Storage and Spatial Variation of Phosphorus in Paddy Soils of China

Pedosphere 19(6): 790–798, 2009 ISSN 1002-0160/CN 32-1315/P c 2009 Soil Science Society of China  Published by Elsevier Limited and Science Press

Storage and Spatial Variation of Phosphorus in Paddy Soils of China∗1 LIN Jin-Shi1,3,4 , SHI Xue-Zheng1,3,∗2 , LU Xi-Xi2 , YU Dong-Sheng1 , WANG Hong-Jie1 , ZHAO Yong-Cun1 and SUN Wei-Xia1 1 State Key Laboratory of Soil and Sustainable Agriculture, Institute of Soil Science, Chinese Academy of Sciences, Nanjing 210008 (China) 2 Department of Geography, National University of Singapore, 119260 (Singapore) 3 Graduate University of the Chinese Academy of Sciences, Beijing 100049 (China) 4 College of Resources and Environment, Fujian Agriculture and Forestry University, Fuzhou 350002 (China)

(Received November 26, 2008; revised July 23, 2009)

ABSTRACT Due to the growing concern about the agricultural phosphorus (P) losses pollution, an in-depth understanding of P in paddy soils of China would be helpful in providing a national perspective of the environmental impact of P cycling and fertility on China’s farms. In this study, we evaluated the P storage and the P density of paddy soils in China, characterized the spatial variations of P among the subgroups of paddy soils and soil regions in China, and evaluated the P data using GIS-based analysis, which included a newly compiled 1:1 000 000 digital soil map of China, and using 1 490 soil profiles. The available and total P densities of paddy soils were 6.7 and 698.5 g m−3 , respectively. Overall in China, the total P storage within 1 m of paddy soils was estimated to be 330.2 Tg. The P density of paddy soils varied substantially with subgroups due to the different soil water regimes such as groundwater table and soil drainage. The P availability in paddy soils, especially in surface layer, was higher in high temperature and precipitation areas. Further research is needed to examine more anthropogenic impact factors, such as increasing use of chemical fertilizer. Key Words:

paddy soil database, paddy soil subgroups, phosphorus density, phosphorus pool

Citation: Lin, J. S., Shi, X. Z., Lu, X. X., Yu, D. S., Wang, H. J., Zhao, Y. C. and Sun, W. X. 2007. Storage and spatial variation of phosphorus in paddy soils of China. Pedosphere. 19(6): 790–798.

INTRODUCTION Phosphorus (P) stored in soils across the world amounts to about 220 Pg (1 Pg = 1015 g) (Schlesinger, 1997). It is one of the essential nutrients for plant growth, and its input is necessary to maintain profitable crop productivity. Using P fertilizer can increase root growth and accelerate nitrogen absorption by the plant (Rao et al., 1986; Bowatte et al., 2006). Soil P can also influence rice field’s methane emission (Rath et al., 2005). Soil carbon and nitrogen cycles seem to be regulated by the pool of P in many agricultural ecosystems. Cultivation of soils can cause leaching of P and soil degradation (Saavedra et al., 2007), and increasing input of P into agriculture soils has become an important environmental problem, including surface water eutrophication. The P losses from agricultural soils have being reported in many parts of China, such as the Taihu Lake region and the Yangtze River Delta (Zhang et al., 2003; Zhou and Zhu, 2003; Shan et al., 2005). There is growing concern about agricultural P losses pollution (He et al., 2006; Penn et al., 2006; Heredia and Cirelli, 2007). Therefore, from both ecosystem research and management perspectives, it is important to investigate the pool of soil P in agricultural ecosystems at regional or national scales. ∗1 Project

supported by the National Key Basic Research Program (973 Program) of China (No. 2007CB407206), the National Natural Science Foundation of China (No. 40621001), and the Frontier Project of the Chinese Academy of Sciences (No. ISSASIP0715). ∗2 Corresponding author. E-mail: [email protected].

