Aluminum associated with a pH-increase in the humus layer of a boreal haplic podzol

10 Pages·1994·2.8 MB·English

by  Jean-François Ponge

ALUMINUM ASSOCIATED WITH A
PH-INCREASE IN THE HUMUS LAYER OF
A BOREAL HAPLIC PODZOL
ULF SKYLLBERG
IQ
luminum (Al) is gener- Al and Al bound to soil organic matter have been kept fixed. To mm1m1ze the
1
~ ally considered to be a (Bloom et al., 1979; Hargrove and Tho unwanted variation associated with large
major problem for the mas, I 982; Cronan et al., 1986). scale factors the experimental area can be
vitality and productivity of acid tropical Several artificial acidifi reduced.
and temperate forest soils. It is the cation experiments have shown that cat The objectives of this
soluble inorganic Al species that seem to ion exchange between W and alkali and study were to examine the influence of
affect the morphology and functioning of alkali-earth cations is the basic pH-buff exchangeable Al on the small-scale vari
roots. The chemical processes involving ering process in naturally acid organic ability of pHcaCli· pHH 0 and ex
2
Al-compounds in the solid phase, in the horizons (James and Riha, 1986; Brown, changeable protons in sliced one-cm-lay
soil solution phase or on the soil adsorp 1987; Kreutzer et al., 1989; Natscher and ers of the 0-horizon (mor layer). An in
tion complex are also coupled to a con Schwertmann, 1991; Vance and David, tensive soil sampling was undertaken
sumption or production of protons. 1991 ). The degree of exchange between within small plots (I to 100 1111) in a
In the spodic horizon of Al and W seems to vary between 0-hori Picea abies (L.) Karst stand. to minimize
podzols the amount of soluble Al is zons, and has been found to be important the influence of larger-scale factors.
regulated by• the solubilization of Al-sili in soils with substantial amounts of ex
cates and AI -hydroxides. Although a sub changeable Al (Natscher and Schwert ;\1aterial and methods
stantial amount of soluble Al occurs as mann, 1991) . Exchange reactions between
organo-metallic complexes (e.g. Cronan Al-cations and H- on organic matter ex Sampling and sample treatment
and Schofield, 1979; David and Driscoll, change sites has in laboratory experi
1984 ), a negative correlation is often ments been shown to be an important The soil was sampled
found between pH and Al in the soil so source of pH-buffering in acid topsails from four plots of different sizes: plot A.
lution of spodic horizons (Reuss et al., (Bloom et al., 1979: Walker et al. . 100 m1 (10 by 10 m); plots Band C. 4
1990; Berggren, 1992). If the reference 1990). Furthermore laboratory experi m: (2 by 2 m); plot D. I m: l I b) I m ).
pH for the acid neutralization capacity of ments have shown that organic matter The B and C plots were randomly placed
tl~e soil is ~et to _4.5 (ANC•.pH 0 ) ?r saturated with Al gives a higher pH in within the A plot, which represented the
higher, AJ3• 1s considered to be an acid soil suspension compared with organic experimental stand (Table I). and plot D
cation, owing to its release of protons matter saturated with H ·. owing to the \1 as random I) placed in one quadrant of
upon hydrolyzation. lower acid strength of Al-organic mailer plot C. Thirteen sampling points \1ere se
In organic horizons the (Hargrove and Thomas, 1982; Natscher lected in a systematic grid in each of the
relationship between pH and soluble Al and Schwertmann. 1991) . plots .. \. B and C. and eight \1ere se
is more puzzling. Soil solution has been Soil chemistr) studies lected in plot D. giving a total of -17
found to be highly undersaturated for dis often include different soil types or soils sam p Ii n g points.
solved Al in relation to pH and possible from different areas in order to obtain Two humus cores and
Al-hydroxides or Al-silicates (David and variation in the factors studied. Results 1110 cores from the E-horizon. each with
Driscoll. 1984; Natscher, 1987). Labora from such studies are difficult to interpret a 70 mm diam .. \\ere tnJ..en at e1 er) sam
tory experiments have shown this to be in terms of chemical processes owing to pling point. sliced into one-cm-la~ crs
caused by an equilibrium between soluble variation in factors that probably should (SJ..) llbcrg. 990) and mixed to gi\c one
J
I KEY WORDS I Exchangeable Aluminum I pH I Humus layrr I Podzol I
Ulf Sk) !Iberg. Soil Scirntist (\I.Sc .. Ph.0- cudcnt). Rcsr:irchrr on soil chrmi~try. forr~t ~oil
acidity and nutrient status. Address: Swedish l!nivcrsity of Agricultural Scirncr5. -901 83. l"mc;i. S11cdrn.
0.'78-1844/94/0b .'~1>-10 ~ ·' 1\0 0 '-<" DI c 1. ;i;J '' 11 . 1' 1 "' ,, mmmrnEiR
$ample per cm-layer. and sampling point. ·--------
In plot A the mineral soil was also
sampled at depths of I 0 to 15, 20 to 25
and 50 to 55cm in the upper and lower ·--.. ....................
Bs- and B/C-horizons, respectively. All
samples were taken from 28 June to I 0
July 1988, and were immediately frozen
until analyzed. Mineral soils were manu
ally homogenized and particles larger 5
0
than 2mm manually separated before
analyses.
