Processos hidro-erosivos em distintas classes de solo sob diferentes tipos de uso em zona semiárida

Abstract

The distribution and expressive occurrence of surface erosive processes caused by hydrological dynamics in tropical semi-arid environments, may vary both spatially and temporally. This feature highlights a serious problem, especially in the development of agricultural practices, as these environments are naturally unstable, because of its natural elements (geology, geomorphology, soils, plant cover and climate) and usually have an unbalanced socioeconomic organization. There are many problems caused by the spread of erosion accelerated in these environments, such as: loss of natural soil fertility, siltation of water bodies, water supplies, decreased productivity of agricultural areas, accelerating the process of desertification, emission CO2 to atmosphere, impoverishment of rural communities, rural exodus, among other problems. Thus, the Brazilian semiarid region which is located in Paraiba (Cariri) includes unique features with regard to its physical environment (natural) and their socioeconomic organization. Therefore, the present study had as objectives: to analyze the hydro-erosive dynamics under different classes of soil (Regosol and Luvisol) and different types of use and management (palm culture and fallow system) in a semiarid zone; characterize and classify soils; evaluate runoff and erosion rates; analyze the selectivity of eroded materials; and understand the dynamics of water infiltration in soils. For this purpose, two trenches were opened for morphological description and collections of soil samples, and soon afterwards the material collected was sent to the laboratory for routine analysis (physical and chemical); 4 hydro-erosive plots were installed (GUERRA, 2005), in a proportion of 10 m2

, with reservoirs in the gutters of all plots, to collect the material carried during the rainy events, in two different types of soils: Regosol and Luvisol (significant for the region studied), under different types of use and management (palm and fallow cultivation), to measure runoff and erosion rates; the sediments collected from the hydro-erosive plots were sent to the laboratory to understand the selectivity of the materials (amount of Carbon (C), Nitrogen (N) and textural class); ring infiltrometers were also positioned in the two soil types, in order to know the speed of water infiltration at different times of the year. The results reported in the portions positioned Regosol (PNRPal and PNRPou) generated more erosion than the portions on Luvisol (PLCPal and PLCPou), which in turn, gave greater values of runoff. Plots with the fallow system (PNRPou and PLCPou) were more efficient in containing hydro-erosive dynamics than plots with palm cultivation (PNRPal and PLCPal). It draws attention the plots on the Regosol, where the erosion measured in the PNRPal plot exceeded by 152 times, the erosion collected in the PNRPou plot, showing that a simple change in use and/or management can strongly unbalance natural systems. The highest runnof and erosion rates were analyzed on days

with consecutive rains, exposing the importance of antecedent humidity in the occurrence of hydro- erosive processes. Statistical correlations show that runoff has a good correlation with rainfall, whereas

erosion is remarkably close to runoff. The analysis of the sediments allowed to identify that the plots with palm plantations (PNRPal and PLCPal) lost about 5,6 times of Carbon (C) and 7,8 times of Nitrogen (N), more than the plots in system fallow land (PNRPou and PLCPou), highlighting the PNRPal plot, which alone lost 139% more C and N than all other plots combined. Regarding the granulometry of the sediments, the strong correlation with the parent material is noteworthy, since the PNRPal plot has about 73,1% of coarse and fine sand (sand textural class), and the PLCPal and PLCPou plots, had more than 92% of silt and clay (textural class from clayey to very clayey), being only the PNRPou plot that did not allow the analysis of its granulometry due to insufficiency of collected material, showing its great efficiency in the control of the hydro-erosive dynamics. The most important variables in the variability of runoff and erosion data and in the selectivity of sediments (nutrients and textural class) were the different soil types (Regosol and Luvisol) associated with the different types of use and management (palm culture and system fallow). The infiltration tests show that the highest values of infiltration speed occur in the driest months (low soil moisture), then in the rainy months (high soil moisture). The tests carried out in the Regosol have a higher constancy in the basic infiltration speed (VIB) than the Luvisol, although the Luvisol at specific times of the year can overcome the infiltration rates of the Regosol. In general, the Regosol showed to be more sensitive to anthropic interferences than the Luvisol, in relation to the hydro-erosive dynamics and in the selectivity of the sediments. When it comes to the infiltration dynamics, Luvisol because it contains high clays activity (type 2:1) in its structure, undergoes more changes throughout the year than the Regosol, showing greater variability in the results. Therefore, it is suggested to expand the measurement of hydro-erosive processes in the Brazilian semiarid region, to improve understanding of the spatial and temporal variability of these

phenomena, such knowledge is essential not only for the progress of erosive studies in a semiarid environment, but also can serve to subsidize public policies that aim at the ordering and management of rural spaces.