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Influence of moist--dry cycles on pH changes in surface soils.

Abstract

It is well established that in the moderately acidic soils of southern Australia, the 0-2 cm layer commonly has a higher pH than soil layers between 2 and 10 cm depth. The surface 2 cm of soil is also exposed to much greater fluctuations of moisture content than deeper soil layers. There are contradictory or speculative reports in the literature on how soil moisture fluctuation affects pH and processes which influence pH. Therefore, the aim of this study was to determine the effect of moist-dry cycles on pH, and on processes involving H+ transformations, in 3 surface soils (0-2 cm) sampled from southern New South Wales.

Following a pre-incubation, the 3 surface soils were incubated for 28 days at 30 [degrees] C and were: (i) maintained continuously dry, (ii) subjected to short (2 days dry, 5 days moist) or long (7 days dry, 7 days moist) moist-dry cycles, or (iii) maintained continuously moist.

During the incubation, the pH of continuously dry soil slightly increased by 0.03-0.10 units, while the pH of continuously moist soil decreased by 0.16-0.39 units. In soils subject to both short and long moist-dry cycles, the pH decreased by 0.06-0.34 units. However, relative to soils maintained moist, exposure to moist-dry cycles suppressed acidification by 0.05-0.26 pH units.

In dry soils the pH increased, since some of the [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] produced by net N mineralisation was not subsequently nitrified, and there was a net reduction of Mn. In soils which received water, acidification was predominately attributed to nitrification. Relative to soils maintained moist, acidification was suppressed by 1.6--6.5 mmol [H.sup.+]/kg due to the 11-35% decrease of nitrification on exposure to moist-dry cycles. In acidic surface soils (pH [is less than] 5.5), acidification rates were further suppressed by 0.1-1.0 mmol [H.sup.+]/kg due to the 1.06-2.06 times greater net Mn reduction in moist-dry soils than in continuously moist soils.

Additional keywords: intermittent drying, soil acidity, surface soil pH gradients.

Introduction

Throughout much of the moderately acidic soil of southern Australia, the pH within the surface 0-2 cm is commonly 0.2-1.4 units higher than in the subsurface layer between 2 and 10 cm depth (Conyers and Scott 1989; Young et al. 1995). The presence of an acidic subsurface soil layer influences soil water (Pinkerton and Simpson 1986a) and nutrient availability, e.g. calcium and magnesium ions ([Ca.sup.2+] and [Mg.sup.2+]) (Conyers and Scott 1989), nitrogen (N) (Young et al. 1995; Evans et al. 1998), phosphorus (P) (McLaughlin et al. 1990; Purnomo and Black 1994); the distribution of Rhizobium trifoli (Richardson and Simpson 1988); and root growth (Pinkerton and Simpson 1986a, 1986b).

Surface soil layers are exposed to frequent drying and rewetting cycles of much greater magnitude than experienced in the deeper soil layers (Campbell and Biederbeck 1976). There are conflicting reports about the effect of rewetting dry soil on pH (Jager and Bruins 1975; Barlett and James 1980; Haynes and Swift 1989; Walworth 1992). Furthermore, these pH changes have not been quantitatively attributed to the various processes within soils, nor have they been investigated within soil from the surface 2 cm.

It has been shown that the pH change within surface soils sampled from southern New South Wales can be predominately attributed to net N mineralisation and subsequent nitrification, net organic anion oxidation, and net Mn reduction (Conyers et al. 1995). However, although many workers (e.g. Soulides and Allison 1961; Seneviratne and Wild 1985) have observed that moist-dry cycles enhance N and C mineralisation, others (e.g. van Veen et al. 1985; van Gestel et al. 1991) have observed that moist-dry cycles suppress or have little effect on these processes. The effect of moist-dry cycles on net Mn reduction remains speculative (Ritchie 1989).

The main objective of the present experiments was to ascertain whether the pH in the 0-2 cm layer of soils sampled from southern New South Wales is subject to less acidification because of soil moisture effects on biological processes (net N mineralisation, nitrification, and net organic anion oxidation) and chemical processes (net Mn reduction), which, in turn, alter net [H.sup.+] production. A second objective was to determine whether the [H.sup.+] budget described by Conyers et al. (1995) accurately calculates the pH change observed in soils subject to various moisture treatments.

Materials and methods

Soil sampling sites

Soil was sampled from 3 sites within the Wagga Wagga region of southern New South Wales during September 1995. The soils sampled were a Yellow Sodosol (YS), Red Chromosol (RC), and Red Kandosol (RK) (Isbell 1996). The YS was sampled from under a grazed annual ryegrass and subterranean clover pasture at Book Book. The other 2 sites were located within the experimental farm of Charles Sturt University. The RK soil was sampled from a plot of direct-drilled wheat, while the RC soil was sampled from a plot of grazed oats. The RK soil had been fertilised with 60 kg/ha of urea in May 1995, and 110 kg/ha of di-ammonium phosphate in September 1995. Characteristics of these soils are given in Table 1.

Table 1. Clay content, [pH.sub.Ca] (1:5 soil:0.01 M Ca[Cl.sub.2]), pH buffering capacity (pHBC), cation exchange capacity (CCC), organic carbon (OC), and gravimetric soil moisture content at field capacity of Yellow Sodosol (YS), Red Chromosol (RC), and Red Kandosol (RK) soils from 0-2 cm depth
Soil Clay [pH.sub.Ca] pHBC CEC OC Field
 (%) (mmol/kg. (cmol(+)/ (%) capacity
 [Delta]pH) kg) (%)

YS 24 4.28 27.0 4.56 3.6 21.3
RC 27 5.98 18.9 9.81 2.6 15.4
RK 31 5.11 21.7 6.81 2.4 13.5


A 4-in (1 in = 2.54 x [10.sup.-2] m) auger was used to expose the soil profile to a depth of approximately 15 cm. The surface 2 cm of soil was then collected using a trowel. At each of the 3 sites, soil was collected from 50 randomly selected locations within a 40-[m.sub.2] area. The soil samples were bulked, air-dried (gravimetric moisture content [is less than] 2.2%), and ground to pass a 2-mm sieve. Soil was then stored at room temperature for 2 months prior to incubation.

