SCIENCE IN CHINA (Series D)
Vol. 44 No. 4
April 2001
The effect of C3 and C4 plants for the magnetic
susceptibility signal in soils
LÜ Houyuan (Ç;) & LIU Dongsheng (LIU Tungsheng žJ])
Institute of Geology and Geophysics, Chinese Academy of Sciences, P. O. Box 9825, Beijing 100029, China
Correspondence should be addressed to Lü Houyuan (email: [email protected])
Received December 27, 1999
Abstract To understand the origin of the ultrafine pedogenic components responsible for the
magnetic susceptibility (MS) enhancement remains a major challenging problem in linking magnetic signal with paleoclimate. Here we examine the effect of the natural fires on the MS signal of
both plants and modern soils and in particular the MS difference between C3 and C4 plant ashes
and their influence on magnetic susceptibility. We also proved the influence of the different floral
root systems on the MS signal of modern soils. We find that the C3 and C4 plants are different in
their ability to enhance MS signal of modern soils. Increased MS signal of modern soils by C4
plants was much greater than that by C3 plants.
Keywords: C3 and C4 plant, soils, magnetic susceptibility.
Heller and Liu are the first to point out that the climatic information recorded in the
loess-paleosol sequence at Luochuan (central part of the Loess Plateau, China) can be retrieved by
using low field magnetic susceptibility[1]. Magnetic enhancement of paleosols compared with intervening loess has been widely considered as a proxy for paleoclimate changes in the Loess Plateau. Several possible mechanisms for the magnetic enhancement of the paleosols have been suggested[2
9]
. Most studies concluded that pedogenic processes are responsible for the observed
variations in magnetic susceptibility (MS) in the Chinese loess-paleosol sequences[4
[8]
7]
. Kletet-
[9]
schka et al. and Meng et al. have considered respectively the possible role of natural fires and
decomposition of vegetation in the generation of the MS signal. However the source and cause of
the MS signal from natural fires and plants remain poorly understood. Furthermore, none of these
studies has considered the MS differences caused by different plants. More specifically, there has
been no systematic comparison between C3 and C4 plants in the generation of the magnetic susceptibility signal. In this paper, we present new evidences for the influence of the different floral
root systems on the MS signal of modern soils and the effect of fire on the MS signal of both
plants and modern soils, and MS differences between C3 and C4 plant ashes.
1 Samples and methods
Twenty-five samples were collected from 24 genera of modern plants including herbaceous
and woody plants in the northern suburb of Beijing, China (table 1). δ 13C measurement was performed at the State Key Laboratory of Gas Geochemistry, Lanzhou, China. After CO2 of selected
No. 4
C3 & C4 FOR MAGNETIC SUSCEPTIBILITY
319
plant samples was extracted, the δ 13C values are measured on a Finnigan MAT252 mass spectrometer with PDB as reference and an average precision less than 0.2‰. Two different methods
were used to burn the plant samples. The first is natural burning, which produced 40% 70%
components of plant incinerated and 30% 60% carbonized. The second is nearly complete burning assisted with ventilation and addition alcohol, which produced about 85%
90% incinerated.
Low field magnetic susceptibility was measured using the Bartington MS2 magnetic susceptibility
meter. The Fe2O3 concentration was determined with X-fluorescence spectrum.
Table 1 Magnetic susceptibility of ashes from different species of C3 and C4 plants
Taxon
Chloris virgataa)
Buchloe dactyoides
δ 13C(‰)
−16.285
/
Cynodon dactylon
/
Eragrostis pilosa
Setaria viridis
/
−17.199
Setaria viridisa)
Imperata cylindrca
/
−18.892
Zea maysa)
Echinochloa crusgallia)
Roegneria kamoji
−15.144
/
−28.456
Triticum aestivum
−28.156
Phragmites communis
Oryza sativa
Pleioblastus amarus
/
−28.163
−28.414
Poa annua
Cyperus sp
/
−29.173
Artemisia sp
/
Brassica campestris
Amaranthus sp
Populuus sp
/
/
/
Salix sp1
/
Salix sp2
Ulmus sp
/
/
Melia sp
Pinus sp
/
/
Burning mode
1
1
2
1
2
1
1
2
1
1
2
1
1
1
2
1
2
1
2
1
2
1
1
2
1
2
1
1
1
2
1
2
2
1
2
1
1
MS/10−8m3 kg−1
217.7
446.6
565.7
192.7
458.6
280
455.5
506.1
224
503.3
596
202
157.1
78
111.6
52
185.7
36.6
73.3
66.6
117.5
72.7
116
259
85.2
90
70
29.2
128.5
122.8
31.9
81.8
51.9
93.5
72.2
12.5
32.8
a) Sample of unmatured plant. Burning mode: 1, natural burning; 2, complete burning.
