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 chronosequences in the western Brazilian Amazon Basin state of Rondonia, to investigate possible mechanisms for changes to soil nitrogen sources and transformations that occur as a result of land use. One chronosequence consisted of forest and 3-, 5- and 20-year-old pasture, the other of forest and 8- and 20-year-old pasture. The delta(15)N values of 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 temperate forest soils. Fractionation associated with nitrification and denitrification and selective losses of N-15-depleted nitrate, could potentially result in a strong enrichment of nitrogen in soil organic matter over the time scale of soil development in highly weathered tropical soils. Pasture surface soils were 1-3 parts per thousand, depleted in 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-depleted atmospheric-derived nitrogen, fixed by free-living bacteria associated 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 forest 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.