Using quantity/intensity relationships to assess the potential for ammonium leaching in a Vertosol.
High concentrations of exchangeable ammonium (N[H.sub.4.sup.+]) (in the order of 0.1 cmol/kg) have been observed below 1.2 m in a Vertosol soil near Warra in south-eastern Queensland (Hossain et al. 1996). Nitrogen is often one of the most limiting nutrients for plant growth. Consequently, this large reservoir of N[H.sub.4.sup.+] would be a valuable resource for the agricultural sector if it could be recycled into the surface of the profile through, for example, the incorporation of deep-rooted species into cropping rotations.
The accumulation of subsoil N[H.sub.4.sup.+]-N observed at this site is unusual for 2 reasons. Firstly, no obvious source of N[H.sub.4.sup.+] production can be immediately identified. Secondly, it is unusual that nitrification has not diminished N[H.sub.4.sup.+] 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 N[H.sub.4.sup.+] 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 N[H.sub.4.sup.+]. Some of the most common pathways of N[H.sub.4.sup.+] 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 N[H.sub.4.sup.+]-based minerals could be detected), and that fixed N[H.sub.4.sup.+] concentrations are low (<0.2 cmol/kg) and not believed to contribute significantly to exchangeable N[H.sub.4.sup.+] concentrations (K. Page, unpublished data). However, in order to be able to successfully manage the subsoil N[H.sub.4.sup.+] at this site, it is important to understand how the N[H.sub.4.sup.+] has formed. This will also allow identification of other sites that may contain high concentrations of subsoil N[H.sub.4.sup.+].
At the Warra site, elevated N[H.sub.4.sup.+] concentrations exist under areas currently used for dryland wheat (Triticum aestivum L.) and sorghum (Sorghum bicolor) cropping, but not in adjacent areas of native vegetation (Page et al. 2002). This suggests that the conversion of land to cultivated agriculture has altered the soil environment and triggered N[H.sub.4.sup.+] production. One of the major changes caused by land clearing is an increase in water movement through the soil profile (Conacher and Conacher 1995). Consequently, it is possible that if increased water movement also increased the rate of N[H.sub.4.sup.+] movement, leaching could be responsible for some or all of the subsoil N[H.sub.4.sup.+] accumulation.
Ammonium leaching seldom occurs to any great extent in soils, due to the low concentrations of N[H.sub.4.sup.+] in most surface soils, and the retention of this N[H.sub.4.sup.+] by cation exchange sites (Mengel 1985). Although significant N[H.sub.4.sup.+] leaching has been observed in some instances, this has occurred in soils that have low CEC/clay contents (Deare et al. 1995; Li et al. 1997; Gundersen 1998), contain large amounts of organic or N[H.sub.4.sup.+] fertiliser (Deare et al. 1995; Li et al. 1997), and/or have environmental characteristics that inhibit nitrification and allow a build-up of N[H.sub.4.sup.+] in the surface soil (Haynes 1986; Wang and Bettany 1995). These conditions are in marked contrast to the soil at Warra, which is characterised by a high clay content [56% at 0-0.10 m, 66% at 1.2-1.5 m (Dalal et al. 1995)], is not fertilised or irrigated, and does not have accumulations of N[H.sub.4.sup.+] in the surface soil (Page et al. 2002).
Obviously, large amounts of N[H.sub.4.sup.+] leaching over short time periods are unlikely to occur at Warra. However, if very small amounts of N[H.sub.4.sup.+] are leached into the subsoil each year, over the 65 years since the site was cleared, a gradual build-up of N[H.sub.4.sup.+] may have occurred (given the absence of a significant nitrification rate). If leaching were the source of the N[H.sub.4.sup.+], it would be expected that other soils with low subsoil nitrification rates would also exhibit N[H.sub.4.sup.+] accumulations. However, it is known that in soils with very similar soil characteristics to Warra, including undetectable subsoil nitrification rates, this is not the case (K. Page, unpublished data). Consequently, if leaching is the source of subsoil N[H.sub.4.sup.+], there must be some difference in N[H.sub.4.sup.+] movement through the Warra soil compared with these other sites.
