GLOBAL BIOGEOCHEMICAL CYCLES, VOL. 19, GB1020, doi:10.1029/2004GB002296, 2005
Pools and distributions of soil phosphorus in China
Chi Zhang,1 Hanqin Tian,1 Jiyuan Liu,2 Shaoqiang Wang,2 Mingliang Liu,1,3
Shufen Pan,1 and Xuezheng Shi4
Received 12 May 2004; revised 6 December 2004; accepted 18 January 2005; published 16 March 2005.
[1] We have investigated the pools and distributions of soil phosphorus (P) in the top
50 cm of soil in China by using a combination of total and available P information from
more than 2400 soil profiles and a map of soil types at a resolution of 1:1,000,000. Our
estimates indicate that the average total P density and available P density in China are
about 8.3 102 g/m3 and 5.4 g/m3, respectively. The total national soil P pool in the
surface half meter is 3.5 Pg (1015 g). The available P density ranges from 0.7 g/m3 in the
Lithosols to 16.7 g/m3 in the Irrigated Silting Soils. The total P density ranges from
1.2 102 g/m3 in the Lithosols to 19 102 g/m3 in the Frigid Desert Soils. The ratio of
available P to total P density ranges from 0.6 103 in Aeolian Soils to 21.6 103 in
Coastal Solonchaks. The available P content and its vertical distribution show a complex
pattern among soil orders of different development stages, possibly indicating the
important role of biota’s control over soil available P content. There are large variations of
P content in different climatic regions. The tropical and subtropical region has the lowest
available P density (4.8 g/m3) and the second lowest total P density (8.2 102 g/m3)
among all climatic regions. The large variation in the soil P content suggests that further
study is needed to investigate climatic and land-use controls over the soil P content.
Citation: Zhang, C., H. Tian, J. Liu, S. Wang, M. Liu, S. Pan, and X. Shi (2005), Pools and distributions of soil phosphorus in China,
Global Biogeochem. Cycles, 19, GB1020, doi:10.1029/2004GB002296.
1. Introduction
[2] In many terrestrial ecosystems, accumulation of C and
N seems to be regulated by the pool of biologically active P
[Walker and Adams, 1958; Tate and Salcedo, 1988; Vitousek,
1998; Lloyd et al., 2001]. The supply of P to plants is
generally constrained by both low total quantity of this
element in the soils and by the very low solubility of the
scarce quantity [Sanchez, 1976; Uehara and Gillman, 1981;
Onthong et al., 1999; Neufeldt et al., 2000]. Nevertheless,
compared to widely comprehensive evaluations of the C and
N cycles, there are only a few studies which have examined
the large-scale P cycles in terrestrial ecosystems [Tiessen,
1995; Reddy et al., 1999; Smil, 2000]. From both scientific
and ecosystem management perspectives, it is important to
investigate the pool and density of soil P in terrestrial
ecosystems at regional and global scales.
[3] Estimates on the pool and density of P in terrestrial
ecosystems at regional and global scales remain largely
1
School of Forestry and Wildlife Sciences, Auburn University, Auburn,
Alabama, USA.
2
Institute of Geographic Sciences and Natural Resources Research,
Chinese Academy of Sciences, Beijing, China.
3
Also at Institute of Geographic Sciences and Natural Resources
Research, Chinese Academy of Sciences, Beijing, China.
4
State Key Laboratory of Soil and Sustainable Agriculture, Institute of
Soil Science, Chinese Academy of Sciences, Nanjing, China.
Copyright 2005 by the American Geophysical Union.
0886-6236/05/2004GB002296$12.00
uncertain [Taylor, 1964; Lerman et al., 1975; Pierrou,
1976; Richey, 1983; Smil, 1990; Mackenzie et al., 1998].