Analyses E He 100m1-plo1
u Ale 100 '1"1.plol •--•
pH110 pHcaClr exchangeable alumi- ...£. LOI •
1
11um (Ale) and exchangeable protons
0
(He) 10
The deep-frozen soil 15
samples were thawed at room tempera
ture overnight. Moist samples were used
for all analyses to avoid chemical
changes upon drying (Bartlett and James,
1980). Due to variation in the water con
tents of the moist samples, measured pH
values and KCl-extracted hydrogen ions Lou on iunition X of dry mou
were corrected with a nonlinear function
to obtain corresponding data for a con Figure I. Mean values of exchangeable hydrogen ions (H.), exchangeable aluminum (Al.) and loss
stant extraction solution:soil mass ratio on ignition (LOI) from plot A (100 m1, n=l3). Nole that 0-levels for the 0-horizon and the mineral
(Skyllberg, 1991; Skyllberg, 1993). This soil differ. This p~per is mainly concerned with interactions between H and Al in the stippled part
is important, especially in organic hori of the profile.
zons where pH is strongly buffered by
organic acids. To be able to express hy
drogen ions in relation to ash-free or pHcvs. Al
0
ganic matter (loss on ignition, LOI), a Correlation
similar correction was undertaken. Ex ~.o -0.8 -0.6 -o. ... -0.2 Coetfioent 0.2 o. ... 0.6 0.8 l.O
changeable cations other than hydrogen
ions were, however, linearly corrected for
water contents and LOI (Skyllberg, 2
1993). 0
J
A mass of 3.0g moist
humus or I O.Og moist mineral soil was
suspended in 30ml of 0.0 IM CaCl• After 51-----------
four hours of shaking followed 2by one 0t---------,
hour of sedimentation, the pHcaCl (ab
2
breviated pHc) was measured. The
pHH,O {abbreviated pHH) was measured
in the similar way, but in distilled water.
The values were corrected to correspond
to a solution:soil mass ratio of 30: I in
p<0.001 -
humus and 4: I in mineral soil.
Effective exchangeable p<001 ~h
acidit\" tH + Al ) was obtained by ex •---------
tracti~g 3~0 g .;,oist humus or 10.0g
moist mineral soil in 25ml of I .OM KCI
for 30 min. The humus suspensions were
iiltrated through a Munktell analytical fil
ter paper no. 3, and the mineral soil sus
pensions were filtrated through paper
OOH. The soil was additionally washed
"ith I .:!5ml I.Of\1 KCI. The filtrate was Dept h(cm)
titrated '' ith NaOH to pH 8.2. Thus the
referen.:e pH for acidit~ was set to 8.2.
Figure 2 Simple linear correlation codficients bet\\een pHc and Al, at different depths in the
The acidity was separated into Al. and or
Haplic Podzol prolilc The hatched areas the bars illustrate the degree of significance. The degree
II b) :idding IOml 1.\1 KF and b:i.ck-ti
of significance varied "ith depth depending on the number of samples (n=27-32 in the 0-horizon
trating :iccording to Thomas ( 1982J. The and n=l3 in the mineral soil horizons}. Note that 0-levels for the 0-horizon and the mineral soil
method docs not distinguish hctwcen Al differ
lnJER(lfn[lij 1'0\ · DEC. 1<;9~. \UL. 19 !'.'! o 357
and Fe and therefore Ale-values contain TABLE I
small amounts of exchangeable Fe. The DESCRJPTION OF lHE EXPERIMENTAL SITE AND lHE SOIL STUDIED AT KULBACKSLIDENS
titrations were performed automatically to
FORSOKSPARK rN NORlHERN SWEDEN
a given pH on a Mettler DL 70 titrator.
The corrected He corresponded to a total
extraction solution:soil mass ratio of
150: I in humus and 20: I in mineral soil L..ocauon 64 10' N lat.. 19 35' 0 long, 285 malt
and was, as well as Ale. expressed as Mc.an annual 1.3 C Mean annual 533 mm
t.cmperarurc January -11.8 C prccipitauon
centimoles of charge per kilogram dry c
(1931-1960) July 15.4 (Ha.Jlnas 1931-1960)
soil (cmolckg·1
). Vcgctauon tree layer 135-ycar-<ild Picea ab1es
field layer dwarf schrub, Vaccimum myrt1/Jus
Effective cation exchange capacity scattered grass, Deschampsia jlexuosa
(CEC.) bonom layer Pleurozfum schreben
Hylocomnium tiplendens
Exchangeable Ca, Mg, Parent glacial till originating rrom predominantly
K, Na and Mn were obtained by extract material gncissic bedrock.
Soil t.cxrure 2.0 -0.05 mm 54%
ing 3.0 g moist humus for two hours in
rractions <2mm 0.05-0.02 mm 20%
30 ml of 0.5 M CuCl2, followed by fil 0.02-0.002 mm 21 %
tration through a Munktell analytical fil <0.002 mm 5%
ter paper no. 3. The soil was washed Soil Haplic Podz.ol according to the FAO system (FAO, 1988).
with another 30 ml of CuCl solution be classification Typic Cryorthod acccrding to Soil taxonomy (Soil Survey Staff.