Experiment 1: Short moist-dry cycles

As soil had been stored in an air-dry state, 30-g soil samples were pre-incubated in sealed 375-mL incubation jars at 80% field capacity and 20 [degrees] C for 7 days. Soils were wet to 80% field capacity by adding deionised water using a fine-tipped pipette. The incubation jars were then allocated to 3 soil moisture treatments: (i) air-dried, (ii) 4 moist-dry cycles of approximately 2 days air-dry and 5 days at 80% field capacity, and (iii) maintained at 80% field capacity. Drying was achieved by removing the lids of the incubation jars. Soil gravimetric moisture content was [is less than] 2% within 20 h of lid removal.

The incubation was conducted at 30 [degrees] C for a period of 28 days. Each soil treatment was replicated 4 times and the incubation jars were completely randomised within a fan-forced incubator. Soil was destructively sampled on Days 0, 7, 14, and 28 of incubation. Therefore, for each soil type there was a total of 48 soil samples incubated (3 treatments x 4 replicates x 4 samplings). Soil samples were analysed for pH and the concentration of [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII], extractable Mn, and dissolved organic carbon (DOC). The moisture content of soil samples was not altered prior to chemical analysis. Measurement of soil pH and the extraction of soil for chemical analysis were performed on the day that the soil was sampled. Soil extracts were stored at 0-4 [degrees] C for 2-10 days prior to chemical analysis.

Experiment 2: Long moist-dry cycles

The second experiment was similar to the first except that the moist-dry soil treatment consisted of 2 cycles of 7 days dry and 7 days moist. There was no continuously dry soil treatment, and sampling took place on Days 0, 7, 14, 21, and 28 of incubation.

Chemical analysis

Soil pH was determined using a 1:5 ratio of soil:0.01 M Ca[Cl.sub.2] (p[H.sub.Ca]) or soil:deionised water (p[H.sub.w]). Samples were shaken end-over-end for 1 h and allowed to settle for 0.5 h. The pH of the supernatant was measured using a combination glass and Ag-AgCl electrode which had been calibrated with buffers of 6.88 and 4.00. The detection limit of the pH meter was within 0.02 units.

Soil mineral N was extracted from soil using a 1:5 ratio of soil: 1 M KCl. Samples were shaken end-over-end for 1 h and then filtered through a Whatman 5 paper. The [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] concentration was determined by the colorimetric analysis of the emerald-green [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]-salicylate complex at 640 nm (Crooke and Simpson 1971). The [NO.sub.3]-N concentration was determined using cadmium reduction and the colorimetric analysis of the pink/purple N-I-naphthylethlenediamine complex at 540 nm (Henriksen and Selmer-Olse 1970). The colorimetric measurements of [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]-N and [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] concentration were made using a continuous flow ALPKEM analyser (Flow Solution IV, USA). The detection limit of this method was 0.20-0.25 mg N/kg.

Soil DOC concentration was determined in a 1: 2 soil: distilled water extract. This extract was shaken for 1 h and filtered through a 0.45-[micro]m pore-size polycarbonate filter. A mixture of 5 mL of soil water extract, 5 mL of 0.006 N potassium dichromate, and 15 mL of concentrated sulfuric acid (Burford and Bremner 1975) was maintained at 170 [degrees] C in a digestion block for 0.5 h (Yeomans and Bremner 1988). Samples were then analysed colorimetrically for DOC concentration, as [Cr.sup.3+] product, using a spectrophotometer at 600 nm (Heanes 1984). We had no method to measure the oxidation of particulate organic anions.

Soil-extractable Mn concentration was measured in 1:5 ratio of soil:0.01 M Ca[Cl.sub.2]. Extracts were shaken end-over-end for 1 h prior to filtering through a Whatman No. 42 paper. The extractable Mn concentration was determined using atomic absorption spectroscopy. The assumption was made in this study that 0.01 M Ca[Cl.sub.2] extracted [is greater than] 80% of the reduced Mn (Conyers 1992).

For each of the 3 soils studied, a pH titration curve was constructed by measuring the P[H.sub.Ca] of soil samples amended with various volumes of 0.01 M Ca[Cl.sub.2] and 0.03 m [HNO.sub.3]. The volumes of these solutions were adjusted in order to obtain a pH range, yet also maintain constant ionic strength and soil:solution ratio. The pH buffering capacity (pHBC, mmol [H.sup.+]/kg soil.[Delta]pH) and pH buffering intensity (pH[Beta]; [Delta]pH/mmol [H.sup.+].kg soil) were estimated from the inverse slope and slope, respectively, of the linear segment of the titration curve between p[H.sub.Ca] 3.0 and 6.0. The standard errors of the pH[Beta] estimates were [is less than] 0.005.