The most frequently used global soil P database, the
WISE (World Inventory of Soil Emission) global soil
profile database [Batjes, 2002], contains P data for only
924 soil profiles of the entire globe. The P is primarily
rock derived, and its spatial distribution is highly heterogeneous. Besides parent material, other state factors such
as climate and biota also play an important role in
controlling soil P content. Together, they generate a very
complex temporal and spatial pattern of different P
fractions during soil development. Although it is generally agreed that the total soil P gradually decreases as the
result of weathering [Walker and Syers, 1976], the available soil P content, which can be utilized by plants, may
not change in the same pattern [Crews et al., 1995;
Frizano et al., 2002]. The plants’ uptake, litterfall and
decomposition rates, and mycorrhizal symbioses all can
modify the available P fraction of the soil [Lajtha and
Harrison, 1995]. In addition, human activity is becoming
an important factor in controlling soil P content. However,
there is a shortage of the quantitative assessment of controls
on the soil P content at a regional scale. To reduce this
uncertainty, it is necessary to have a more accurate estimate
of the pool and density of soil P under different climates
and land uses.
[4] China has an area of about 9,600,000 square kilometers, covering about 50° of latitude and 60° of longitude. Within the vast territory, there are various soil
types, which are developed under different bioclimatic
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ZHANG ET AL.: POOLS AND DISTRIBUTIONS OF SOIL PHOSPHORUS
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Figure 1. Distribution of soil profiles in five climatic zones across China: zone A, temperate desert;
zone B, cool temperate zone; zone C, warm temperate zone; zone D, frigid highland; and zone E, tropical
and subtropical zone.
conditions and derived from various parent materials in
diversified topographical environments. The large-scale
land transformation in China in recent years has significantly influenced biogeochemical and hydrological cycles
[Liu et al., 2002; Tian et al., 2003]. Previous studies have
indicated that land-use induced loss of soil P could result
in land degradation and then affected forest regeneration
and reforestation [He and Yue, 1984]. Concerns regarding
a P limitation of net primary production in terrestrial
ecosystems, particularly tropical forests, also arise. In
addition, lakes throughout the country are commonly
undergoing eutrophication partly because of the input of
excessive P from the land [Jin, 2003]. To understand the
P cycle and its impacts on both terrestrial and aquatic
ecosystems in China, baseline estimates on the pool and
density of both total and available soil P across the nation
need to be developed. Since the 1960s, the Chinese
government has conducted several soil surveys nationwide, including total and available P. The major objectives of our study are to (1) estimate both total and
available soil P content for different soil types in China
and (2) investigate patterns of soil P over soil development stages and under different climatic zones. Special
attention is paid to the tropical and subtropical region
where terrestrial ecosystem productivity is suggested to be
generally limited by soil P nutrient availability [Sanchez,
1976; Lu, 1990; Brady and Weil, 2002]. We also identify
uncertainty in existing information that needs to be
investigated in the future to improve our understanding
of the P cycle and its control on the C and N cycles in
this region.
2. Materials and Methods
2.1. Data Sources
[5] The data we used for this study are from the second
national soil survey of China [National Soil Survey Office,
1993, 1994a, 1994b, 1995a, 1995b, 1996, 1998]. This
database includes 2473 typical soil profiles, each of which
represents soil species in the Genetic Soil Classification of
China (GSCC) [Shi et al., 2004b]. Each soil profile is
divided into A, B, and C horizons as depth increased. Each
horizon may be further divided into several subhorizons.
