Perda de solo e nutrientes em voçorocas com diferentes níveis de controle e recuperação no Médio Vale do Rio Paraíba do Sul, RJ

Abstract

As a consequence of the natural resources exploitation cycle, which started with coffee plantations in the 19th century, and the hilly landscape, Paraíba do Sul river watershed is presently under advanced degree of soil degradation, evidenced by advanced stage of erosion forms such as gullies, causing sedimentation in the Paraíba do Sul River. The objective of this study was to characterize soil attributes, landscape and climate, as well as, to evaluate the efficiency of practices of soil amelioration in stabilizing and recovering gullies in the Pinheiral municipality, Rio de Janeiro State, Brazil. Soil physical and chemical attributes, slope, and soil and nutrient loss were evaluated in areas with gullies under different stages of activity and erosion control: a) stabilized gully with physical barriers, inside and outside the gully, and planted with nodulated and mycorrhizal legume trees; b) gully stabilized, with physical barriers, and; c) gully without intervention. It was also included in the study an area of a 20 years old secondary forest. For each gully a sediment collector was installed. The areas under study presented a strongly hilled landscape, with C horizons reaching up to 15 m. Samples of soils collected inside and around de gullies were analyzed for physical and chemical properties. Bulk density, aggregates stability, soil permeability and soil fertility demonstrated to be similar in the areas of the studied gullies. The carbon and nitrogen stocks were very low at all sites when compared with values found in the literature for similar soils types. Revegetation with legume trees showed high potential in elevating soil C stocks. In relation to soil losses, 33.9, 13.2 and 18.7 Mg.ha-1 of sediments was found in the gully without intervention. In contrast there was a loss of 1.3, 2.2 and 2.3 Mg.ha-1 in the gully with physical protection, a reduction of 96, 83 and 88%, respectively. The gully stabilized with physical protection and revegetation showed practically no soil loss. Considering the total loss in the three gullies (not including catchments area) the loss reduction was of 98% in the gully with physical protection, and 99% in the gully with physical protection and vegetated, when compared to the gully without intervention. In relation to particle size the sediments tended to be sandy in the gully without intervention and the one with physical and biological measures, and silty in the gully with physical protection only, that is probably related to the effect of the physical barriers. Regarding the nutrient loss, the amount of Ca, Mg, K, P (pseudo-total) and N (total) lost in the sediments of the gully without intervention was of 1811 kg in 1145 m2 of gully area. The biological and physical measures of erosion control (gully with physical barriers and vegetation) resulted in a decreasing of nutrient losses up to 90% in the first three evaluations, and of 96% for all the rainy period, when compared to the gully without intervention, which would result in an economy of US$1,000.00, when taken the direct replacement of those nutrients. The results show that the methodology utilized was very efficient in the reduction of the soil and nutrient loss in gullies and it contributed for the increase of soil C and N.