Annals of Botany 84 : 401–410, 1999
Article No. anbo.1999.0941, available online at http:\\www.idealibrary.com on
Influence of Soil Moisture on Soil Solution Chemistry and Concentrations of
Minerals in the Calcicoles Phleum phleoides and Veronica spicata Grown on a
Limestone Soil
A P A R N A M I S R A* and G E R M U N D T Y L ER
Department of Ecology, Soil-Plant Research, Lund UniŠersity, Ecology Building, S-223 62 Lund, Sweden
Received : 4 May 1999
Returned for revision : 24 May 1999
Accepted : 21 June 1999
Veronica spicata and Phleum phleoides are calcicole plants, mainly occurring on neutral or alkaline soil. An
experiment of 16 weeks duration was performed in a glasshouse with the objective of elucidating the influence of soil
moisture level on soil solution chemistry, and biomass concentrations and uptake of mineral nutrients by the plants.
Seven levels of moisture, corresponding to 35–85 % of the water holding capacity (WHC) of the soil, were tested. Soil
solution HCO , P and Mn concentrations, and pH, increased, whereas Ca, Mg and Zn concentrations decreased, with
$
increasing soil moisture. Concentrations of K were highest at 50–70 % WHC. Concentrations and amounts of P, Zn
and Mn in the two species were usually related to soil solution concentrations ; these are elements with low solubility
and availability in calcareous soils. Concentrations of nutrients in biomass were more influenced by soil moisture in
V. spicata than in P. phleoides. This indicates that P. phleoides is more capable of controlling its uptake of mineral
nutrients, whereas V. spicata is sensitive to variations in soil moisture. It is concluded that variation in soil moisture
regime may greatly influence concentrations of mineral nutrients in calcareous soil solutions and their uptake by
plants. Species able to utilize these solubility fluctuations may have an advantage in competition for nutrients.
Variation in soil moisture content might even be a prerequisite for adequate acquisition of mineral nutrients and
growth of plants on limestone soils, thereby influencing the field distribution of native plants among habitats.
# 1999 Annals of Botany Company
Key words : Calcareous, calcicole, concentration, mineral, moisture, nutrient, Phleum phleoides, soil, soil solution,
uptake, Veronica spicata, water.
I N T R O D U C T I ON
Water content is an important property of soils, influencing
soil solution chemistry and nutrient uptake by plants.
Morphology and other specific properties of the root,
nutrient concentration in the soil solution, the mobility of
nutrients in the soil, and supply from solid phases, affect
nutrient uptake (Nye and Tinker, 1977 ; Barber, 1995).
Consequently, there are consistent differences in concentrations of elements near the rhizoplane at a range of soil
water contents (Dunham and Nye, 1976). Soil chemical
properties may exert a profound influence on growth and
performance of plants (Grime and Curtis, 1976), and soil
concentrations of several elements may be closely related to
floristic composition (Tyler, 1996 a). Under field conditions,
soil moisture fluctuates with temperature and rainfall. By
changing soil solution chemistry, moisture fluctuations
could regulate the availability of nutrients, and the field
distribution of plant species.
Scarcity of readilykavailable forms of several plant
nutrients constitutes a severe problem for plant growth on
limestone soils. ‘ Calcifuge ’ vascular plants are unable to
develop successfully on these soils as they are incapable of
supplying enough available P, or sometimes Fe, for uptake
(Tyler, 1992, 1994). Limestone soil species may also differ in
their capacity to render elements soluble for uptake, and
* For correspondence.
Fax j46 46222 3742 e-mail : aparna.misra!planteco.lu.se
0305-7364\99\090401j10 $30.00\0
such differences may be influenced by soil moisture
conditions. Phosphorus uptake by plants is greatly
influenced by soil moisture, being largely controlled by
diffusion rates, and P depletion in the rhizosphere
(Gahoonia, Raza and Nielsen, 1994).
Limestone soils are generally characterized by high
concentrations of CaCO and soil solution HCO −, high pH
$
$
and almost no exchangeable H+. Many studies (Inskeep and
Bloom, 1986 ; Mengel, Breininger and Bu$ bl, 1984 ; McCray
and Matocha, 1992) have demonstrated that an increase in
soil moisture causes an increase in HCO− concentration.
