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Carbon turnover in two soils with contrasting mineralogy under long-term maize and pasture.

Introduction

Maintenance of organic matter levels is a key factor in both the sustainable use of soils and the sequestration of carbon (C). Cultivation and cropping of soils often reduce soil C levels, because the input of C from the crop is usually less than the output from mineralisation (Balesdent et al. 1988). Mineralisation of C can be reduced in soils where the soil organic matter is stabilised both within small pores and by chemical sorption. Recent studies have shown that the stabilisation of total soil organic C is influenced by soil aluminium extractable in pyrophosphate ([Al.sub.py]) (Percival et al. 2000). In laboratory experiments (weeks duration), however, the turnover of C is influenced by clay minerals such as allophane (Saggar et al. 1994, 1996; Parfitt et al. 1997), but the influence of allophane and [Al.sub.py] on the stabilisation of C in a 25-year time frame has not been investigated.

The turnover of C in the 10-30-year time frame can be investigated using natural variations in stable isotopes (Balesdent et al. 1987). Isotopes of C are fractionated during photosynthesis and C-4 plants (e.g. maize) have a different isotopic composition from C-3 plants (e.g. ryegrass and New Zealand's indigenous forests). Comparison of the isotopes in soils of contrasting clay mineralogy under maize and pasture, therefore, provides additional information on the effect of clay on soil organic matter stabilisation.

In this paper we measured C isotopes in adjacent fields under maize and ryegrass--clover pasture where 2 soil types occurred in each field. Our overall goal was to assess whether C mineralisation differed within a 25-year time frame in soils with different clay mineralogies that give different specific surface areas, and whether this could provide information which has application to modelling C dynamics.

Materials and methods

Site descriptions

Mineral soil (0-10 cm) was sampled from a farm in New Zealand that had 2 soil types and 2 land uses, i.e. pasture and maize that existed on both soil types. The farm was located in the central North Island at 175 [degrees] 25'E, 37 [degrees] 56'S. Annual precipitation is approximately 1230 mm, and mean annual air temperature is 13.4 [degrees] C. The soils were Horotiu sandy loam, classified as a Typic Hapludand (Vitric Orthic Allophanic Soil in the New Zealand Soil Classification; Hewitt 1998) and Te Kowhai silt loam, an Aerie Endoaquept (Typic Orthic Gley Soil; Hewitt 1998). Both were formed in rhyolitic alluvium and tephra, and had different particle size distributions and different specific surface areas (Table 1). The Horotiu soil (Andisol) contained allophane as the dominant clay mineral, whereas the Te Kowhai soil (Inceptisol) was dominated by halloysite (Table 2). Since the soils were formed in alluvium it was not possible to obtain an exact match between the soil pairs. Both the Horotiu soils were on a levee and were 200 m apart. The Horotiu soil under maize was slightly higher in the landscape, and the soil profile contained more coarse sand below 50 cm depth. The clay mineral contents (on a whole soil basis) were broadly similar, and the soil under pasture contained slightly more clay (Table 2). The Te Kowhai soil pairs were both in low positions in the landscape and were closely matched.

The farm had previously been under native broadleaf/podocarp forest that was cleared for pastoral agriculture around 1850. The perennial ryegrass (Lolium perenne L.)--clover (Trifolium repens L.) pasture had not been ploughed within the last 50 years. The pasture was grazed regularly and fertilised (30 kg P and 20 kg K/ha.year). The maize (Zea mays L.) field, which had been under permanent pasture, was first ploughed and planted in 1971, then was cropped with maize each year, and left fallow in winter. In late winter the residue was ploughed in, using a 4-furrow semi-mounted plough set to a depth of 20 cm. A second field had been planted in 1986. Agricultural lime (0.5 t/ha) and 55 kg N, 63 kg P, and 60 kg K were applied before planting, and the maize was later side-dressed with 185 kg N/ha as liquid urea. The yield of grain was 13.6 t/ha in 1996.