C3/C4
C4
C4
C4
C4
C4
C4
C4
C4
C4
C4
C4
C4
C4
C3
C3
C3
C3
C3
C3
C3
C3
C3
C3
C3
?
?
C3
?
C3
C3
C3
C3
C3
C3
C3
C3
C3
320
SCIENCE IN CHINA (Series D)
Vol. 44
In order to characterize MS changes of modern natural soils as a result of surface vegetation
burnt, we designed the following experiment. In the autumn of 1996, 10 samples of modern soils
were collected from 10 sites in an area of about 0.5 km2 wasteland, located in the northern suburb
of Beijing, where Setaria viridis, Imperata cylindrca and Cyperus sp. as well as other grass plants
were luxuriantly growing. Each sample is a mixture of the top 2 cm in the soil profile. We burned
the vegetation and then collected another 10 samples of 2 cm thick soils including ashes on the top
from the neighboring sites.
Forty-four samples from both rhizosphere soils which adhere to the surface of plants root
[10]
axis and the rhizosphere outside modern soil were collected from 22 modern plants including
14 samples of C3 plants and 8 samples of C4 plants in the Malantai, Zhaitang of Hebei Province
and northern suburb of Beijing, China.
2
2.1
Results
Variations in MS of C3 and C4 plant ashes
Table 1 lists 25 plants used in this study. Among them, the photosynthetic pathways of 23
plants in China have been identified by their PEPC/RuBPC activities and δ 13C value, and also by
studying their leaf anatomy[11
. The δ 13C content of plant material is correlated with the type of
14]
photosynthetic cycle conducted by the plant organisms, i.e., δ 13C value of C3 plants falls around
−26‰, while C4 plants around −13‰[15]. The photosynthetic types of Artemisia sp. and Amaranthus sp. are not identified due to the lack of PEPC/RuBPC and δ 13C value.
The average MS value of C4 plant ashes is about 370
average MS value of 88
10−8 m3
kg−1, much higher than the
kg−1 of C3 plant ashes. The average MS value of C4 plant
ashes by completely burning is about 532 10−8 m3
value of 121 10−8 m3
10−8 m3
kg−1, also much higher than the average MS
kg−1 of C3 plant ashes by complete burning. It can be seen from table 1
that the ashes of C4 plants have MS values 4
5 times higher than those of C3 plants.
2.2
The relationship between iron oxides and MS values of both C3 and C4 plant ashes
Table 2 gives major chemical components (%) and MS values of both C3 and C4 plant ashes.
The average content of Fe2O3 of C4 plant ashes is about 2.10%, much higher than the average
Fe2O3 content of 0.94% of C3 plant ashes. It is clear that the ashes of C4 plants have the content
of Fe2O3 two times higher than that of C3 plants and the C4 plant ashes have MS values 4 5
times higher than that of C3 plants. Fig. 1 shows that the MS and the contents of Fe2O3 of plant
ashes are well correlated.