2
fore an elemental analysis of the filtrate 1990).
was made on a Perkin-Elmer ICP-AES 0-horizon A typical mor layer consisting of L (S), F and H subhorizons (thickness in parentheses):
L (2-3 cm) -Better classified as an S-laycr (Forsslund, 1943), with living mosses and liner
instrument. Natscher ( 1987) found a good
mixed togctber; a decomposition z.one "~th hyphae demarcates the border between the S-and
I: I correspondence between the amounts
F-laycrs; abrupt.. smooth boundary.
of Ca, Mg, K, Na and Mn obtained in F (1-3 cm) -Slightly de::omposed residues from mosses and needles interwoven with fine roots
0.33 M CuCl and the amounts obtained and hyphae, giving a mat-like structure; fibrous; gradual, wavy boundary.
2
in 1.0 M NHCI, in mor layers. Total H (1-4 cm) -Predominantly strongly decomposed organic matter; some samples were black.
4
amounts of organically bound Al (AlcJ greasy, massive structureless, others were dark brown with fine structure; many fine roots;
and Fe (Fe ) were also determined in the rhizomes from Vaccinium myrti/lus abundant within the abrupt.. wavy boundary.
above mentioned CuCl filtrates (Juo and Ah-horizon 0.5-1.5 cm thick
2 E-horizon 0-8.5 cm depth, color: light grey (10 YR 7/1, dry)
Kamprath, 1979; Hargrove and Thomas,
Bs-horizon 8.5-35 cm depth, color: light olive brown (2.5 Y 5/6, dry)
1984 ). The effective cation exchange ca
pacity (CEC) was calculated as cmolckg·1
(Ca+Mg+K+Na+Mn)cuc1 + (H+Al)~ci and
2
the effective base saturation (Ve%) as
I OO(Ca+Mg+K +Na)/CEC_.
Table II presents chemi
cal properties of the experimental site,
expressed on an areal basis (kmol<ha·1).
TABLE II
Total acidity (TA) was titrated up to pH
SOME CHEMICAL PROPERTIES (N=I3) AND THE BULK DENSITY (BD, N=6-13) OF THE
8.2 in accordance with Thomas ( 1982).
HAPLIC PODZOL PROFILE, FROM PLOT A. THE CECE-VALUES OF THE 0-HORIZON
The loss on ignition (LOI) was expressed
WERE TAKEN FROM PLOT B (N=l3) AND THE CECE-VALUES OF THE MrNERAL SOIL
in relation to dry mass. The samples
c HORIZONS WERE TAKEN FROM PLOT A (MIXED SAMPLES)
were combusted 4-6 h at 500° until no
traces of carbon were left.
Estimation of the acid strength
of humus samples pHcac12 He Ale CECe TA Ye LOI BD
Horizon ---kmolcha-1 ----- ----% ---- mgm-3
Mixed humus samples
were formed to represent each of the five
cm-layers from plot B. For each of the 0- 2],I:, ?5
1 cm and 4-5 cm-layers only one mixed 0 0-1 cm 3 40 68 5.6 ~ 102 yt 83.6 0 15
sample was formed. whereas two mixed CV% 3 2 18 68 II 12 60 15
samples with different amounts of Al
E 0-5 cm 3 73 3.5 8 7 14 8 40 18 5 5 0 90
were forrned for each of the 1-2, 2-3 and _,_,
3-4 cm-layers. Five grams of each mixed CV% 3 2 25 38 24 '' 13
sample was H+-saturated by shaking it in Bs 10-15 cm 4 29 3.4 15 4 20 6 133 9 67 0 96
50 ml of 1.2 M HCI for 24 ·h. before
CV% 44 56 33 32 35 29
centrifugation at 13000 rpm for 10 min.
The centrifuged soil was washed twice Bs 20-25 cm 4 62 0.8 66 7.8 IOI 5 6.0 0 82
with 50 ml distilled water, with each CV% 25 45 45 28 2R i:;
washing followed by centrifugation at
8/C 50-55 cm 4 84 06 2 5 35 19 I 01
13000 rpm. Washings with I 00 ml dis
tilled water were alternated with suction CV% l.4 20 30 28 32 10
1'0\ . 1>1 < 1<,<;J. 'rn . '" r-" i. HUE!lOEr\[JR
filtration through a OOH-filter (MunJ.:tcll
TABLE Ill
analytical filter papers). until no traces of
SIMPLE LINEAR CORRELATION COEFFICIENTS (R) BE1WEEN pHCACL2 AND EXTRACTABLE
Cl· were indicated by AgNO dissolved
in 10 ml water. The H'-saturat3ed samples POOLS OF Al AND Fe AS WELL AS BElWEEN pHH2o AND POOLS OF Al AND Fe, IN TI-IE MOR
were titrated with NaOH. Duplicates cor LAYER. THE INDEX DM DESIGNATES CORRELATIONS WITI-l VARJABLES EXPRESSED AS cmolckg·
responding to 0.7 g dry mass were sus
I DRY MASS WHEREAS LOI DESIGNATES THE UNIT cmolckg-11 -1_ THE pH-VALUES WERE
01
pended in 30 ml of 0.0 I M CaCl with 0.