Samples were also obtained from a second farm at 175 [degrees] 10'E, 37 [degrees] 46'S with similar soils and similar management. The soil pairs were closely matched on this farm. Only [sup.13]C data are presented for this second farm.

Sample collection and preparation

Using a 25-mm-diameter corer, samples of mineral soil (0-10 and 10-20 cm depth) were taken from the 4 sites in December 1996, when soil moisture was close to field capacity. Three replicate samples, each containing 10 pooled cores, were collected from random positions under maize and under pasture in both the Andisol and Inceptisol. All samples were then sieved moist through a 5-mm sieve, and stored at 4 [degrees] C, and a subsample was dried at 35 [degrees] C. Soils were also sampled in February 1998 from every 5 cm to a depth of 60 cm from the 4 faces of 8 soil pits (2 pits for each combination of soil and crop), and the samples were bulked for each pit. Samples from each pit were taken for measurement of bulk density using 5-cm-diameter brass rings. The soils were sampled at these depths, rather than by horizon, so that the data could be modelled by depth increment.

Samples of maize (leaves, stalks, husks, and grain) were collected from both the fields before harvest in May 1997 and were dried at 60 [degrees] C. Pasture herbage samples were also taken. Soil cores were collected to 20 cm depth from all pasture and maize plots and roots were extracted from the soil by washing.

One of the replicate samples of each moist soil, equivalent to 20 g oven dry weight, was fractionated into a free light fraction and a dispersed light fraction following the method of Golchin et al. (1994). Briefly, the free light fraction was separated by centrifugation after shaking the soil in a sodium polytungstate solution of density 1.6 Mg/[m.sup.3]; the dispersed light fraction was separated after ultrasonic dispersion. The samples were washed on GFC filter paper and dried at 35 [degrees] C. The remainder (heavy fraction) was washed and separated into <2 [micro]m, 2-60 [micro]m and >60 [micro]m fractions by repeatedly dispersing in water, centrifuging and sedimentation, and then flocculating by freezing and thawing. The suspensions were air-dried at 35 [degrees] C. The air-dry water content was determined at 105 [degrees] C, and the mass of the samples determined. The recovery of soil C was 90-95%.

Analytical procedures

Soil pH was measured on a 1:2.5 w/v mixture of soil and water, and pyrophosphate-A1 ([Al.sub.py]) by extraction at pH 10 (Blakemore et al. 1987). Total C and nitrogen were measured on a LECO FP-2000 CNS analyser. The total soil C in t/ha on a soil volume basis was determined from the bulk density (BD) (Table 3) and the C concentration of the respective soil layer (BD.%C.depth). The total soil C in t/ha for each pasture site also was calculated on an equivalent mass basis by multiplying by the ratio of maize soil BD:pasture soil BD for each layer. The sum for each layer gave the total pasture soil C for the same mass of maize soil (Arrouays et al. 1995). Particle size was measured by dispersing the field-moist soil in water with an ultrasonic probe and separating the <2 [micro]m, 2-60 [micro]m and >60 [micro]m fractions by sedimentation. Clay minerals were estimated by acid-oxalate extraction and differential thermal analysis (Parfitt and Wilson 1985). The specific surface area was measured from water adsorption at P/[P.sub.0] = 0.42, which is correlated with the BET water surface area (Parfitt et al. 2001 b).

Values for [sup.13]C natural abundance were determined on a mass spectrometer. The natural abundance of heavy isotopes was expressed as parts per thousand relative to a standard using delta units ([delta]). The per cent of organic C derived from maize residues within the last 25 years was calculated from:

([delta] - [[delta].sub.pa])/([[delta].sub.m] - [[delta].sub.pa]) x 100

where [delta] is the [delta][sup.13]C value of the maize soil sample, [delta.sup.pa] is the [delta][sup.13]C value of the pasture soil sample, and [[delta].sub.m] is the [delta][sup.13]C value of the maize residues (here the value -12.3 was used in the calculations).