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Total P (TP) gradually decreases as the results of weathering and soil erosion (Onthong et al., 1999; Neufeldt et al., 2000). Soil P is primarily derived from rock and phosphate; hence parent material is a major impact factor of soil P content. In addition, other factors, such as climate and biota, also play an important role in controlling the soil P content, creating a highly heterogeneous temporal and spatial pattern of soil P. Compared to comprehensive evaluations of soil carbon and nitrogen, there are only a few studies which examined the large-scale P cycles in agricultural ecosystems (Smil, 2000; Zhang et al., 2005). At the global scale, Stevenson and Cole (1999) assumed that the average TP concentration of global soil was 0.05% and estimated that the storage of P in 0–50 cm global soil was 50 Pg and the average TP density was 375 g m−3 . Smil (2000) estimated that P storage in 1.5 × 103 Mha of global arable soils was 5–6 Pg. At the regional level, Zhang et al. (2005) used the data from 2 400 soil profiles and the 1:1 000 000 soil map to estimate the TP storage of 3.5 Pg for the upper 0.5 m soils across China. As for paddy soils, Zhang et al. (2005) used the data from 421 soil profiles and estimated the TP storage to be 98 Tg (1 Tg = 1012 g). At the field level, Jiang et al. (2005) examined the spatial distribution of TP and Olsen-P by using traditional statistics in combination with geostatistics methods. They concluded that Olsen-P had higher variation coefficients than TP, though both TP and Olsen-P had similar spatial patterns, and that the variables, such as parent material, terrain and underground water level, were major impact factors for soil phosphorus spatial distribution. Although all these studies provided useful information about soil P storage in China, there are some uncertainties. For example, the previous studies used limited soil profile numbers, which can be problematic, given the heterogeneity of soil P as affected by parent material, climate, biota, and human activity. To reduce such uncertainty, further research on P storage at the regional scale is necessary. This study used the newly compiled 1:1 000 000 digital soil map of China (Shi et al., 2004) and more soil profiles than earlier studies. The aims of our study were to estimate P storage and P density in Chinese paddy soils and characterize the spatial variations of P among the subgroups of paddy soils and soil regions across China. Special attention was paid to the investigation of the availability of P in Chinese paddy soils and different soil regions. MATERIALS AND METHODS Chinese paddy soils As major cultivated soils in China, the total area of paddy soils is 46 Mha (Liu et al., 2006). Paddy soils are very important food source worldwide and produce about 30.6% of total grain production (Yu, 2004). With 7 000 years of rice cultivation in China, human cultivation has caused a wide variety of characteristics of paddy soils due to frequent manipulation of water movement. As a unique type of anthropogenic soil, paddy soil was categorized into eight subgroups in Genetic Soil Classification of China (Li, 1992) (Table I). The soil map of China (1:1 000 000) was used in this study. Categories of the map consist of eight soil regions which were classified on the basis of the environmental factors, such as climate and other factors of influencing soil pedogenesis (EBPGC, 1981). Given that a small number of the isolated paddy fields are located in North China, only six major paddy soil regions were shown in Fig. 1 (Li, 1992). Data source In this study, the data used was obtained from the second national soil survey of China conducted in the 1980s. The paddy soil polygons were digitized from the 1:1 000 000 soil map of China (Shi et al., 2004). The precision of the database is two orders higher than that of the 1:4 000 000 soil map, which was used in earlier studies (Pan, 1999). The mapping units of paddy soils on the 1:1 000 000 soil map are soil families. In total, 18 162 polygons of paddy soils were identified on the 1:1 000 000 soil map, which are much more than the 238 polygons found on the 1:4 000 000 soil map. The soil attribute data of 1 490 paddy soil profiles was used in this study, doubling the number of soil profiles used in comparable studies

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TABLE I The subgroups of paddy soils in China Subgroup

Reference to World Reference Basea)

Horizonsb)

Descriptions

Hydromorphic

Hydragric Anthrosols

Aa-Ap-P-W-G-C

Submergenic

Hydragric Anthrosols

Aa-Ap-P-C

Percogenic

Hydragric Anthrosols

Aa-Ap-C

Gleyed

Gleyic-Hydragric Anthrosols

Aa-Ap-G-C

Degleyed

Gleyic-Hydragric Anthrosols

Aa-Ap-Gw-G

Bleached

Hydragric Anthrosols

Aa-P-E-C

Salinized

Hydragric Anthrosols

Aa-Ap-G

Acid sulfate

Fluvic Cambisols

Aa-Ap-Ds-G

Mainly distributed in floodplain, long cultivation history, well-drained, underground water level was below 90 cm, neutral soil reaction Mainly distributed in alluvial plain or low flat ground, moderate drainage, underground water level was below 60 cm, neutral soil reaction Mainly distributed on gentle hill slopes, no underground water, associated with rain-fed paddy fields, neutral to slightly acidic soil reaction Mainly distributed in depression areas, high underground water level, poorly drained, distinct gleyization, slightly acidic soil reaction Same distribution area as gleyed paddy soils, the manmade drainage land, the lower underground water level leading to degley processes, slightly acidic soil reaction Mainly distributed in foothills, usually no underground water, impervious layer at 60 cm depth, neutral or slightly acid soil reaction Mainly distributed in coastal lands, high underground water level, slightly alkaline soil reaction Mainly distributed in alluvial lands, surface layer is stream sediment, subsoil is acid sulfate soil, acidic soil reaction

a) Cited

from Shi et al. (2004, 2006). to Genetic Soil Classification of China, Aa is arable layer, Ap is plow pan, C is undeveloped parent material, Ds is fragmental deposit horizon, E is bleached horizon, G is gley horizon, Gw is degley horizon, P is percogenic horizon, and W is waterlogged horizon (Xi, 1994). b) According

Fig. 1 Location of soil profiles and six paddy soils regions of China (Liu et al., 2006). Numbers 1 to 6 represent south, southeast, Yangtze River, central, north, and southwest regions of China, respectively. This basic map was downloaded from the State Bureau of Surveying and Mapping of China.