[H.sup.+] budget

To ascertain the soil processes contributing to p[H.sub.Ca] change ([Delta]p[H.sub.Obs]), the expected changes in soil p[H.sub.Ca] ([Delta]p[H.sub.Calc]) were calculated using a modified version of the [H.sup.+] budget equation described by Conyers et al. (1995):

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]

The [H.sup.+] budget equation is based on the understanding that: (i) 1 mole of [H.sup.+] is consumed for each mole of [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] produced due to net N mineralisation (min.), (ii) 2 moles of [H.sup.+] are produced for each mole of [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] produced due to nitrification (nit.), (iii) 2 moles of [H.sup.+] are consumed for each mole of [Mn.sup.2+] produced due to net Mn reduction (red.), while 2 moles of [H.sup.+] are produced for each mole of [Mn.sup.2+] consumed due to net Mn oxidation (ox.), and (iv) the [H.sup.+] change due to net [OA.sup.-] oxidation or accumulation is dependent on the soil p[H.sub.w] and the pK of the organic acid such that (Bisogni and Arroyo 1991; Conyers et al. 1995):

[[OA.sup.-]] = ([10.sup.-pK] x [DOC] mg C/L x 6.5 [micro]mol/mg C x 0.02 L/10 g)/([10.sup.-pK] + [10.sup.-pHw])

and (Oliver et al. 1983; Conyers et al. 1995)

pK = 0.96 + 0.90p[H.sub.w] - 0.039[(p[H.sub.w]).sup.2]

It was expected that negligible denitrification occurred during the aerobic incubation. However, net N mineralisation and subsequent nitrification followed by denitrification was assumed to have a net neutral effect on soil [H.sup.+] concentration.

Data analysis

A 1-way analysis of variance was used to estimate the effect of soil moisture treatments at each sampling time on the concentration of [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII], DOC, and extractable Mn, and on P[H.sub.Ca]. Also, 1-way analysis of variance was used to estimate the effect of soil moisture treatments over the total incubation period on net N mineralisation, nitrification, net [OA.sup.-] oxidation, net Mn reduction, and [Delta]p[H.sub.Obs]. A 1-way analysis of variance was also used to estimate the effect of soil moisture treatments over the total incubation period on the net [H.sup.+] production resulting from the pH-influencing processes and [Delta]p[H.sub.Calc].

Results and discussion

Net N mineralisation and nitrification

Figs 1 and 2 show the changes in the concentration of soil [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] and [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] during incubation which reflect net N mineralisation and nitrification. Over the 28-day incubation period, net N mineralisation and nitrification were consistently greatest in soils maintained moist (Table 2). In these soils, up to 106 mg N/kg was mineralised, and up to 132 mg N/kg was nitrified. Exposure of soils to moist-dry cycles significantly decreased net N mineralisation and nitrification to 67-89% and 51-84% of that observed in continuously moist soils, respectively. In soils maintained dry, net N mineralisation and nitrification were less than 13% of that observed in soils maintained moist.

[Figures 1-2 ILLUSTRATION OMITTED]

Table 2. Cumulative net N mineralisation, nitrification, net [OA.sup.-] oxidation, and net Mn reduction (observed over the 28-day incubation of all moisture treatments of Yellow Sodosol (YS), Red Chromosol (RC), and Red Kandosol (RK) soils in Expts 1 and 2

For comparisons of soil moisture treatments within columns, means followed by the same letter are not significantly different at P = 0.05. Dry treatments not represented in Expt 2
Treatment Net N Nitrification
 mineralisation (mg N/kg)
 (mg N/kg) (mg Mn/kg)

Expt: 1 2 1 2

 YS soil

Dry 12a -- 7a --
Moist-dry 69b 55a 73b 71a
Moist 87c 98b 90c 106b

 RC soil

Dry 9a -- 6a --
Moist-dry 84b 87a 85b 86a
Moist 95c 106b 96c 102b

 RK soil

Dry -1a -- 4a --
Moist-dry 60b 35a 115b 82a
Moist 89c 69b 132c 127b

Treatment Net [OA.sup.-] Net Mn
 oxidation reduction
 (mg N/kg)

Expt: 1 2 1 2

 YS soil

Dry -48a -- 21.3a --
Moist-dry 14b 205a 55.3b 43.9a
Moist 14b 210a 41.2c 41.4a

 RC soil

Dry -23a -- 2.8a --
Moist-dry 42b 126a 0.5b 0.1a
Moist 44b 135a 0.0c -0.1a

 RK soil

Dry -58a -- 14.9a --
Moist-dry 6b 94a 49.0b 51.0a
Moist 15b 109b 24.1c 24.8b


When soils from the long moist-dry treatment were dry (days 0-7 and 14-21), [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] tended to accumulate (Fig. 1), and [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] production was 7.4-51.4 mg N/kg less than in soils maintained moist (Fig. 2). When soils from this treatment were moist (Days 7-14 and 21-28), [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] utilisation and [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] production were only 2.2-20.9 and 0.6-26.6 mg N/kg greater, respectively, than that observed in continuously moist soil. Intermittent drying therefore delayed mineralisation of organic N since the suppression of net N mineralisation and nitrification during dry periods was not counterbalanced by the small flush of these processes on rewetting.

Consistent with the present study, other studies of surface soils sampled from southern Australia (van Veen et al. 1985; van Gestel et al. 1991, 1992) have consistently demonstrated relatively small net N mineralisation and nitrification responses to rewetting dry soil. These findings contrast with the enhanced N mineralisation and nitrification observed in moist-dry treated soils sampled from regions of high rainfall (Birch 1958, 1960; Soulides and Allison 1961; Seneviratne and Wild 1985; van Gestel et al. 1993).

It is considered that, like other surface soils from southern Australia, the soils studied had a high proportion of refractory organic matter (Spain et al. 1983). In a Western Australian sandy lateritic podsol, Degens and Sparling (1995) found that refractory organic C was not decomposed even when aggregates were disrupted by moist-dry cycles. Perhaps the little available substrate that was present was utilised in a flush of mineralisation during the pre-incubation period.