Statistical analyses

Differences between treatments (pasture and maize) and soil type were tested using 2-way analysis of variance. Pair-wise comparisons were made using the Bonferroni procedure. All statistical analyses were performed using SYSTAT7.

Results

The specific surface area of the Andisol under maize, which contained 100 g/kg of allophane in the 0-20 cm layer, was 130 [m.sup.2]/g (Tables 1 and 2), while the specific surface area of the halloysitic Inceptisol was 60 [m.sup.2]/g.

The total soil C (0-60 cm depth) in t/ha, on an equivalent mass basis, was significantly higher (P = 0.001) for the Andisol than for the Inceptisol (Table 4), but there was no significant difference between each pasture soil and maize soil. When the comparisons were made on the basis of soil volume, similar results were obtained for the Inceptisol. Because of the lower bulk density, a lower C value (t/ha) was obtained for the Andisol pasture soil, but again there was no significant difference between the pasture soil and maize soil.

The distribution of C in the pasture and maize soils differed with depth. The soils under maize had been mixed to about 25 cm, and the C was therefore fairly uniformly distributed between 0 and 25 cm. On the other hand, the pasture soil C decreased with depth.

The measured annual inputs of C under maize were 2.5 t/ha of leaves, 3 t/ha of stalks, 1.5 t/ha of husks, and 2 t/ha of roots, whereas the estimated C inputs into the soil under pasture were 2 t/ha pasture litter, 2 t/ha dung, and 5.5 t/ha roots (Saggar et al. 1999a). Therefore, the gross annual inputs of C to soil (9 t/ha) were similar under pasture and maize, and were consistent with the total soil C summed from 0-60 cm.

The [delta][sup.3]C values of the maize stalks, leaves, and husks were -12.3 [+ or -] 0.1, and the value for the grain was -11.5 [+ or -] 0.3; the value for the roots was -12.7 near to the stem, and -14.7 at the mid-row position. The [sup.13]C value was -29.4 [+ or -] 0.1 for the pasture herbage, and -28.7 [+ or -] 0.3 for the pasture roots.

The soil [delta][sup.13]C values were significantly different with both crop (P < 0.001) and soil type (P = 0.001). The [delta][sup.13]C values for soil under pasture showed an increase with depth from -27.8 to -25.1 for the Inceptisol (Table 5), as observed previously for other pasture soils (O'Brien and Stout 1978; Gregorich et al. 1995); the data for the Andisol were similar. The [delta][sup.13]C values for the soils under maize were between -21.8 and -24.8, and showed the influence of the addition of C from the maize. The total C at 0-20 cm depth that came from maize (25 years) was calculated to be 21-23% in the Andisol, and 28-31% in the Inceptisol (Table 6). The retention of old pasture and forest C was therefore 77-79% for the Andisol and 69-72% for the Inceptisol. Similar results were obtained for a second farm (Andisol-2 and Inceptisol-2 in Table 6). The distribution of carbon from the C-4 maize, and from the C-3 pasture and indigenous forest vegetation, is shown in Fig. 1 for both soils under maize. The Andisol contained 112 t C/ha (0-35 cm) from [C.sub.3] plants and 27 t C/ha from the C-4 maize (total = 139 t C/ha); the values for the Inceptisol were 70 t/ha from C-3 plants and 31 t/ha from the C-4 maize (total = 101 t/ha). Since this result only uses total C data for the soils under maize, it is not influenced by the total C in the pasture soils.

[FIGURE 1 OMITTED]

The [DELTA][sup.14]C values for both soils are also reported in Table 5. The values for the Andisol are lower than those for the Inceptisol, and are consistent with greater retention of old pasture and forest C in the Andisol.

The Inceptisol had [delta][sup.13]C values for the >60 [micro]m, 2-60 [micro]m, and <2 [micro]m fractions that were similar to those reported by Gregorich et al. (1995) for another soil under maize. There was more fresh maize C in the >60 [micro]m fractions than the other fractions of both soils, whereas the 2-60 [micro]m fractions contained the least maize C (Table 6).

