chemical fractionation to characterize changes in sulphur and carbon in soil caused by management
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Chemical fractionation to characterize changes insulphur and carbon in soil caused by management
B HUP INDER PAL - S I NGHa , M . J . H EDLEY
a , S . S AGGARb & G . S . F RANC I S
c
aInstitute of Natural Resources, Massey University, Private Bag 11222, Palmerston North, bLandcare Research, Private Bag 11052,
Palmerston North, and cCrop and Food Research, Private Bag 4704, Lincoln, Canterbury, New Zealand
Summary
Classical chemical fractionation of soil sulphur (S) into HI-reducible S and carbon-bonded S does not
separate S in soil into fractions that have differing mineralization potentials. Other techniques are needed
to separate organic S into more labile and less labile fractions of biological significance, irrespective of
their bonding relations. We have sequentially fractionated soil S and carbon (C) into their ionic forms
released onto ion-exchange resins and organic S and C extracted in alkali of increasing concentration.
We evaluated the technique on pasture and arable soils that had received various fertilizer and cultivation
treatments. Total S and C were greater in the soil of the fertilized pasture than in that of the unfertilized
pastures. Continuous arable cropping decreased total soil S and C, whereas restoration to pasture caused
an accumulation.
Resin, 0.1M NaOH, 1M NaOH and residual fractions accounted for between 1–13%, 49–69%, 4–16%
and 19–38% of total soil S and between 5–6%, 38–48%, 5–7% and 46–53% of total soil C, respectively.
Among different S and C fractions, the size of the 0.1M NaOH and residual fractions changed more with
the change in land use and management. The 0.1M NaOH fraction had a narrower C:S ratio (50–75:1) than
did the residual fraction (96–141:1). The significant degree of change in these two fractions, caused by
differences in land management, indicates that they may be useful indicators of change in ‘soil quality’.
Introduction
Maintaining and improving soil quality is essential if agricul-
ture and environmental quality are to be sustained. The
organic matter content of the soil has been widely used as a
measure of soil quality because it serves numerous functions
important to crop production and is an important reservoir of
the essential macronutrients such as nitrogen (N), sulphur (S)
and phosphorus (P). It also contributes significantly to the
formation and stabilization of soil structure. Losses in organic
matter content caused by intensive tillage and reduced input of
crop residues are well documented. Identifying changes in
organic matter content and quality is important in defining
sustainable agricultural practices over the longer term. Short-
term changes in the quantity of total organic matter in soil
induced by management are difficult to detect because a large
spatial variation obscures small changes in time. Measurement
of smaller, more labile fractions of organic matter in soil have,
however, been proposed to identify early changes caused by
management (Gregorich, 1996; Haynes, 2000).
The labile component of soil organic matter is a primary
source of mineralizable N, P and S, and therefore plays a
prominent role in nutrient dynamics and nutrient sustainabil-
ity of agricultural systems. As with C, changes in the contents
of these elements associated with the labile organic matter can
be important indicators of soil degradation or restoration.
More accurate estimates of ‘potentially’ dynamic fractions of
organic matter are required if changes in soil quality are to be
characterized over time. In contrast to N and P, information
on the effect of short- and long-term changes in land use and
management on labile soil organic matter fractions that supply
S to plants is scarce.
The classical chemical fractionation of S in soil according to
reactivity with reducing agents into HI-reducible S and
C-bonded S (Freney et al., 1969) does not characterize organic
fractions of S that have distinct mineralization potentials
(McLaren & Swift, 1977; Ghani et al., 1992). Some workers
have developed physical (Anderson et al., 1981) and a mixture
of chemical and physical (Bettany et al., 1979; Eriksen et al.,
1995) separation techniques to characterize the complex nature
of S in soil irrespective of its bonding relations to C. Again, theCorrespondence:Bhupinderpal-Singh.E-mail: bpsingh@agric.uwa.edu.au
Received 8 August 2002; revised version accepted 5 March 2003
European Journal of Soil Science, March 2004, 55, 79–90 doi: 10.1046/j.1365-2389.2003.00552.x
# 2003 Blackwell Publishing Ltd 79
fractionations were not entirely satisfactory, perhaps because
of the heterogeneity of S substrates in the organic matter. The
ideal solution to these problems might be provided by other
techniques that divide the organic S into a few biologically
meaningful labile fractions important for short- and long-term
supply of S to plants.
To characterize dynamic soil organic matter, Murata et al.
