Contribuição de resíduos de milho e de soja para o carbono e suas frações em amostras de solos submetidas a cultivos sucessivos e à aplicação de fósforo

• Autor(a) principal: Schulthais, Fernanda
• Data de Publicação: 2012
• Tipo de documento: Tese
• Idioma: por
• Título da fonte: LOCUS Repositório Institucional da UFV
• Texto Completo: http://locus.ufv.br/handle/123456789/1644
• topic: Matéria orgânica; Fósforo; Estabilização de C; Cultivos sucessivos; Organic matter; Phosphorus; C Stabilization; Successive crops; CNPQ::CIENCIAS AGRARIAS::AGRONOMIA::CIENCIA DO SOLO

The substitution of natural vegetation by crops can induce a reduction in the levels of soil organic matter (SOM) and its chemical, physical and biological quality properties. This study analyzed the factors involved in the maintenance and/or increases in SOM levels to ensure sustaintability over successive crop cycles. For this purpose, two experiments were conducted to: evaluate changes in soil chemical properties as a result of the entry of C from residues of successive corn or soybean crops in samples of soil previously under native forest and evaluate the effect of corn and soybean root and shoot residues in soil samples preincubated with a soluble P source on the stabilization of applied C. For experiment 1, samples were collected from the 0-20 and 40-60 cm layers of an Oxisol under native forest. Pots were filled with 3 dm³ subsamples to plant corn or soybean and as control (without plant); the pots were maintained for eight successive crop cycles in a greenhouse. Prior to cultivation (cultivation 0) and after the 2nd, 4th, 6th, and 8th crop, plants and soil were sampled for analysis. In the plant material, the levels of P, K, Ca, Mg, S, Fe, Zn, Mn, and Cu were determined. In the soil samples, the levels of P were determined by Mehlich-1, Bray-1 and resin, remaining P, available K, Ca2+, Mg2+, Al3+, H+Al, total C and N and labile C and N. The experiment was arranged in a 2x3 factorial design, corresponding to the two soil layers and the species (corn and soybean) and control (without plant) arranged in randomized blocks with six replications in a split-plot design. After analysis of variance, orthogonal contrasts were tested for each study variable. The dry matter production and nutrient levels in corn and soybean shoots and roots were affected by temporary nutrient immobilization in the residues of these crops. The addition of corn or soybean residues during the crop sequence raised the levels of total C (CT) in the soil compared to the control (without plant). The highest levels of available P by Mehlich-1 and Bray-1 in the 40-60 cm layer of cultivated soils demonstrated the existence of plant mechanisms that increase available P levels in the soil. The higher levels of labile C (CL) in cultivated soils seemed to contribute to the blocking of adsorption sites of Fe and Al oxyhydroxides by the applied P. Moreover, despite the inclusion of residues in the soils at each harvest, the K levels dropped drastically, especially in soils under maize, while the levels of Ca2+ and Mg2+ were not affected. For experiment 2, the same sample was used as in experiment 1 (40-60 cm layer). This sample was divided into sub-samples and incubated at increasing graded P rates, up to the maximum P adsorption capacity (MPAC). After 120 days, these subsamples were treated with increasing doses of C residue in the form of corn or soybean roots or shoots. The treatments consisted of the factorial 4x[(2x2x3)+1], corresponding to four P levels (0, 0.25, 0.5 and 1.0 times MPAC), the two tested species (corn and soybean), and their compartments (roots and shoots), the three residue doses (5, 10 and 25 g kg-1 C) plus the control (0 g kg-1 C), arranged in a randomized block design with three replications. During the incubation period, the CO2 efflux from the subsamples was measured. After incubation, the CL of these subsamples was determined and they were physically fractionated in particulate organic matter (MOP) and mineral-associated organic matter (MAM). The total C and N levels of these fractions were determined. The incorporation of root residues promoted a greater accumulation of soil C than the addition of shoot residues. The application of increasing P associated with increasing residue doses favor increased levels of soil CT and of the MOP and MAM fractions. Even after the application of the highest C doses, the soil samples were not saturated by organic carbon, with reference to the C content in the surface layer under native forest (0-20 cm) as a reference of saturation.