title: Net nitrogen mineralization and net nitrification rates in soils following deforestation for pasture across the southwestern Brazilian Amazon Basin landscape.
abstract: Previous studies of the effect of tropical forest conversion to cattle pasture on soil N dynamics showed that rates of net N mineralization and net nitrification were lower in pastures compared with the original forest. In this study, we sought to determine the generality of these patterns by examining soil inorganic N concentrations, net mineralization and nitrification rates in 6 forests and 11 pastures 3 years old or older on ultisols and oxisols that encompassed a wide variety of soil textures and spanned a 700-km geographical range in the southwestern Brazilian Amazon Basin state of Rondônia. We sampled each site during October-November and April-May. Forest soils had higher extractable NO3--N and total inorganic N concentrations than pasture soils, but substantial NO3--N occurred in both forest and pasture soils. Rates of net N mineralization and net nitrification were higher in forest soils. Greater concentrations of soil organic matter in finer textured soils were associated with greater rates of net N mineralization and net nitrification, but this relationship was true only under native forest vegetation; rates were uniformly low in pastures, regardless of soil type or texture. Net N mineralization and net nitrification rates per unit of total soil organic matter showed no pattern across the different forest sites, suggesting that controls of net N mineralization may be broadly similar across a wide range of soil types. Similar reductions in rates of net N transformations in pastures 3 years old or older across a range of textures on these soils suggest that changes to soil N cycling caused by deforestation for pasture may be Basin-wide in extent. Lower net N mineralization and net nitrification rates in established pastures suggest that annual N losses from largely deforested landscapes may be lower than losses from the original forest. Total ecosystem N losses since deforestation are likely to depend on the balance between lower N loss rates from established pastures and the magnitude and duration of N losses that occur in the years immediately following forest clearing.
authors.affiliation: The Ecosystems Center, Marine Biological Laboratory, Woods Hole, MA 02543. USA fax: 508 457 1548; e-mail: cneill@lupine.mbl.edu, , , , , , US.
authors.forename: Christopher
authors.initials: C
authors.lastname: Neill
authors.affiliation: Centro de Energia Nuclear na Agricultura (CENA), Universidade de São Paulo, Avenida Centenário 303, Caixa Postal 96, CEP 13416000, Piracicaba, SP, Brazil, , , , , , BR.
authors.forename: Marisa C
authors.initials: MC
authors.lastname: Piccolo
authors.affiliation: Centro de Energia Nuclear na Agricultura (CENA), Universidade de São Paulo, Avenida Centenário 303, Caixa Postal 96, CEP 13416000, Piracicaba, SP, Brazil, , , , , , BR.
authors.forename: Carlos C
authors.initials: CC
authors.lastname: Cerri
authors.affiliation: The Ecosystems Center, Marine Biological Laboratory, Woods Hole, MA 02543. USA fax: 508 457 1548; e-mail: cneill@lupine.mbl.edu, , , , , , US.
authors.forename: Paul A
authors.initials: PA
authors.lastname: Steudler
authors.affiliation: The Ecosystems Center, Marine Biological Laboratory, Woods Hole, MA 02543. USA fax: 508 457 1548; e-mail: cneill@lupine.mbl.edu, , , , , , US.
authors.forename: Jerry M
authors.initials: JM
authors.lastname: Melillo
authors.affiliation: Centro de Energia Nuclear na Agricultura (CENA), Universidade de São Paulo, Avenida Centenário 303, Caixa Postal 96, CEP 13416000, Piracicaba, SP, Brazil, , , , , , BR.
authors.forename: Marciano
authors.initials: M
authors.lastname: Brito
keywords: Key words Brazilian Amazon ; Tropical pasture ;  Nitrification ;  Nitrogen mineralization;  Tropical forest 