2 CORRECTED FOR LOI BY APPLYING SPECIFIC FUNCTIONS FOR EACH cm-LAYER AND ASSUMING
1.92, 3.84, 5.75. 7.67 and 11.5 mmoJL·1
THATTI-IE HYDROGEN IONS IN THE MOR LAYER ARE BOUND TO ORGANIC CATION EXCHANGE
NaOH. The pHcaCJ was measured in
the suspensions after 1I 0 h of shaking fol SITES, ACCORDING TO SKYLLBERG (1993). CORES \VERE TAKEN FROM PLOT B THE p
lowed by one hour of sedimentation. DESIGNATES THE R-VALUE FOR SIGNIFICANCE AT THE 99% LEVEL.
The influence of ionic strength
on the pH of "Al-rich" pH\ pHH
and "Al-poor" samples vs. vs.
The two samples with lgAlc lgAJc" lgFcc11 lgA!e lgAlcu lgFcc11 p n
the largest amounts of Al. from 2-3cm
depth in plot 8, were mixed to form an DM 0-1 cm 0.07 0 31 0.40 0 08 0.28 0.30 0.68 12
"Al-rich" sample (Al.=7.5 cmol,kg·1). and LOI 0 07 0.31 0.39 0 08 0 27 0.29 0.68 12
the samples with the smallest amounts
were used to form an "Al-poor" mixed
DM 1-2 cm 0.39 0.66 0.74 0.43 0.70 0.76 0.66 13
sample {Al,=0.6 cmol,kg"). Ten replicates
LOI 0.38 0.65 0.73 0 43 0.69 0.75 0.66 13
of each of these two mixed samples, each
corresponding to I.Og dry soil, were sus
pended in 25ml distilled water and DM 2-3 cm 0 79 0.84 0.83 0 81 0.81 0.73 0.66 13
shaken for three hours. The pH was LOI 0 76 0 81 0 80 0 78 0 78 0 68 0.66 13
measured after one hour of sedimenta
tion. The "Al-rich" suspensions were ti
trated down to the pH-value of the "Al DM 3-4 cm 0.83 0.73 0.69 0.84 0.58 0.44 0.71 II
poor" ones, by adding a few droplets of LOI 0 87 0 84 0.81 0 91 0 79 0 65 0 71 II
HCI, whereupon 5.0ml of KCI at each of
five different concentrations (0.6, 0.03, DM 4-5 cm 0.48 0 43 0.59 0.39 0.30 0.56 0.80 8
0.06, 0.003 and 0.006M KCI) was added
LOI 0 85 0 92 0.93 0 92 0 88 0.90 0 80 8
to the suspensions, giving five duplicates
for each of the mixed samples. The pH~ci
was measured after three hours of shak
ing and one hour of sedimentation.
TABLE IV
Results and discussion
Table 4. Calculated pK°PP(CEC,) and sample size adjusted coefficients of determina
0
Depth-related variation in tion (r) for equations (1) and (2) with pH measured in 0.01 M CaCl and in distilled
2
pH,, H, and Al, water. The samples were taken from plot B. Samples taken at 2-4 cm were divided
Loss on ignition (LOI) into two populations that differed in their amounts of CECJ LOI: S 1 with 39.4 to
was clearly related positively to both H< 45.6 cmol.kg·1 and 82 with 47.4 to 56.5 cmol.kg·1
•
and Al, in the soil profile as a whole
(Fig. I), owing to an increase in the
number of cation exchange sites with in pHC•ClZ pHHIO
creasing LOI in the mineral soil. In the
Humus Eq. Cl) Eq. C2) Eq. Cl) Eq. C2)
mor layer Al. increased continuously
with depth until the point at which the cm-layer n rZ r1 rz rz
organic matter content dropped drasti
cally, in the E-horizon. The exchangeable
hydrogen ions (H ), on the other hand,
showed a maxim~m already at 2-3 cm
depth, approximately at the boundary be 0-1 12 C0.68) 2.86 C0.78) co. 63) 3. 54 CO. 71)
tween the F- and H-horizons. The contri 1-2 13 C0.62) 2.89 co. 88) co. 58) 3. 72 co. 87)
bution of Al to the CEC, relative to that
of H , increa"sed with depth in the humus 2-3 12 C0.05) 2.85 C0.74) C0.03) 3.80 co. 62)
layer~ The pH,-value (not shown) was 3-5 18 -C0.14) 2.89 C0.55) -C0.23) 3.81 co. 49)
negatively correlated (p<0.00 I) with H,
in all cm-layers and decreased between
0-1 cm (3.52) and 2-3 cm (3.32). before 2-4 Sl 13 2.86 C0.94) 3.75 co. 76)
increasing to a value of 3.42 at 4-5 cm
depth. This paper focuses mainly on the 2-4 S2 12 2. 72 C0.90) 3.68 co. 61)
relationship between pH and A I in the
ln1EIWEl\[IR f'JOV - DEC 199J. VOL. t9 N" 6 J:;C)
Note that Tabl e 4 is cor rected for printing error~
I
lo\\er part of the mor layer. stippled in -1
Alecmolckg
Figure I.