Discussion

The total C in the 0-20 cm soil depth under pasture was greater in the Andisol than in the adjacent Inceptisol (Table 4); the [Al.sub.py] values were also greater in the Andisol than in the Inceptisol (Table 2). Since these 2 soils had been under similar management, these results are consistent with the finding that the level of organic C is strongly correlated with A1 in the 0-20 cm soil depth in New Zealand (Percival et al. 2000). Arrouays et al. (1995) have suggested that this is an important mechanism in stabilising soil organic matter in acid forest soils, and most New Zealand soils, until about 150 years ago, were under forests that produce acid litter. Generally, they have not been heavily limed.

The specific surface area of the Andisol 0-20 cm layer under maize was 130 [m.sup.2]/g, while the specific surface area of the Inceptisol was 60 [m.sup.2]/g, which reflects the influence of allophane on specific surface area. Saggar et al. (1994, 1996, 1999b) have shown that the turnover time of freshly added C substrates is greater with soils that have greater specific surface area and a greater capacity to sorb organic matter. Thus, soil specific surface area also may influence the soil organic matter levels in these soils.

Soil organic matter decreased with depth under pasture, since most inputs of organic matter under grazed pasture occur close to the soil surface. In contrast, the plough layer under maize contained relatively uniform amounts of organic matter to 25 cm depth because cultivation had altered the distribution of C with depth (Fig. 1). For the Inceptisol, the total C (t/ha) to 60 cm depth under maize and pasture was similar on the basis of both equivalent soil mass (Table 4) and equivalent soil volume; for the Andisol there was also no significant difference in total C under maize and pasture. The maize had been grown for grain on both soils for 25 years, and the residue was returned to the soils. The annual addition of C from tops and roots was estimated to be 9 t/ha. The annual addition of C from pasture to soil was also estimated to be about 9 t/ha. The similarity of C inputs under maize and pasture partly accounts for the maintenance of soil organic matter levels with maize cropping on these soils. This result contrasts with the normal finding that soil organic matter levels decrease with cropping (e.g. Balesdent et al. 1988; Arrouays et al. 1995; Liang et al. 1998). Since the soil total C content did not appear to change with time, the gross C mineralization was also about 9 t C/ha each year.

Our data showed there was more fresh maize C in the >60 [micro]m fractions than the other fractions of both soils (Table 6). Similar results were obtained by Balesdent et al. (1987) and other workers cited in this reference. Balesdent (1996) also showed that C turnover was slowest in the <50 [micro]m fraction. Although our data for the particle size fractions were not replicated, they tended to show that the C derived from maize was lowest in the 2-60 [micro]m fractions (Table 6). This is consistent with the finding that the C mineralisation for the 2-60 [micro]m fraction is lower than for the >60

[micro]m fraction or <2 [micro]m fractions of these soils (Parfitt and Salt 2001) and suggests that C is physically protected in the micropores within the silt fraction (Gregorich et al. 1989).

Although they had different clay mineralogy and specific surface area, the Andisol and the Inceptisol (0-35 cm depth) retained similar amounts of maize C (27 t/ha and 31 t/ha) after 25 years of cropping (Fig. 1). In contrast, Liang et al. (1998) showed that the rate of increase in the proportion of maize C increased with clay content. The amounts of old pasture and forest C retained, however, were greater for the Andisol than the Inceptisol, suggesting that the old C was turning over more slowly in the Andisol. This is consistent with the [DELTA][sup.14]C data, and suggests allophane and Al reduce the decomposition rate of old C in soil. Our data are also consistent with the results of Balesdent et al. (1988), who showed that at least 50% of prairie C remained in a soil after 100 years of cultivation, and that most loss of C occurred in the labile pool.