The identification of different rates of N[H.sub.4.sup.+] leaching between soils that do/do not contain subsoil ammonium through field trials would require extensive long-term studies. However, the quantity/intensity (Q/I) relationship is one technique that can relatively quickly describe the adsorption capacity of a soil for N[H.sub.4.sup.+], and thus indicate the ability of a soil to retain N[H.sub.4.sup.+] against leaching. This technique does not provide realistic data of the amount of N[H.sub.4.sup.+] that would be leached in the field, and does not take into account site effects such as preferential leaching. However, it is useful as a preliminary method to identify differences in the exchange capability of different soils to determine whether long-term field trials may be warranted. The aim of this study was therefore to use the Q/I technique as a preliminary method to determine whether there are any differences between the exchange characteristics of N[H.sub.4.sup.+] in the Warra soil, compared with 2 other soils that did not contain elevated levels of subsoil N[H.sub.4.sup.+].
Materials and methods
Soils from 3 sites located in south-eastern Queensland at Warra (26[degrees]47'S, 150[degrees]53'E), Billa Billa (28[degrees] 26'S, 150[degrees] 18'E), and Condamine (27[degrees]00'S, 149[degrees]56'E) were used. All 3 soils were located on the riverine floodplain and characterised by neutral to alkaline surface horizons that became acidic (pH <5) below 1 m depth. Both the Warra and Billa Billa soils were Grey Vertosols, whereas the Condamine soil was a Brown Vertosol (Isbell 1996). The Warra and Condamine sites were originally dominated by brigalow vegetation (Acacia harpophylla), whereas the Billa Billa soil was dominated by Belah (Casuarina cristata). When sampling for this study was conducted, ammonium concentrations in the subsoil between 1.2 and 3 m averaged ~0.09 cmol/kg at Warra and 0.01 cmol/kg at Billa Billa and Condamine. Dryland wheat (Triticum aestivum L.) is the dominant crop grown on each of the sites sampled.
Soil sampling and characterisation
Soil was collected from the 0-0.3 m layer of the profile using a hydraulically operated soil sampler that extracted soil cores of 0.042 m diameter. Five soil cores were taken evenly spaced along a 10-m transect at each of the study sites. There was no change in elevation along the transect. Once collected, all soil cores were placed in plastic bags, sealed, and transported back to the laboratory where they were air-dried at 40[degrees]C and ground to <2 mm.
The various chemical characteristics of the soil layers used in the experimental work are presented in Table 1. The pH and EC values presented in this table were obtained from a 1:5 soil:water extract using standard combination reference electrodes, and exchangeable cations were analysed from a 1: 10 soil: 0.1 m Ba[Cl.sub.2]/N[H.sub.4]Cl extract using an ICP-AES. Exchangeable N[H.sub.4.sup.+] was analysed from a 1:10 soil:KCl extract using the colorimetric technique referred to below. 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. (2002, 2003).
All soils were initially treated to replace existing exchangeable cations with calcium ([Ca.sup.2+]). This was achieved by weighing 4.5 g of air-dried soil (sieved to <2 mm) into 50-mL centrifuge tubes and washing individual soil samples 3 times each with solutions of 1 M, 0.1 M, and then 0.01 m Ca[Cl.sub.2] in a 1:10 soil:solution ratio. Once soil washing was complete, solutions of 0.01 mol/L Ca[Cl.sub.2] containing 8 different concentrations of N[H.sub.4]Cl (0, 0.025, 0.05, 0.1, 0.15, 0.2, 0.3, and 0.4 mmol/L) were prepared. This range of ammonium concentrations was used because, although it was lower than commonly used in Q/I studies, it more realistically represented the N[H.sub.4.sup.+] concentrations that would be expected to exist in soils that do not regularly receive organic/nitrogen-based fertilisers, such as the soils being studied. Ammonium solutions were then added to soil samples in a 1:10 soil:solution ratio along with nitrapyrin (2-chloro-6-(trichlormethyl) pyridine) at a rate of 2 mg/L to inhibit nitrification (Thompson and Blackmer 1992). Samples were then shaken on a rotary shaker at 22[degrees]C for 1 h. It should be noted that a preliminary experiment was conducted using the Warra soil to determine that a 1-h shaking period was sufficient for all exchange reactions to occur in these Vertosol soils. Analysis from this testing revealed no significant difference between 1- and 24-h shaking times. After shaking, samples were centrifuged and the supernatant solution analysed for N[H.sub.4] using a colorimetric procedure based on the indophenol blue technique (Henzell et al. 1968), and for [Ca.sup.2+] using a GBC Avanta atomic absorption spectrophotometer. All experiments were replicated 5 times.