The amount of Al was 2 4 6 B 10 12 14 16 18 20
positively correlated between all c~-lay­ 9J.o to.a' I I I I I I
";~·
ers of a humus core (p<0.00 I, p<0.0 I be ~
tween 0-1 and 4-5 cm) and the mean Al, -
value of the mor layer was positively
correlated (p<0.00 I) to each of the up 3.50 '''' ', 82.6±11
permost two centimeters of the E-hori
/
zon. This means that the spatial variation ""'·' 1 /
showed a similar panern at all depths in /
/
the mor layer and in the upper part of -·- /
the E-horizon. Such correlations were not ---">!!''.':'] ---·- 89.8 t2.1
3.40 -
as strong for H,. but pH, was positively
correlated (p<0.00 I) between the 0- (0-5
N I
cm) and the E-horizons (0-5 cm). i3
<tl
Spatial variation did not (.)
differ significantly among the plots, de Ia .
spite their different sizes. Calculated 3.30 - nmo -
semi-variances suggested that auto-corre 80.7t15 • o -1 cm depth
lations within the range 0.5 to I 0 m were .. 1 -2cm
small in all cm-layers or horizons. As a o 2-3cm
consequence of the relatively large, but • 3-4 cm
" 4-Scm
uniform, variation the humus cores were
regarded as being independent of each 3.20- -
other and could be statistically treated as
belonging to the same sample population.
l I l I I l I I I I
Correlations between pH
and pools of extractable aluminum Figure 3. Mean pHc and Al, values for two subpopulations of mor samples from plot A. The dotted
line represents a subpopulation of cores with Al more than 3.0 cmol kg·• at 1-2 cm (n=6}, whereas
the other sub-populations had lower amounts or' Al, (n=4-6). Loss o~ ignition + CV% is given for
The negative correla
each depth. The bars illustrate the 95-% conlidence intervals for pH<. and Al,.
tions between pH, and Al, in the 8,- and
8/C-horizons (Figure 2) could be ex
plained by an increase in the solubiliza
tion of aluminum hydroxides with de
creasing pH. The exchangeable Al pool
pHCaCl
is. however. much larger than A I in solu 2
tion, and is dependent on the availability
of cation exchange sites. Therefore the 2.40 2.60 2.BO 3.00 3.20 3.40 3.60
relationship between pH< and A I, is af
c
fected by an indirect component related A B
10 the variation in CEC . •41.3 I• 2 .99 •0.1
1
The p~sitive correlation 1 / cmolcAl0 kg ·1
(p<0.00 I) between pH< and A I, in the cmolc CEC kg •
0
mor layer (Figure 2) has not been re :,:f"
ported before. One reason for this could 43.5/./ 40.7 0.4.\ •\ 3 6
be the lack of small-scale studies of or
E 2
ganic horizons with relatively uniform
u
CEC,-values among samples. With a sub
stantial variation in CEC the cation ex .i::: 38.3 ••40.3 2.83. • 2.90
1.81 ,4.5
change processes would 'be obscured by a. ''l \
an overall positi\e correlation between 3 \
<D \ I
the amount of cations and the number of "T'' \
0
cation exchange sites. Tamminen and 2.84 •• 2 88 3.4• • 7.4
Starr ( 1990) re-ported a negative correla
tion between pHH.Q and Al extracted in 4
0.1 f\.t BaCl froni 65. mainly Podzols.
humus layers1 in southern Finland. Look •33.9 • 2.88 5.9 .
ing al their results and restricting the
samples lo humus la) ers with a pH 11,0 5
below 4.5 there \\as no relationship l:ie
l\1een pHH,Q and Al. Figure 4 l\li'<cd humus san:plcs from the cm-la~er< l>f plot 13 13roken anti s!ll1d line' rcrrc<ctlt rcla
- Figure 3 furthermore il- ti\cl] 1\1,-rich and Al,-p!lor samples. respcctl\d~ .\: rH measured in O 01 ,\f l'aCI: i'l'r 11·-,atu·
rated samples. \\ith amounts of CEC "''tetl fM each sample ll: The acid strcn!!th 1.11' the 11·-,atu
lustrates the pH;Al-relationship. The sub
ratcd samples. npressed as pi I, m neutraliLati1>n l'r 112 CIT,. with the pK;•·•tCl·C, )·' alue< 11111,·d
population \\ilh small amounts of Al, C: l\kan pllcaCI: for inJi,idual. natural sampks. \\ith amounts of Al,. tHllctl.
Jl>O t--0\ - DEC 19'.I~. \CJL. l'.1 I'" 6 Jn7Efl[}En[IR
showed a pH,-gradicnt typical for most
studied mor layers. with pH being sig
nificantly higher in the upper part (F 20 30 40 50
layer) than in the lower part (H-layer;
Hesselman. 1926: ~lattson and Koutler •
Andersson. 1941; Skyllberg. 1990). By 3_40 ~
contrast. the "A I -rich" cores did not °'-.o • •
show any depth-gradient at all. Al 2-J "
cm depth, and below. LOI was lower in "
the "Al-rich" cores than in the "Al-poor" 3.30 ....
cores but, as pointed out earlier, no sig ~ 0 • •
0
nificant correlation (-0.l<r<O.I) was "
found between LOI and Al, in any cm
layer. (\,) 3.20 100·H9/CEC9 o "" •
0
The proportion of the (.) Ve % • " 1-2cm
total organic pool of A I (Ale.; Hargrove CU
(.)
and Thomas, 1984) that was KCl-ex I
changeable increased from 26% at 0-1 a. 3.10
cm to 61 % at 4-5 cm. The linear regres
sion between these pools showed that Al.
reflected the total amount of Al bound in
the mor layer (Ale. = -0.3035 + 1.6338 3.00
Al,, r2=0.91; 0-5 cm; n=57). This was in
•
agreement with Natscher ( 1987) who
found a strong relationship between Al
extracted in 0.33 M CuCl and in 1.0 M
2 50 70
NH,CI in L- and 0-horizons. The correla
tion between A le. and Fee. was also very
strong (r2=0.82; 0-5 cm; n=57). Table III
shows that all of the measured A I- and
Fe-pools were similarly correlated to both
pH, and pH,,.
