ABSTRACT: Soil organic matter (SOM), which influences chemical, physical and biological soil attributes, is the main form of C found in the soil which can also be used as a soil sustainability index. The aim of this study was to use the quantity and quality of SOM as an indicator to determine the sustainability of different land uses (native vegetation, secondary vegetation, fruit orchards, horticultural areas, degraded pasture, improved pasture, and fields with annual crops) in the eastern Amazon. Improved pasture had higher soil C stock than the other land uses and was similar to the native vegetation, and also presented the highest quantity of C in a stable form in the soil (fraction < 53 μm). According to the C management index, improved pasture is the most similar in use to native vegetation. Changes in land use reduced the soil microbial C content, although the more conservationist systems (fruit orchards, secondary vegetation, and improved pasture) had contents similar to those of the native vegetation. The highest soil microbial quotients were found in fruit orchards and horticultural areas. Well-managed pastures were effective in accumulating C as stable forms in the soil, which demonstrates the sustainability of this land use in the region studied.

ABSTRACT Viticulture is an activity of great social and economic importance in the lower-middle region of the São Francisco River valley in northeastern Brazil. In this region, the fertility of soils under vineyards is generally poor. To assess the effects of organic and nitrogen fertilization on chemical properties and nitrate concentrations in an Argissolo Vermelho-Amarelo (Typic Plinthustalf), a field experiment was carried out in Petrolina, Pernambuco, on Syrah grapevines. Treatments consisted of two rates of organic fertilizer (0 and 30 m3 ha-1) and five N rates (0, 10, 20, 40, and 80 kg ha-1), in a randomized block design arranged in split plots, with five replications. The organic fertilizer levels represented the main plots and the N levels, the subplots. The source of N was urea and the source of organic fertilizer was goat manure. Irrigation was applied through a drip system and N by fertigation. At the end of the third growing season, soil chemical properties were determined and nitrate concentration in the soil solution (extracted by porous cups) was determined. Organic fertilization increased organic matter, pH, EC, P, K, Ca, Mg, Mn, sum of bases, base saturation, and CEC, but decreased exchangeable Cu concentration in the soil by complexation of Cu in the organic matter. Organic fertilization raised the nitrate concentration in the 0.20-0.40 m soil layer, making it leachable. Nitrate concentration in the soil increased as N rates increased, up to more than 300 mg kg-1 in soil and nearly 800 mg L-1 in the soil solution, becoming prone to leaching losses.