The rate of decomposition of ryegrass C and pine needle C in New Zealand is rapid, with 50-80% decomposing within the first 12 months (Parshotam et al. 2000; Parfitt et al. 2001a); decomposition then proceeds at a lower rate. This can be modelled with a 3-compartment system and mean residence times of individual C atoms: 0.2 year for ryegrass substrate, 0.1 year for microbial biomass, and 3 years for humus in Andisols (Saggar et al. 1996). Our data for the Andisol under maize included the annual C input, and the maize C remaining in the soil at 10 years and 25 years (Table 6). This is a limited data set, so that mean residence times cannot be calculated for a 3-compartment system. Since maize residues generally contain more recalcitrant materials than ryegrass, it is anticipated that mean residence times will be greater for maize than for ryegrass.

The comparatively high total C content of the Andisol would appear to arise from stabilisation of old soil organic matter by allophane and Al (Percival et al. 2000). Both the Andisol and Inceptisol, however, retained similar amounts of maize C after 25 years of cropping. This suggested that the stabilisation of maize C is not influenced by the large amounts of allophane and Al in the Andisol. Possibly, the allophane and Al sites are already blocked by the pasture and forest C.

Conclusions

Although the distribution of C with depth differed under pasture and maize, the total C in the soil under each land use appeared to be the same. This is consistent with the similar gross inputs of C under both management systems. In the 25-year time frame, the net input of maize C was 27 t/ha for the Andisol soil and 31 t/ha for the Inceptisol, and this result is not influenced by the total C in the soils under pasture. The mineralisation of maize C did not differ greatly in these two soils, which had different specific surface areas. The Andisol had higher total C than the Inceptisol, and the old pasture and forest C is probably stabilised by allophane and Al in the Andisol. However, the allophane and Al in the Andisol did not stabilise the maize C, and their sorption sites may be already blocked. Based on the ryegrass and pine needle models, there is rapid decomposition of maize residues in the first few years. These results fill a gap in the data required to model decomposition of C from crop residues in New Zealand soils over the 25-year time frame.
Table 1. Selected soil characteristics

LZ, loamy silt; SL, sandy loam; LS, loamy sand; ZL, silt loam;
SA, specific surface area

 Depth Colour Texture
 (cm)

 Andisol
Pasture
 Ap 0-18 10YR 3/2 LZ
 Bw1 18-34 10YR 5/8 SL
 Bw2 34-54 2.5Y 5/6 LS
 Bw3 54- 2.5Y 7/4 LS

Maize
 Ap 0-24 10YR 3/2 + 5/3 LZ
 Bw1 24-40 10YR 6/6 SL
 Bw2 40-50 2.5Y 7/6 SL
 Bw3 50- 2.5Y 7/3 LS

 Inceptisol
Pasture
 Ap 0-18 10YR 3/2 ZL
 Bw(g) 18-36 10YR 5/6 + 6/8 ZL
 Bw(g) 36-55 2.5Y 7/4 + 10YR 7/6 ZL
 Bw(g) 55- 2.5Y 7/6 + 8/6 ZL

Maize
 Ap 0-24 10YR 3/2 ZL
 Bw(f) 24-40 5Y 7/2 + 2.5Y 7/6 ZL
 Bw(f) 40-50 2.5Y 7/4 ZL
 Bw 50- 2.5Y 7/4 LS

 SA pH Stones
 ([m.sup.2]/g) ([H.sub.2]O) (%)

 Andisol
Pasture
 Ap 174 5.5 0
 Bw1 171 5.9 0
 Bw2 n.d. 6.3 0
 Bw3 n.d. 6.4 0

Maize
 Ap 127 6.2 0
 Bw1 134 6.5 3
 Bw2 n.d. 6.5 6
 Bw3 n.d. 6.6 8

 Inceptisol
Pasture
 Ap 93 5.4 0
 Bw(g) 76 5.4 0
 Bw(g) n.d. 5.6 0
 Bw(g) n.d. 5.6 5

Maize
 Ap 59 6.3 0
 Bw(f) 61 6.5 0
 Bw(f) n.d. 6.4 0
 Bw n.d. 6.5 0

n.d., not determined.
Table 2. Analysis of soil C, N, pyrophosphate-Al, and clay mineral
composition

 Depth C N C/N [Al.sub.py]
 (cm) (g/kg)

 Andisol

Pasture 0-5 121 11 11.5 7.9
 10-15 69 6 11.8 7.3
 25-30 22 1.5 14.5 n.d.
 55-60 5 0.3 18.4 n.d.