(1995) and Murata & Goh (1997) fractionated soils using
various pre-extractions such as shaking with cold water and
hot water followed by seven shakings with a mixture of acids
(0.1MHCl and 0.3MHF). They considered hot-water-extractable
C an indicator of labile organic matter, whereas HCl–HF-
extractable C represented an indicator of change in more stable
organic matter. They subsequently separated unextracted C into
fulvic acids, humic acids and humin using a series of five 0.1M
Na4P2O7 and three 0.5M NaOH extractions, and considered the
final residual C fraction (humin) to include a mixture of stable
organic forms suchashumic acidmaterials, andalso the remainsof
crop residues and dead microorganisms. By using this detailed,
multistep, fractionation, the scientists hoped to produce quality
indicators to compare the effects of (i) long-term superphosphate
application to irrigated pasture (Murata et al., 1995), and
(ii) cropping systems (Murata & Goh, 1997) on the quantity and
quality of soil organic matter. Long-term superphosphate applica-
tion to irrigated pasture increased hot-water-soluble C, but it had
no significant effects on the distribution of fulvic and humic acid
fractions of C. In the sequence of cropping and pasture rotations
studied by Murata & Goh (1997), the hot-water-soluble C
decreased as a proportion of the total C after 6 years of continuous
cultivation. In thepasturephase, however, over the sameperiod, all
C fractions – hot water soluble, HCl–HF, fulvic and humic acids
(Na4P2O7 or NaOH) and humin – increased in some soils. So
despiteusing thesepretreatments in a soil organicmatter extraction
scheme,Murata et al. (1995) andMurata&Goh (1997) found that
labile soil organic matter still appeared in both the extracted and
unextracted fractions.
The sequential extraction procedure for soil P, introduced by
Hedley et al. (1982), is being successfully used to characterize
changes in labile organic P fractions with change in soil
management and cropping (Tiessen et al., 1983; Huffman
et al., 1996) and plant uptake (Hedley et al., 1994). It relies on
differing strengths of reagents to extract soil organic phosphate
of differing lability. Recently, Lilienfein et al. (2000) also used a
sequential P fractionation, similar to that of Hedley et al.
(1982), to characterize the influence of different forms of land
use on P and S pools in savannahOxisols. They found that land
use influenced the amounts of P and S in the ‘plant available
fractions’ (0.5M NaHCO3, 0.1M NaOH). Nguyen & Goh
(1992) have reported that the alkaline solutions (0.5M
NaHCO3 (pH 8.5) and 0.1M NaOH) could extract consider-
able proportions of total soil S. Therefore these extractants
might be suitable for use in a sequential fractionation of S and
for assessing their ability to characterize the availability of S in
organic matter.
We have investigated whether a scheme similar to the P
fractionation of Hedley et al. (1982) can be used to character-
ize changes in organic S and C fractions on a range of pasture
and arable soils with varied management histories. If this was
possible then data on the dynamics of P and S in the soil could
be obtained simultaneously.
Materials and methods
Soil samples were obtained from three experimental sites
representing a range in fertilizer applications of S (pasture
soils, Mt Thomas), period of cultivation (Kairanga) and pasture
restoration crops (Wakanui) (Table 1). A detailed description of
these sites, experimental treatments, management history and
soils is presented elsewhere (Giddens et al., 1995; Francis et al.,
1999; Shepherd et al., 2001). Table 1 summarizes the soil types,
agronomic treatments and fertilizer applications, and we briefly
describe the more important aspects below.
Description of field sites
Mt Thomas site (fertilized pasture). The fertilizer treatments
on this initially S-deficient permanent pasture were SO42–-S
applied as single superphosphate (SSP) or SSP and gypsum
(SSP–G), a mix of sulphate and elemental S in ‘sulphur super
extra’ (SSX), and elemental S0 granulated with bentonite
(Ben28, Ben56) applied tri-annually over 6 years (Table 1).
There were four replicates of each fertilizer treatment. Soil
samples (0–7.5 cm depth) were collected with a 2.5-cm diameter
probe with 20 soil cores from each replicate, which were bulked.
All S treatments produced marked growth responses in legume
yield in the first 4 years of the trial (Craighead et al., 1990).
Kairanga site (short- to long-term cultivation). This site car-
ried a permanent pasture (PP) and several continuous cultivation
systems with different times since last in pasture, i.e. 4 years of
maize cultivation (M4Y), 11 years of maize cultivation (M11Y)
and 30years of barley cultivation (B30Y). Also included was a
site that had been ploughed for 11 years for maize and then
returned to permanent pasture for 10years (M11YþP10Y)
(Table 1). Soil samples were taken from 0–10 cm depth of soil
(from each paddock) with a 2.5-cm diameter probe.