$
The concentration of HCO− interacts strongly with the
$
availability of several ions, especially Fe#+, and it is often
considered to be the primary factor responsible for chlorosis
of plants on calcareous soils (Coulombe, Chaney and
Weibold, 1984 ; Kolesch, Oktay and Ho$ fner, 1984 ; Mengel
et al., 1984 ; Ao et al., 1987 ; White and Robson, 1990 ; Shi
et al., 1993 ; Mengel, 1994). Bicarbonate ions also interfere
with the uptake of other elements, e.g. K and Mg (Pissaloux,
Morarad and Bertoni, 1995).
The objective of this study was to demonstrate the
importance of soil moisture regime on soil solution chemistry
and mineral nutrient concentrations in native calcicole
species grown on a limestone soil. It is hypothesized that
calcicole species differ in mineral nutrient uptake according
to soil moisture conditions. It is further hypothesized that,
in particular, pH and concentration of HCO− , which might
$
have great effects on plant uptake of essential nutrients, are
# 1999 Annals of Botany Company
402
Misra and Tyler—Soil Moisture and Nutrient Concentrations
influenced by soil moisture conditions. One herb, Veronica
spicata L. and one grass, Phleum phleoides (L.) Karst., both
widely distributed in semi-natural calcareous sites, were
chosen for study. Mineral nutrient concentrations in shoots
and roots, and soil solution concentrations of the same
elements, were measured at different soil moisture levels
using a limestone soil from the ‘ alvar ’ of O$ land (56m 25h N,
16m 37h E), southern Sweden, where both species are common.
MATERIALS AND METHODS
The soil used for the experiment was an Ordovician
limestone soil (Rendzic leptosol), about 10 cm deep, resting
directly on limestone bedrock. The soil was sieved through
a 6 mm screen and well mixed. Water holding capacity
(WHC) was determined before further treatment, as follows :
about 1n5 dm$ of soil was placed in preweighed 2 dm$ plastic
pots (five replicates) ; after weighing, these were placed in
water, about 1 cm deep, for 24 h, to cause water saturation
from below, and held under free drainage for 24 h ; the pots
were then dried at 85 mC and reweighed.
For plant growth, 56 pots of volume 2 dm$ were used.
The weight of soil used per pot was the same as in the WHC
determination, although 25 g was reserved to cover the
seeds after sowing. Seven different soil moisture levels were
included : 35 %, 43 %, 52 %, 60 %, 68 %, 77 % and 85 %
WHC (four replicate pots for each level and species).
Seeds of V. spicata or P. phleoides, were sown (3 Mar.
1997) at a rate of 20p0n2 mg per pot. The pots were kept in
a computer-controlled glasshouse, in which temperature
was controlled at 14n8p1n8 mC (night), and 23n5p3n2 mC
(day). Extra light, at 70 W m−#, was provided by highpressure sodium lamps during the day if total solar radiation
fell below 100 W m−#. All pots were adjusted to 68 % WHC
at the start of the experiment by the addition of water as
required. During the next 7 d, water content was gradually
adjusted to the desired levels. Loss of water, usually 5–10 g
per pot d−", was compensated for, at intervals of 24 h, by the
addition of water to the surface of the soil. Soil moisture
fluctuations would inevitably have been greatest near the
soil surface, though few roots developed there.
Seeds started germinating 5 d after sowing. The final
number of plants per pot was recorded. Leaf colour was
estimated using Swedish Standard Colour Atlas (sheet 2,
SIS, 1989 ; a 15 degree scale from yellow to deep green). The
plants were harvested 90 d after they were sown. After
harvest, the plants were washed several times by spraying,
and adhering water was removed using filter paper. Shoots
and roots were separated and dried at 85 mC for 3 d. The
biomass produced was expressed as g dry weight per pot.
Shoot (leaf) and root biomass of both species was wet
digested using conc. HNO . Residues were made up to
$
volume and solutions analysed for K, Mg, Ca, Mn, Fe, Zn
and P by plasma emission spectrometry (ICP-ES). Concentrations were calculated as µmol g−" dry weight, and amounts
as µmol per pot and nmol per plant. Five blanks (without
sample) gave negligible values for all elements.
The concentrations of Ca, Fe, Mg, Mn and Zn in the soil,
which were exchangeable by shaking with 0n2  BaCl (10 g
#
soil at field moisture and 100 ml extractant, 1 h), were
determined by ICP-ES, and K by flame atomic absorption
spectrophotometry (AAS) in 1000 mg l−" of Cs added as
CsCl. For readily-exchangeable phosphate, the Sb\Mo blue
method with ascorbic acid reductant was used on filtered
and centrifuged extracts obtained by shaking 10 g soil at
field moisture content for 20 min with 100 ml unbuffered
neutral 0n05  Na SO j0n02  NaF solution (Tyler, 1992).
