Nitrate leaching stimulates subsurface root growth of wheat and increases rhizosphere alkalisation in a highly acidic soil

Abstract

Subsurface acidity is a major factor limiting crop yield in some agricultural soils. The surface application of lime has limited effect on the subsurface acidity due to the slow downward movement, while deep incorporation of lime is costly. This paper tested the concept of biologically ameliorating subsurface acidity in a highly acidic soil through the net uptake of anions by plant roots. Nitrogen was supplied to the top soil (0-10 cm) as Ca(NO 3 ) 2 at rates equivalent to 30-240 kg N ha −1 . Four water levels were imposed (40, 60, 80 and 100% of field capacity). Aluminiumtolerant wheat was grown for 58 days. The high N and high water treatments stimulated root growth below 15 cm, which in turn increased N capture, resulting in a greater excess anion uptake over cations and thus alkalisation of subsurface soil layers. This study suggests that it is feasible to exploit the process of nitrate uptake by an aluminium-tolerant wheat genotype to increase pH in acidic subsoil.

Root system-based limits to agricultural productivity and efficiency: the farming systems context

“…Most studies have been conducted using very high concentrations and young plants in artificial systems. Cover crop experiments in the field, where cover crops leave the sub-soil with low N content, have demonstrated that roots of crop plants can proliferate when they encounter high levels of residual N remaining in the deep soil layers (Kristensen and Thorup-Kristensen, 2007;Weligama et al, 2010a), but also that rooting depth may be reduced if they encounter deep layers with low N concentrations. Assuming a crop is adequately supplied with N to satisfy healthy root and shoot growth, species or varietal differences in response to deep soil N concentrations are of interest (NK Ytting, unpubl.…”

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

“…Conventional surface application of lime is known to be ineffective in combating subsurface acidity largely due to slow downward movements of lime (Conyers and Scott 1989;McLay et al 1994;Conyers et al 2003). New strategies are being tested to combat subsurface acidity (Liu and Hue 1996;Hue and Licudine 1999;Weligama et al 2008Weligama et al , 2010.…”

“…One strategy to combat subsurface acidity is to supply a large amount of NO 3 − in the subsurface rhizosphere alkalisation (Weligama et al 2008(Weligama et al , 2010. When plant uptake of one type of charge exceeds the other, the plant maintains electroneutrality by extrusion of H + or OH − leading to acidification or alkalisation of rhizosphere soil Indeed, the magnitude of pH change depends on the amount of acid or alkali excreted and the pH buffering capacity of the soil.…”

“…Thus, the plants supplied with the NO 3 − form of N-fertilizers can result in alkalisation in the rhizosphere and the supply of NH 4 + form of N-fertilizers can lead to acidification of rhizosphere or exacerbate acidity of an acid soil. A supply of NO 3 − at the soil surface in a leaching environment can stimulate root growth and N capture, resulting in rhizosphere alkalisation in acidic subsoil (Weligama et al 2010). Nitrification of ammoniacal fertilizers has been recognised as a major contributor to soil acidification of agricultural soils (Conyers et al 1996;Hu et al 2002, Sumner andNoble 2003).…”

“…Further, in soils especially with low CEC, the surface application of lime can increase the possibility of downward movement of exchangeable Ca and Mg, which accompanies anions such as nitrate and sulphate (Ridley et al 2001). The uptake of nitrate by plants increases rhizosphere pH in subsoil layers provided that roots are available in that layer (Poss et al 1995;Noble et al 1997;Tang et al 2000;Weligama et al 2008Weligama et al , 2010.…”