Soil properties examined include the thickness of the
horizons, soil total P content, soil available P content, and
soil bulk density. Perchloric acid digestion followed by
molybdate colorimetric test was used for total P analysis
[Smith, 1965]. The Olsen method was used for available P
analysis [Olsen et al., 1954]. Bray I method was recommended for acidic soil samples [Bray and Kurtz, 1945]. Of
all the 2473 soil profiles, 2451 have total P content records
and 2174 have available P content records. Soil bulk density
was determined by obtaining a known volume of soil,
drying it to remove the water, and weighing the dry mass
[Brady and Weil, 2002]. There are 1594 profiles with
geographic location information and 2335 soil profiles with
information on the area of each soil species according to the
estimation of national soil survey [National Soil Survey
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Table 1. P Density, Total Soil P Pool Size, and Available P:Total P Ratio of Soil Great Groups
Great Groups in Chinese
Soil Taxonomy
U.S. Soil
Taxonomy
Number
of
Profiles
Available
P, g/m3
Total P,
102 g/m3
Available
P:Total P,
103
China’s Soil
P Pool, 102 Pg
Brown coniferous forest soils
Yellow brown soils
Yellow cinnamon soils
Brown soils
Dark-brown soils
Bleached Bejiang Soils
Torrid red soils
Cinnamon soils
Gray-cinnamon soils
Black soils
Gray forest soils
Chernozems
Castanozems
Castano-cinnamon soils
Dark loessial soils
Brown caliche soils
Sierozems
Gray desert soils
Gray-brown desert soils
Brown desert soils
loessial soil
Red primitive soils
Neo-alluvial soils
Aeolian soils
Limestone soils
Volcanic soils
Purplish soils
Lithosols
Skeletal soils
Meadow soils
Sajiang black soils
(Lime concretion black soils)
Mountain meadow soils
Shrub meadow soils
Fluvi-aquic soils
Bog soils
Peat soils
Solonchaks
Desert solonchaks
Coastal solonchaks
Frigid plateau solonchaks
Solonetzs
Paddy soils
Irrigated silting soils
Irrigated desert soils
Felty soils (Alpine meadow soils)
Dark felty soils (subalpine meadow soils)
Frigid calcic soils
Cold calcic soil
Cold brown calcic soils
Frigid desert soils
Cold desert soils
Frigid frozon soils
Latosols
Latosolic red soils
Red soil
Yellow soils
Inceptisols
Alfisols
Inceptisols
Alfisols/Inceptisols
Alfisols/Inceptisols
Alfisols
Inceptisols
Inceptisols
Alfisols
Udolls
Alfisols
Cryolls
Ustolls
Ustolls
Calci-great groups
Calci-great groups
Inceptisols
Calci-great groups
Calci-great groups
Salic great groups
Orthents, Psamments
Alfisols
Fluvents
Psamments
Orthents, Psamments
Andisols
Orthents, Psamments
Orthents, Psamments
Orthents, Psamments
Aquic suborders/Udolls
Vertisols
9
29
22
80
57
20
15
116
19
32
6
58
47
3
18
13
32
11
10
8
33
21
48
45
47
12
83
14
42
95
22
5.9
6.0
4.6
5.5
2.1
6.6
2.3
7.2
4.6
2.3
1.0
3.9
3.1
2.8
2.9
7.2
4.5
5.4
3.9
4.7
3.6
2.7
5.2
1.0
2.8
4.6
2.5
0.7
2.7
3.4
3.8
10.0
9.4
5.7
5.5
9.3
8.6
2.9
7.6
2.4
9.8
9.2
7.8
8.7
10.6
10.2
9.0
10.7
8.7
6.8
5.1
7.7
7.4
7.6
15.4
7.0
16.0
7.5
1.2
5.1
8.2
6.6
5.9
6.4
8.1
10.0
2.2
7.7
7.9
9.4
3.7
2.4
1.0
5.0
3.6
2.7
2.8
8.0
4.2
6.2
5.8
9.2
4.7
3.7
6.9
0.6
4.1
2.9
3.4
5.6
5.4
4.2
5.7
5.8
8.4
1.0
5.5
18.8
2.3
0.1
9.7
3.8
3.6
1.4
5.2
15.4
1.0
0.4
11.6
2.9
1.3
10.4
1.1
4.7
0.7
1.6
45
3.8
0.2
7.1
1.1
6.6
10.3
1.2
Inceptisols
Inceptisols
Fluvents
Aquic suborders
Histosols
Salic great groups
Salic great groups
Salic great groups
Salic great groups
Salic great groups
Fluvents/Inceptisols
Fluvents
Fluvents
Inceptisols
Inceptisols
Inceptisols
Inceptisols
Inceptisols
Psamments
Psamments
Psamments
Ultisols
Ultisols
Ultisols
Ultisols
14
2
182
34
8
19
5
20
4
11
421
24
16
20
20
8
24
4
3
2
5
23
29
122
73
4.1
1.4
4.7
3.4
5.0
3.5
8.0
15.0
5.0
5.2
6.2
16.7
7.1
3.2
3.4
3.0
7.9
9.1
4.1
6.5
3.8
4.1
3.2
6.4
6.6
8.4
7.7
17.7
9.4
8.0
8.9
6.9
7.0
9.3
5.4
6.6
12.8
9.7
6.9
8.0
8.0
8.9
11.2
19.1
5.2
5.6
4.0
5.0
6.1
4.5
4.8
1.8
2.6
3.6
6.3
4.0
11.6
21.6
5.4
9.8
9.4
13.0
7.4
4.6
4.2
3.8
9.0
8.0
2.2
12.5
6.7
10.2
6.4
10.6
14.6
1.8
0.2
22.8
6.0
0.5
4.6
0.9
0.7
0.3
0.2
9.8
0.8
0.4
18.6
7.8
27.7
4.9
0.1
8.6
1.4
8.7
0.8
4.4
17.3
5.2
Office, 1993, 1994a, 1994b, 1995a, 1995b, 1996, 1998].