2.3
MS changes of modern soils as a result of surface vegetation burning
The MS values of all modern soils with plant ashes on the top of soil profile are greater than
those of modern natural soils at the same site. Apparently, the ashes of plants make the MS of
soils enhanced after surface vegetation burning. Additionally, fire heating on soils also has the
No. 4
C3 & C4 FOR MAGNETIC SUSCEPTIBILITY
321
Table 2 Major chemical components (%) and MS values of C3 and C4 plant ashes
Taxon (C3\C4)
Roegneria kamoji (C3)
SiO2
TiO2
Al2O3
Fe2O3
MnO
CaO
MgO
K2O
Na2O
P2O5
LOIa)
MSb)
42.56
0.08
1.57
1.24
0.04
5.88
2.88
20.22
0.76
3.29
24.19
111.6
Triticum aestivum (C3)
31.06
0.08
1.24
0.75
0.00
3.18
1.55
5.04
0.35
0.58
56.17
52
Oryza sativa (C3)
63.43
0.04
0.65
0.83
0.26
3.61
1.37
2.12
0.11
0.93
26.59
73.3
Cyperus sp (C3)
27.75
0.08
1.54
0.91
0.04
6.10
2.79
16.52
0.70
2.74
38.99
116
Pleioblastus.amarus (C3)
64.08
0.06
1.05
0.95
0.09
5.68
1.61
2.49
0.00
0.65
22.29
117.5
Setaria viridis (C4)
56.48
0.30
6.22
2.30
0.13
8.33
2.84
5.85
1.00
2.07
13.57
506.1
Zea mays (C4)
22.46
0.14
2.89
1.32
0.07
6.94
8.72
16.66
0.72
5.46
34.59
202
Chloris virgata (C4)
26.62
0.11
2.29
1.23
0.06
3.95
3.41
21.72
0.97
3.58
39.52
217.7
Imperata cylindrca (C4)
57.92
0.31
6.22
3.02
0.11
9.13
2.58
6.86
0.84
2.74
8.68
596
Buchloe dactyoides (C4)
47.07
0.32
5.95
3.07
0.15
10.04
2.41
6.22
0.97
2.86
19.74
565
Eragrostis pilosa (C4)
24.88
0.14
2.79
1.64
0.07
5.66
2.75
14.9
5.08
3.65
43.15
280
a) LOI: Loss of ignition. b) MS: Magnetic susceptibility (10−8m3
kg−1) of plant ashes.
potential to enhance the MS significantly[8]. The
average MS values of modern natural soils and
burnt soils vary from 68
124
10−8 m3
213 10−8 m3
kg−1 and 87
10−8 m3
10−8 m3
kg−1 to
kg−1 to
kg−1, respectively (fig. 2). The rela-
tive contributions of MS of ashes and heating to
total MS of soils after surface vegetation burning are
about an average of 30% 40%.
To distinguish the heating from ashes for the
contribution of MS enhancement of soils, Lü1) indicated that the relative contribution of heating (exFig. 1. Relationship between MS values and Fe2O3
cluding the ashes) to total MS of soils at nature fir- concentrations in the ashes of C3 and C4 plants.
ing is as much as 16% in the paleosols and about 11% in loess via simulation burning experiment.
This means that the ashes produced by one burning of the grassland, mainly consisting of C4
plants, can enhance MS of surface soil by about 20%
25% if taking no account of wind erosion,
leaching of the ashes and possible transformation of the magnetic minerals during pedogenetic
processes.
2.4
The relationship between MS values of modern soils and floral root system of both C3 and
C4 plants
The rhizosphere soils take on better MS values than the rhizosphere outside modern soil. The
MS values of all rhizosphere soils are comparatively increased averagely by 33%. But the C3 and
1) Lü, H.Y., Quaternary environmental changes recorded by magnetic susceptibility and plant fossils: quantitative estimates of paleoclimates, Ph. D. Thesis (in Chinese with English abstract), Institute of Geology, Chinese Academy of Sciences,
China, 1998, 1 145.
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SCIENCE IN CHINA (Series D)
Vol. 44
C4 plants rhizosphere soils are different in their ability to enhance MS signal of modern soils. Increased MS signal of rhizosphere soils by C4 plants was much greater than that by C3 plants. Fig.
3 shows that the MS values of the C4 plant rhizosphere soils are comparatively increased averagely by 56.7%, and the MS values of the C3 plant rhizosphere soils are comparatively increased
averagely by 12.5%.
Fig. 2. Comparison of MS values from modern soils
with and without surface vegetation burning. A, MS
value in modern soils with surface vegetation burning,
B, MS value in modern soils without surface vegetation
burning.
Fig. 3. Comparatively increased MS percentage of
both C3 and C4 plants rhizosphere soils.