Wakanui site (short-term pasture cropping). This site had a
short-term mixed cropping trial, which included a range of
permanent pastures, annual pastures and arable crops started
in 1989 on a Wakanui silt loam soil after 11 years of arable
cropping (Francis et al., 1999). Four treatments providing dif-
ferences within and between the annual (ryegrass grazed and
conventionally cultivated, ARc; ryegrass grazed and direct
drilled, ARd) and perennial (ryegrass grazed, PRg; ryegrass
mown, PRm) pastures (Table 1), replicated four times, were
included in the present study because different crops in the
trial confound any other comparisons. In the sixth year of the
80 Bhupinderpal-Singh et al.
# 2003 Blackwell Publishing Ltd, European Journal of Soil Science, 55, 79–90
Table1Soilclassificationanddescriptionoftreatm
ents
fortheMtThomas,KairangaandWakanuisites
Soilgroupa
Applicationrate
/kgha�1application�1
Soilname
(Application)
New
Zealand(G
enetic)
FAO
Treatm
ent
Treatm
entcode
SO
42–
S0
MtThomas
Allophanic
brownsoil
DystricCambisol
Control
Ct
00
siltloam
(HighcountryYellow
Single
super
phosphate
andgypsum
SSP–G
28
0
(Triannual)
BrownEarth)
Bentonitic
S0
Sulphursuper
extra
Bentonitic
S056
Sulphursuper
extra(reapplied)
Ben28
SSX
Ben56
SSXr
0
19 0
27
28
37
56
91
Kairangasilty
Typic
orthic
gleysoil
Eutric
Gleysol
Permanentpasture
PP
22b
0b
clayloam
(RecentGley)
Maize4years
M4Y
35b
0b
(Annual)
Maize11years
Barley
30years
Maize11years
followed
by10years
pasture
M11Y
B30Y
M11YþP10Y
NA
b
NA
b
44b
NA
b
NA
b
0b
Wakanui
Immature
pallic
soil
Gleyic
Cambisol
Annualryegrass
grazedandconventionallycultivated
ARc
17
0
siltloam
(Yellow
GreyEarth)
Annualryegrass
grazedanddirectdrilled
ARd
17
0
(Annual)
Perennialryegrass
grazed
Perennialryegrass
mown
PRg
PRm
17
17
0 0
aHew
itt(1992).
bTheSregim
esgiven
are
those
applied
only
duringthesamplingyear.AlthoughShasbeenpreviouslyapplied
inthesetreatm
entseither
asammonium
sulphate
orasSSP,thedata
forthose
applicationswerenotavailable
from
thefarm
ers.
NA,notavailable.
Characterization of soil organic sulphur and carbon 81
# 2003 Blackwell Publishing Ltd, European Journal of Soil Science, 55, 79–90
trial, we sampled the topsoil (0–5 cm depth) by taking 10 cores
from each replicate using a 2.5-cm diameter probe.
Preparation of soil samples
Soil sampling depths for the root zone of each site varied for
each site as they were dictated by the previous researchers (see
above). Comparisons of C and S content in the soil across
different treatments were therefore restricted to within a single
site, where the depth of soil sampled was the same.
From these soil samples, organic material such as visible
roots was removed. The soils were sieved (< 2mm) and air-
dried at 30�C� 2�C. The air-dried samples of soil were then
finely ground for 10 s in a ring grinder (Rocklabs,Auckland,New
Zealand) for fractionation and other soil analyses. Unless other-
wise stated, all soil samples were analysed in duplicate in the
various extractions described below.
A sequential soil S and C fractionation technique
The procedure used to fractionate soil S and C sequentially is
summarized in Figure 1. It is similar to that of Hedley et al.
(1982) for soil P. As in that fractionation, the initial design of
the sequential fractionation was to determine first the
exchangeable ionic forms of S and C in the soil (both soluble
and adsorbed and simple organic esters) extracted on to ion-
exchange resins [resin-S(C)]. Strips of both anion- and cation-
exchange resin membranes were used because in soils of
variable charge, extraction of anions may be more effective
when cations contributing to surface positive charge are
removed (Saggar et al., 1992). Solid-phase non-exchangeable
forms of S and C were extracted in alkali extractions of two
strengths. Hence, conceptually ‘labile’ organic S and C fractions
would be extracted by 0.1MNaOH [0.1MNaOH-S(C)] thatwould
meet medium- to long-term plant nutrient availability, and less
‘labile’ fractions extracted by 1M NaOH [1M NaOH-S(C)]
(Hedley et al., 1982). The S and C fractions not extracted with
either resin (representing short-term nutrient availability) or the
NaOH extractants were termed as the residual fraction [residual-
S(C)]. The initial concept viewed the unextracted residual fraction
as a recalcitrant, chemically stable fraction.