The soil data were integrated into a geographical information system (GIS) database with their geographical locations
(Figure 1). China’s soil P pools were estimated by combining the P content information of each soil type with the
distribution information of each soil type in China. After
removal of the land area associated with water bodies,
glaciers, perennial snow, bare rock, and gravel hills the
total investigated soil area is 839.36 M ha in this study.
[6] Since the GSCC [National Soil Survey Office, 1998]
was used in the soil surveys, we utilized the same soil
classification system in this study. In addition, we have
compared our system with the USDA soil taxonomy [Shi et
al., 2004b] (Table 1). The Chinese soil taxonomy system we
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used has a hierarchical structure, with 12 orders, 61 great
groups, 235 sub-great groups, 909 families, and more than
2473 soil species.
2.2. Estimation of the Soil P Density and Pools
[7] We estimated the size of China’s soil P pool to the
depth of 50 cm based on the P density of each soil great
group. Soil depth of each horizon based on thickness and
vertical locating sequence of horizons was calculated as
H¼
X
ð1Þ
hi ;
where H is the depth of the horizon; h is the thickness of
each horizon above the current horizon, including the
thickness of the current horizon; and i is the number of
profile layers. Then the P density of the soil was calculated
as
X
SPDH ¼
hi BDi Pi
H
;
ð2Þ
where SPDH is the average soil P density of the soil to the
depth of a certain horizon, H is the depth of the horizon; and
hi, BDi and Pi are the thickness, bulk density, and P content
of each horizon above current horizon, respectively. In this
way, we calculated the soil P density of different depths. In
addition, for each profile we further selected the soil P
density above the horizon whose depth (H) mostly
approximates to 50 cm as the density of the soil species
(SPDsp) it represents. Then we estimated the P density of
different soil great groups with the distributional area and P
density of each soil species as following:
XAsp
SPDsp
Agt
X
Asp ;
Agt ¼
SPDgt ¼
ð3Þ
ð4Þ
where SPDgt is the soil P density of the great group; Asp is
the area of each soil species that belong to current great
group, and Agt is the total area of the great group.
[8] China’s soil P pool of 50 cm depth is the summary of
the P pool of all the soil great groups. The pool size of each
great group is calculated by multiply its soil P density to its
area,
SPCN ¼
X
Agt SPDgt ;
ð5Þ
where SPCN is the soil P pool of China. Soil P density of
China is calculated by dividing SPCN with the total
investigated soil area of 839.36 M ha in this study.
[9] Finally, using GIS technology, we assigned the soil P
density value to each sub-great group on the 1:1 million soil
map of China [Shi et al., 2002, 2004a], and then generated
the China’s soil P density map.
2.3. Estimation of Soil P Density Under Different
Climatic Zones
[10] To investigate the soil P density under different
climatic conditions, we first divided China into five biocli-
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matic zones: Frigid Highland, Cool Temperate Zone, Warm
Temperate Zone, Temperate Desert, and Tropical and Subtropical Zone. Then, by using a geographic information
system, we located the soil profiles on the map of climatic
zones, and calculated the average soil P density for each
bioclimatic region (Figure 1).
3. Results and Discussion
3.1. Density and Pool of Total and Available Soil P
[11] Our results showed that the available P density
ranges from 0.7 g/m3 in the Lithosols (Lithic suborders,
Entisols) to 16.7 g/m3 in the Irrigated Silting Soils (Table 1).
The total P density ranges from 1.2 102 g/m3 in the
Lithosols to 19.1 102 g/m3 in the Frigid Desert Soils. The
ratio of available to total P ranges from 0.6 103 in
Aeolian Soils to 21.6 103 in Coastal Solonchaks. The
averaged total and available P densities are 8.3 102 g/m3
and 5.4 g/m3, respectively.