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(Pan et al., 2004; Zhang et al., 2005). A total of 525 of the 1 490 profiles were obtained from the book “Soil Species of China” (National Soil Survey Office, 1993, 1994a, 1994b, 1995a, 1995b, 1996). The remaining profiles were from soil survey of individual provinces. The soil attribute data include extensive information, such as soil names, profile locations, bulk density, horizon depth, total nitrogen, organic matter, and TP. Sample analysis In accordance with the 1980 technical specifications of the second national soil survey (Technical Committee of Soil Survey, 1986), each soil sample was collected in 1980. According to the soil analysis methods (Institute of Soil Science, 1978), perchloric acid digestion followed by molybdate colorimetric test was used for TP analysis. In addition, the Olsen method was used to analyze available P. The Bray I method was included for the assessment of acidic soil samples. Bulk density was determined using a soil cylinder with a fixed volume. There is a geographic location in latitude and longitude for each paddy soil profile according to the national soil survey data (National Soil Survey Office, 1993, 1994a, 1994b, 1995a, 1995b, 1996). Using ArcGIS 9.0, the 1:1 000 000 paddy soil database of China was developed by linking the soil attribute database with the soil spatial database using a pedological professional knowledge-base method (PKB) (Zhao et al., 2006; Yu et al., 2007). Phosphorus storage was estimated by combining the P concentration of each soil type with the distribution area of each paddy soil type in China. For an individual soil profile with n layers, P density was calculated using Eq. 1: n 

Pj =

Hi Bi Oi

i=1

n 

(1) Hi

i=1

where Pj is the P density of profile j, Hi is the thickness (cm) of horizon i, Bi is the bulk density (g cm3 ) of horizon i, and Oi is the TP concentration (g kg−1 ) of horizon i. The P storage of paddy soils was calculated using Eq. 2: PS =

n 

Pj Sj Hj

(2)

j=1

where PS is P storage of paddy soils, n is profile number of paddy soils, Sj is the distribution area (ha) of profile j, and Hj is the thickness (cm) of profile j. The P density of paddy soils was calculated using Eq. 3: PDk =

PSk Sk

(3)

where PDk is the P density (g cm−3 ) of paddy soil k, PSk is the P storage (Tg) of paddy soil k, and Sk is the distribution area (ha) of paddy soil k. Eq. 1 was used to calculate the soil P density of each soil profile while Eqs. 2 and 3 were used to calculate available P density (APD), total P density (TPD), and total P storage (TPS). The P storage and P density of paddy soils were estimated at the depths of 0–20 and 0–100 cm. GIS and statistical software were used for spatial and statistical analyses for different soil subgroups and various soil regions. RESULTS AND DISCUSSION Phosphorus density and storage in Chinese paddy soils Statistical results showed a large variation of TPD and APD, as indicated by the coefficient of

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variation (CV) calculated from polygons of paddy soils (Table II). The difference was over 1 400 times between the highest and lowest values of APD at the 0–20 cm depth. The mean APD for the 0–100 cm depth was 6.7 g m−3 . This was close to Zhang’s result of 6.2 g m−3 (Zhang et al., 2005). As for the TPD, the difference between the highest and the lowest values in 0–20 cm depth was about 190 times. The mean TPD for the 0–100 cm depth was 698.5 g m−3 , which was slightly higher than the results obtained from Zhang’s study (660 g m−3 ). TABLE II Descriptive statistical for the total P density and available P density calculated from polygons of Chinese paddy soils P density Available Total

