Biological amelioration of subsoil acidity through managing nitrate uptake by wheat crops

Abstract

Subsoil acidity occurs in many agricultural lands in the world, and is considered to be an irreversible constraint due to amelioration difficulties. This field study aimed to develop a biological method to ameliorate subsoil acidity through the rootinduced alkalisation resulting from nitrate uptake. Aluminium (Al)-tolerant wheat variety Diamondbird and Al-sensitive variety Janz (Triticum aestivum L.) were grown at two contrasting field sites with mild and severe subsurface acidity, respectively, and were supplied with either Ca(NO 3 ) 2 at the soil surface, Ca (NO 3 ) 2 at 10 cm depth or urea at 10 cm depth. Application of nitrate increased rhizosphere pH up to 0.5 units and bulk soil pH to 0.3 units, and to a depth >30 cm in the Kandosol. The placement of nitrate at 10 cm increased subsoil pH more than the surface application. Nitrate application increased nitrate concentration in soil profiles as expected, whereas urea application increased NH 4 + concentration which in turn favored acidification processes. Diamondbird generally produced more tillers and shoot biomass at anthesis but the two varieties did not differ in grain yield or rhizosphere alkalisation. Similar grain yields were achieved under supply of nitrate and urea. The results suggest that biological amelioration through managing nitrate uptake is possible as part of an integrated approach to combat subsoil acidity in farming systems.

A combination of biological activity and the nitrate form of nitrogen can be used to ameliorate subsurface soil acidity under dryland wheat farming

“…The application of nitrate at 10 cm depth tended to produce the largest pH increases. At the Chromosol site the direction of treatment effects was similar but the magnitude smaller, despite the Chromosol having a generally weaker buffering capacity (Tang et al 2011). Though statistically significant at 10-25 cm depth, the increase in pH with nitrate fertilization was only 0.1 to 0.2 pH units.…”

“…Exchangeable Al and Mn were both less than 1% of the ECEC at the Kandosol site but reached a maximum of 27% and 6% of the ECEC, respectively, at the Chromosol site. The sites had similar pH buffering capacity in the top 10 cm but the Chromosol site was more weakly buffered at 10-30 cm depth (Tang et al 2011).…”

“…The chemical properties of the two soils from 0 to 100 cm depth in 10-cm intervals were reported in Tang et al (2011). In brief, both sites had their lowest pH and effective cation exchange capacity (ECEC) between 10 and 30 cm depth, within the A horizon.…”

“…The wheat cultivar was the Al-sensitive Janz (Table 2), as had been used in 2006 (Tang et al 2011). Due to the dry conditions of the previous 2 years no further N fertiliser was applied in 2008; rather, we attempted to complete the uptake of residual N fertiliser from the previous seasons.…”

“…However, every problem carries its own solution. In 2011 we reported the first year's results of our field work attempting to minimise or reverse acidification using uptake of the nitrate form of nitrogen (N) (Tang et al 2011). …”