Infodoc : Sciences, Techniques et Culture

The natural abundance of 15N was examined in soil profiles from forests and pastures of the Brazilian Amazon Basin to compare tropical forests on a variety of soil types and to investigate changes in the sources of nitrogen to soils following deforestation for cattle ranching. Six sites in the state of Randönia, two sites in Para and one in Amazonas were studied. All sites except on were chronosequences and contained native forest and one or more pastures ranging from 2 to 27 years old. Forest soil 15N values to a depth of 1 m ranged from 8%oo to 23%oo and were higher than values typically found in temperate forests. A general pattern of increasing 15N values with depth near the soil surface was broadly similar to patterns in other forests but a decrease in 15N values in many forest p[...]

The natural abundance of 15N was examined in soil profiles from forests and pastures of the Brazilian Amazon Basin to compare tropical forests on a variety of soil types and to investigate changes in the sources of nitrogen to soils following deforestation for cattle ranching. Six sites in the state of Randönia, two sites in Para and one in Amazonas were studied. All sites except on were chronosequences and contained native forest and one or more pastures ranging from 2 to 27 years old. Forest soil 15N values to a depth of 1 m ranged from 8%oo to 23%oo and were higher than values typically found in temperate forests. A general pattern of increasing 15N values with depth near the soil surface was broadly similar to patterns in other forests but a decrease in 15N values in many forest profiles between 20 and 40 cm suggests that illuviation of 15N depleted nitrate may influence total soil 15N values in deeper soil where total N concentrations are low. In four
chronosequences in Randônia, the 15N values of surface soil from pastures were lower than in the original forest and 15N values increasingly depleted in older pastures. Inputs of atmospheric N by dinitrogen fixation could be an important N source in these pastures. Other pastures in Amazonas and Para an Randônia showed no consistent change from forest values. The extent of fractionation that leads to 15N enrichment in soils was broadly similar over a wide range of soil textures and indicated that similar processes control N fractionation and loss under tropical forest over a broad geographic region. Forest 15N profiles were consistent with conceptual models that explain enrichment of soil 15N values by selective loss of 14N during nitrification and denitrification. "(Résumé des auteurs)".