Biological amelioration of subsoil acidity via excess anion uptake by wheat

Excess anion uptake in the form of nitrate has been shown to reduce soil acidification at depth. This root-induced alkalization of rhizosphere soil can be extended to bulk soil. This study investigated ways to maximize the alkalinizing effect of calcium nitrate in reducing subsoil acidity in wheat (ET8) and canola (44Y90). A controlled environment experiment was carried out over 35 days. A Chromosol with topsoil (0-8 cm, pHCaCl25.4 and 1.5 mg Al kg-1) and subsurface soil (8-15 cm, pHCaCl2 4.8 and 2.9 mg Al kg-1) layers was used. The soil was reconstructed as 0-10 cm topsoil and 10-50 cm subsurface soil in columns (15 cm in diameter, 60 cm in height). Air-dried soils (<2 mm) were treated with three N fertilizers: 1) control, 2) urea and 3) Ca(NO3)2 with and without phosphorus fertilizer at three depths: 1) 0-10 cm, 2) 10-20 cm and 3) 20-30 cm. All N and P fertilizers were applied at 134 kg N ha-1 and 56 kg P ha-1 at sowing. Uptake of Ca(NO3)2 increased pH up to 0.2 units of bulk soil in the 0-10 cm layer compared with the urea application regardless of the placement of the treatments. Rhizosphere alkalization was greater at the depth where nitrate and P were combined compared with those with the urea treatment. We highlighted the importance of balancing nutrient supply at depth in encouraging anion uptake by plants, which enhances rhizosphere alkalization in acid subsoil.