The fractionation procedure for C in the soil was slightly
modified from that of S (Figure 1) in the following way. The
anion and cation resins were charged as in the procedure
described by Saggar et al. (1990), except that the anion-
exchange resin was charged as OH– with 0.1M NaOH instead
Soil
Elution in 10 mlResin-S(C)
2 g soil (ring ground) in 40 ml polypropylenecentrifuge tubes, add 18 ml deionized water and resins (both cation and anion). Shake for 2 hoursat room temperature.
Soil residue+ water
Ion-exchange resin strips
of 0.5 M NaCl or0.5 M NaNO3
0.1 M NaOH-S(C)
1 M NaOH-S(C)
Add 2 ml of 1 M NaOH (to generate 0.1 M NaOH), shake for 16 hours at room temperature, centrifuge and filter supernatant through Whatman No 42 filter paper or glass microfibre filter (GF,C).
Add 20 ml of 1 M NaOH, shake for 16 hours at room temperature, centrifuge and filter supernatantthrough Whatman No 42 filter paper or glass microfibre filter (GF,C).
Residual-S(C)
Dry the soil residue at 35°C and ring grind.
Remove strips, rinse with deionized water and elute them together.
Filtrate
Soil residue
Soil residue
Filtrate
Figure 1 A sequential fractionation technique for soil S and C.
82 Bhupinderpal-Singh et al.
# 2003 Blackwell Publishing Ltd, European Journal of Soil Science, 55, 79–90
of HCO3– resin (i.e. by using 0.5M NaHCO3). This would avoid
any interference in the estimation of C from HCO3– ions. Both
cation and anion resins were eluted in 0.5M NaNO3 (this eluted
the same amount of C from the cation- and anion-exchange resins
as was eluted in 0.5M NaCl) to avoid interference by the Cl– ion
during dichromate digestion for the estimation of total C in the
extract (see below). Also, alkaline extracts were filtered through
Whatman glass microfibre (GF,C) so that no cellulose came out in
the alkaline extracts from paper filters (Whatman No 42).
Analysis of S in different fractions
Resin-extractable S. We analysed an aliquot of 0.5M NaCl
or NaNO3 elutant for resin-S by reducing S to H2S using HI
mixture and measuring it as bismuth sulphide turbidimetric
finish (Dean, 1966) using a Technicon II autoanalyser. Any
C-bonded S in this extract would not be measured.
Sulphur in NaOH extracts. The alkaline extracts were
digested with freshly prepared sodium hypobromite (NaOBr)
as in the procedure described by Tabatabai & Bremner (1970).
However, the amount of NaOBr used in this digestion was
4ml instead of the 3ml recommended in the original method.
The greyish digests were redissolved in deionized water and
analysed for SO42–-S on an autoanalyser as described above
for resin-S.
Total S in soil and residual fraction. The total S in soil and
residual fraction was oxidized to SO42–-S by the ignition
method of Steinberg et al. (1962). The digests were analysed
for SO42–-S on an autoanalyser as described above.
Analysis of C in different fractions
Total C in soil samples was determined by combustion on a
Leco FP-2000 CNS analyser. The oxidation and digestion
technique of Amato (1983) as described by Sparling et al.
(1991) was used to determine C in the 0.5M NaNO3 elutant,
0.1M and 1M NaOH extracts and the residual fraction of
Kairanga soil.
Chloride (Cl–) interference in the resin extract
Extracts from Kairanga soils contained various concentrations
of C. However, Cl– interfered (Quinn & Saloman, 1964) during
the dichromate digestion (for total C determination, Sparling
et al., 1991) of resin extracts from the Kairanga soils. A recov-
ery of 99.5% C of a glucose spike was obtained for Cl–-free
extracts. The interference effect from chloride was determined
to be linearly related to chloride concentration by spiking a
standard glucose solution with different concentrations of
chloride (0, 0.5, 1, 2, 3, 4 milliequivalent). A linear relation
was used to correct any chloride interference. The Cl– concen-
tration in a sample was determined with chloride analyser.
Statistics
The significance of a linear relation between amounts of S and
C in the soil and in extracts of soil (referred to as S and C
fractions) was expressed by the product-moment correlation
coefficient. Simple linear regression was used to determine the
rate of change of different soil S (or C) fractions with changes
in total soil S (or C) induced by management. The F test was
used to test the significance of difference between the rates of
change (slopes). The significance of differences between
treatment means in each replicated experiment (i.e. Mt Thomas
and Wakanui sites) was tested (at 5% level of significance) by
analysis of variance (ANOVA), and the treatment means for each
site were separated by Tukey’s Honest Significant Difference
procedure.
