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

Abstract

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 N 2 gas or taking up NH 4 ions as the major source of N, (ii) the net nitrification of N derived from fixation or from NH 4 and R -N H 2 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, NH 4 or R -N H 2. 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 ( O H -) ions during subsequent plant decomposition. Nitrification of fixed N 2 or NH 4 and R -N H 2 based fertilizers, and loss of N from the soil either by removal of products or by leaching of NOa-N with a companion basic cation, lead to 'permanent' acidification.

Increase of Soil pH in a Solar Greenhouse Vegetable Production System

“…It has been stated that the rate of soil acidification generated from N input appears to depend largely on the form of N and, more impor- CK, no N treatment; RN, recommended N management; CN, conventional N management. tantly, the amount of NOj" leaching (Bolan et al, 1991;Matson et al, 2002). Since urea hydrolysis does not release protons, the release of NH^''' from urea will behave similarly to NH^"""-based fertilizers; therefore, the fate of NOj~ is of crucial importance in a system where amide-based urea is the major fertilizer (Reuss and Johnson, 1986), as is the case in the present study.…”

“…If all NO3-is absorbed by plants or microorganisms, the effect on the H"*" balance is nil. However, when NO^^" leaches from the system, net H"*" is generated (Bolan et al, 1991). Therefore, the amount of NO^" leached determines H"'' production rate.…”

Seasonal Changes and Treatment Effects on Soil Inorganic Nutrients Following a Decade of Fertilizer Addition in a Lowland Tropical Forest

“…Soil acidification by urea addition is well understood and also occurs to different extents with other N fertilizers (Bolan et al, 1991). A number of factors influence the decline in pH following urea application.…”

“…Acidification is promoted by proton release during the oxidation of urea to nitrate and by the loss of nitrate by leaching. At the same time, acidification is reduced by the consumption of protons during denitrification and by the release of hydroxyl ions during nitrate uptake by plants (Bolan et al, 1991). The strongly-weathered nature of many tropical forest soils, including the soil on Gigante Peninsula, means they are more sensitive to N-induced acidification than less weathered soils that have a greater capacity to buffer the decline in pH through carbonate and cation exchange reactions (Matson et al, 2002).…”

Association of aluminium tolerance with morphological traits in lucerne (<i>Medicago sativa</i> L.)

“…A possible negative influence was induced by other reasons. Also, positive correlations of Al tolerance with height (r = 0.353*) and seed yield (r = 0.339*) were possibly due to soil acidification around the roots of lucerne accessions, which later starts to depress the development of plants [24][25][26]. Such rela tions deserve attention as an adapted investigation technique could help select lucerne cultivars possessing some Al tolerance without scrupulous selection for Al tolerance under laboratory or greenhouse conditions [7,8].…”