Due to the high initial [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] concentration (64 mg N/kg), substrate availability was not limiting nitrification in RK soil. Despite this, nitrification was significantly less in moist-dry relative to moist-treated RK soil, by up to 46 mg N/kg (Table 2). Furthermore, in the 3 surface soils studied, long moist-dry cycles did not substantially increase net N mineralisation or nitrification relative to short moist-dry cycles. In soil sampled from the high rainfall regions of Kenya (Birch 1958, 1960), north-eastern USA (Soulides and Allison 1961), and the UK (Seneviratne and Wild 1985), the longer the soil remained in an air-dry state the greater was the mineralisation on rewetting. We concluded that in our study, the main cause of the insubstantial flush of net N mineralisation and nitrification on rewetting dry soils was the adaptation of microbes to the relatively dry and highly variable climate experienced within the surface 2 cm of arable soils in southern Australia.

It has been shown (West et al. 1989; van Gestel et al. 1993) that biomass from low rainfall regions resists desiccation better than biomass from high rainfall regions. It seems that since there is less available substrate released from biomass killed on drying, the flush of mineralisation on rewetting is relatively low in soil sampled from low rainfall regions. In Western Australia, Murphy et al. (1998) simulated a summer rainfall event. They suggested that microbes had adapted to desiccation, as N was found to be immobilised within 4 h after dry soil was rewet.

It is possible that the net N mineralised and nitrified in moist-dry treated soils would have been even less than that observed had these soils been undisturbed, or if they had a coarser texture. There is some evidence that the flush of C and N mineralisation on rewetting is greater in disturbed than undisturbed soil (van Gestel et al. 1992). Perhaps time may be too short for all soil biota to adjust during the rapid drying experienced in disturbed soils. Also, the release of substrate from non-living organic matter in response to drying is lower in soils of coarse texture which are not well aggregated and have a low cation exchange capacity (van Gestel et al. 1991).

Net [OA.sup.-] oxidation or accumulation

The change in DOC concentration (Fig. 3) reflected the net [OA.sup.-] oxidation or accumulation. Over the 28-day incubation period, net accumulation of [OA.sup.-] in continuously dry soils was 23-58 mg C/kg (Table 2). Also, there was a net [OA.sup.-] accumulation of 2-30 mg C/kg when long moist-dry treated soils were dry (Days 7-14 and 21-28). In soils that received water, there was a net oxidation of [OA.sup.-]. The [OA.sup.-] which had accumulated during intermittent dry stages was oxidised on rewetting. Hence, in both experiments the difference in the net [OA.sup.-] oxidised between moist-dry and moist treated soils was [is less than] 15 mg C/kg. As observed with net N mineralisation, other workers (van Veen et al. 1985; van Gestel et al. 1991, 1992) in southern Australia have also demonstrated that soil heterotrophic microbial activity is not enhanced by exposure to moist-dry cycles. This provides further evidence that in the soils studied, there had been an adaptation of heterotrophic microbes to a relatively dry and highly variable climate (West et al. 1989, 1992; van Gestel et al. 1993).

[Figure 3 ILLUSTRATION OMITTED]

Net Mn reduction or oxidation

Fig. 4 shows the changes in the concentration of soil-extractable Mn during incubation, which reflect net Mn reduction or oxidation. There was negligible change in extractable Mn concentration ([is less than] 3 mg Mn/kg) during incubation of RC soils. This may be attributed to the relatively high initial soil p[H.sub.Ca] of 5.98. Many of the organisms involved in Mn oxidation are most active at p[H.sub.w] 5.8-7.8 (Bromfield 1978). Thus, in the RC soil, Mn reduction may have been counterbalanced by Mn oxidation. Additionally, at soil pH [is greater than] 5-6, Mn adsorption onto aluminium and ferric oxides and birnessite (Mn oxide) increases (McBride 1978; McKenzie 1980).

[Figure 4 ILLUSTRATION OMITTED]

In all treatments of YS and RK soils in Expts 1 and 2, there was a net reduction of Mn (Table 2). Net Mn reduction was lowest in soils maintained dry. This contrasts with reports (Bartlett and James 1979, 1980) that extractable Mn accumulation was lower in moist soils than in dry soils. In moist soils in the present study, microbial Mn oxidation may have been suppressed due to a relatively low availability of organic C substrate, while Mn reduction may have been enhanced due to the development of both low pH and reducing conditions at microsites in which high microbial activity had depressed the oxygen concentration.

During both Expts 1 and 2, there was a total net Mn reduction of 44-55 mg Mn/kg in YS or RK soils exposed to moist-dry cycles (Table 2). This was 2.60-3.29 times the total net Mn reduced in continuously dry soils, and 1.06-2.06 times the total net Mn reduced in continuously moist soils. Hence, unlike other pH-influencing processes measured, net Mn reduction was enhanced by moist-dry cycles.

When YS and RK soils from the long moist-dry treatment were dry (Days 0-7 and 14-21), the net accumulation of extractable Mn was less than that observed in continuously moist soils by only 0.9-7.4 mg Mn/kg (Fig. 4). During the moist stages (Days 7-14 and 21-28), the net accumulation of extractable Mn was greater than that in continuously moist soils by 2.9-18.2 mg Mn/kg. Therefore, total net Mn reduction was enhanced on exposure to moist-dry cycles in response to the large flush of net Mn reduction on rewetting.

