Distribution of water-stable aggregates and aggregating agents in oxisols of the Brazilian cerrados

The effects of land-use change on the structure of Oxisols in the Brazilian savannas (also known as the Cerrados) are still insufficiently understood. We therefore studied loamy and clayey Oxisols under natural savanna, crop, pasture, and reforestation to (1) quantify management-induced changes in the quantity of water-stable aggregates, (2) identify the main aggregating agents, and (3) correlate aggregation with changes in pore-size distribution. Clayey soils showed a significantly higher macroaggregation than did loamy soils. Compared with natural savanna, macroaggregation was clearly reduced under crops, whereas aggregation of soils under pasture and tree plantations was only slightly affected. In both clayey and loamy soils, polysaccharides formed the main aggregating agent. In the clayey soils, lime very effectively disaggregated the soils by weakening the electrostatic forces between positively and negatively charged soil compounds. In the loamy soils, the role of roots in binding macroaggregates was significant. Because pastures provide strong rooting and high polysaccharide production, we recommend introducing crop/pasture rotations. Management-induced disaggregation strongly affected the pore-size distribution by compacting the soils, and thus reducing macroporosity and increasing mesoporosity. Microporosity, however, was unaffected by management and differed only between the two substrates. Considering the Oxisols` typically low pore space at plant-available matrix potentials, the increase in mesoporosity may be important for annual crops during the frequent dry spells in the rainy season

Soil organic matter in oxisols of the Brazilian cerrados

Little is known about the sustainability of cultivation systems in the Brazilian savannas, also known as the Cerrados, despite its increasing significance for that country`s agriculture. To characterize management effects and follow alterations of organic compounds in different fractions, we studied whole-soil samples and particle-size separates from clayey and loamy Oxisols under crops, pastures, reforested sites, and savanna. We assessed soil organic carbon (SOC), polysaccharides, and CuO oxidation products (VSC-lignin). Few changes were found in SOC contents of topsoil (0-12 cm) under different land uses after 10-20 years. But organic carbon clearly diminished under continuous cropping on the loamy soil and under reforestation with pine on the clayey soil. Management effects on SOC were more apparent in sand fractions, suggesting that particulate organic matter (POM) was affected most. In the clay fraction, only minor effects were noted. Carbon-normalized polysaccharide contents were enriched under pastures and depleted under pine, but generally followed a similar distribution to that of SOC. Overall, both polysaccharides and VSC-lignin were closely related to soil porosity. Plant-derived polysaccharides and lignin contents were probably regulated by water availability to soil microbes, so that decomposition was usually more advanced in the clayey soils. Ten years of continuous cropping lowered litter inputs, thus reducing POM, whereas humified organic matter (OM) was unaffected. Planting eucalyptus or well-managed pastures, which produce high amounts of POM, would thus rapidly reverse soil degradation. Continuous cropping does eventually reduce the humified fraction and results in a substantial loss of soil fertility, which is only slowly reversible

Microbial biomass, microbial activity, and carbon pools under different land-use systems in the Brazilian cerrados

In the Brazilian savannas, or Cerrados the rapid advance in agriculture and cattle ranching is affecting soils through, for example, accelerated erosion and depletion of soil organic matter (SOM). Changes in soil microbial biomass are good indicators of changes in SOM. We therefore assessed the effects of agricultural and pastoral use of a clayey Oxisol in the Cerrados on soil microbial biomass, and evaluated the usefulness of this parameter in studying SOM dynamics in savanna ecosystems. Surface soil horizons under a pine forest and different crop and pasture treatments were compared with the control soil under native savanna. Soil microbial carbon (C mic), potential microbial activity, pH, organic C, water-extractable organic carbon (WEOC), and total N were assessed for the different systems. Compared with native savanna, crop cultivation and reforestation depleted C mic. The C mic,/C quotients indicated that C might continue to decline in these two systems. Changing from conventional to no tillage appears to slow down the depletion of topsoil C mic, C, and other parameters measured. Pasture establishment in native savanna did not clearly change C mic, but stimulated microbial activity. The ratio of microbial activity to Cmic was higher under pastures than under the other systems. Soil microbial carbon was shown to be closely related to the soil carbon cycle. Water-extractable organic carbon, possibly the most important source of C for microbes, consists of root exudates and litter degradation products. Root density, together with organic matter (OM) input and soil cover, was therefore assumed to be a major factor controlling the amount of C mic. Microbial growth was hypothesized to be C-limited in the crop systems and possibly N-limited in the pastures. The results indicate that the C mic/C ratio can be used as an indicator of OM dynamics in highly weathered tropical soils

Contribution to the discussions on the origin of the cerrado biome: Brazilian savanna

Theories that attempt to explain the origin of the cerrado biome are mostly based on the isolated action of three major factors: climate, fire and soil. Another factor that has been mentioned is that of human interference. We hypothesise that the evolutionary origin of this biome resulted from the complex interaction of climate, fire and soil, with climate being the triggering agent of this assumed interaction. Fire, as well as acid and dystrophic soils, would be factors involved in the selection of savanna species throughout climatic events, during the Tertiary and the Quaternary, e.g. Pliocene and Pleistocene. The genesis of the physiognomies that would give rise to cerrado sensu lato, rather than forest formations, could have occurred due to the strong pressure exerted by the reduction in water availability, and the selection of the species adapted to the new conditions imposed by the environment. The characteristics of cerrado sensu lato soil, originated from edaphic impoverishment caused by lixiviation and successive past fires, would remain, even after hydric availability increased following the Pleistocene glaciations