A stronger relationship
was found between pH and pools of Al
and Fe when the 'variation in organic 20 30 40 50
matter content was excluded by express
ing the variables in relation to the loss •
•
on ignition. This was evident at 3-5 cm 3.40 0
depth where the variation of LOI was
relatively large. These relationships indi
cate some type of competition between
Al (and Fe) and H for organic cation ex 3.30 \ 0 •
change sites as an important pH-buffering •
0 '\ 100 H/CECe o
process in the mor layer. \
The role of exchange \ 100 Al/ CECe •
3.20
able Al for pH-buffering in the mor layer (\,) \
in relation to the effective cation ex- (C.U) • 0 \ 2- 3cm
change capacity (.) '\
The pH-value in a hu I "
a.
mus sample is a function of: 3.10
'\, 0
'-a
(I) the acid strength ( pK,"PP) of the buff
........
ering organic acids/bases "
3.00 '
(2) the amount of the buffrring acids/ ........
bases (cmol,CECkg·1) 0 " 0
(3) the protonation of the buffering acids/
10 20 30
bases
To determine whether
the pos111ve relationship between pH
Figure 5. Relationships bct\\e.:n pi I( and effective base saturation
and Al was a direct one, caused by an
(V,%). pH, and aluminum saturation (lf cation exchange sites (Al/
acti\'I! 'pH-buffering of Al compounds,
CEC) ~nd between pll, and the prCltonat1on of cation exchange
or an indirect one caused by covaria sites (H /CEC) at 1-2 and 2·> rn1 depth 111 the humus layer of
tion "ith some of the factors (I). (2) or plot B ' '
ln7ER(lffi(IR NOV . DEC 199d, VOL. 19 N9 6 361
----- --------- ---
---
Note that Table 5 is corrected for printing er rors
• 3) these factors \\ere examined in sam TABLE V
rles from plot B with different amounts
EXCHANGEABLE CATIONS IN 1WO MLXED SAMPLES FROM 2-3 CM DEPIB IN PLOT
of Al.
c The acid strength (I) B. EXCHANGEABLE ACIDITY (EA) REPRESENTS THE SUM OF fie AND Ale.
\\aS estimated for H·-saturated samples in
section i). In ser:tion ii) the degree of
Al. H. EA Ca+Mg+ CEC.
proton at ion of the exchange sites (3) was
Mn+Na+K
calculated as the amount of protons in re
lation to the amount of organic acids Mixed sample --------------- cmol,kg·1
---- ----- ------
tCEC ). with and without the inclusion of
proto~s bound to Al-ions. In section iii)
the inability of Al bound to organic mat
ter to release protons (hydrolyze) was
shown in a laboratory experiment. "A1 -ri eh" 8.2 8.9 17.1 24.8 41. 9 59
"Al-poor" 1.0 16.4 17.4 25.4 42.8 59
1) Esrimation of the acid strength of the
humus samples
The pH-dependent
charges of organic matter are often re
garded as a mixture of weak acid func
tional groups with overlapping pK,-val
ues that can be described in terms of a among the pK,""P(CEC.) values from dif relevant in describing the buffering of
mean apparent acid strength (pK;PP, e.g. ferent depths were in good agreement pH, only at 0-1 and l-2cm (Figure 5).
Stevenson, 1982). A! though titration with the relations among pH,-values at Further down in the mor layer substantial
curves of solid phase humus often are H·-saturation. In an estimation of pK;PP, amounts of Al. were pre~ent and the rela
practically linear and do not show any with pH 4.0 and 5.0 chosen as reference tion between pH, and V<% weakened and
distinct inflection points (Hargrove and pH values for complete dissociation of eventually turned negative at 3-4 cm
Thomas, 1982; Federer and Hornbeck, the organic acids, the acid strength rela (p<O.O I). If A 1• • on the other hand. was
1985; Natscher and Schwertmann, 1991 ), tions obtained among samples were simi included as a nonacidic cation, the HI
the CEC at pH 7.0 or 8.2 has been used lar to those obtained for pK,""P(CECJ CEC. quotient (= 1-(A l+Ca+Mg+K+ Na)/
to estimate the amount of negative The results suggest that CEC) accurately described the pH-buff
charges at complete dissociation of the pH-differences among samples with dif ering by cation exchange in all cm-layers
organic acids (e.g. Bloom and Grigal, ferent amounts of Al were not caused by (Figure 5). The important role of Al as a
1985; Walker et al. . 1990). a covariation with th~ amounts of organic neutralizing cation in this quotient was
In this study the acid acids (CECJ or the acid strength of the shown by the positive relationship be
strength of the organic acids was esti acids/bases. tween A IJCEC< and pH, below 1-2 cm
mated for mixed humus samples from (Figure 5).