The flush of net Mn reduction on rewetting dry YS and RK soils was concurrent with the small flush of heterotrophic activity, and thus possibly the creation of reducing conditions within microsites where soluble C was suddenly brought into contact with Mn oxides. The Mn reduced in moist-dry treated soils was apparently not quickly oxidised. Perhaps the activity of soil microbes which oxidise [Mn.sup.2+] was suppressed by rapid moisture content fluctuation. It did not appear that the increase in extractable Mn concentration on exposure to moist-dry cycles was counterbalanced by increased retention of Mn by clay minerals (Ritchie 1989).

Change of soil pH

There was a significant influence of soil moisture treatments on p[H.sub.Ca] during both Expts 1 and 2 (Fig. 5). In soil maintained dry, p[H.sub.Ca] increased by up to 0.10 units over the incubation period. In contrast, soil p[H.sub.Ca] decreased in those treatments where water was added. Acidification was greatest in continuously moist soils. During both experiments, p[H.sub.Ca] declined by 0.16-0.17, 0.19-0.39, and 0.35-0.38 units in moist YS, RC, and RK soils, respectively. Relative to soil maintained moist, p[H.sub.Ca] of YS, RC, and RK soil was significantly higher, by 0.09-0.10, 0.05-0.06, and 0.15-0.26 units, respectively, on exposure to moist-dry cycles.

[Figure 5 ILLUSTRATION OMITTED]

Soil p[H.sub.Ca] increased by 0.03-0.08 units when long moist-dry treated soils were dry (Days 0-7 and 14-2l) (Fig. 5). After the initial rewetting (Days 7-14), there was a 0.19-0.21 unit decrease of p[H.sub.Ca]. However, by the second rewetting (Days 21-28), soil p[H.sub.Ca] was maintained or decreased only slightly. Relative to soil maintained moist, net acidification was lower in moist-dry treated soils since acidification on rewetting was not much greater than alkalisation observed during intermittent dry stages.

Jager and Bruins (1975) also observed suppressed acidification with intermittent drying of an acidic soil (p[H.sub.w] 5.2). They observed a p[H.sub.w] decline of 0.2 and 0.3 units in moist-dry soils (3 days dry, 4 weeks moist at 30 [degrees] C) and continually moist soils, respectively. After 20 moist-dry cycles, p[H.sub.w] had declined by 0.8 units in the continually moist treatment, but only 0.4 units in moist-dry treated soil. However, under more severe drying conditions (85 [degrees] C for 1 day), nitrification, and thus acidification, were inhibited. In contrast, other workers (Bartlett and James 1980; Walworth 1992) have reported increases in p[H.sub.w] on rewetting very acidic (p[H.sub.w] [is less than] 5), dry soils.

Contradictory reports on how moist-dry cycles influence soil pH may be related to the variation in the initial pH of the soils studied. Haynes and Swift (1989) dried (48 h at 22 [degrees] C) and rewet (70% of field capacity) 8 New Zealand soils prior to incubation for 10 weeks at 25 [degrees] C. They noted that although p[H.sub.w] decreased on rewetting soils of relatively high pH, in soils with p[H.sub.w] values [is less than] 5.0, p[H.sub.w] increased (by almost 1 unit), since N mineralisation was not followed by appreciable nitrification. In previous studies (Haynes and Swift 1989; Walworth 1992), whole soil horizons had been bulked. Since we used a 2-cm depth increment, nitrifiers had probably adapted to the average initial pH of the bulked soil which was incubated. Therefore, in the present study, there was no relationship between the observed soil p[H.sub.Ca] change and the initial soil p[H.sub.Ca].

[H.sup.+] budget

From the pH titration curves, it was ascertained that the pH[Beta] was 0.04, 0.05, and 0.05 [Delta]p[H.sub.Ca]/mmol [H.sup.+].kg for YS, RC, and RK soil, respectively.

By using the [H.sup.+] budget for all soil treatments in both experiments, there was a significant relationship between [Delta]p[H.sub.Obs] and [Delta]p[H.sub.Calc] (Fig. 6):

[Delta]p[H.sub.Calc] = 0.99[Delta]p[H.sub.Obs]-0.10 ([r.sup.2]=0.77, P [is less than] 0.001)

[Figure 6 ILLUSTRATION OMITTED]

However, the [H.sup.+] budget overestimated acidification observed, by an average of 0.10 units. The overestimation of acidification was greatest in the RK soil (Tables 3 and 4). This may have been related to errors in the calculation of net N mineralisation in a soil of high (64 mg N/kg) initial [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] concentration. The overestimation of acidification also tended to be high in moist-dry treated soils (Fig. 6). Perhaps an underestimation of pHBC (i.e. overestimation of pH[Beta]) may have been responsible for this error. It is possible that pHBC of the soil may have increased when exposed to moist-dry cycles. There are numerous accounts of moist-dry cycles shattering soil aggregates (e.g. Soulides and Allison 1961; Adu and Oades 1978). The break-up of soil aggregates may increase the exposure of organic matter exchange sites, and therefore increase soil pHBC. It is also possible that acidification was overestimated due to unaccounted for oxidation of particulate [OA.sup.-], and due to an underestimation of net reduction Mn as a result of some of the Mn reduced being retained by the clay fraction of the soil.

