Atributos edáficos em áreas de pastagem plantada em relevo movimentado no noroeste do Rio de Janeiro

Abstract

One of the largest challenges of the world agriculture is to increase the productivity of several crops, to obtain larger return on the investments, with sustainability. For that, it is necessary to study how to minimize negative effects of agricultural handling and alterations in the soil functional and structural properties, and the behavior of the system as a whole. The study was carried out in the Fazenda Pau Ferro, located in Itaperuna municipality, Rio de Janeiro State, and had as objective to evaluate soil attributes as indicators of soil quality in cultivated pastures areas, Braquiaria (Brachiaria brizantha (Hochst. ex A. Rich.) Stapf), Suazi (Digitaria swazilandensis Stent) and Tifton 85 (Cynodon spp. cv Tifton 85) cultivated in a hilly landscape. The experimental design was completely randomized in subdivided blocks with three treatments (grasses) and two replications. Soil samples were taken at 0-10 and 10 -30cm depth, in two seasons - dry period (August of 2004, May and August of 2005) and in the rainy period (December of 2004) on different sections: at the hillside shoulder (TS), back slope (TM) and foot slope (TI) positions. The physical attributes evaluated were: soil aggregate stability, being used the indexes mean weight diameter (MWD), mean geometric diameter (MGD) and percentage of stable aggregates in water> 2,0mm; the bulk density (BD) and the total volume of pores (TVP). The chemical attributes analyzed were: total organic carbon (TOC), C in the humic substances (COH) and soil fertility. Samples of aerial part and grass root system were also collected. To study the multidimensional effect of soil attributes the Principal Component Analysis (PCA) was applied. The aggregates weight distribution as a function of diameter, specially the >2.0mm class, the total organic carbon content, and the amount of carbon in the humic fractions (C-HA and C-FA) showed to be sensitive to the effect of the treatments, grass types and season of the year. In general, the Braquiária showed higher values of MWD, MGD, and percentage of stable aggregates in water> 2.0mm. It also had higher values of TOC and COH, with the C content in the humin fraction (C-HUM) higher than C-HA and C-FA. Among the fractions of soil organic matter, the humin fraction was the one that presented higher values of carbon. The Tifton 85 grass area also presented high values of MWD, MGD, percentage of stable aggregates in water> 2.0mm, TOC, and COH. The PCA analyses allowed evaluating the multivariate effects of the soil chemical and physical attributes, separating the areas as a function of seasonal variation and slope positions. The physical and chemical characteristics that stronger correlated with the first order axe were: MWD; percentage of stable aggregates in water> 2.0mm; micro aggregates, in the classes of 0.50-0.25 and 0.25-0.10mm; C-HA; C-FA; TOC; S Value, and T Value. The areas with Tifton 85 (TS) and Suazi (TS and TM) were more influenced by the physical characteristics, showing higher correlation values with the total porosity, and the weight of aggregates in the class 2.0-0.50mm. The Braquiaria and Tifton 85 grasses showed the best soil quality indexes, showing a potential as soil coverage in recovery projects of degraded lands.