Maize 0-25 56 4.8 11.7 5.5
 25-30 24 2.0 11.9 n.d.
 40-45 13 0.9 13.7 n.d.
 55-60 6 0.4 14.9 n.d.

 Inceptisol

Pasture 0-5 75 74 11.5 4.0
 10-15 38 38 11.3 3.3
 25-30 11 11 13.3 n.d.
 55-60 4 4 15.4 n.d.

Maize 0-25 46 4.1 11.5 1.3
 25-30 8 0.7 11.7 n.d.
 40-45 3 0.2 13.9 n.d.
 55-60 2 0.1 18.5 n.d.

 Depth Allophane Halloysite Gibbsite
 (cm) (g/kg)

 Adnisol

Pasture 0-5 110 0 0
 10-15 130 0 0
 25-30 150 20 0
 55-60 100 30 0

Maize 0-25 100 0 tr
 25-30 100 0 3
 40-45 150 0 0
 55-60 100 0 0

 Inceptisol

Pasture 0-5 20 220 6
 10-15 10 200 6
 25-30 10 280 0
 55-60 10 280 0

Maize 0-25 20 200 5
 25-30 10 200 4
 40-45 10 300 0
 55-60 10 250 0

n.d., not determined; tr, trace.
Table 3. Bulk densities (g/[cm.sup.3]) for each layer
Mean of two pit samples

 Andisol Inceptisol
Depth Pasture Maize Pasture Maize
(cm)
 0-5 0.57 0.82 0.80 0.83
 5-10 0.67 0.90 0.86 0.79
10-15 0.70 0.93 0.97 0.85
15-20 0.75 0.86 1.02 0.82
20-25 0.67 0.83 0.98 0.90
25-30 0.66 0.88 0.93 1.09
30-35 0.64 0.87 0.93 1.14
35-40 0.66 0.90 0.96 1.11
40-45 0.70 0.98 0.91 0.95
45-50 0.71 0.98 0.95 0.84
50-55 0.82 1.11 0.94 0.91
55-60 0.88 1.21 0.99 0.96
Table 4. Total carbon (t/ha) in each layer based on equivalent soil
mass

Mean of two pits with four pooled samples.
Standard errors are in parentheses

 Andisol Inceptisol
Depth Pasture Maize Pasture Maize
(cm)

 0-5 48.5 (4.5) 23.0 (0.8) 30.7 (1.5) 18.4 (0.1)
 5-10 40.7 (3.2) 25.1 (1.9) 23.7 (0.3) 18.7 (0.8)
10-15 33.7 (1.3) 26.0 (0.2) 18.0 (1.8) 20.4 (0.4)
15-20 22.1 (3.7) 24.6 (1.0) 10.9 (2.3) 19.0 (1.9)
20-25 13.0 (1.9) 23.0 (2.3) 5.6 (0.4) 17.8 (2.3)
25-30 10.0 (3.5) 10.6 (0.1) 4.9 (0.2) 4.3 (0.1)
30-35 8.6 (2.9) 7.3 (0.1) 4.6 (0.7) 2.7 (0.2)
35-40 7.6 (3.5) 5.7 (0.0) 3.7 (0.4) 2.6 (0.3)
40-45 5.8 (3.5) 5.9 (0.2) 3.2 (0.3) 1.7 (0.1)
45-50 4.6 (2.4) 4.8 (0.1) 2.5 (0.4) 1.3 (0.1)
50-55 3.5 (2.0) 4.5 (0.1) 2.7 (0.1) 1.2 (0.1)
55-60 3.0 (1.1) 4.1 (0.5) 2.6 (0.5) 1.1 (0.1)