Table 3. Change in cumulative [H.sup.+] and the [PH.sub.Ca] change observed ([pH.sub.Obs]) and calculated ([pH.sub.calc]) due to net N mineralisation (Nmin), nitrification (Nit), net [OA.sup.-] oxidation or accumulation ([OA.sup.-] ox/acc), and net Mn reduction or oxidation (Mn red/ox) of Yellow Sodosol (YS), Red Chromosol (RC), and Red Kandosol (RK) soils during Expt 1

For comparisons of soil moisture treatments within columns, means followed by the same letter are not significantly different at P = 0.05
Treatment (mmol [H.sup.+]/kg)
 Nmin Nit [OA.sup.-] Mn
 ox/acc red/ox

 YS soil
Dry -0.8(a) 1.0(a) 0.2(a) -0.8(a)
Moist-dry -4.9(b) 10.4(b) -0.1(b) -2.0(b)
Moist -6.2(c) 12.8(c) -0.1(b) -1.5(c)

 RC soil
Dry -0.7(a) 0.9(a) 0.2(a) -0.1(a)
Moist-dry -6.0(b) 12.2(b) -0.3(b) 0.0(b)
Moist -6.8(c) 13.8(c) -0.3(b) 0.0(b)

 RK soil
Dry -0.3(a) 0.8(a) 0.3(a) -0.5(a)
Moist-dry -4.3(b) 16.4(b) -0.1(b) -1.8(b)
Moist -6.4(c) 18.8(c) -0.1(b) -0.9(c)

Treatment [Delta][pH.sub.Ca]
 Sum [pH.sub.Obs] [Delta][pH.sub.Calc]

 YS soil
Dry -0.4(a) 0.06(a) 0.01(a)
Moist-dry 3.4(b) -0.08(b) -0.13(b)
Moist 5.0(c) -0.17(c) -0.18(c)

 RC soil
Dry 0.3(a) 0.03(a) -0.02(a)
Moist-dry 5.9(b) -0.34(b) -0.31(b)
Moist 6.7(c) -0.39(c) -0.35(c)

 RK soil
Dry 0.3(a) 0.10(a) -0.01(a)
Moist-dry 10.2(b) -0.23(b) -0.47(b)
Moist 11.5(c) -0.38(c) -0.53(c)

Treatment
 [MATHEMATICAL EXPRESSION NOT
 REPRODUCIBLE IN ASCII]

 YS soil
Dry -0.05
Moist-dry -0.05
Moist -0.01

 RC soil
Dry -0.05
Moist-dry 0.03
Moist 0.04

 RK soil
Dry -0.11
Moist-dry -0.24
Moist -0.15


Table 4. Change in cumulative [H.sup.+] and the [pH.sub.Ca] change observed ([pH.sub.Obs]) and calculated ([pH.sub.calc]) due to net N mineralisation (Nmin), nitrification (Nit), net [OA.sup.-] oxidation or accumulation ([OA.sup.-] ox/acc), and net Mn reduction or oxidation (Mn red/ox) of Yellow Sodosol (YS), Red Chromosol (RC), and Red Kandosol (RK) soils during Expt 2

For comparisons of soil moisture treatments within columns, means followed by the same letter are not significantly different at P = 0.05
Treatment (mmol H+/kg)
 Nmin Nit OA- Mn
 ox/acc red/ox

 YS soil
Moist-dry -3.9(a) 10.1(a) -1.1(a) -1.6(a)
Moist -7.0(b) 15.1(b) -1.1(a) -1.5(a)

 RC soil
Moist-dry -6.2(a) 12.2(a) -0.8(a) 0.0(a)
Moist -7.5(b) 14.6(b) -0.S(a) 0.0(a)

 RK soil
Moist-dry -2.5(a) 11.7(a) -0.6(a) -1.9(a)
Moist -5.0(b) 18.2(b) -0.7(b) -0.9(b)

Treatment [Delta][pH.sub.Ca]
 Sum [pH.sub.Obs] [pH.sub.calc]

 YS soil
Moist-dry 3.5(a) -0.06(a) -0.13(a)
Moist 5.6(b) -0.16(b) -0.21(b)

 RC soil
Moist-dry 5.2(a) -0.13(a) -0.28(a)
Moist 6.2(b) -0.19(b) -0.33(b)

 RK soil
Moist-dry 6.7(a) -0.09(a) -0.31(a)
Moist 11.7(b) -0.35(b) -0.54(b)

Treatment
 [pH.sub.Calc-]
 [pH.sub.Obs]

 YS soil
Moist-dry -0.07
Moist -0.05

 RC soil
Moist-dry -0.15
Moist -0.14

 RK soil
Moist-dry -0.22
Moist -0.19


The sum of the [H.sup.+] change in continuously dry soil was less than 0.4 mmol [H.sup.+]/kg (Table 3). There was a slight p[H.sub.Ca] increase observed in these soils since what little N was mineralised was often not subsequently nitrified, and there was a net reduction of Mn.

Net N mineralisation accounted for 24-33% of the absolute [H.sup.+] change in YS and RC soil treatments which received water (Tables 3 and 4). In these soils, nitrification accounted for 60-66% of the absolute [H.sup.+] change. There was an approximate 2:1 ratio of [H.sup.+] change due to net N mineralisation to [H.sup.+] change due to nitrification. As nitrification was not dependent on net N mineralisation in RK soil of high initial NHJ-N concentration, net N mineralisation and nitrification contributed 15-24% and 70-74% to absolute [H.sup.+] change, respectively. It was therefore found that in YS, RC, and particularly RK surface soils which received water, nitrification had a predominant influence on net acidification. The suppression of acidification observed with intermittent drying was largely attributable to the suppression of nitrification in moist-dry relative to continuously moist soils. There was a 1.6-2.5 and 2.4-6.5 mmol [H.sup.+]/kg decrease in [H.sup.+] production resulting from the suppression of nitrification during short and long moist-dry cycles, respectively (Tables 3 and 4).