Quantity/intensity graphs were constructed by plotting the change in the N[H.sub.4.sup.+] concentration of the extracting solution after shaking, against the activity ratio for N[H.sub.4.sup.+] ([AR.sup.NH4]). The [AR.sup.NH4] in equilibrium solutions was calculated using the following equation (Pasricha 1976):
[AR.sup.NH4] = [mN[H.sub.4]([gamma]N[H.sub.4])]/[[(mCa).sup.1/2][([gamma]Ca).sup.1/2]]
where m is the equilibrium concentrations (mol/L), [gamma]N[H.sub.4] is the single ion activity coefficient for N[H.sub.4.sup.+], and [gamma]Ca is the single ion activity coefficient for [Ca.sup.2+]. Activity coefficients were calculated using the Davis equation by the computer program PHREEQC (Parhurst 1995).
It should be noted that the Q/I technique was originally developed using potassium ([K.sup.+]). However, because of the similar physical properties of [K.sup.+] and N[H.sub.4.sup.+] (Bohn et al. 1979), this technique has also been widely applied to the study of N[H.sub.4.sup.+] (Pasricha 1976; Lumbanraja and Evangelou 1990, 1994; Thompson and Blackmer 1992). A typical Q/I curve generally has a small curvilinear section at low AR, which then becomes linear as the AR increases (Fanning et al. 1989). The work originally conducted for the development of the Q/I technique determined that the non-linear response observed in the [K.sup.+] Q/I curve could be attributed to adsorption by high affinity sites (including sites where [K.sup.+] had become 'fixed' in the clay interlayer), whereas the linear portion of the curve could be attributed to adsorption by less selective sites (planar or edge sites) (Beckett 1964). Consequently, it is possible to estimate the number of different adsorptions sites present in a soil from the Q/I curve. The amount of N[H.sub.4.sup.+] held on high affinity sites in this study was estimated by calculating the distance between the y-intercept of the linear portion of the curve, and the Q/I plot value obtained at AR = 0 [determined by extrapolation using a curve-plotting program (Jandel Scientific 1994)]. The linear section of the curve was assumed to be that section whose points fell within a 90% confidence interval for linearity. It is also possible to estimate the potential buffer capacity (PBC) of the soil for N[H.sub.4.sup.+], or the ability of the soil to maintain a certain concentration of N[H.sub.4.sup.+] in solution against removal by plant uptake or leaching. This was calculated using the gradient of the linear portion of the curve (Fanning et al. 1989).
Analysis of any differences in PBC and high affinity sites between sampling locations was determined using 1-way ANOVA. Differences were considered significant if P < 0.05.
Graphs illustrating the average shape of Q/I isotherms for each soil are shown in Fig. 1. For all the soils examined, the Q/I plot exhibited minimal curvilinear behaviour at low [AR.sup.NH4]. An estimation of the number of high affinity sites present at each location is given in Table 2. Statistical analysis revealed that there was no significant difference between the amounts of ammonium adsorbed at high affinity sites in Warra compared with Condamine or Billa Billa soils, but that the Billa Billa soil did have more high affinity sites than Condamine (Table 2). The PBC of the soil for ammonium was significantly different in all the soils examined, with the buffering capacity of the soil being greatest at Warra followed by Billa Billa, then Condamine (Table 2).