plot B by litratior. of H •-saturated ii) The relevance of considering Al,. as
samples with NaOH in 0.03 M ionic a neutrali=ing cation or as an acidic The extended Henderson-1-lasselbach
strength medium (see Material and meth cation contributing to the protonation equation
ods). The titration curves for the humus of CEC,
one-cm-layers were closely parallel to The effective cation ex pH = pK,'PP + nlog(V/(1-V,) (I)
each other in the pH, interval from 2.5 to change capacity was positively correlated
5.0 and did not show any distinct inflec with the sum of the cations has been used to describe the pH-buffer
tion points up to pH, 7.0 (data not Ca+Mg+K+Na (cmol);g·1• p<0.001 below ing by cation exchange in soils on a
shown). To examine whether the pH/Al< 1-2 cm) and with H, (p<O.OI below 2-3 thermodynamical basis (Bloom and
relationship was caused by covariation cm) at all depths. Exchangeable Al and Grigal. 1985). The pK;PP is the apparent
with the acid strength, the negative pH, were not significantly correlated with acid strength. n is an empirical constant
charges included in the CEC< were as CEC. at any depth in the mor layer. It and \1 the base saturation at pH 7.0.
1
sumed to be more relevant for the pH can be assumed that amounts of \\ ith exchangeable A I included with three
buffering in the pH,-interval from 2.5 to Ca+Mg+K+ Na and H. are more closely charges (equivalent to protons) in the de
.J.0 than were functional groups contrib related to decomposition and humifica nominator (cmol, (Ca+Mg+K+Na)/(H+AI)).
uting with negative charges at higher pH. tion processes than is the A I<. \\ hich is In this study equation
Therefore CEC was chosen as an estima derived mainly from the mineral soil. in (I) was modified and based on CEC in
tion of the amount of organic acids/bases dependently of these processes. To ex stead of CECr '' b) changing \f lo
71, ( 1
to be used \\hen determining the acid clude the influence of variation in CEC V,). To evaluate the degree to which Al.
strength pK;r<1CEC). among samples, variables were expressed contributed to the protonalion of the or
The pK;PP(CEC) -values in relation to CEC . ganic acids at natural pH-values in the
\\t:rc measured as the pH, with half of Tl1e effect ivc base satu mor layer. equation (I) was compared to
the CEC neutralized !Fic.ure 4). The 0-1 ration is the percentage of the CEC. that equation (2). in \\hich Al was included
.:m shO\\:ed a slightly higl1cr pK/Pr(CEC) is balanced by neutralizing nonacidic as a neutralizing cation co~sidered not to
than the other cm-la) ers otherwise there cations V< %=I OO(Ca+rvtg+K +Na)/CEC). h~ drolyze.
\\ere small di ffcrences in acid strength The base saturation is invcrselv related to
among the cm-la) ers and among samples the degree of prolonation. wi~h A I, in
"ith different amounts of Al . Because cluded as three protonated charges ( \·, =
\ariation in CEC, \'<IS low th~ relations 1-(11.+Al)ICEC). In this study V,% was nlog (Ca·~1g+K+0:n+AI. l H.) (2l
'-0' · Dt c t99.i. \ 01 . 19 N" 6 H\JEfl[IEf\[IR
Table IV lists the coeffi
cients of determination (r2) for the regres
sions between exchangeable cations and
pfl. Equation (I) was only relevant for 1
8.2 cmolcAle kg - • ,..
describing the pH-buffering in the upper 4.20 ,,
most two cm-layers, where Al. was of •"
1.0 cmolcAle kg -1 o
minor importance. Equation (2) was, /
/
however, superior to equation (I) at all /
depths, both in 0.0 I M CaCl and in 4.00 / /
2 :"' /
Hp. The intercept (pK,""P(CEC.)) showed
/
relatively small differences among the
u
cm-layers.
This was in accordance with I ~ 3.80
c.
the laboratory estimations (Figure 4). The /
/
equations could only be used properly on /
a uniform population of samples of simi
3.60
lar acid strength. Therefore populations
were restricted to samples within a speci
fied cm-layer.
To get even more uni 3.40
form populations, with a restricted
amount of organic acids/bases, the
samples from 2-3 and 3-4 cm depth were
classified according to amount of. CEC I 3.20
LOI (SI and S2, Table IV). As much ;s
95% (r2=.95) and 91 % of the variation of
1.0 2.0 3.0
pH< was accounted for by equation (2)
when AI. was included as a nonhydro
pKCI (Ionic strength)
lyzed neutralizing cation (Al3') in SI and
S2 (equation 2). When Al. was assumed
to be slightly hydrolyzed and bound as
Al(OH),.n· to the organic acids r2 de Figure 6. The pH~Cl in two humus samples whose properties are described in Table V. The Al-rich
1 samples were titraled down from 4.37 (I) to 4.27 (2) before the ionic strength was increased wilh
creased slightly in both SI and S2, indi KCI 10 0.0005, 0.00 I, 0.005, 0.0 I and 0.1 M.
cating that Al. was acting as a neutraliz
ing cation with no substantial degree of
hydrolyzation.
iii) The influence of ionic strength on the
pH of humus samples differing in the
amo11111s of exchangeable Al
Figure 6 illustrates
the effect of ionic strength on the pH B) , indicating that the input and/or pro in the lower part of the mor layer.
value of two samples differing in Al. duction of acidity were spatially rather The main source of pro
content. The samples were taken at 2-3 uniform in the mor layer. The relative tons in humus layers in areas receiving
cm depth in the mor layer from plot B contributions of Al and H to the acidity little atmospheric acid deposition is the
and had very similar contents of CEC<, showed a much l~rger spatial variatio~ uptake of nutrient cations by roots, lead
EA and V<% (Table V). Since no sub and were decisive for the realization of ing to a successive protonation of CEC• .