Results and discussion
Changes in total soil S and C
Improved permanent pasture. Fertilizer-S applied to the
S-deficient pasture soil at Mt Thomas over 6 years significantly
increased the amount of S in the soil (Figure 2). The greatest
accumulation of total S was in the SSXr treated soil, and the
amount decreased in the order of Ben56> SSX¼ SSP–G
>Ben28>Ct (Figure 2). This indicates a better conservation
of elemental S0 into organic S or an accumulation of residual
elemental S0. There was a concurrent increase in the total C
content with increasing rates of fertilizer-S (Figure 2). However,
the relationship (r¼ 0.43) between total soil S and C with
increasing rates of S application was not significant because
although SSXr treatment resulted in considerably more accu-
mulation of S than did the Ct treatment, the corresponding
increase in C was small. Nguyen & Goh (1990) also found
significantly greater organic C in the soil of irrigated, grazed
pastures receiving long-term application of superphosphate
than in the unfertilized pasture soil. They attributed this
increase in C to an increased biological fixation of N2 by
legumes that were no longer deprived of S. This led in turn to
an increase in pasture dry matter and hence greater organic C
input to soil. At the Mt Thomas site, S fertilization markedly
increased pasture production (Craighead et al., 1990), and this
has resulted in most of the applied S accumulating in organic
forms. After many years of pasture improvement, the rate of
accumulation of organic S in the soil decreases, and mineraliza-
tion becomes a significant source of S for pasture plants
(Sakadevan et al., 1993). Accumulated organic S therefore
improves the fertility and economic value of a pasture soil.
Cropping and pasture restoration phases. At the Kairanga
site (short- to long-term cultivation) the soil on the PP farm
contained more total S and C than the soils cultivated for
maize (M) or barley (B) (Figure 2). There was a greater
depletion of total S and C in the medium- to long-term culti-
vation treatments (M11Y and B30Y) than in the short-term
Characterization of soil organic sulphur and carbon 83
# 2003 Blackwell Publishing Ltd, European Journal of Soil Science, 55, 79–90
cultivation (M4Y). Decreases in soil S were significantly
(P< 0.01) related to decreases in soil C (r¼ 0.98). A similar
significant linear relationship between the change in the total
soil S and C was also observed in the short-term cropped
Wakanui site (Figure 2). The Kairanga soil, however, had a
narrower C:S ratio (67–83:1) compared with the wider C:S
ratio (93–105:1) of the Wakanui soil (Figure 2).
Long-term cultivation had considerably decreased organic
S and C in the soil, probably as a result of enhanced decompos-
ition of organic matter and decreased organic matter inputs
under cropping (Saggar et al., 2001). We think the enhanced
decomposition is due to mechanical stress breaking aggregates
bound by organic matter and exposing the organic matter to
microbial decomposers, which leads to faster mineralization in
warmer soil. Murata & Goh (1997) also found that increased
period of cultivation and cropping decreased total C and total
N, whilst under pasture following cultivation, total C and
total N, and aggregate stability, of soils increased with time
(Shepherd et al., 2001). We found in the present study that
conversion of 11-year arable cropping sites (Kairanga or
Wakanui) to perennial pasture management for 10 years
(Kairanga, compare M11Y with M11YþP10Y) or 6 years
(Wakanui, PRg and PRm treatments) caused a greater accu-
mulation of total S and C in the soil than in the annually
cultivated pasture (ARc and ARd treatments) (Figure 2).
The comparison of unit change in total S and C across all
the three sites (Figure 2) showed that changes in total S
content in relation to unit changes in C were not statistically
different. This suggests that although management changes
considerably, changes in S in the soil follow changes in C.
Sulphur and carbon fractions sensitive to changes in
management practices
The chemical fractionation appeared to recover between
90 and 115% of the total S in the soil and between 102
and 110% of the total soil C. Apparent recoveries greater
than 100% presumably result from either summation of
errors in the fractionation of total S or underestimation of
soil total S content by the NaHCO3–AgO oxidation (i.e. sum of
S fractions> total S).
In all the three experimental sites (Table 1), 0.1M NaOH-S
constituted the major portion (49–69%) of total soil S, fol-
lowed by residual-S (19–38%), 1M NaOH-S (4–16%) and
resin-S (1–13%) (Figures 3, 4a and 5). However, in the
Kairanga soil, which was also subjected to sequential C frac-
tionation, the major portion of total C was recovered in resi-
dual-C (46–53%) followed by 0.1M NaOH-C (38–48%), 1M
NaOH-C (5–7%) and resin-C (5–6%) (Figure 4b).