The presence or absence of a large number of high affinity N[H.sub.4.sup.+] retention sites can be useful for estimating the relative ability of 2 soils to retain N[H.sub.4.sup.+]. This is because in soils with a large number of high affinity sites, a greater proportion of the N[H.sub.4.sup.+] is held tightly on the exchange, and thus should be more resistant to leaching. Comparison of the 3 soils examined revealed that over the concentration range studied, the number of high affinity sites calculated to be present in each soil was low, and there was no significant difference between the amount of N[H.sub.4.sup.+] retained on high affinity sites in the Warra soil relative to the Condamine or Billa Billa soils (Table 2). This result indicates that there is no evidence to suggest that N[H.sub.4.sup.+] would be more likely to leach through the Warra profile relative to the other 2 soils because of differences in the amount of N[H.sub.4.sup.+] retained via high affinity sites. Indeed, the fact that a significantly lower amount of N[H.sub.4.sup.+] was calculated to have been retained via high affinity sites in the Condamine relative to the Billa Billa soil (Table 2) indicates that it would be Condamine rather than Warra that would allow a greater amount of N[H.sub.4.sup.+] leaching due to less N[H.sub.4.sup.+] retention at high affinity sites.
The trend in PBC between the 3 sites also failed to indicate that N[H.sub.4.sup.+] would be more likely to leach in the Warra soil. The PBC is a measure of the soil's ability to buffer the soil solution against depletion of an ion, and is largely a function of the soil's cation exchange capacity (Fanning et al. 1989). It has been found that for the same [K.sup.+] saturation [and therefore for N[H.sub.4.sup.+] saturation, given the similar behaviour of these ions (Bohn et al. 1979)], a soil with a higher PBC will retain a greater proportion of any [K.sup.+] present on the exchange, and less in the soil solution (Fanning et al. 1989). Consequently, at any one time, there will be less N[H.sub.4.sup.+] available for leaching in soils with a greater PBC.
In the soils examined in this study it was revealed that the Warra site had the greatest PBC, followed by Billa Billa, and then Condamine (Table 2). These data indicate that, all things being equal, the Warra soil is likely to have a lower concentration of N[H.sub.4.sup.+] in the soil solution at any given time compared with the other 2 soils examined. Consequently, there is no evidence to suggest that the exchange complex of the Warra soil is likely to allow a significantly greater amount of N[H.sub.4.sup.+] leaching than the other 2 soils examined. If anything, the evidence indicates that the Warra soil has the greater capacity to retain N[H.sub.4.sup.+] against leaching.
It should be noted that the N[H.sub.4.sup.+] concentration used in this study was lower than is often used when employing the Q/I technique to study N[H.sub.4.sup.+] adsorption. As a result, it is possible that the reason that a low number of high affinity sites were calculated to be present in these soils was that the N[H.sub.4.sup.+] added was insufficient to completely occupy all the high affinity sites available. If this were the case, the top of the curvilinear section of the Q/I curve would never have been reached, and the point at which N[H.sub.4.sup.+] retention was dominated by less specific exchange sites (i.e. the linear section of the Q/I curve) would never have been observed. In other studies that have used the Q/I technique to examine N[H.sub.4.sup.+] adsorption at higher N[H.sub.4.sup.+] loadings, the linear section of the Q/I curve did not begin until an [AR.sup.NH4] of between approximately 0.002 and 0.015 was reached (Pasricha 1976; Lumbanraja and Evangelou 1990). This is markedly above the maximum [AR.sup.NH4] of approximately 0.002 observed in this study (Fig. 1). Consequently, it is possible that if greater N[H.sub.4.sup.+] loadings had been used in the current study, a change in the gradient of the Q/I curve may have occurred at higher [AR.sup.NH4], and thus a greater number of high affinity sites would have been calculated to be present [although it should be noted that studies using much higher loadings of N[H.sub.4.sup.+] have also found a complete absence of high affinity sites in soils with smectite mineralogy, similar to the Warra site (Egashira et al. 1998)].