Total 201 (35) 164 (5) 113 (5) 110 (5)
Table 5. [delta][sup.13]C and [DELTA][sup.14]C [per thousand] of
the Andisol and Inceptisol under maize and pasture in February 1998

 Andisol
Depth
(cm) [delta][sup.13]C [DELTA][sup.14]C
 Pasture Maize Pasture Maize

 0-5 -27.5 -23.2 119.5 68.2
 5-10 -27.0 -23.2 89.6 68.2
10-15 -26.4 -23.2 26.9 68.2
15-20 -25.7 -23.2 -57.0 68.2
20-25 -25.2 n.d. -120.9 n.d.
25-30 -24.8 -24.1 -149.9 -9.7
40-45 -25.0 -24.1 n.d. n.d.
55-60 -25.2 -24.1 n.d. n.d.

 Inceptisol
Depth
(cm) [delta][sup.13]C [DELTA][sup.14]C
 Pasture Maize Pasture Maize

 0-5 -27.8 -21.8 141.1 79.4
 5-10 -27.3 -21.8 146.2 79.4
10-15 -26.5 -21.8 102.5 79.4
15-20 -25.1 -21.8 33.4 79.4
20-25 -25.8 n.d. -43.8 n.d.
25-30 -25.5 -24.1 -85.0 -49.4
40-45 -25.3 -23.2 n.d. n.d.
55-60 -25.1 -24.8 n.d. n.d.

n.d., not determined.
Table 6. [[delta].sup.13]C under pasture and maize for two farms,
proportion of old pasture carbon remaining in maize soil, and
annual increase in maize-derived carbon in December 1996

Standard errors are in parentheses; n = 3

Sample Pasture Maize C derived
 from maize
 (%)

 Andisol, 10 years

0-10 cm -27.3 (0.1) -25.4 (0.1) 12 (0.9)
10-20 cm -26.7 (0.1) -25.4 (0.03) 9 (1.1)

 Andisol, 25 years

0-10 cm -27.3 (0.1) -23.7 (0.1) 23 (0.6)
10-20 cm -26.7 (0.1) -23.7 (0.1) 21 (1.4)
>60 [micro]m fraction -27.5 -22.4 33
2-60 [micro]m fraction -27.1 -24.8 15
<2 [micro]m fraction -26.6 -22.6 27

 Inceptisol, 25 years

0-10 cm -26.8 (0.3) -22.7 (0.1) 28 (1.2)
10-20 cm -26.9 (0.1) -22.4 (0.4) 31 (2.3)
>60 [micro]m fraction -29.8 -22.1 43
2-60 [micro]m fraction -28.0 -24.4 23
<2 [micro]m fraction -27.4 -23.4 26

 Andisol-2, 25 years

0-10 cm -25.9 (0.1) -22.9 (0.1) 22 (0.6)
10-20 cm -25.3 (0.3) -22.5 (0.1) 21 (1.2)

 Inceptisol-2, 25 years

0-10 cm -26.2 (0.3) -21.3 (0.03) 33 (1.4)
10-20 cm -25.6 (0.1) -21.5 (0.3) 31 (1.7)


Acknowledgments

We are grateful to N. Fisher and P. Hooker for generous assistance and access to their farms, to S. Saggar for valuable assistance, and to D. J. Ross, K. R. Tate, and referees for comments on the manuscript. The work was supported by the Foundation for Research, Science and Technology under contract C09X0016.

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Manuscript received 4 December 2000, accepted 4 May 2001

R. L. Parfitt (A), A. Parshotam, and G. J. Salt

Landacre Research, Private Bag 11052, Palmerston North, New Zealand.

(A) Corresponding author; email: parfittr@landacre.cri.nz
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Author:Parfitt, R.L.; Parshotam, A.; Salt , G.J.
Publication:Australian Journal of Soil Research
Article Type:Statistical Data Included
Geographic Code:8AUST
Date:Jan 1, 2002
Words:5428
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