Subsoil nitrogen mineralisation and its potential to contribute to NH4 accumulation in a Vertosol

Abstract

High concentrations of NH 4 + (up to 270 kg N/ha) have been observed in a Vertosol below 1 m depth in south-east Queensland. This study examined the possibility that mineralisation associated with the removal of native vegetation (Acacia harpophylla) for cropping was responsible for the production of NH 4 + . Particularly, the potential contribution of decomposing root material and/or dissolved organic nitrogen (DON) leached into the subsoil after clearing was investigated. The amount of N that was contained within native vegetation root material was determined from an area of native vegetation adjacent to the cleared site containing elevated NH 4 + concentrations. In addition, the amount of NH 4 + that could be mineralised in the native vegetation soil was determined by monitoring NH 4 + concentrations over 360 days in intact cores, and by conducting waterlogged incubations. To determine the rate at which a source of DON leached into the subsoil would mineralise, soil was amended with glutamic acid at a rate of 250 mg N/kg and placed under waterlogged incubation. The possibility that the acidic pH of the subsoil, or the lack of a significant subsoil microbial population, was inhibiting mineralisation was also examined by increasing soil pH from 4.4 to 7.0, and inoculating the subsoil with surface soil microorganisms during waterlogged incubations. Low concentrations of N, approximately 90 kg N/ha between 1.2 and 3 m, were found in the native vegetation root material. In addition, no net N mineralisation was observed in either the extended incubation of intact cores or in the control samples of the waterlogged incubations. Net N mineralisation was also not detected when the subsoil was amended with a source of organic N. Results indicate that this lack of mineralisation is largely due to pH inhibition of the microbial population. It is concluded that the mineralisation of either in situ organic material, or DON transported to the subsoil during leaching events, is unlikely to have significantly contributed to the subsoil NH 4 accumulation at the study site. S R 0 2 0 3 8 M i n e r a l i s a t i o n i n a V e r t o s o l s u b s o i l K . L . P a g e e t a l .

Crop management effects on soil carbon sequestration on selected farmers? fields in northeastern Ohio

“…Wider C:N ratio in the subsoil layers in wooded than in cropland fields may be due to high level of SOC attributed to deep root system of native perennials. Page et al (2003) reported that removal of deep-rooted native vegetation (Acacia harpophylla Benth.) and introducing shallow-rooted crops changed the soil environment, lowered SOC, and contributed to subsoil NH 4 concentration.…”

Vertical distribution of soluble organic nitrogen, nitrogen mineralization, nitrification, and amidohydrolase activities in a manure‐treated soil

“…Jones and Hodge (1999) reported that 10% of amino acids were adsorbed to the solid phase. Page et al (2003) showed that net N mineralization was not detected in the subsoil (2.7-3 m) of a Vertisol amended with a source of organic N. The nitrification rate in the subsoil of the Vertisol was extremely low due to lacking of an active nitrifying population in deep soil layers (Page et al, 2002). They believed that the microbial population of the subsoil had lower capacity for N mineralization than surface soil organisms.…”

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

“…Since this study has clearly failed to identify the source of the subsoil NH 4 + at the Warra site, further hypotheses regarding the mechanism of NH 4 + formation need to be identified. As discussed previously, it is already known that mineralisation (Page et al 2003) and fixed NH 4 + release (K. Page, unpublished data) are not capable of significantly contributing to exchangeable NH 4 + concentrations. However, it is known that NO 3 will readily leach through Vertosol soils such as Warra (Waring and Saffigna 1984;Catchpoole 1992;Ridge et al 1996), and the possibility that any NO 3 leached could be reduced to NH 4 + is another pathway yet to be investigated.…”

“…Total carbon and nitrogen were analysed using a LECO CNS 2000 combustion analyser. A more detailed description of the Warra soil down to 3 m is given in Page et al (2002Page et al ( , 2003.…”

“…However, studies have so far failed to determine the original source of the NH 4 + . Some of the most common pathways of NH 4 + formation, such as net N-mineralisation, are undetectable in the subsoil (Page et al 2003). Mineralogical analysis has also revealed that the soil is dominated by smectite minerals to 3 m (no NH 4 + -based minerals could be detected), and that fixed NH 4 + concentrations are low (<0.2 cmol/kg) and not believed to contribute significantly to exchangeable NH 4 + concentrations (K. Page, unpublished data).…”