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 NO-N and total inorganic N concentrations than pasture soils, but substantial NO-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.

Nitrogen dynamics in Amazon forest and pasture soils measured by 15N pool dilution

“…These ®ndings, which tend to be consistent across a range of sites and soils (Neill et al, 1997;Verchot, pers. comm.…”

“…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, inorganic 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).…”

“…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, 1995Neill et al, , 1997Verchot, pers. comm.).…”

Disponibilidade de nitrogênio e fluxos de N2O a partir de solo sob pastagem após aplicação de herbicida

“…Isto é consistente com os relatos de baixos fluxos de N 2 O a partir de solos sob pastagens em diversos locais nesta região (Garcia-Montiel et al, 2003). As pastagens da Amazônia apresentam concentrações tipicamente baixas de N-NO 3 -no solo, bem como baixas taxas de mineralização e nitrificação líquida do N. Gramíneas, como as braquiárias, apresentam altas taxas de crescimento e são (Neill et al, 1997). Alguns autores, tais como Neill et al (1995, 1997) e Verchot et al (1999, mostraram que a quantidade de N que cicla nas pastagens é bem menor quando comparadas com valores observados nas florestas.…”

Nitrogênio e enxofre na recuperação de pastagem de capim-Braquiária em degradação em Neossolo Quartzarênico com expressiva matéria orgânica

“…Isto tem sido atribuído à inibição da nitrificação devido a substâncias excretadas por raízes de gramíneas e monoterpenos liberados pelas árvores em solos florestais, sendo que o baixo pH também contribui para isso (NIELL, 1997;VASCONCELLOS, 2001;MOREIRA;SIQUEIRA, 2002). Entre os fatores que influenciam o processo de nitrificação, existem aqueles que controlam os teores de N-NH 4 + e N-NO 3 -no solo e são os inibidores da atividade da urease e da ação dos microorganismos nitrificadores (CLARK ; PAUL, 1970;VICTORIA et al, 1988).…”