necessary to break down the soil macroaggregates. For the Red Yellow Ultisol evaluated, the adequate energy to disperse the soil aggregates was 600 J ml-1. The separation using different densities of NaI solution was a significant factor in the amount, in the organic carbon content and in the data of d13C in the free light fraction and intra-aggregate light fraction of organic matter, extracted in all soil coverage.