In order to assess the relative likelihood of N[H.sub.4.sup.+] leaching in these 3 soils, it is considered important to examine N[H.sub.4.sup.+] behaviour within a concentration range that could realistically be expected to exist in the field. Although the N[H.sub.4.sup.+] concentrations used in this study are lower than commonly employed for Q/I work, they are considered appropriate because none of the soils examined is regularly fertilised. The data obtained provide evidence that none of the soils showed preferential adsorption of ammonium at low [AR.sup.NH4] over the concentration range studied. This is considered sufficient evidence to indicate that N[H.sub.4.sup.+] leaching would not differ markedly between soils because of the presence/absence of preferential exchange sites, over the range of ammonium concentrations likely to be present in the field. Obviously, this study is not capable of predicting whether these are differences in N[H.sub.4.sup.+] movement between these soils under field conditions, especially given the potential for macropore flow in these Vertosol soils. However, given the lack of any marked difference in their N[H.sub.4.sup.+] exchange characteristics, and given the low N[H.sub.4.sup.+] concentrations in the surface of these soils (Table 1), N[H.sub.4.sup.+] leaching would appear to be an implausible hypothesis to account for the presence of subsoil N[H.sub.4.sup.+] accumulations at Warra but not other similar soils.
Since this study has clearly failed to identify the source of the subsoil N[H.sub.4.sup.+] at the Warra site, further hypotheses regarding the mechanism of N[H.sub.4.sup.+] formation need to be identified. As discussed previously, it is already known that mineralisation (Page et al. 2003) and fixed N[H.sub.4.sup.+] release (K. Page, unpublished data) are not capable of significantly contributing to exchangeable N[H.sub.4.sup.+] concentrations. However, it is known that N[O.sub.3.sup.-] will readily leach through Vertosol soils such as Warra (Waring and Saffigna 1984; Catchpoole 1992; Ridge et al. 1996), and the possibility that any N[O.sub.3.sup.-] leached could be reduced to N[H.sub.4.sup.+] is another pathway yet to be investigated. This possibility will form the basis of future studies.
The experiments conducted in this study failed to provide any evidence that ammonium was more likely to leach through the surface horizon of the Warra soil compared with other similar soils where accumulations of subsoil ammonium have not been observed because of marked differences in soil exchange characteristics. Obviously, these results are not representative of field conditions, especially given the potential for macropore flow in these Vertosol soils. However, given the lack of nitrogen fertilisation, the moderate rainfall (630 mm/year), and the high clay content of the Warra soil (>50%), it is believed to be highly unlikely that leaching of ammonium is the primary cause of the subsoil accumulation in this soil.
Table 1. Summary of relevant site characteristics for the 0-0.3 m layer of the study soils Values in parentheses are standard errors Site pH Organic C Organic N (%) (%) Warra 8.8 0.54 0.05 Billa Billa 8.4 0.64 0.05 Condamine 8.6 0.73 0.05 Values in parentheses are standard Exchangeable cations errors (cmol/kg) EC (dS/m) N[H.sub.4] Ca Warra 0.2 0.02 (0.002) 12.7 (0.14) Billa Billa 0.1 0.02 (0.002) 7.9 (0.36) Condamine 0.2 0.03 (0.001) 8.8 (0.41) Exchangeable cations (cmol/kg) K Mg Na Warra 0.4 (0.01) 4.5 (0.03) 4.5 (0.12) Billa Billa 0.5 (0.06) 3.3 (0.05) 4.3 (0.27) Condamine 0.2 (0.02) 2.2 (0.15) 1.0 (0.06) Table 2. Average potential buffer capacity (PBC), high affinity sites (HAS), and [R.sup.2] values for the linear regression curve measured from Q/I plots for the Warra, Billa Billa, and Condamine soils Within a column, values followed by the same letter are not significantly different (P > 0.05) Site [R.sup.2] PBC HAS (cmol/kg) Warra 0.99 141a 0.042ab Billa Billa 0.98 108b 0.061b Condamine 0.98 76c 0.033a
The authors thank Dr Wayne Strong for his advice throughout the project, and Denise Orange and John Cooper for their assistance with soil sampling.
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Manuscript received 23 May 2002, accepted 12 November 2002
K. L. Page (A), N. W. Menzies (A), and R. C. Dalal (B)
(A) School of Land and Food Sciences, The University of Queensland, St Lucia, Qld 4072, Australia.
(B) Department of Natural Resources and Mines, Indooroopilly, Qld 4068, Australia.
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|Author:||Page, K.L.; Menzies, N.W.; Dalal, R.C.|
|Publication:||Australian Journal of Soil Research|
|Date:||Mar 1, 2003|
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