Resin-S, which constituted only a small part of total soil S
(1–13%), was greater in the recently fertilized pasture soils
(Mt Thomas) than in the soils under short- and long-term
cultivated (Kairanga) and short-term pasture cropping (Wakanui)
(cf. Figures 3, 4a and 5). This might reflect the fact that the soil at
6
Total soil C /%
200
300
400
500
600
700
To
tal s
oil
S /m
g k
g–1
PP
M4Y
M11Y+P10Y
B30YM11Y
PRmPRg
ARc
ARd
SSXr
Ben56
SSP/GSSX Ben28Ct
0 2 3 4 50
Mt Thomas, r = 0.43
Kairanga, r = 0.98
Wakanui, r = 0.99
Figure 2 Relations between total soil S and soil C
as influenced by varied management. The total S
at theWakanui site is the sum of all the S fractions
recovered by the sequential fractionation.
Treatment codes are given in Table 1.
84 Bhupinderpal-Singh et al.
# 2003 Blackwell Publishing Ltd, European Journal of Soil Science, 55, 79–90
Mt Thomas (high country Yellow Brown Earth) has a larger
anion exchange capacity (% phosphate retention, see Metson,
1979) than the Kairanga (Recent Gley) or Wakanui silt loam
(Yellow Grey Earth), and so more SO42� can be retained on the
greater hydrous oxide surfaces.
Improved permanent pasture. In fertilized pasture soils (Mt
Thomas), S in all the fractions increased with the increase in
total S in soil from increasing amounts of fertilizer-S (Figure 6).
The rate of increase in 0.1M NaOH-S was significantly greater
(P< 0.001) than that in residual-S, and both rates of increase
were significantly (P< 0.001) greater than increases in resin-S
and 1M NaOH-S (Figure 6). When applied at the same rate, the
different forms of fertilizer had no significant effect on any
particular fraction of S, except resin-S (Figure 3). For example,
treatment Ben56 increased resin-S significantly in excess of that
obtained from SSX treatment (Figure 3) although these two
sources supplied equal amounts of S. When applied at the
different rates (cf. 56 against 112 kgSha�1, Figure 3), Ben56
increased 0.1M NaOH-S significantly more than that obtained
from SSP–G and Ct treatments. However, treatment SSXr
resulted in significantly larger resin-S and 0.1M NaOH-S than
did the other treatments. The amount of S in the residual
fraction was significantly greater in the SSXr than in the Ct
treatment (Figure 3). This may be attributed to the unoxidized
elemental S0 that remained from fresh top dressings. The 1M
NaOH-S was not influenced significantly by the various fertil-
izer treatments (Figure 3).
Cropping and pasture renovation phases. At the Kairanga
sites, cultivation (M4Y, M11Y or B30Y) caused 0.1M NaOH-
S, 1M NaOH-S and residual-S in soils to decrease. However,
resin-S increased in theM4YandM11Y treatments, anddecreased
in the B30Y treatment and also when the M11Y site was restored
back to pasture for 10 years (M11YþP10Y) (Figure 4a). In
terms of proportional change, 0.1M NaOH-S was 50% smaller
at theM11YorB30Y sites than at the PP site. Restoration of the
M11Y site with pasture (M11YþP10Y), however, markedly
increased 0.1M NaOH-S to within 12% of that at the original
PP site. Similarly, residual-S in the M11Y and the B30Y was
33% and 50%, respectively, of that in the PP site. When the
cultivated soil (M11Y) was returned to pasture (M11YþP10Y)
the increases in the residual-S fraction were smaller (i.e.
increased from 33% to 47% of that in the PP site) relative to
changes in the 0.1MNaOH-S fraction (Figure 4a). Trends in the
change of 1M NaOH-S, with the change in duration of cultiva-
tion, were irregular and changed little when the cultivation site
(M11Y) was restored to pasture (M11YþP10Y). The rates of
change in both 0.1MNaOH-S and residual-S with the change in
total S were similar and were significantly greater (P< 0.01)
than the rates of change in resin-S and 1M NaOH-S (data not
shown).
All C fractions were smaller in the cultivated (M4Y, M11Y
or B30Y) soils than in pasture soils (PP). Short-term
cultivation (M4Y) was associated with marked reductions in
0.1M NaOH-C and residual-C fractions and small reductions
in resin-C and 1M NaOH-C (Figure 4b). In the medium- to
long-term cultivated treatments (M11Y or B30Y), however, all
C fractions were about one-third less than in the PP phase. The
pasture restoration phase (M11YþP10Y) contained twice as
much 0.1M NaOH-C than the medium-term cultivated site
(M11Y), but smaller differences existed in resin-C, 1M
NaOH-C and residual-C (Figure 4b). The rates of change in
both 0.1M NaOH-C and residual-C with the change in total
soil C were similar and were significantly (P< 0.001) greater
than the rates of change in resin-C and 1M NaOH-C (data not
shown).