# %
pH was determined after extraction (2 h) of 10 g soil with
50 ml 0n2  KCl. Organic matter content was estimated as
loss on ignition (550 mC, 3 h, a temperature at which CaCO
$
is stable) and calculated as % dry weight.
The same soil (without plants) was also held at different
moisture levels (30 %, 40 %, 50 %, 60 %, 70 %, 80 % and
90 % WHC, to obtain a slightly greater moisture range than
in the plant experiment ; four replicate pots of each level),
and water content adjusted as in the main experiment. After
2 weeks, soils were centrifuged at 12 000 rpm (equivalent to
13 900 mean relative centrifugal field ; RCFig) for 1 h in a
specially equipped cooling centrifuge (Falkengren-Grerup
and Tyler, 1993). pH was determined electrometrically and
HCO− concentration by titration with 5 m HCl. The
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remaining solution was ultrafiltered (0n2 µm) and concentrations of Ca, Fe, K, Mg, Mn, P and Zn determined by
plasma emission mass spectrometry (ICP-MS). Data were
treated statistically by correlation and regression analysis,
and by calculating means and standard errors, using the
program SPSS 8.0 for Windows.
RESULTS
Soil chemical properties
The pH of the soil was 7n2 when determined with 0n2  KCl.
Exchangeable Ca was high (146 µmol g−") (Table 1) and
maintained by the high levels of CaCO , as estimated by
$
CO evolution on HCl addition, which released Ca to the
#
soil solution and the exchangeable pools. Concentrations of
micronutrients (exchangeable Fe, Zn and Mn), but also of
phosphate, were low (compared with a typical acidic silicate
soil, data not shown). The concentration of exchangeable
Mg was higher than that of K (Table 1).
pH and HCO− , P, and Mn concentrations in the soil
$
solution increased with soil moisture from 30–90 % WHC.
T     1. Chemical characteristics ( pH, organic matter
content, and exchangeable concentrations of nutrients) at field
moisture, of the soil used for the experiment. Meansps.e.
(n l 5)
Soil property
pH
Organic matter, % dry weight
Phosphate
Calcium
Magnesium
Potassium
Iron
Manganese
Zinc
Concentration (nmol g−" d.wt)
7n2p0n1
12n3p0n8
5n0p1n6
146n10$p20n10$
940p50
520p50
1n2
0n65p0n07
1n5p0n3
403
Misra and Tyler—Soil Moisture and Nutrient Concentrations
1000
7·6
HCO3 µmol l–1
800
pH
7·2
600
6·8
400
6·4
6
20
200
30
40
50
60
70
80
90
0
20
100
30
40
50
60
70
80
90
100
30
40
50
60
70
80
90
100
30
40
50
60
70
80
90
100
30
40 50 60 70 80
Soil moisture, % WHC
90
100
200
3500
150
2500
Mg µmol l–1
Ca µmol l–1
3000
2000
1500
1000
100
50
500
0
20
30
40
50
60
70
80
90
0
20
100
25
35
30
20
P µmol l–1
K µmol l–1
25
20
15
15
10
10
5
5
30
40
50
60
70
80
90
0
20
100
0·6
1·2
0·5
1
0·4
0·8
Mn µmol l–1
Zn µmol l–1
0
20
0·3
0·2
0·4
0·2
0·1
0
20
0·6
30
40 50 60 70 80
Soil moisture, % WHC
90
100
0
20
F. 1. Second order regressions of soil moisture content on pH and on HCO , Ca, Mg, K, Mn, Zn and P concentrations ( µmol l−") in soil solution.
$
Meansps.e. (s.e. smaller than dot is invisible).
For HCO −, P, and Mn most of the increase occurred above
$
70 % WHC. Concentrations of Ca and Mg decreased above
50 % WHC, and of Zn over the entire moisture range (Fig.
1). Concentrations of K were highest at 50–70 % WHC.
Concentration differences between low and high soil
moisture regimes exceeded or approached one order of
magnitude for HCO −, Zn and Mn. Phosphorus, Ca and Mg
$
concentrations varied 3–5 fold according to soil moisture.
Misra and Tyler—Soil Moisture and Nutrient Concentrations
ObserŠations and measurements on plants
Germination of the two species varied with soil moisture
level. In P. phleoides, germination was almost the same at
different moisture levels whereas fewer seeds of V. spicata
germinated above 77 % WHC. At 21 d after sowing, a redpurple colour was visible on the lower and upper epidermis
of leaves of the seedlings, which might have been due to P
deficiency. This colour, which was most apparent in plants
growing at low soil moisture, disappeared as the growth of
the plants proceeded.
