Heinrich Wilhelm Scherer, Yongsong Zhang:
Fixation and Release of Ammonium in Paddy Soils after Flooding

HEINRICH WILHELM SCHERER$1$, YONGSONG ZHANG$2$
$1$University of Bonn, Institute of Plant Nutrition, Germany
$2$Zhejiang University, Institute of Agrochemistry, China

Poor utilization of N fertilizers by rice plants seems to be largely due to N losses from the soil plant system and through denitrification, leaching, runoff and NH$$₃ volatilization. Therefore, the primary aim of improved N management is to minimize transformation processes that result in N losses and to maximize N uptake. One important transformation process, which leads to a temporary immobilization of fertilizer N as well as of NH$$₄⁺ from the deamination of organic N compounds, is the fixation of NH$$₄^<#1923#>+ ions by 2:1 clay minerals. However, factors which may have an impact on fixation and release of ammonium in paddy soils are still poorly understood. It is assumed that NH$$₄⁺ fixation might be promoted by the decreasing redox potential in flooded soils, because of the reduction of structural FeIII, causing an increase of the negative charge of certain clay minerals. However, since rice plants are able to secrete O$$₂ into the rhizosphere, which may oxidize FeII to FeIII, it seems possible that the release of fixed NH$$₄⁺ might be promoted by rising the redox potential in the vicinity of the roots.

A series of model experiments were conducted to establish the effect of O$$₂ secretion from imitated rice roots on the availability of non-exchangeable NH$$₄^<#1929#>+ and N uptake by imitated rice roots under flooding conditions. The soil was flooded and incubated at 30°C. A special apparatus was used to imitate O$$₂ secretion and N uptake by rice roots. A PVC pipe sealed with a nylon net at one end was inserted into the soil. One week after flooding air was pumped into the PVC pipe by a gas dispenser to imitate O$$₂ secretion from rice roots. To simulate N uptake a nylon net bag filled with an ion exchange resin mixture was placed in the PCV pipe. Results on the influence of the redox potential on the fixation and release of NH$$₄⁺ and N uptake will be discussed.

Keywords: N-utilisation, redox potential, rice

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Contact Address: Heinrich Wilhelm Scherer, University of Bonn, Institute of Plant Nutrition, Karlrobert-Kreiten-Strasse 13, 53115 Bonn, Germany, e-mail: h.schereruni-bonn.de