Application of nitrogen in NO 3 - form increases rhizosphere alkalisation in the subsurface soil layers in an acid soil

Leaching of NO (3) (-) derived from ammoniacal fertilizers in the topsoil and subsequent uptake of NO (3) (-) by plants from deeper layers may be used as a method of biological amelioration of subsurface soil acidity. This paper reports a glasshouse column experiment testing the above concept. Nitrogen with labelled N-15 was supplied with and without lime to the surface soil (0-10 cm) as urea, (NH4)(2)SO4 or Ca(NO3)(2) at a rate equivalent to 120 kg N ha(-1). Soil columns were regularly watered from the top to facilitate NO (3) (-) leaching. An aluminium-tolerant wheat genotype was grown for 38 days. The application of lime with nitrogen fertilizers increased growth of shoot and roots in all soil layers. The application of Ca(NO3)(2) resulted in about 66% of recovery efficiency irrespective of whether lime was applied in the surface. This in turn resulted in about 0.2 units increase in rhizosphere pH in the subsurface (10-15 cm) soil layer compared to the same layer of the unlimed control. The supply of urea and (NH4)(2)SO4 alone or with lime did not increase rhizosphere pH in the subsurface soil layers. Importantly, this study indicates that it is possible to exploit the process of nitrate uptake by wheat to increase pH in acidic subsurface soil.