0
100
200
300
400
500
600
Ct SSP–G Ben28 SSX Ben56 SSXr
S fertilizer treatments
S e
xtra
cted
/mg
S k
g–1
so
il
Resin 0.1 M NaOH 1 M NaOH Residual
b
a
d
c
ab ab ab ab
a
a
a aa a
a
ac c b
bbccd bc
56 kg S ha–1 112 kg S ha–1
236 kg S ha–1
c
Figure 3 Effect of S fertilizer application to
pasture on different S fractions in the 0–7.5 cm
depth of Mt Thomas soil. Treatment codes are
given in Table 1. Error bars: one standard error
(SE) of treatment means. Comparing across
fertilizer treatments, fractions in the figure
followed by the same letter (a, b, c, d) are not
significantly different at P< 0.05.
Characterization of soil organic sulphur and carbon 85
# 2003 Blackwell Publishing Ltd, European Journal of Soil Science, 55, 79–90
Comparison of the amounts of soil S and C recovered in the
0.1M NaOH and residual fractions indicates that in both these
fractions, losses and gains of S and C with the change in
cultivation practices (Figure 7) were significantly correlated
(P< 0.05).
At the Wakanui site, the short-term annual and perennial
pasture cropping caused insignificant changes in the size of 1M
NaOH-S and resin-S (Figure 5). Within either annual (ARc,
ARd) or perennial (PRm, PRg) pasture treatments, the size of
0.1M NaOH-S fractions were not significantly different. This
contrasts with residual-S (Figure 5). Between annual and
perennial treatments there were differences in S fractions.
The 0.1M NaOH-S fraction was significantly smaller in the
ARc treatment than in the PRm treatment only, whereas
residual-S in the ARc treatment was significantly smaller
than in the other treatments (ARd, PRm, PRg). The largest
amounts of 0.1M NaOH-S and residual-S were in the PRm
and PRg treatments, respectively. The rate of change in 0.1M
NaOH-S with the change in total soil S was significantly
(P< 0.01) greater than that in residual-S and both rates of
change were significantly greater (P< 0.001) than changes in
resin-S and 1M NaOH-S (data not shown).
The above-mentioned results from the three sites indicate that
the 0.1M NaOH and residual fractions changed more than
other fractions as pastoral or cropping management changed
in the short or long term. They show that the residual fraction,
which represents the soil organic matter not extracted with
alkali (humin fraction), also contains a relatively labile organic
S and C. McLaren & Swift (1977) also found that residual
humin fraction, which is often regarded as being fairly inert,
cannot in the long term be neglected as a potential source of
mineralizable S. Using ‘bomb’ 14C enrichment, Goh et al. (1976)
also showed that the residual humin fraction contained both old
(i.e. stable) and young (i.e. labile) forms of soil organic matter.
Short- to long-term cultivation
M4Y M11Y B30Y M11Y+P10Y
PP
PP
M4Y M11Y B30Y M11Y+P10Y
(a)
(b)
0
100
200
300
400
500
600
700
0
10
20
30
40
50
60
70
S e
xtra
cted
/mg
S k
g–1
so
ilC
ext
ract
ed /g
C k
g–1
so
il
Resin 0.1 M NaOH 1 M NaOH Residual
Figure 4 Effect of short- to long-term cultivation
on (a) different S and (b) different C fractions
in the 0–10 cm depth of Kairanga soil. Treatment
codes are given in Table 1. Error bars: 1 SE of
sample means.
86 Bhupinderpal-Singh et al.
# 2003 Blackwell Publishing Ltd, European Journal of Soil Science, 55, 79–90
The 0.1M NaOH fraction had a narrower C:S ratio (50–75:1)
than the wider C:S ratio (96–141:1) of the residual fraction
(Figure 7). This is because the 0.1M NaOH recovered a greater
proportion of total S than did the residual fraction (Figure 7),
thus suggesting that 0.1M NaOH could have hydrolysed a
greater proportion of S in protein, and therefore resulted in a
0
100
200
300
400
ARc ARd PRm PRg
Short-term pasture cropping
a a a a
a ababb
aa
a a
ab
bc
S e
xtra
cted
/mg
S k
g–1
so
il
Resin 0.1 M NaOH 1 M NaOH Residual
Figure 5 Effect of short-term pasture cropping
on different S fractions in the 0–5 cm depth of
Wakanui soil. Treatment codes are given in
Table 1. Error bars: 1 SE of treatment means.
