Nitrogen dynamics in Amazon forest and pasture soils measured by 15 N pool dilution Christopher Neill a, *, Marisa C. Piccolo b , Jerry M. Melillo a , Paul A. Steudler a , Carlos C. Cerri b a The Ecosystems Center, Marine Biological Laboratory, Woods Hole, MA 02543, USA b Centro de Energia Nuclear na Agricultura, Avenida Centena Ârio 303, Caixa Postal 96, CEP 13416000, Piracicaba, SP, Brazil Accepted 20 September 1998 Abstract Clearing tropical forests of the Amazon Basin for pasture alters rates of soil nitrogen cycling. Previous studies have shown that rates of soil net N mineralization and net nitri®cation are lower in established pastures than in forests. We compared soil inorganic N concentrations, rates of net and gross mineralization and net and gross nitri®cation in a chronosequence and an experimental slash-and-burn plot in RondoÃnia. Soils of pastures 4, 10 and 21-yr-old contained more NH 4 + and less NO 3  than soils of forest. Soil NH 4 + and NO 3  concentrations were elevated for 2 months after burning but were similar to pools in the forest after 8.5 months. Rates of net N mineralization and net nitri®cation decreased from forest to 21-yr-old pasture. Rates of gross N mineralization were similar in forest, 4- and 10-yr-old pasture then declined in 21-yr-old pasture. These ®ndings indicate that when forests are converted to pasture, soil N turnover is maintained for a period of a decade or longer, but N turnover eventually slows in old pastures. As older pastures come to dominate deforested regions of the Amazon, the total N cycled in soils of the region is likely to decrease, but not as quickly as studies based on net mineralization and net nitri®cation alone would indicate. # 1999 Elsevier Science Ltd. All rights reserved. 1. Introduction Clearing of tropical forest can have important eects on soil physical and chemical characteristics, long term soil storage of carbon and soil cycling of mineral nutri- ents. The world's highest rates of clearing occur in the Brazilian Amazon Basin, where 11,000±29,000 km 2 of forest are cleared each year, primarily for cattle pas- tures (INPE, 1998). Understanding the changes to soil biogeochemistry that in¯uence soil fertility after clear- ing will be important for implementing eective man- agement of pasture agriculture and for predicting the regional ecological consequences of deforestation. The replacement of forest vegetation with grasses and the maintenance of cattle pastures can in¯uence rates of soil N turnover. Two patterns of changes of N turnover following forest clearing and pasture creation are apparent from previous studies. First, rates of net mineralization and net nitri®cation are consistently lower in soils of pastures 3 yr old or older than in soils of the forests from which they were created (Reiners et al., 1994; Neill et al., 1995, 1997; Verchot, pers. comm.). In the Amazon, this pattern occurs over a wide area on Oxisols and Ultisols (Neill et al., 1997). Second, although rates of net N mineralization and net nitri®cation are lower in established pastures, inor- ganic N pools, net mineralization rates and net nitri®- cation rates may be elevated in the immediate postclearing period (Matson et al., 1987; Montagnini and Buschbacher, 1989). We currently interpret these patterns to indicate that after a brief period of elevated N turnover following the disturbance of clearing, N turnover gradually declines as pastures age. Net N mineralization provides an index of plant available N in many systems (Nadelhoer et al., 1983), but does not re¯ect the total amount of N cycling between organic matter and soil inorganic N. Gross rates of N mineralization and nitri®cation measured by 15 N dilution (Davidson et al., 1991) provide measured of the total amount of N mineralized and nitri®ed that Soil Biology and Biochemistry 31 (1999) 567±572 0038-0717/99/$19.00 # 1999 Elsevier Science Ltd. All rights reserved. PII: S0038-0717(98)00159-X PERGAMON * Corresponding author. Tel.: +1-508-289-7481; fax: +1-508-457- 1548; e-mail: cneill@mbl.edu.