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Efeitos da extrus?o de H+/OH- em plantas de feij?o crescidas com diferentes fontes de nitrog?nio sobre o in?cio da forma??o de n?dulos radiculares. / Effects of extrusion of H+/OH- in bean plants grown with different nitrogen sources on the early formation of root nodulesMacedo, Ricardo Ant?nio Tavares de 29 October 2010 (has links)
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Previous issue date: 2010-10-29 / Bean plants can get nitrogen from soil organic matter, fertilizer or biological fixation of
atmospheric N2 (BNF). However, the biological process in this culture has shown low
efficiency under field conditions. As a result, many farmers do not inoculate the seeds with
efficient strains of rhizobia, being fertilizer the main source of N to this culture. Besides the
absence of inoculation, high doses of N applied to the soil inhibit nodulation and BNF.
However, the reasons for this inhibition are not fully understood, especially because the N is
uptaked as nitrate (NO3
-) and ammonium (NH4
+). Whereas the rhizosphere of plants grown
with nitrate is alkalized and ammonium is acidified, the aim of this study was evaluate the
effect of pH changes in rhizosphere (pHR), caused by the metabolism of these N sources, in
start of nodules formation. The experiments were conducted in growth chamber with average
brightness of 400 lux, photoperiod of 12/12 hours (light/dark) and temperature 25?C ? 2?C.
Plants were grown in pots with sand, soils with low (8%) and medium (25%) clay content and
nutrient solution. In the 1st test (salinity) was found that ammonium had the greatest increase
in electrical conductivity of the sand (EC), obtained with a 60 KgN ha-1 the value of 1.655 mS
cm-1 with ammonium and 1.301 mS cm-1 with nitrate. In the following assay were compared
two contrasting cultivars in capacity of nodulate, which confirmed the high (Ouro Negro) and
low (Rio Tibagi) capability. As among cultivars were not significant differences in the values
of pHR, in others experiments was evaluated Ouro Negro only. In this assay and in others
ammonium was the source of N with more capacity of rhizosphere acidify and inhibit
nodulation. Through regression analysis for each N source, was given the dose of total
inhibition of nodulation (critical level), pHR at this dose and the dose which was 80% of
maximum nodulation (dose of coexistence). In general, was verified the lowest doses of
critical level and coexistence using ammonium than with nitrate.
In the sand test (Ouro Negro sampled 20 DAE) for ammonium and nitrate the doses of
coexistence was 13 and 58 KgN ha-1 and the values of pHR was 3,95 and 5,59, respectively.
In test with different soil types (20 DAE), the dose of coexistence was lower with ammonium
than nitrate, being in soil with lower clay content 18 e 41 KgN ha-1 and soil with higher clay
content 15 e 27 KgN ha-1, respectively. The values of pHR for ammonium and nitrate were
4,69 and 6,08 in soil with lower clay content and 4,23 and 4,63 in soil with higher clay
content, respectively. In this test the neutralizers alleviated acidification of the rhizosphere
and nodulation was optimized. In test with leaf application of N, ammonium also more acidify
the rhizosphere and promoted less nodulation (pHR 4,15 and 14 nodules plant-1) compared to
nitrate (pHR 5,03 and 27 nodules plant-1). In the 6th experiment (20 DAE) the dose of 115
kgVO4 ha-1 allowed that highest dose of ammonium did not make changes in pHR, allowing
also the dose of coexistence of 46 KgN ha-1 for ammonium as source of N. In all tests, the
highest N rates from all sources completely inhibited nodulation, suggesting that these doses
were the occurrence of others inhibiting factors independent of pH and not measured. / O feijoeiro pode adquirir nitrog?nio da mat?ria org?nica do solo, de adubos ou da fixa??o
biol?gica do N2 atmosf?rico (FBN). Entretanto, nesta cultura o processo biol?gico tem
apresentado baixa efici?ncia nas condi??es de campo. Em fun??o disso, muitos produtores
n?o inoculam as sementes com estirpes eficientes de riz?bio, sendo a aduba??o a principal
fonte de N para esta cultura. Al?m da aus?ncia da inocula??o, altas doses de N aplicadas no
solo inibem a nodula??o e a FBN. No entanto, as causas para esta inibi??o n?o s?o totalmente
esclarecidas, especialmente pelo fato do N ser absorvido na forma de nitrato (NO3
-) e am?nio
(NH4
+). Considerando que a rizosfera da planta crescida com nitrato ? alcalinizada e com
am?nio ? acidificada, este trabalho teve o objetivo de avaliar o efeito das altera??es de pH
rizosf?rico (pHR), causadas pelo metabolismo destas fontes de N, no in?cio da forma??o de
n?dulos. Os experimentos foram realizados em c?mara de crescimento com luminosidade
m?dia de 400 Lux, fotoper?odo de 12/12 horas (luz/escuro) e temperatura de 25?C ? 2?C.
