Processes of soil acidification during nitrogen cycling with emphasis on legume based pastures

In areas that remain unaffected by industrial pollution soil acidification is mainly caused by the release of protons (H⁺) during the oxidation of carbon (C), sulphur (S) and nitrogen (N) compounds in soils. In this review the processes of H⁺ ions release during N cycling and its effect on soil acidification are examined. The major processes leading to acidification during N cycling in soils are: (i) the imbalance of cation over anion uptake in the rhizosphere of plants either actively fixing N₂ gas or taking up NH₄⁺ ions as the major source of N, (ii) the net nitrification of N derived from fixation or from NH₄⁺ and R-NH₂ based fertilizers, and (iii) the removal of plant and animal products containing N derived from the process described in (i) and losses of NO₃-N by leaching when the N input form is N₂, NH₄⁺ or R-NH₂. The uptake of excess cations over anions by plants results in the acidification of the rhizosphere which is a "localized" effect and can be balanced by the release of hydroxyl (OH^-) ions during subsequent plant decomposition. Nitrification of fixed N₂ or NH₄⁺ and R-NH₂ based fertilizers, and loss of N from the soil either by removal of products or by leaching of NO₃-N with a companion basic cation, lead to 'permanent' acidification.