NATURAL-ABUNDANCE OF N-15 IN SOILS ALONG FOREST-TO-PASTURE CHRONOSEQUENCES IN THE WESTERN BRAZILIAN AMAZON BASIN

We examined the natural abundance of N-15 in soil profiles along two c hronosequences in the western Brazilian Amazon Basin state of Rondonia , to investigate possible mechanisms for changes to soil nitrogen sour ces and transformations that occur as a result of land use. One chrono sequence consisted of forest and 3-, 5- and 20-year-old pasture, the o ther of forest and 8- and 20-year-old pasture. The delta(15)N values o f surface soil and soil to 1 m depth in the native forest ranged from 9.8 to 13.6 parts per thousand and were higher than reported for tempe rate forest soils. Fractionation associated with nitrification and den itrification and selective losses of N-15-depleted nitrate, could pote ntially result in a strong enrichment of nitrogen in soil organic matt er over the time scale of soil development in highly weathered tropica l soils. Pasture surface soils were 1-3 parts per thousand, depleted i n N-15 compared with forest soils. Lower delta(15)N values in 20-year- old pastures is consistent with greater cumulative inputs of N-15-depl eted atmospheric-derived nitrogen, fixed by free-living bacteria assoc iated with planted pasture grasses in older pastures, or differential plant utilization of soil inorganic N pools with different delta(15)N values. The pattern of delta(15)N values following conversion of fores t to agricultural use differs from the pattern in the temperate zone, where pasture or cultivated soils are typically more enriched in N-15 than the forest soils from which they were derived.