Comparing across pasture cropping treatments,
fractions in the figure followed by the same
letter (a, b, c) are not significantly different at
P< 0.05.
350 400 450 500 550 650600 700
Total soil S /mg kg–1soil
0
50
100
150
200
250
300
350
400
S e
xtra
cted
/mg
S k
g–1
so
il
0
0.1 M NaOH-S, y = 0.42x + 88.31, R
2 = 0.81
Residual-S, y = 0.25x – 26.99, R
2 = 0.70
Resin-S, y = 0.10x + 0.88, R
2 = 0.72
1 M NaOH-S, y = 0.05x + 7.30, R
2 = 0.24
Increasing fertilizer S
Figure 6 Changes in amounts of S fractions, y,
as total soil S, x, increases from increasing
amounts of fertilizer application to the
permanent pasture soil (Mt Thomas) over
6 years.
Characterization of soil organic sulphur and carbon 87
# 2003 Blackwell Publishing Ltd, European Journal of Soil Science, 55, 79–90
narrower C:S ratio of the 0.1M NaOH fraction. The residual
fraction probably included remains of undecomposed or partly
decomposed roots and crop residues, and dead microbial tissues
rich in cellulose and lignin (Murata & Goh, 1997) that are not
soluble in alkali (Lowe, 1978). The residual fraction also
includes a mixture of complex organic forms (humic acid),
which could be strongly complexed with clays and hydrous
oxides (Schulten & Schnitzer, 1997), and were not extracted
even with strong alkali. However, Hatcher et al. (1985), using
solid-state 13C-NMR, found that the spectra of humin (the
alkali-insoluble fraction) differed dramatically from those of
their respective humic acids, suggesting that humin is not a
clay–humic acid complex. They also showed the presence of
polysaccharide, aromatic (lignin) and paraffinic C in humin.
Clearly, there is a need to examine the origin of C appearing
in the residual fraction.
The amounts of S and C in the 1M NaOH fraction changed
little with change in management and contribute only a very
small proportion of the total S and C in the soil. The 1M
NaOH fraction might represent relatively humified organic
matter. The contribution of this fraction to available S in the
soil might be small and therefore of little value in predicting
the S requirement of plants.
Conclusions
Pastures, renowned for increasing soil organic matter and
aggregate stability, increased both S and C content of the
soil when S-deficient permanent pasture was fertilized with S
or when previously tilled soils were returned to pasture.
Our study has shown that the P fractionation scheme
extracts from soils S and C fractions that vary in their response
to change. Under conditions of organic S and C accumulation
or depletion, both the 0.1M NaOH fraction (representing
49–69% soil S and 38–48% soil C) and the residual fraction
(representing 19–38% soil S and 46–53% soil C) showed sig-
nificant changes and can be used as indicators of the quantity
and quality of the soil organic matter.
The novelty of our study is that S in the soil has been
determined in fractions usually used to characterize soil P.
We could not, however, separate organic S and C into
fractions of decreasing lability by sequential extractions with
alkali of increasing concentration as Hedley et al. (1982) did
for P. The reason for this difference in the fractionation of S
and P is perhaps their presence in different compounds in the
organic complexes within the soil organic matter. As the
residual-S (or C) appeared to contain both labile and stable
fractions of organic matter, in subsequent research we shall
examine how C in recent plant root material is distributed
amongst the fractions of this sequential extraction procedure.
Acknowledgements
We thank the farmers who gave us access to their land,
T.G. Shepherd (Landcare Research, New Zealand) and
M. Craighead (Ravensdown Fertilizer Co-op. Ltd) for supplying
soil samples, R. Webster, Soren Holm and Anne West for
providing valuable comments and checking statistics on an
earlier version of the script, and the New Zealand Ministry
of Foreign Affairs and Trade for awarding a NZODA–PGS
fellowship to Bhupinderpal-Singh to study for a PhD.
0
50
100
150
200
250
300
350
400
0 5000 10 000 15 000 20 000 25 000 30 000
Amount of soil C in the fractions /mg kg–1
Am
ou
nt
of
soil
S in
th
e fr
acti
on
s /m
g k
g–1
0.1 M NaOH r = 0.92
Residual r = 0.95
M11Y+P10Y
M11Y+P10Y
M11Y
M11Y
B30Y
M4Y
M4Y
PP
PP
B30Y
Figure 7 Comparison of amounts of S and C
recovered in 0.1M NaOH and residual fractions
from 0–10 cm depth of Kairanga soil. Treatment
codes are given in Table 1.
88 Bhupinderpal-Singh et al.
# 2003 Blackwell Publishing Ltd, European Journal of Soil Science, 55, 79–90
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