[12] According to Walker and Syers [1976], most of the P
in the Lithosols, a Lithic Entisol at its early stage of soil
development, is in the primary mineral form, and the
available P content is quite low. This prediction was proved
by several chronosequence studies, in which available P in
the youngest soil was more than an order of magnitude
lower than the one in the intermediate-aged soil [Crews et
al., 1995; Frizano et al., 2002]. The long cultivation history
on Irrigated Silting Soils may have improved the soil
productivity and increased its available P content [Zhu
and He, 1992].
[13] Total P pool of the top 50 cm soil in China is
estimated at 3.5 Pg, which is much lower than Richey’s
estimation of 13.4 Pg P in Chinese soil [Richey, 1983]. The
difference between the two may be due to several reasons:
(1) In our research, all of the water bodies, glaciers,
perennial snow, bare rock, and gravel hills were excluded
from the calculation of the total P pool, so the total
terrestrial area (839.36 M ha) in our calculation is only
about 77% of the area Richey used; (2) our estimate of soil
P density is only two thirds of Richey’s value of 12.31 102 g/m3; and (3) the soil depth used in our estimation is
only half of what Richey used. Our estimation of total soil P
density is 8.3 102 g/m3, or 0.42 kg P per square meter in
the top 50 cm soil, which is close to Smil’s estimation of
7.5 102 g/m3 [Smil, 2000].
3.2. Changes in Soil P Content Over Different Soil
Development Stages
[14] To further analyze the changes of soil P content over
time, we assigned the soils to different USDA soil orders
(Figure 2). According to the weathering regime [Smeck,
1985; Brady and Weil, 2002], we divided soil orders into
three weathering stages: slight (Entisols, Gelisols, Histosols,
Inceptisols, Andisols), intermediate (Aridisols, Vertisols,
Alfisols, Mollisols), and strong (Ultisols, Spodosol, Oxisols). Results indicated that as the soil ages, the total P
content decreases from 9 102 g/m3 (or about 0.64 mg P/g)
to 4.9 102 g/m3 (or 0.35 mg P/g) (Table 2), which is in the
range of total soil content of eight orders (from 0.684 to
0.200 mg P/g) in another study based on 88 soil profiles
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Figure 2. Changes in soil P content among seven soil orders in China. The soil orders were arranged in
an increased development sequence from left to right.
[Cross and Schlesinger, 1995]. The available P content,
however, remains unchanged with soil age.
[15] The pattern of decreased total P content with soil age
agrees with the Walker and Syers model [Walker and Syers,
1976], which predicts that as soil ages, the loss of P through
weathering will cause a decrease of total soil P content. The
unchanged pattern of soil available P with soil aging is
unexpected (Table 2). We attributed this pattern of available
P fraction during soil development to the biota’s control
over the soil P content [Lee et al., 1989; Schneider et al.,
2001]. Many researchers have suggested that biological
processes regulate the movement and distribution of labile
forms of P, and intrasystem P cycle is important to the
availability of soil P [Smeck, 1985; Stewart and Tiessen,
1987; Schlesinger, 1991]. The biota tunes the P mineralization rate depending on phosphorus availability and tries to
maintain soil available P content to meet the nutrient
requirements of the organisms [Cross and Schlesinger,
1995]. It is also suggested that the biota’s control over the
P cycle becomes more and more important with soil age.
[16] As ecosystems develop, more soil available P is
needed to support the higher productivity of the vegetation
and the increased amount of soil organisms. Also, more
energy can be allocated by plants to exploit P in the deep
soil by enhancing root system activity, or through symbioses with mycorrhizal fungi [Tiessen et al., 1994]. We further
calculated the ratio between the available P density of the
top 50 cm and the available P density of the total sample
depth, and found this ratio increased from 1.1 in slightly
weathered soil types to 1.2 in intermediately weathered soil
types to 1.4 in strongly weathered soil types. This pattern
suggests that biota may modify the soil P fractions by
uptaking P from the deep soil and then moving it to the
surface during soil development. Also, this control mechanism may generate a considerable upward translocation of
available P that counteracts the downward P movement
through leaching during soil development. Furthermore, a
productive vegetation cover can prevent loss of P in the top
layer of soil by reducing soil erosion rate and by storing P
in living biomass. The results of the chronosequencelandslide-scars study conducted in the Luquillo Mountains,
Puerto Rico, also showed that available P in the topsoil
increased with an increase in landslide age while the total P
tended to decrease over time [Frizano et al., 2002]. Frizano
and his colleagues further suggested that vegetation growth
on highly leached soils may release some occluded P to the
available pool. Similarly, investigators in the Hawaiian
Chronosequence study suggested that the occlusion of P
by secondary Fe and Al minerals in strongly weathered soil
is not entirely permanent [Crews et al., 1995].