In P. phleoides, root (r l 0n801) and shoot (r l 0n772)
biomass production was linearly correlated with soil
moisture. In V. spicata, biomass production of roots and
shoots, calculated as g dry weight per pot, was highest at
68 % WHC and decreased with increasing soil moisture
above this value (Fig. 2). Biomass per plant (not illustrated,
but can be recalculated from Fig. 2 and Table 2) was linearly
correlated with soil moisture in both species.
Soil moisture content influenced the shoot : root biomass
ratio differently in the two species (Fig. 3). The shoot : root
biomass ratio indicated a shift towards shoot biomass from
60 % to 85 % WHC in V. spicata, whereas the shoot : root
ratio of P. phleoides decreased with increasing soil moisture
from 35 % to 60 % WHC, being consistently low at higher
moisture levels.
At high soil moisture, interveinal chlorosis was initially
observed in V. spicata. In P. phleoides, chlorosis was
observed at the tip of leaves at high soil moisture. This
chlorosis turned into necrotic spots after some weeks of
growth. Individual plants of V. spicata were larger at high
soil moisture but had a more yellow (lower) colour value.
1·0
Total biomass of V. spicata g per pot
The pH of the soil solution was 6n2 at 30 % WHC but
increased to almost 7n6 at 90 % WHC. The increase in Mn
concentration followed the same pattern as HCO− with
$
increasing soil moisture, being closely correlated with
HCO− concentration (r l 0n955).
$
Shoot biomass
0·5
0·0
Root biomass
0·5
30
50
70
Soil moisture, % WHC
90
0·6
Total biomass of P. phleoides g per pot
404
0·4
Shoot biomass
0·2
0·0
0·2
30
Root biomass
50
70
Soil moisture, % WHC
90
Mineral nutrient concentrations and amounts
F. 2. Shoot and root biomass produced (g d. wt per pot) by V. spicata
and P. phleoides at different soil moisture levels. Meansps.e.
The concentrations as well as amounts of mineral nutrients
per pot in root and shoot biomass varied with both species
and soil moisture regime. In V. spicata, increasing soil
moisture influenced shoot and root biomass concentrations
of K, Mg, P and Mn positively, whereas Ca concentration
was influenced positively in shoots, but showed a small but
significant decrease in roots (Figs 4 and 5). Iron concentrations in V. spicata were little affected by soil moisture
regime. Zinc concentration of V. spicata shoots decreased
above 50 % WHC, and that of roots decreased over the
whole range studied with increasing soil moisture. It is
noteworthy that regression curves of P concentrations in
shoots (Fig. 4) and roots (Fig. 5) were similar to those of K
in each species, but differed consistently between species.
Root as well as shoot biomass concentrations of K in V.
spicata increased with soil moisture, being highest at 85 %
WHC, but they varied little with soil moisture in P.
phleoides. When considered as amount per plant (Table 2),
uptake of all elements (except Fe in P. phleoides) by shoots
and roots of both species was higher at high soil moisture
levels (77–85 % WHC) than at lower moisture levels
(35–45 % WHC) (Table 2). Total uptake of all elements in
roots and shoots of V. spicata, calculated as amounts per
pot, exhibited near-bell shaped curves, giving good fits with
second order regressions (not illustrated, but may be
recalculated from Table 2) at a significance level of P 0n001
(P 0n01 for Fe) ; they usually peaked at 60 % WHC.
In roots and shoots of P. phleoides, concentrations of Ca
and Fe were influenced negatively by increase in soil
moisture. Manganese concentration tended to be influenced
negatively in root biomass but positively in shoot biomass
with increasing soil moisture (Figs 4 and 5). Potassium, Mg
and P concentrations of shoots and root biomass of P.
phleoides did not vary with variation in soil moisture.
405
Misra and Tyler—Soil Moisture and Nutrient Concentrations
T     2. Shoot and root amount (nmol per plant) of elements and number of plants per pot in V. spicata and P. phleoides at
different soil moisture content. Values represent means (ps.e.)
