Mudanças na matéria orgânica do solo causadas pelo tempo de adoção de um sistema agrossilvopastoril com eucalipto

• Autor(a) principal: Vergütz, Leonardus
• Data de Publicação: 2007
• Tipo de documento: Dissertação
• Idioma: por
• Título da fonte: LOCUS Repositório Institucional da UFV
• Texto Completo: http://locus.ufv.br/handle/123456789/5394
• topic: Carbono orgânico; Matéria orgânica leve; Índice de manejo do carbono; Organic carbon; Light organic matter; Carbon management index; CNPQ::CIENCIAS AGRARIAS::AGRONOMIA::CIENCIA DO SOLO

Soil organic matter (SOM) is the main property that indicates the sustainability of a cropping system, especially in tropical soils. Additionally, SOM is the major global organic C pool, making this pool a very important sink or source of C to the atmosphere, depends on the soil management. Agrossilvipastoral (AGSP) systems known to impove diversity and sustainability of cropping systems. Furthermore, it is expected that this system maintain or increase C stocks in soil when compared with soils under native Cerrado (Brazilian savannah) vegetation. One simple way to compare cropping systems is through the C management index (CMI), proposed by Blair et al. (1995). It compares cropping systems to a reference area. This index is based in a measure of C lability (C oxidizable by potassium permanganate), which isn t very reliable. The objectives of the present work are to study the impact of eucalypt cultivation under agrossilvipastoral systems on C and N stocks of different SOM fractions in an AGSP chronosequence, and to compare these results with those from soils under native vegetation (Cerrado). Additionally, is proposed a modification in the CMI proposed by Blair et al. (1995). A chronosequence compound of six implantation times of an AGSP system was sampled. The zero time is represented by the soil under Cerrado, followed by 2, 3, 4, 7 and 10 year-old AGSP systems. The soil layers sampled in each area were from 0-10, 10-20, 20-40, 40-60 e 60-100 cm deep, taken in the eucalypt planting line and between lines. The soil samples were chemically and physically characterized. It was also determined the total organic carbon (TOC), total nitrogen (TN), C and N in the light organic matter fraction (LOM) and C oxidizable by potassium permanganate, hereafter called labile carbon (LC). Based on these data, the CMI proposed by Blair et al. (1995) was calculated (CMILC), as well as a modified CMI, which is based on the use of C in the LOM (CMILOM). The TOC stocks showed an initial decrease in the 0-10 and 10-20 cm soil layers in the planting line and in the 0-10 and 20-40 cm soil layer between the eucalypt planting line, with a trend to recover in the older stands (10 years). Deeper soil layers (60- 100 cm) showed an initial increase in C stocks, more in the line than in between line, but this increase was not sustainable. Thus, the C losses would be greater for longer rotation cycles. The total C stock up to 100 cm deep, was similar to the initial TOC stock in the planting line, while in the between line the trend was to loose C without returning to the levels observed in the reference (Cerrado) at the end of the cycle. The main change in the N stocks occurred in deeper soil layers (20-40 and 60-100 cm). In these layers, N stocks showed a rapid initial decrease, followed by a great increase. But this last increase was not sustained and by the end of the cycle the N stocks were less than the original ones in the soil under native vegetation. The same trend was observed for the N stocks at 0-100 cm soil layer. The free LOM fraction presented the major changes, what confirms its great sensibility to management changes. The free LOM C and N losses were greater in soils to between planting lines than in the planting line. Labile C, as well as the CMILC, was not a good indicator of the changes that occurred after the implantation of the AGSP system. But the modified CMI, calculated based on the LOM C (CMICLOM) fitted well for all soil layers analyzed. In addition, the CMICLOM showed the best correlation with TOC, thus been more indicated to compare the cropping system studied. These results show the importance to study deeper soil layers; moreover, they can show relatively large losses of C, which are difficult to recover. The AGSP systems caused a decrease of the soil organic C in upper soil layers and the time that takes to it recover to the original stocks are beyond the time frame of the present study. The free light organic matter was the fraction more sensitive to the changes in soil use and management.