Rate of soil acidification under wheat in a semi-arid environment

The rate of acidification under wheat in south-eastern Australia was examined by measuring the fluxes of protons entering and leaving the soil, using the theoretical framework of Helyar and Porter (1989). Monthly proton budgets were estimated for the root zone (0–90 cm layer) and for the 0–25 and 25–90 cm layers. After an annual cycle, the root zone was alkalinized by 0.5 to 3.1 kmol OH^- ha^-1. The alkalinity originated from the mineralization of the organic anions contained in the organic matter. The budget was near neutrality in the 0–25 cm layer (range: –1.0 to 1.4 kmol H⁺ ha^-1), whereas there was net alkalinization in the 25–90 cm layer (1.7 to 2.3 kmol OH^- ha^-1). In the 0–25 cm layer, the acidity produced in autumn by mineralization of organic nitrogen was counterbalanced by the alkalinity released from crop residues. The main acidifying factor in this layer was leaching of NO₃^- during early winter (2.4 kmol H⁺ ha^-1). Nitrate added through leaching was the main alkalinizing factor in the 25–90 cm layer, as added NO₃^- was taken up by the roots or denitrified in this layer. Urea fertilization had almost no effect on the rate of acidification, as little NO₃^- was leached out of the root zone. The factors acidifying the soil under wheat were limited in this environment because of the small amout of NO₃^- leached and the retention of the crop residues.