Soil moisture (% WHC)
V. spicata
Ca
Shoot
Root
Shoot
Root
Shoot
Root
Shoot
Root
Shoot
Root
Shoot
Root
Shoot
Root
Mg
K
P
Fe
Mn
Zn
Number of
plants per pot
P. phleoides
Ca
Shoot
Root
Mg
Shoot
Root
K
Shoot
Root
P
Shoot
Root
Fe
Shoot
Root
Mn
shoot
Root
Zn
Shoot
Root
Number of
plants per pot
35
43
52
60
68
77
85
536(p106)
136(p18)
91n4(p18)
42(p9)
706(p123)
151(p23)
56(p10)
21n1(p4)
3n1(p0n6)
5n4(p1n5)
1n7(p0n3)
0n65(p0n14)
2n8(p0n6)
0n76(p0n13)
69(p12)
514(p29)
130(p9)
84(p5)
48(p1n7)
663(p51)
165(p8n4)
62(p3)
21n4(p0n77)
2n5(p0n2)
6n7(p0n77)
1n6(p0n1)
0n81(p0n05)
2n8(p0n2)
0n76(p0n03)
173(p11)
925(p65)
300(p25)
185(p18)
104(p3n4)
1318(p93)
364(p11)
107(p5)
45(p2n21)
6n9(p0n7)
18(p2)
3n6(p0n3)
2n1(p0n15)
5n1(p0n5)
1n5(p0n1)
165(p12)
1353(p171)
335(p90)
242(p39)
144(p32)
2271(p241)
538(p73)
157(p15)
66(p14)
8n5(p1n2)
17n1(p5)
5n3(p0n6)
3n0(p0n8)
6n5(p0n8)
1n9(p0n4)
127(p13)
1731(p125)
329(p17)
356(p31)
155(p12)
2943(p253)
600(p38)
209(p14)
75(p4n2)
10n2(p0n8)
16(p2)
7n2(p0n4)
3n04(p0n26)
7n0(p0n5)
1n8(p0n1)
117(p8)
3981(p455)
575(p53)
939(p104)
382(p69)
7559(p992)
1276(p147)
624(p87)
188(p33)
15n2(p2n4)
22(p3)
16n1(p2n2)
8n1(p1n6)
10n4(p1n4)
3n09(p0n6)
31(p7)
4274(p501)
517(p106)
991(p216)
357(p40)
8502(p1301)
1349(p260)
766(p125)
197(p27)
20n2(p3n4)
19(p1n6)
16n2(p2n7)
8n7(p1)
10n3(p1n8)
2n7(p0n6)
12(p2)
460(p56)
303(p97)
106(p16)
19(p5)
988(p100)
95(p24)
89(p10)
22(p4)
4n21(p0n7)
22(p6n8)
4n0(p0n54)
1n7(p0n5)
5n0(p0n6)
0n67(p0n12)
36(p7)
716(p150)
501(p135)
154(p24)
41(p11)
1626(p278)
245(p59)
156(p31)
53(p14)
10n6(p5)
42(p13n7)
7n1(p1n6)
3n5(p1)
7n6(p1n3)
2n1(p0n5)
39(p11)
848(p80)
570(p313)
219(p31)
79(p46)
2233(p264)
637(p379)
182(p18)
121(p68)
7n0(p0n7)
40(p22)
10n0(p1n5)
5n2(p3)
9n94(p0n9)
5(p3)
39(p8)
1384(p294)
671(p153)
426(p85)
85(p23)
4021(p773)
789(p209)
360(p90)
132(p36)
10n9(p2n5)
32(p6)
20n2(p3n8)
6n7(p1n6)
17n1(p3n0)
6n1(p2)
36(p8)
1144(p106)
1166(p209)
332(p32)
95(p17)
3537(p210)
707(p109)
305(p33)
131(p22)
11n0(p2n1)
60(p20)
17n4(p1n3)
10n1(p3)
12n0(p1n2)
7(p1)
36(p5)
1385(p219)
827(p272)
455(p78)
118(p34)
4378(p565)
876(p195)
367(p67)
176(p50)
10n7(p3n2)
31(p8)
20n5(p2n4)
9n2(p3)
12n6(p2n2)
9n4(p3)
34(p8)
1450(p267)
803(p194)
455(p93)
90(p17)
4198(p913)
642(p122)
352(p54)
142(p27)
11n1(p1n9)
20n4(p6)
19n1(p4n2)
7n6(p1n5)
11n1(p2n7)
7n7(p1n5)
47(p5)
increasing biomass production with increasing soil moisture
(not illustrated, but can be recalculated from Table 2). The
significance level in the case of Ca, K, Mg, P, Mn in shoots
was P 0n001, the exceptions being Fe and Zn (P 0n01).
In roots, the significance level for positive correlation of
amounts per pot with soil moisture was P 0n001 in K, Mg,
P and Zn, P 0n01 in Mn, P 0n05 in Ca and P 0n05 in
Fe.
