Data/hora: 18/06/2024 11:26:51

Biblioteca(s): | Embrapa Cerrados.
Data corrente: | 30/03/1999
Data da última atualização: | 30/03/1999
Autoria: | NEUFELDT, H.
Título: | Land-use effects on soil chemical and physical properties of cerrado oxisols.
Ano de publicação: | 1998
Fonte/Imprenta: | Bayreuth: [Universitat Bayreuth], 1998.
Páginas: | 174p.
Série: | (Bayreuther Bodenkundliche Berichte, Band 59).
Idioma: | Inglês
Conteúdo: | RELEVANCE: The cerrado region is among the world's most highly productive agricultural frontiers. The development will and must continue at high pace in order to keep up with the increasing food demand. But future development will be possible only at the cost of cultivating less accessible soils or by intensifying land use on soils already under cultivation. Moreover, the techniques in use have proven unsuitable for sustainable management due to the loss of organic matter and soil compaction which might promote severe erosion. The main questions studied within the scope of this work were: How do typical land-use systems affect the most important physical and chemical properties of Cerrado soils and there sensible pools that allow to monitor the impact of land-use systems at an early stage. APPLIED METHODOLOGY: In the southern part of Central Brazil's Cerrado region with an average temperature of 22oC and annual rainfall of 1650 mm, fine and coarse-textured Oxisols with the most representative land-use systems were selected: Conventional maize/soybean crop rotations, degraded Brachiaria decumbens pastures, and tree plantations (Pinus caribaea on the clayey substrate and Eucalyptus citriodora on the loamy sbstrate). The selected on-farm treatments were chosen because of their comparatively long management history (10-20 years) and the small distance between them. Native Cerrado savanna was used as control. After establishing the background for a comparative study by characterising the soils with physical and chemical methods, surface soil samples were taken on all treatments and fractionated into particle-size separates and aggregate classes. The samples were subsequently analysed for organic carbon, polysaccharides, and lignin as well as pedogenic iron and aluminum oxides and various phosphorus fractions. RESULTS: Soil organic carbon (SOC) appeared to be key element in all soils because it controlled both physical and chemical processes: SOC provided the majority of the exchange sites and controlled the formation of stable micro-aggregates which give Oxisols their favourable structure and unique water retention characteristics. Cropping reduced pore volume in the whole solum but increased the amount of meso-pores which are important for the plant water supply due to seedbed preparation and possibly liming effects. Whole soil SOC contents were approximately three times higher in the clayey than in the loamy soils. Similar relationships were found for all organic constituents, pedogenic oxides, and most phosphorus fractions. The potentially labile carbon stock in the soils was estimated to approximately 20 Mg ha-1, corresponding to around 20% and 50% of total organic carbon in the clayey and the loamy soils, respectively. Management effects on SOC were small except on the loamy crop and the pine reforestation. It appeared that plant derived polysaccharides and VSC-lignin were related to the porosity of the soils, especially to pores that hold capillary water and therefore control microbial activity. Soil aggregation as characterised by the geometric mean diameter (GMD), was clearly reduced under crops and the pine reforestation in function of depleted organic compounds, mainly polysaccharides which bond small aggregates together, but the enmeshing abilities of roots were also important, whereas pedogenic oxides showed no close correlations to GMD. Phosphorus dynamics in the whole soil were characterised by the strong contribution of organic P forms to plant supply under P deficiency conditions. The ratio of bioavaiable inorganic to organic P reflected the P status of the studied land-use systems well and deficiency increased in the order crop<< pasture < reforestation < Cerrado. Independent of soil texture, between 60% and 80% of SOC, organic constituents, and phosphorus were bound to the clay fraction. Organic compounds including organic phosphorus followed a biological gradient from weakly to highly humified organic matter with decreasing particle-size, which is consistent with the microbial decomposition of plant tissue and the subsequent build-up of microbially mediated organic structure. Inorganic phosphorus additionally followed a mineralogical gradient of increasingly stronger P adsorption to oxyhydroxides with decreasing particle-size. Management effects on SOC and P fractions were more apparent in the particle-size separates, especially in the 20-50 um and the 50-2000 um fraction. The results showed that mainly particulate organic matter was affected by land use, whereas humified matter bound to the clay fraction showed only minor effects. Polysaccharide contents were enriched under pastures and depleted under pine. The 20-50 um fraction showed the accumulation of residual fertiliser P and was apparently enriched in small charcoal pieces that indicate past burning. CONCLUSIONS: Management systems on substrate show a lower degree of degradation than on loamy substrate due to the stabilising effects of clay. But continued cropping will eventually lead to a similar decrease of organic matter on clayey soils as is already visible on the loamy soils. The sand fractions, especially the 20-50 um is most sensitive to land-use changes and represents an indicator of dynamics that will become apparent in the future. Since the separation of the sand fractions is simple, the characterisation of organic carbon in these fractions for routine analysis is proposed. Future research should concentrate on the impact of land use on water retention characteristics in Oxisols and consider the interactions with organic matter dynamics. This is important because dry spells during the rainy season impose a serious limitation for rain fed agriculture but a management induced increase in meso-pores might be manageable without the concomitant loss of organic matter.
Palavras-Chave: | Propriedade fisico-quimica do solo; Soil chemicophysical properties.
Thesagro: | Cerrado; Uso da Terra.
Thesaurus Nal: | land use.
Categoria do assunto: | --
Registro original: | Embrapa Cerrados (CPAC)
Exemplares: | CPAC - (11669-1) - ADD - TS - -- - CRI4876 -  - CRI4876 - DIS