During both Expts 1 and 2, the oxidation of [OA.sup.-] accounted for [is less than]6% of the absolute [H.sup.+] change in continuously moist and moist-dry treated soils (Tables 3 and 4). It appears that the labile organic C pool of the soils studied was too low to produce significant effects of net [OA.sup.-] oxidation on soil p[H.sub.Ca].

In relatively acidic (p[H.sub.Ca] [is less than] 5.5) soils, net Mn reduction accounted for 3-12% of the absolute [H.sup.+] change in continuously moist and moist-dry treated soils (Tables 3 and 4). Enhanced [H.sup.+] consumption by net Mn reduction in moist-dry YS and RK soils contributed to the higher final p[H.sub.Ca] than observed when those soils were maintained moist. The enhancement of net Mn reduction on exposure to moist-dry cycles accounted for a 0.1-1.0 mmol [H.sup.+]/kg increase of [H.sup.+] consumption. Net Mn reduction accounted for less than 1% of the absolute [H.sup.+] change observed in continuously moist and moist--dry RC soils. The lack of net Mn reduction and concomitant [H.sup.+] consumption was partly responsible for the often insignificant difference in p[H.sub.Ca] between continuously moist and moist-dry RC soil (Fig. 5).

Conclusions

Soil p[H.sub.Ca] slightly increased in soils maintained dry due to net N mineralisation and net Mn reduction. The p[H.sub.Ca] declined in soils which received water. This was predominantly due to nitrification. Relative to soils maintained moist, exposure to moist-dry cycles decreased net acidification because nitrification was suppressed, and in relatively acidic soils (p[H.sub.Ca] [is less than] 5.5), net Mn reduction was enhanced. It therefore appears that the 0-2 cm layer is subject to less acidification because of intermittent drying. However, further work is required to ascertain the effect of moist-dry cycles on pH and pH-influencing processes in undisturbed surface soils of various textures.

Acknowledgments

We thank Dr Keith Helyar for allowing us to sample soil from the field site at Book Book. The project was funded by Land and Water Resources Research and Development Corporation and a Charles Sturt University Postgraduate Writing-Up Award.

References

Adu, J. K., and Oades, J. M. (1978). Physical factors influencing decomposition of organic materials in soil aggregates. Soil Biology and Biochemistry 10, 109-15.

Bartlett, R., and James, B. (1979). Behavior of chromium in soils: III. Oxidation. Journal of Environmental Quality 8, 31-5.

Bartlett, R., and James, B. (1980). Studying dried, stored soil samples--some pitfalls. Soil Science Society of America Journal 44, 721-4.

Birch, H. F. (1958). The effect of soil drying on humus decomposition and nitrogen availability. Plant and Soil 10, 9-30.

Birch, H. F. (1960). Nitrification in soils after different periods of dryness. Plant and Soil 12, 81-96.

Bisogni, J. J., and Arroyo, S. L. (1991). The effect of carbon dioxide equilibrium on pH in dilute lakes. Water Research 25, 185-90.

Bromfield, S. M. (1978). The oxidation of magnanous ions under acidic conditions by an acidophilous actinomycetes from acid soil. Australian Journal of Soil Research 16, 91-100.

Burford, J. R., and Bremner, J. M. (1975). Relationships between the denitrification capacities of soils and total, water-soluble and readily decomposable soil organic matter. Soil Biology and Biochemistry 7, 389-94.

Campbell, C. A., and Biederbeck, V. O. (1976). Soil bacteria changes as affected by growing season weather conditions: a field and laboratory study. Canadian Journal of Soil Science 56, 293-310.

Conyers, M. K. (1992). Seasonal variation in soil acidity on the south-western slopes of New South Wales. PhD Thesis, LaTrobe University, Melbourne.

Conyers, M. K., and Scott, B. J. (1989). The influence of surface incorporated lime on subsurface soil acidity. Australian Journal of Experimental Agriculture 29, 201-7.

Conyers, M. K., Uren, N. C., and Helyar, K. R. (1995). Causes of changes in pH in acidic mineral soils. Soil Biology and Biochemistry 27, 1383-92.

Crooke, W. M., and Simpson, W. E. (1971). Determination of ammonium in Kjeldahl digests of crops by an automated procedure. Journal of the Science of Food and Agriculture 22, 9-10.

Degens, B. P., and Sparling, G. P. (1995). Repeated wet-dry cycles do not accelerate the mineralisation of organic C involved in the macro-aggregation of a sandy loam soil. Plant and Soil 175, 197-203.

Evans, C. M., Conyers, M. K., Black, A. S., and Poile, G. J. (1998). Effect of ammonium, organic amendments, and plant growth on soil pH stratification. Australian Journal of Soil Research 36, 641-53.

Haynes, R. J., and Swift, R. S. (1989). Effect of rewetting air-dried soils on pH and accumulation of mineral nitrogen. Journal of Soil Science 40, 341-7.

Heanes, D. L. (1984). Determination of total organic-C in soils by an improved chromic acid digestion and spectrophotometric procedure. Communications in Soil Science and Plant Analysis 15, 1191-213.

Henriksen, A., and Selmer-Olsen, A. R. (1970). Automatic methods for determining nitrate and nitrite in water and soil extracts. Analyst 95, 514-18.

Isbell, R. F. (1996). `The Australian Soil Classification.' (CSIRO Publishing: Melbourne.)

Jager, G., and Bruins, E. H. (1975). Effect of repeated drying at different temperatures on soil organic matter decomposition and characteristics, and on the soil microflora. Soil Biology and Biochemistry 7, 153-9.