Shoot:root biomass ratio
5
4
3
D I S C U S S I ON
2
1
30
50
70
Soil moisture, % WHC
90
F. 3. Shoot : root biomass ratios of V. spicata (
) and P. phleoides
(8) at different soil moisture levels. Meansps.e.
However, there was a clear negative relationship between
increasing soil moisture and shoot concentration of Zn.
Amounts per pot of all elements in roots and shoots of P.
phleoides were influenced positively and linearly by the
The results reported here indicate that concentrations in soil
solution, and concentrations and amounts (uptake per plant
and per pot) in plants, can vary, sometimes considerably,
with soil moisture. Differences between amounts and
concentrations of nutrients in roots and shoots of the two
species were, however, to a large extent, caused by
differential variation in biomass and number of plants per
pot with soil moisture content.
Not being a nutrient originating from the mineral soil,
nitrogen was not studied ; it was not considered to be a
limiting factor in this experiment, as previous glasshouse
studies, using the same soil for other plant cultivation
406
Misra and Tyler—Soil Moisture and Nutrient Concentrations
400
5
d. wt)
300
V. spicata
–1
250
Fe (µmol g
Ca (µmol g–1 d. wt)
350
200
150
P. phleoides
100
4
3
V. spicata
2
1
P. phleoides
50
0
30
40
50
60
70
80
0
30
90
800
Zn (µmol g–1 d. wt)
K (µmol g–1 d. wt)
V. spicata
600
500
400
P. phleoides
300
200
100
40
50
60
70
80
70
80
90
P. phleoides
1·5
V. spicata
1
0·5
40
50
60
70
80
90
2·5
80
Mn (µmol g–1 d. wt)
Mg (µmol g–1 d. wt)
60
2
0
30
90
100
V. spicata
60
40
P. phleoides
20
0
30
50
2·5
700
0
30
40
40
50
60
70
80
90
P. phleoides
2
1·5
V. spicata
1
0·5
30
40
50
60
70
80
90
Soil moisture, %WHC
70
V. spicata
P (µmol g–1 d. wt)
60
50
40
P. phleoides
30
20
10
0
30
40
50
60
70
Soil moisture, %WHC
80
90
F. 4. Second order regressions for concentrations of elements in shoot biomass of V. spicata and P. phleoides on different soil moisture levels.
Meansps.e.
purposes, had demonstrated some accumulation of NO − at
$
the end of growth experiments (e.g. Tyler and Olsson, 1993 ;
Tyler, 1996 b).
Increase in concentration of K, Mg, P and Mn, with
increased soil moisture, in both shoots and roots of V.
spicata, indicate that high soil moisture changes the
407
Misra and Tyler—Soil Moisture and Nutrient Concentrations
50
640
Fe (µmol g–1 d. wt)
Ca (µmol g
–1
d. wt)
560
480
P. phleoides
400
320
240
160
V. spicata
40
30
20
10
80
0
30
40
50
60
70
80
0
30
90
Zn (µmol g–1 d. wt)
K (µmol g–1 d. wt)
V. spicata
400
300
200
P. phleoides
100
40
50
60
70
80
50
60
70
80
90
V. spicata
0·5
160
4
3·5
120
V. spicata
100
80
60
40
20
50
60
70
40
50
60
70
80
90
70
80
90
3
P. phleoides
2·5
2
1·5
V. spicata
1
0·5
P. phleoides
40
P. phleoides
1
0
30
90
Mn (µmol g–1 d. wt)
Mg (µmol g–1 d. wt)
40
1·5
140
0
30
V. spicata
2
500
0
30
P. phleoides
80
90
0
30
40
50
60
Soil moisture, %WHC
P (µmol g–1 d. wt)
80
V. spicata
70
60
50
40
30
P. phleoides
20
10
0
30
40
50
60
70
Soil moisture, %WHC
80
90
F. 5. Second order regressions for concentrations of elements in root biomass of V. spicata and P. phleoides on different soil moisture levels.
Meansps.e.
availability of these nutrients to this species. In contrast, the
comparatively high concentrations of Ca and Fe at low
moisture in P. phleoides indicate that low soil moisture
favours uptake of these elements per unit of biomass
produced by this species. Increased amounts of elements in
shoot and root of both species at higher (75–85 % WHC),
408
Misra and Tyler—Soil Moisture and Nutrient Concentrations
than at lower (35–43 % WHC), moisture levels were mainly
related to differences in number of plants and biomass
production per pot, whereas consistent and element-specific
relationships with soil moisture were found for concentrations in shoots and roots of both species.