stantial differences were found in the exchangeable acidity into hydrogen ions In combination with an increasing CEC<
acid strength (see section i), the distribu in solution. Thus it can be concluded that and a decreasing pK;PP of the organic
tion of exchangeable acidity into A I< and the difference in pH between samples matter during the initial stages of decom
H was the only difference between sam mainly was due to whether the protons position (i.e. 0-1 to l-2 cm in Fig. 4),
ples. were bound directly to cation exchange this gives rise to a pH decrease with
The Al-poor sample re sites or bound stronger in aluminum ions. depth in the upper part of the mor layer
leased about twice as much H" into solu Sources and reactions of aluminum in l Sky II berg, 1990; and Figure 3 ). With in
tion (0.3 pH-units) compared with the the humus layer - theory creasing depth, however, silicate weather
Al-rich one, at all levels of ionic ing can be assumed to increase its contri
strength. This was in agreement with the In sections i) to iii) it bution to the acid/base status.
rcl3tive amounts of H in the untreated was shown that the positive correlation If root uptake is effec
samples (Table V) a~d indicated that between pH and Al. was not caused b) ti' e. the consumption of protons associ
1here was no subs1an1ial release of pro variation in acid strength or in the a1ed with the release of nutrient cations
tons owing to hydrolyzation of organi amount of organic acids. Exchangeable from silicates is balanced by an equiva
call' bound A I . Al did not hydrolyze substantially, thus it lent release of protons. Aluminum- and
· ' Exchangeable acidity (H< behaved like a nonacidic cation in the Fe-cations are, however, not taken up
+A I ) showed relatively low spatial varia mor layer. These findings, however, do and bind strongly to exchange sites of
tion' 3t all depths (CV% 13-15% from O not explain the processes involved in the org:inic matter (and root surfaces). Alu
J to 3-4 cm and 31 % at 4-5 cm in plot competitive interaction between H and Al minum and Fe can be assumed to accu-
ln7EWEl\[IR :-.O\ . DEC 199J, \'Ol.. t9 N" <> J(,J
mulate in the lower part of the 0-hori latcd A I with depth in the mor layer sources of A I and processes behind the
zon, balancing CEC negative charges in (Figure 2 and 3) in combination with a competitive interaction between II and Al
an equivalent amount to protons con significant (p<0.00 I) positive correlation for cation exchange sites remain to be
sumed upon weathering of Al(OH) and between A I. in the mor layer and A 1. in further determined.
3
Fe(OH)r Although the degree of Al. hy the elluvial horizon, suggests that soluble In any analysis of ex
drolyzation in this study appeared to be A I is either transported upwards from the changeable acidity in naturally acid or
insignificant, substantial hydrolyzation mineral soil, or that Al accumulates in, ganic horizons. one should distinguish
has been reported in peat soils even at or in association to, roots and is subse between protons bound to organic ex
pH-values below 4.0 (Hargrove and Tho quently incorporated into relatively resis change sites and those derived from alumi-
mas, 1982). The number of protons that tant humus compounds during decompo num.
will be consumed, or leached, from the sition. Because of the weak relation be The usefulness of a
humus layer, for each bound monomer tween LOI and Al-pools, the absolute small-scale sampling design for studies of
aluminum cation varies depending on amount of mineral particle~ in the mor soil chemical processes in natural samples
whether or not the neutralization of layer does not seem to be of significance was shown.
Al(OH) is coupled to a direct H/AI cat for the amount of accumulated Al. Even
ion exchange on soil organic matter and small amounts of mineral particles could
on the degree of hydrolyzation of bound probably release large amounts of Al de
aluminum. pending on mineral composition, the
ACKNOWLEDGEMENTS
Schematically, positively quality of the organic acids and time of
charged Al-species (Al(OH) 13·•1·), which exposure. A subjective examination of
can be supposed to be bala0nced by or the cores from plot B with the naked eye I am very grateful to M
Martensson at the Environmental Re
ganic anions in solution (R ·coo·(aq)), indicated that cores with a more well de
search Laboratory, who conducted a large
may neutralize or exchange protons veloped H-layer, i.e. black with a greasy
part of the laboratory work. I also would
bound to cation exchange sites consistency, had larger amounts of Al.
like to thank N Nykvist for valuable sup
(RCOOH): and Al,. than did cores with more finely
port and for comments "on the manuscript
structured H-layers. The massive weak
and G Briinnstrijm for drafting all the
(RCOOH)3 + Al(OH).'1. ..1 · + structure may reflect more wet condi· figures.
tions, older, more decomposed humus or
This study was sup
(3-n)R'COO·(aq) <-> Al(OOCR) + humus derived from a slightly different
1 ported by grant from the Swedish Envi
substrate (Klinka et al. 1981 ).
ronmental Protection Agency.
(3-n)R 'COOH(aq) Cycling of Al-species by
vegetation varies in magnitude between
The reaction is promoted coniferous ecosystems. Vogt et al. ( 1987)
by the presence of protonated organic ex found that of the total amount of cycled
change sites with relatively low pK -val Al in an Abies amabilis stand, 97% was
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