Nitrification in a Vertisol subsoil and its relationship to the accumulation of ammonium-nitrogen at depth

Abstract

Unusually high concentrations of ammonium have been observed in a Vertisol below 1 m depth in south-east Queensland. This study investigated the possibility that an absence of nitrification is allowing this ammonium to accumulate and persist over time, and examined the soil environmental characteristics that may be responsible for limiting nitrifying organisms. The possibility that anaerobiosis, soil acidity, soil salinity, low organic carbon concentrations, and/or an absence of active nitrifying microorganisms were responsible for limiting nitrification was examined in laboratory and field studies. The presence/absence of anaerobic conditions was determined qualitatively using a field test to give an indication of electron lability. In addition, an incubation study was conducted and soil environmental conditions were improved for nitrifying organisms by adjusting the pH from 4.4 to 7, adjusting the electrical conductivity from 1.6 to 0.5 dS/m, amending with a soluble carbon substrate at a rate of 500 mg/kg, and using microorganisms from the surface horizon to inoculate to the subsoil. Over a 180-day period no nitrification was detected in the control samples from the incubation study, indicating that an extremely low rate of nitrification is likely to be responsible for allowing ammonium to accumulate in this soil. Analysis of the effect of soil environmental conditions on nitrification revealed that anaerobic conditions did not exist at depth and that pH, EC, organic carbon, and inoculation treatments added in isolation had no effect on nitrification. However, when inoculum was added to the soil in combination with pH, a significant increase in nitrification was observed, and the greatest amount of nitrification was observed when inoculum, pH, and EC treatments were added in combination. It was concluded that the reason for the low rate of nitrification in this soil is primarily the absence of a significant population of active nitrifying microorganisms, which may have been unable to colonise the subsoil environment due to its acidic, and to a lesser extent, its saline environment.

Spatial distribution of inorganic nitrogen contents of marsh soils in a river floodplain with different flood frequencies from soil-defrozen period

“…Zhu and Ehrenfelc (1999) found that soils with higher plant root biomass had more chances to occur nitrification. Page et al (2002) reported that nitrifying organisms were likely to be absent in the subsoil due to inhibition of high alkalinity of subsoils (pH=9.0). Hence high NH þ 4 À N contents in deep soil depths were most likely caused by less plant uptake and nitrification.…”

Subsoil constraints to grain production in the cropping soils of the north-eastern region of Australia: an overview

“…5). Highly mobile ions, such as NO 3 -, have been shown to concentrate deeper within the soil profile (Thomas et al 1995;Turpin et al 1998) concentration of NH 4 + has also been reported between soil depths of 1.2 and 3 m; however, in some Vertosols, the source of this deposit is unknown (Page et al 2002).…”

“…Brigalow (Acacia harpophylla), an N 2 -fixing tree species, tends to overlay acidic subsoils (Ahern et al 1993). However, it is unclear whether these Ntransformation processes cause subsoil acidification, as the organisms involved in the N-cycle are either absent or inhibited by soil environmental conditions in the subsoil (Page et al 2002).…”

Using quantity/intensity relationships to assess the potential for ammonium leaching in a Vertosol

“…Secondly, it is unusual that nitrification has not diminished NH 4 + concentrations, given that this usually occurs rapidly in most agricultural soils (Tate 2000). Previous studies of the Warra soil have determined that the reason that nitrification has not diminished NH 4 + concentrations is that the nitrification rate is extremely low (undetectable over a 180-day period), and limited by the lack of an active nitrifying population (Page et al 2002). However, studies have so far failed to determine the original source of the NH 4 + .…”