V. spicata and P. phleoides are able to take up and retain
Fe in a form which may be utilized by the plant when
growing naturally on calcareous soil. However, interveinal
chlorosis still developed initially in V. spicata and some
chlorotic, and later necrotic, spots were observed at the leaf
tips of P. phleoides, especially at high soil moisture. These
symptoms suggested a deficiency of Fe under relatively high
soil moisture conditions in both species, and biomass
concentration of Fe in both species tended to be lowest at
77–85 % WHC. The explanation may be that Fe is
immobilized due to interactions with CaCO , HCO −, CO
#
$
$
and pH in the soil or plant systems, but mechanisms
responsible for such immobilization of Fe in plant tissues
are not well known. High concentrations of CaCO and
$
HCO − may be the main factors responsible for chlorosis
$
associated with Fe deficiency in Strategy I plants
(Marschner, Ro$ mheld and Kissel, 1986) when grown on
poorly-aerated calcareous soils (Awad, Ro$ mheld and
Marschner, 1994), and this could be the cause of chlorosis
in V. spicata when grown on calcareous soil under conditions
of high soil moisture.
High, but varying, concentrations of HOC − in the soil
$
solution are typical of calcareous soils (Mengel et al., 1984).
Bicarbonate is produced by root and microbial respiration,
but it may also be formed by the dissolution of CaCO . The
$
soil used in this experiment was highly calcareous and, when
more water was supplied, the HCO − concentration would
$
have risen owing to increased microbial respiration and
decrease in gas exchange, resulting in an increase in the
partial pressure of CO . The HCO − concentration is partly
#
$
controlled by the partial pressure of CO , which may be
#
high at microsites in soils (Mengel, 1994). A positive
regression was obtained between soil moisture and HCO −
$
concentration, between soil moisture and pH, and between
pH and HCO − (r l 0n912). Bicarbonate may be taken up by
$
plants and, due to its strong buffer action, the pH of the root
apoplast can increase, causing an inhibition of the Fe$+
reductase activity of the plasmalemma. Zohlen and Tyler
(1997) showed that, in Veronica officinalis (usually a calcifuge
species), grown on a calcareous soil, much of the Fe actually
taken up by the plant is immobilized in the leaf tissues in
metabolically-inactive forms ; this is not the case when it is
grown on an acid soil.
As the solubility of P is particularly low in limestone soils,
it is of interest that high soil moisture resulted in a
significant increase in the P concentrations in the soil
solution, and in the shoots and roots, of V. spicata. A high
P concentration in the leaves is sometimes related to Fe
chlorosis and a high P : Fe ratio can be an additional factor
in inducing chlorosis (Mengel et al., 1984 ; Inskeep and
Bloom, 1986). At low soil moisture, the diffusion path is
impeded, which makes P less available to plants. The stage
of plant development is also important, as seedlings are
particularly sensitive to low phosphate availability (Tyler,
1992) ; the purple pigmentation (often considered a symptom
of P deficiency) in V. spicata disappeared with age. In P.
phleoides, biomass concentration of P did not vary
appreciably with WHC, suggesting that regulation of P
uptake may be different in the two species.
Phytosiderophores (Fe-chelating compounds), which may
have been exuded from the roots of the grass P. phleoides,
were not measured in this experiment, but it was observed
that concentrations of Fe in the shoots were highest at
35–43 % WHC and decreased with increasing soil moisture.
This might have been caused by the degradation of
phytosiderophores by microbes at high soil moisture
content, as found by Marschner et al., (1986).
At high pH, dissolution of CaCO may control the
$
soluble Ca level in soils. This may raise the phosphate
concentration near the root surface and result in increased
P uptake (Ao et al., 1987). In the present experiment, the Ca
concentration of the soil solution decreased greatly with
increasing soil moisture above approx. 70 % WHC, whereas
HCO − and P concentrations increased. This decrease in the
$
solubility of Ca may have been a result of precipitation.
Phosphate concentration in solution will increase if the Ca#+
activity is depressed (Knight, Dudley and Jurinak, 1992).
Since the availability of P to plants on calcareous soil may,
therefore, be increased if soil moisture is in excess of 70 %
WHC, occasional or transient wet conditions may be of
great importance to the P acquisition of some plants on
normally dry calcareous soils.
Place and Barber (1964) investigated the effect of soil
water on ion diffusion, using )'Rb to simulate soil K.