[17] Because of multiple controls from the parent material,
soil weathering regime, and biota [Smeck, 1985], and
because of changes in the relative importance of these
controls during soil development [Cross and Schlesinger,
1995], the pattern of soil available P content is usually quite
complex. Figure 2 shows that during the soil development,
the available P fraction increases from Entisols to Inceptisols, reaching the first peak at Aridisols. Then the fraction
decreases at Vertisols and Mollisols. After that, its fraction
increases again at Alfisols and finally reaches the peak at
Table 2. P Density of Soils of Different Development Stages in
China
Weathering Status
Available P,
g/m3
Total P,
102 g/m3
Available P:Total P,
103
Slighta
Intermediateb
Strongc
4.7 ± 1.2
4.8 ± 1.3
5.1 ± 1.4
9.0 ± 1.6
8.4 ± 0.7
4.9 ± 0.7
5
6
10
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a
Including soil orders Entisols, Gelisols, Histosols, Inceptisols, Andisols;
Including soil orders Aridisols, Vertisols, Alfisols, Mollisols;
Including soil orders Ultisols, Spodosol, Oxisols.
b
c
ZHANG ET AL.: POOLS AND DISTRIBUTIONS OF SOIL PHOSPHORUS
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Figure 3. Spatial distribution of total soil phosphorus density in China (g/m3). See color version of this
figure at back of this issue.
the Ultisols. This pattern is consistent with the observation
of resin and bicarbonate fraction among soil orders on 88
soil profiles in another study [Cross and Schlesinger, 1995],
implying that there may be some general patterns of soil
available P fraction among soil orders of different weathering stages. This pattern, however, cannot simply be described as being increased or decreased with soil age.
3.3. Variations in Total and Available Soil P Across
Climatic Zones
[18] The spatial distribution of total soil P density shows
substantial variation across China, but the total P content in
the tropical and subtropical soils of the southeast China is
generally lower than other parts of the nation (Figure 3).
The content of soil P density is dependent on the degree of
soil weathering, which is controlled by the hydrothermal
conditions, soil age, and parent material [Gardner, 1990].
China is characterized by a great spatial variability in
climate, ranging from tropical to cool temperate zones [Tian
et al., 2003; Wu et al., 2003]. The southern part of China is
strongly humid due to the influences of the Asian monsoon
circulations [Zhang, 1991; Tian et al., 2003], while in
northwest China, the barrier effect of the Tibetan Plateau
to moisture and the long distance from the ocean result in an
arid climate. Thus the mean annual temperature of China
increase from about 6.5°C to 23.5°C with the decrease of
latitude, and the annual precipitation decreases from about
2500 mm to 15 mm along southeast to northwest China.
Cold conditions prevail across the Tibetan Plateau owing to
the high elevation.
[19] To investigate the effects of climate on soil P pools,
we analyzed the soil P distribution patterns in five different
climatic zones (Table 3, Figure 1). The results showed
considerable variation of soil P density in the top 50 cm
of soils among different climatic zones. The warm temperate zone has the highest soil available phosphorus density
Table 3. Soil P Density of Different Climatic Zones of China (g/m3)
Total P
Available P
Climatic
Zone
Number
of Profiles
Mean,
102 g/m3
95%
Confidence
Interval
Number
of Profiles
Mean,
g/m3
95%
Confidence
Interval
Frigid Highlands
Cool Temperate Zone
Warm Temperate Zone
Temperate Desert Zone
Tropical and Subtropical Zone
113
237
461
92
681
9.7
8.0
8.8
10.0
8.2
1.0
1.0
1.3
1.7
1.4
102
193
413
82
594
5.8
5.2
8.9
6.3
4.8
1.4
2.1
1.4
2.1
0.9
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ZHANG ET AL.: POOLS AND DISTRIBUTIONS OF SOIL PHOSPHORUS
(8.9 ± 1.4 g/m3). The temperate desert zone in northwest
China has the highest total P density (10 ± 1.7 102 g/m3).