As plantas foram crescidas em vasos com areia, solos com baixo (8%) e m?dio (25%) teor de
argila e em solu??o nutritiva. No 1? ensaio (salinidade) foi verificado que o am?nio causou
maior eleva??o da condutividade el?trica da areia (CE), obtendo-se na dose de 60 kgN ha-1 o
valor de 1.655 ?S cm-1 com am?nio e de 1.301 ?S cm-1 com nitrato. No ensaio seguinte foram
comparadas 2 cultivares contrastantes quanto ? capacidade nodular, que confirmaram a alta
(Ouro Negro) e a baixa (Rio Tibagi) capacidade. Como entre as cultivares n?o ocorreram
diferen?as expressivas nos valores de pHR, nos demais experimentos avaliou-se apenas a
Ouro Negro. Neste ensaio e nos demais o am?nio foi a fonte de N que mais acidificou a
rizosfera e inibiu a nodula??o. Atrav?s de an?lises de regress?o para cada fonte de N foi
determinada a dose de inibi??o total da nodula??o (n?vel cr?tico), pHR nesta dose e a dose
onde ocorreu 80% da m?xima nodula??o (dose de conviv?ncia). Em geral, foram observadas
as menores doses de NC e de conviv?ncia com uso do am?nio do que com nitrato.
No ensaio em areia (cv Ouro Negro coletada aos 20 DAE), para am?nio e nitrato as doses de
conviv?ncia foram 13 e 58 kgN ha-1 e os valores de pHR 3,95 e 5,59, respectivamente. No
ensaio em diferentes tipos de solo (20 DAE), a dose de conviv?ncia foi menor com am?nio do
que com nitrato, sendo no solo com menor teor de argila 18 e 41 kgN ha-1 e no solo com
maior teor de argila 15 e 27 kgN ha-1, respectivamente. J? os valores de pHR para am?nio e
nitrato foram 4,69 e 6,08 no solo com menor teor de argila e 4,23 e 4,63 no solo com maior
teor de argila, respectivamente. Neste ensaio os neutralizantes aliviaram a acidifica??o da
rizosfera e otimizaram a nodula??o. No ensaio com aplica??o de N via foliar, o am?nio
tamb?m acidificou a rizosfera e promoveu menor nodula??o (pHR 4,15 e 14 n?dulos planta-1)
em rela??o ao nitrato (pHR 5,03 e 27 n?dulos planta-1). No 6? experimento (20 DAE) a dose
de 115 kgVO4 ha-1 permitiu que na mais alta dose de am?nio o pHR n?o sofresse altera??es,
permitindo tamb?m a dose de conviv?ncia de 46 kgN ha-1 para o am?nio. Em todos os
ensaios, as mais altas doses de N de todas as fontes suprimiram a nodula??o, sugerindo que
nestas doses ocorreram fatores inibidores independentes do pH e n?o mensurados.
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