Though these ions do not behave identically in soil (Drobner
and Tyler, 1998), the diffusion rate of Rb increased with
water content, the rate of increase becoming greater the
higher the level of exchangeable Rb. Plant uptake of Rb
increased linearly with soil moisture level. These investigations support the idea that diffusion controlled K uptake
in V. spicata, whereas P. phleoides, as with P, seemed to
exert greater control of its uptake of K.
The decrease in soil solution Mg concentration observed
with rising soil moisture and pH levels may be due to Mg
immobilization as a result of the increase in pH with soil
moisture. Chan, Davey and Geering (1979) showed that Mg
becomes much less exchangeable in soils above pH 6n5. It is,
however, noticeable that shoot concentrations of both Ca
and Mg increased with soil moisture level in V. spicata, but
not in P. phleoides. The control of the uptake of these
elements in the two species studied appears to be different.
The decrease in Zn concentration in the soil solution at
high moisture content is related to the low solubility of
transition metal ions at higher pH. Other antagonistic
effects could be the increased solubility of P (Norvell,
Dabkowska and Cary, 1987 ; Tagwira, Piha and Mugwira,
1992) or Mn, and increased HCO − (Sajwan and Lindsay,
$
1986) in the soil solution with increasing soil moisture.
Warncke and Barber (1972) stated that the influence of soil
moisture on Zn diffusion in soil is dependent on its effect on
Zn concentration in soil solution.
In most elements studied, the concentrations in shoots
tended to be higher in V. spicata than in P. phleoides. One
exception was Zn, where concentrations in roots and shoots
were consistently higher in P. phleoides. Opposite trends for
Misra and Tyler—Soil Moisture and Nutrient Concentrations
Zn concentrations in roots and shoots of P. phleoides (Figs
4 and 5) show that, with increasing soil moisture, relatively
less Zn was transported to the shoots. The decreasing soil
solution concentration of Zn with moisture correlated well
with shoot concentrations in both V. spicata and P.
phleoides. High pH and presence of CaCO can be considered
$
major factors associated with Zn deficiency (Cakmak et al.,
1996).
The decrease of Mn concentration in roots and increase in
shoots indicates that Mn transport from roots to shoots was
favoured by increasing soil moisture in P. phleoides. Once
again, a different relationship was observed in V. spicata, in
which the concentration was more closely related to soil
solution concentration in both biomass fractions. Higher
Mn concentrations in shoots of P. phleoides than V. spicata
may imply a higher demand of Mn in P. phleoides.
Root biomass concentration of Mg in V. spicata increased
with soil moisture but remained relatively constant in
P. phleoides. The two species also seem to have different
patterns of Ca uptake. Supply of Ca and Mg to roots
depends on root interception, mass flow and diffusion, but
mass flow is the major mechanism at low levels of Ca and
Mg supply.
The relationship between shoot : root biomass ratio and
soil moisture differed between the two species. A high
shoot : root ratio may indicate high nutrient and water
availability, as plants do not need to invest so much energy
in root production. Further studies are necessary to elucidate
the considerable differences in shoot : root ratio.
C O N C L U S I O NS
Moisture exerts a considerable influence on soil solution
chemistry, mineral uptake and tissue concentrations of
plants on calcareous soil. Biomass concentrations of mineral
nutrients in the two species studied, cultivated from seeds
originating from populations on a (limestone) Rendzic
leptosol, varied, sometimes greatly, according to soil
moisture. The results support the hypothesis that calcicoles
vary in mineral nutrient uptake according to soil moisture
and that variation in pH and HCO − concentration in soil
$
solution as a result of soil moisture regime, may have a great
effect on plant uptake of essential nutrients. The results
reported in this study could have a high ecological relevance,
because, under field conditions, soil moisture fluctuates
considerably during the growing season. Dry periods favour
the solubility and availability of some nutrients to plants,
wet periods favour others. Soil solution chemistry, as
influenced by soil moisture regime, should be studied in
order to increase the understanding of mechanisms of
observed changes. The changes in soil solution concentrations, created by soil moisture regime, could become
both beneficial (increased Mn, P availability) and harmful
(decreased Zn availability) to the nutrition of plants,
depending on the demand of the species. Plants which are
able to utilize these solubility fluctuations might have a
competitive advantage. Soil moisture variability might even
be a prerequisite for adequate mineral nutrient acquisition
and growth of at least some plants on limestone soils.
409
A C K N O W L E D G E M E N TS
We thank Maj-Britt Larsson for laboratory assistance. The
study was financially supported by the Swedish Natural
Science Research Council (NFR) and the Swedish Research
Council for Forestry and Agriculture (SJFR).
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