2.5 Variations in percentage of dumbbell phytolith of the C4 plant and values of MS of modern
soils with precipitation in China
In the recent investigation of distribution of phytolith assemblages in the modern soils in
China, we found that the variations in dumbbell phytoliths are consistent with those of MS of
modern soil[16] (fig. 4). The dumbbell phytolith is from the typical C4 plant, subfamily Panicideae.
The abundance of this kind of phytolith in the modern soils increases with mean annual precipitation in the temperate semi-arid regions of North China. While a contrary relationship exists in
southern China where mean annual precipitation exceeds 1000 1200 mm. The same as the above
phenomena, the highest MS values present in modern soil under the mean annual precipitation
1100
1200 mm, indicating that the high MS values correspond to dominated C4 plant soils.
3 Discussion and conclusion
Biogenic magnetites have been found in many life forms[17] and discovered in a wide variety
of environments, including marine, lacustrine sediments and land soils[18
20]
. The process of re-
lease and concentration of iron oxides during plant decay has long been known[21]. The average
iron content in plants is 100
composed plants are 0.1
300 mg/kg[22]. Most of these iron oxide grains releasing from de-
1.0 µm in diameter, falling within the single domain category or, more
No. 4
C3 & C4 FOR MAGNETIC SUSCEPTIBILITY
323
Fig. 4. Variations in percentage of dumbbell phytolith (a) and values of MS (b) of modern soils with present-day mean
annual precipitation in China.
properly, pseudo-single domain[23,24]. It has been shown that MS enhancement in the paleosols was
caused principally by fine-grain magnetic minerals[4], and one of the major sources of the ultrafine
magnetic minerals appears to be decomposed plant litter[9].
The enhancement of MS in paleosols is associated with higher organic matter content in
Chinese Loess Plateau[25]. However, our recent work revealed that the MS of modern soils change
across the different climatic zones, showing an increase with decreasing organic matter content
from northeastern forest steppe area to middle and lower reaches of Yangtze River subtropical
broadleaved forest, China. One of the possible reasons is that the variations in MS of modern soils
are related to contribution of C3 and C4 plants in different areas. In China, C4 plants occupy about
13% 14% of the total plants in northeastern forest steppe area and about 61% in the subtropical
broadleaved forest area of middle-lower reaches of Yangtze River, China[13]. This may interpret
MS increase with decreasing organic matter content from northeast to the Yangtze area.
Many reports claimed that C3 and C4 plants are different in their ability to absorb iron from
the root zone. Concentration of iron required by C4 plants was much greater than that of C3
plants[26,27]. Based on our result, the content of Fe2O3 in C4 plant ashes is higher than that of C3
plants. The contents of C3 and C4 plant ashes have nearly the same values at about 10%
accounting for total dry matter
15%
[28]
, suggesting that C4 plants have larger potential to provide mag-
netic minerals on soils.
The possibility of C3 and C4 plant to produce MS signal of soils has not previously been
discussed, and our experimental data suggest that burning of C3 and C4 plants can enhance MS
signal of modern soils. We show that C4 plants have greater potential to enhance MS signal. The
average MS value of C4 plant ashes is about 532 10−8 m3
MS value of 121
10−8 m3
kg−1, much higher than the average
kg−1 of C3 plant ashes. Fe2O3 concentration in C4 plant is up to two
324
SCIENCE IN CHINA (Series D)
Vol. 44
times higher than that of C3 plant. One burning of the grassland, mainly consisting of C4 plants,
can enhance MS value of the surface soil up to about an average 30% 40%. C3 and C4 plants
rhizosphere soils are different in their ability to enhance MS signal of modern soils. Increased MS
signal of rhizosphere soils by C4 plants was much greater than that of C3 plants. In the recent investigation of distribution of phytolith assemblages in the modern soils in China, we found that
the variations in C4 plant phytoliths are consistent with those of MS of modern soils.
Acknowledgements This work was supported by the Nation Natural Science Foundation of China (Grant Nos.
400242002 and 49894170-04), project on Formation and Evolution of Tibetan Plateau with its Environment and Resource Effect
(Grant No. 1998040800) and Chinese Academy of Sciences (CAS KZ951-A1-402). The authors thank Prof. Han, J. M., Prof.
Zhou, L. P., Dr. Gu, Z. Y. and Dr. Liu, B. Z. for critically reviewing the manuscript and providing many helpful suggestions.
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