McBride, M. B. (1978). Retention of [Cu.sup.2+], [Ca.sup.2+], [Mg.sup.2+],

and Mn2+ by amorphous alumina. Soil Science Society of America Journal 42, 27-31.

McKenzie, R. M. (1980). The adsorption of lead and other heavy metals on oxides of manganese and iron. Australian Journal of Soil Research 18, 61-73.

McLaughlin, M. J., Baker, T. G., James, T. R., and Rundle, J. A. (1990). Distribution and forms of phosphorus and aluminium in acidic topsoils under pastures in south-eastern Australia. Australian Journal of Soil Research 28, 371-85.

Murphy, D. V., Sparling, G. P., Fillery, I. R. P., McNeil, A. M., and Braunberger, P. (1998). Mineralisation of soil organic nitrogen and microbial respiration after simulated summer rainfall in an agricultural soil. Australian Journal of Soil Research 36, 231-46.

Oliver, B. G., Thurman, E. M., and Malcolm, R. L. (1983). The contribution of humic substances to the acidity of colored natural waters. Geochimica et Cosochimica Acta 47, 2031-5.

Pinkerton, A., and Simpson, J. R. (1986a). Responses of some crop plants to correction of subsoil acidity. Australian Journal of Experimental Agriculture 26, 107-13.

Pinkerton, A., and Simpson, J. R. (1986b). Interactions of surface drying and subsurface nutrients affecting plant growth on acidic soil profiles from an old pasture. Australian Journal of Experimental Agriculture 26, 681-9.

Purnomo, E., and Black, A. S. (1994). Wheat growth from phosphorus fertilizers as affected by time and method of application in soil with an acidic subsurface layer. Fertilizer Research 39, 77-82.

Richardson, A. E., and Simpson, R. J. (1988). Enumeration and distribution of Rhizobium trifolii under a subterranean clover-base pasture growing in an acid soil. Soil Biology and Biochemistry 20, 431-8.

Ritchie, G. S. P. (1989). The chemical behavior or aluminium, hydrogen and manganese in acid soils. In `Soil Acidity and Plant Growth'. pp. 1-60. (Ed. A. D. Robson.) (Academic Press: Marrickville, NSW.)

Seneviratne, G., and Wild, A. (1985). Effect of mild drying on the mineralization of soil nitrogen. Plant and Soil 84, 175-9.

Soulides, D. A., and Allison, F. E. (1961). Effect of drying and freezing soils on carbon dioxide production, available mineral nutrients, aggregation, and bacterial population. Soil Science 91, 291-8.

Spain, A. V., Isbell, R. F., and Probert, M. E. (1983). Soil organic matter. In `Soils: an Australian Viewpoint'. pp. 551-63. (CSIRO Publishing: Melbourne.)

van Gestel, M., Ladd, J. N., and Amato, M. (1991). Carbon and nitrogen mineralization from two soils of contrasting texture and microaggregate stability: influence of sequential fumigation, drying and storage. Soil Biology and Biochemistry 23, 313-22.

van Gestel, M., Ladd, J. N., and Amato, M. (1992). Microbial biomass responses to seasonal change and imposed drying regimes at increasing depths of undisturbed topsoil profiles. Soil Biology and Biochemistry 24, 103-11.

van Gestel, M., Merckx, R., and Vlassak, K. (1993). Microbial biomass responses to soil drying and rewetting: The fate of fast- and slow-growing microorganisms in soils from different climates. Soil Biology and Biochemistry 25, 109-23.

van Veen, J. A., Ladd, J. N., and Amato, M. (1985). Turnover of carbon and nitrogen though the microbial biomass in a sandy loam and a clay incubated with [[sup.14]C(U)]glucose and [[sup.15]N] [([NH.sub.4]).sub.2][SO.sub.4] under different moisture regimes. Soil Biology and Biochemistry 17, 747-56.

Walworth, J. L. (1992). Soil drying and rewetting, or freezing and thawing, affects soils solution composition. Soil Science Society of America Journal 56, 433-7.

West, A. W., Sparling, G. P., and Speir, T. W. (1989). Microbial activity in gradually dried or rewetted soils as governed by water and substrate availability. Australian Journal of Soil Research 27, 747-57.

West, A. W., Spading, G. P., Feltham, C.W., and Reynolds, J. (1992). Microbial activity and survival in soils dried and different rates. Australian Journal of Soil Research 30, 209-22.

Yeomans, J. C., and Bremner, J. M. (1988). A rapid and precise method for routine determination of organic carbon in soil. Communications in Soil Science and Plant Analysis 19, 1467-76.

Young, S. R., Black, A. S., and Conyers, M. K. (1995). Nitrogen mineralisation and nitrification in acidic subsurface layers of soil. In `Plant and Soil Interactions at Low pH'. (Eds R. A. Date, N. J. Grundon, G. E. Rayment, and M. E. Probert.) pp. 111-15. (Kluwer Academic Publishers: Netherlands.)

Manuscript received 23 October 1998, accepted 6 July 1999

K. I. Paul(AC), A. S. Black(A), and M. K. Conyers(B)

(A) School of Agriculture, Charles Sturt University, Locked Bag 677, Wagga Wagga, NSW 2678, Australia.

(B) Agricultural Institute, NSW Agriculture, PMB, Wagga Wagga, NSW 2650, Australia.

(C) Present address of corresponding author: CSIRO Forestry and Forest Products, PO Box E4008, Kingston, ACT 2604, Australia. Email: keryn.paul@ffp.csiro.au3
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Author:Paul, K.I.; Black, A.S.; Conyers, M.K.
Publication:Australian Journal of Soil Research
Article Type:Statistical Data Included
Date:Nov 1, 1999
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