Acidification in the Rhizosphere of Rape Seedlings and in Bulk Soil by Nitrification and Ammonium Uptake

Ammonium salts used as fertilizers may cause soil acidification by two different processes: nitrification in soil and net release of protons from roots. Their influence on soil pH may vary depending on the distance from root surface. The aim of this study was to distinguish between these two processes. For this purpose rape seedlings were grown 10 d in a system which separated roots from soil by a fine-meshed screen. As a function of distance from the plane root layer formed on the screen, pH, titratable and exchangeable acidity and NO₃- and NH₄-nitrogen were determined. The soil, a luvisol from loess, was supplied with no N or (NH₄)₂SO₄ either with or without a nitrification inhibitor (DCD). The bulk soil pH remained unaffected when no N or 400 mg NH₄? N kg^?1 soil plus DCD was applied but it decreased from 6.6 to 5.8 without DCD. In contrast, rhizosphere pH decreased in all cases, mainly within a distance of 1 mm from the root plane only, but with gradients extending to between 2 and 4 mm into the soil. The strongest pH decrease, from 6.6 to 4.9, occurred at the root surface of plants treated with both NH₄-N and DCD where most of the mineral N remained as ammonium. In this case Al was solubilized in the rhizosphere as indicated by exchangeable acidity. Total soil acidity produced in the NH₄ treatment without DCD was mainly derived from nitrification compared to root released protons. However, acidification of the rhizosphere was diminished by nitrification because nitrate ions taken up by the roots counteracted net proton release. It is concluded that nitrification inhibitors may reduce proton input from ammonium fertilizers but enhance acidification at the soil-root interface which may cause Al toxicity to plants.