N-15 NATURAL-ABUNDANCE IN FOREST AND PASTURE SOILS OF THE BRAZILIAN AMAZON BASIN

The natural abundance of N-15 was examined in soil profiles from fores ts and pastures of the Brazilian Amazon Basin to compare tropical fore sts on a variety of soil types and to investigate changes in the sourc es of nitrogen to soils following deforestation for cattle ranching. S ix sites in the state of Rondonia, two sites in Para and one in Amazon as were studied. All sites except one were chronosequences and contain ed native forest and one or more pastures ranging from 2 to 27 years o ld. Forest soil delta(15)N values to a depth of 1 m ranged from 8 part s per thousand to 23 parts per thousand and were higher than values ty pically found in temperate forests. A general pattern of increasing de lta(15)N values with depth near the soil surface was broadly similar t o patterns in other forests but a decrease in delta(15)N values in man y forest profiles between 20 and 40 cm suggests that illuviation of N- 15-depleted nitrate may influence total soil delta(15)N values in deep er soil where total N concentrations are low. In four chronosequences in Rondonia, the delta(15)N values of surface soil from pastures were lower than in the original forest and delta(15)N values were increasin gly depleted in older pastures. Inputs of atmospheric N by dinitrogen fixation could be an important N source in these pastures. Other pastu res in Amazonas and Para and Rondonia showed no consistent change from forest values. The extent of fractionation that leads to N-15 enrichm ent in soils was broadly similar over a wide range of soil textures an d indicated that similar processes control N fractionation and loss un der tropical forest over a broad geographic region. Forest delta(15)N profiles were consistent with conceptual models that explain enrichmen t of soil delta(15)N values by selective loss of N-14 during nitrifica tion and denitrification.