There may be many factors that control the total soil P
density. The hydrothermal conditions in the temperate
desert zone (low precipitation and temperature) may contribute to the preservation of total soil P content by limiting
the degree of soil weathering and P loss through surface soil
erosion.
[20] The tropical and subtropical regions in southeast
China have the second lowest total soil phosphorus density
(8.2 ± 1.4 102 g/m3), and the lowest soil available
phosphorus density (4.8 ± 0.9 g/m3). The low soil P density
in southeast China can also be observed from the total soil P
density map (Figure 3). Many previous investigators have
suggested that high temperature and precipitation in tropical
and subtropical regions enhance the soil weathering and the
P loss through soil erosion [Vitousek and Walker, 1987; Lu,
1990; Onthong et al., 1999; Neufeldt et al., 2000; Lehmann
et al., 2001]. In the Hawaii tropical forests, the P fertilization increased the forest net primary productivity by about
25% [Herbert and Fownes, 1995]. There were also reports
that the ecosystem productivity of tropical and subtropical
China is limited by its low soil P content [Peng and Zhao,
2000].
[21] The dramatic land-use change in recent years may be
an important control in the soil P content of tropical and
subtropical China where soil P content is relatively low [Liu
et al., 2002; Tian et al., 2003, 2005]. Previous analyses
showed that soil erosion could lead to a low P content in the
dry cultivated field, despite the P fertilizer applied into the
soil [He and Yue, 1984]. The nutrient loss through soil
erosion is significant in tropical and subtropical China
where annual precipitation is high [Yu and Peng, 1995].
In Hainan, a tropical province of China, the annual soil
erosion in a fallowed land is as high as 3200 g/m2 while the
erosion rate of a forest land is only 5 g/m2 per year [Peng
and Zhao, 2000]. The high soil erosion after deforestation
may deplete the total P budget. In Xiaoliang Tropical
Chinese Ecological Research Station, the P content of forest
soil is 9 times higher than that of deforested soil. From 1995
to 2000, about 90,724 hectares of forests are converted to
agriculture lands in southeast, most of which occurred in the
tropical-subtropical zone of China [Liu et al., 2002]. The
high erosion rate of deforested lands may threaten soil
productivity [Saugier et al., 2001; Davidson et al., 2002]
and even cause reforestation to be impossible without P
fertilization on such poor-P soils [Peng and Zhao, 2000].
Furthermore, the P input from the land has become a major
problem of eutrophication of aquatic ecosystem in China
[Jin, 1990, 2003].
4. Conclusion
[22] Our findings show that the average total P density
and available P density of the China’s soils are about 8.3 102 g/m3 and 5.4 g/m3, respectively. The total national soil
P pool in the surface half meter is 3.5 Pg. A general pattern
of decreased total soil P density with increased soil weathering state was apparent while little difference in available
soil P density was observed. A comparison of total soil P
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content in the upper 50 cm relative to total P in soils below
50 cm also varied with soil weathering state, demonstrating
an increasing relative amount of P in the upper soil with
weathering. General patterns of P content with climatic zone
also were apparent in the analysis, with tropical-subtropical
zone being relatively lower than most of the other zones.
Deforestation in the tropical-subtropical zone could lead to a
decrease in soil P.
[23] It is also recognized that since the structure of soil is
highly heterogeneous, estimations and comparisons of soil
P density may contain noticeable errors. The large variation
in the soil P content suggests that further study is needed to
determine the climatic and land-use controls over the soil P
content.
[24] Acknowledgments. This study has been supported by grants
from the Chinese Academy of Sciences (CAS), National Science Foundation of China (40128005), and NASA Interdisciplinary Science Program
(NNG04GM39C). We thank A. Chappelka, H. Chen, M. Andreae, and two
anonymous reviewers for very helpful comments and suggestions.
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ZHANG ET AL.: POOLS AND DISTRIBUTIONS OF SOIL PHOSPHORUS
Figure 3. Spatial distribution of total soil phosphorus density in China (g/m3).
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