Net nitrogen mineralization and net nitrification rates in soils following deforestation for pasture across the southwestern Brazilian Amazon Basin landscape

• Published in: Oecologia Vol. 110; no. 2; pp. 243 - 252
• Main Authors: Neill, C, Piccolo, M.C, Cerri, C.C, Steudler, P.A, Melillo, J.M, Brito, M
• Format: Journal Article
• Language: English
• Published: Berlin Springer-Verlag 01.04.1997; Springer
• Subjects: Agricultural soils; Agronomy. Soil science and plant productions; Animal and plant ecology; Animal, plant and microbial ecology; Biological and medical sciences; Chemical, physicochemical, biochemical and biological properties; deforestation; Ecosystoms Ecology; Forest soils; Fundamental and applied biological sciences. Psychology; grassland soils; losses from soil; Nitrification; Nitrogen; Organic matter; Organic soils; Pastures; Physics, chemistry, biochemistry and biology of agricultural and forest soils; Soil science; Soil texture; Soil water; Synecology; Terrestrial ecosystems; Tropical forests; Tropical soils; South America; Brazil; Amazon Basin; America; Process dynamics; Organic matter; Rain forest; Nitrates; Forest soil; Inorganic element; Tropical soil; Mineralization; Ecosystem; Soil science; Ultisols; Anthropogenic factor; Intertropical zone; Deforestation; Landscape; Drainage basins; Rate; Tropical forest; Nitrogen; Oxisols; Biogeochemistry; Nitrogen cycle; Regional study; Pasture; Nitrification; Soil biology; Environment; GRASSLAND SOILS; Nitrogen mineralization; Key words Brazilian Amazon; Tropical pasture
• ISSN: 0029-8549; 1432-1939
• DOI: 10.1007/s004420050157

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.