Depth cm 0–20 0–100 0–20 0–100

Minimum

Maximum

Mean g

0.1 0.1 50.0 28.9

Standard deviation

Coefficient of variation

7.0 5.8 555.6 480.0

% 78.3 86.0 75.7 68.7

m−3

147.6 215.9 9 640 6 477

8.9 6.7 734.1 698.5

The TPS was 68.2 Tg for the 0–20 cm depth and was 330.2 Tg for the 0–100 cm (Table III). The surface layer’s TPS accounted for 20.6% of the 0–100 cm TPS. This indicated that no remarkable concentration of TP resided in the surface layer. Our estimation of TPS in the top 1 m soils was much higher than the estimate of 98 Tg obtained by Zhang et al. (2005). Two possibilities help explain the differences between the studies: 1) different estimate depth. Our estimate depth was 1 m, but Zhang et al. (2005) only used 50 cm; 2) the number of soil profiles used in our estimation is about three times that used by Zhang et al. Researches by Batjes (2000) and Liu et al. (2006) also indicated that diverse methods and different data sources could cause variability. TABLE III Total P and available P storage in subgroups of paddy soils in China Paddy soil

Area

Available P storage 0–20 cm

106

Hydromorphic Percogenic Submergenic Gleyed Degleyed Bleached Salinized Acid sulfate Total

× ha 23.87 7.93 7.11 3.27 1.70 1.07 0.64 0.10 45.69

× 45.5 12.4 14.1 4.3 3.8 1.0 1.4 0.2 82.7

10−2

Total P storage 0–100 cm

0–20 cm

176.1 44.0 53.9 20.1 14.5 4.3 7.7 0.7 321.3

38.3 10.8 11.1 4.0 2.0 1.1 0.8 0.1 68.2

Tg

0–100 cm Tg 185.6 51.8 53.1 21.0 9.4 4.5 4.3 0.5 330.2

Phosphorus density and storage in subgroups of paddy soils Fig. 2 indicated that all eight subgroups had a higher APD in 0–20 cm than in 0–100 cm, and that the APD varied with subgroups of paddy soils. The hydromorphic, percogenic, submergenic, and degleyed paddy soils in 0–20 cm had a higher APD than other subgroups, and the variation between the surface layer and the whole profile was also higher. The APD variation between the surface and the whole profile, however, was quite small for gleyed and salinized paddy soils. Previous investigators suggested that waterlogged condition could enhance soil reduction and high soil reducibility increased release of available P (Miller et al., 2001; Liu et al., 2003). Therefore, in waterlogged gleyed and salinized paddy soils (mainly distributed in depression area), the APD distributions in the whole profile were homogeneous. Submergenic and bleached paddy soils, mainly distributed in foothill belt without groundwater

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Fig. 2 Phosphorus density varied in the eight subgroups of paddy soils in China. The box plot shows 5th/95th percentile of P density in paddy soils and bold black bar represents mean value of P density in paddy soils. Letters a to h represent hydromorphic, percogenic, submergenic, gleyed, degleyed, bleached, salinized, and acid sulfate paddy soils, respectively.

interaction, had a lower APD in 0–100 cm than other subgroups. There was no obvious relationship between TPD and underground water regime of paddy soils. The hydromorphic, percogenic, and degleyed paddy soils had a higher TPD values and variations than other subgroups. The highest mean TPD of 0–20 cm paddy soils was found in hydromorphic paddy soil (840.7 g m−3 ). Other than percogenic and degleyed paddy soils, the other six subgroups had a higher TPD in 0–20 cm than in 0–100 cm. The TPD’s difference between surface and subsurface soils was less than the APD’s. The hydromorphic, submergenic, percogenic, and gleyed paddy soil subgroups accounted for 92.3% of the total paddy soils area (Table III). Thus, the TPS of these four subgroups accounted for a large majority of TP (94.4%) in Chinese paddy soils. Phosphorus density in soil regions China is characterized by a variable climate, ranging from tropical to cool temperate zone (Yu et al., 2007). The P density varied substantially with soil regions (Fig. 3). The surface available P was lower in the middle Sichuan basin. The majority of paddy soils had a higher APD in 0–20 cm than 0–100 cm. In the middle Yangtze River and part of the southwest region, the APD in 0–100 cm was higher. The TPD was higher in the middle Yangtze River, Dongting and Poyang Lake areas, and the scattered areas in Sichuan basin and the southwest region. Lower TPD (below 500 g m−3 ) regions were mainly distributed in coastal areas and most of central south region. There was no obvious difference in the TPD distribution pattern between the 0–20 and 0–100 cm layers. The spatial distribution of P density showed substantial variation across China. Summary statistics also indicated that the south region had the highest mean APD (10.9 g m−3 ) in the depth of 0–20 cm, and the central south region had the lowest APD (6.7 g m−3 ) (Fig. 4). The variation ranges for APD in both profiles were different among the six soil regions. In the south, southeast, and central regions, APD in 0–20 cm was over 20% higher than that in 0–100 cm, but in north region, it was only 6% higher. As for TPD, the southwest and north regions had higher TPD than other regions. There are many factors that control soil TPD, such as degree of soil weathering controlled by the hydrothermal conditions, soil age, and parent material (Gardner, 1990). The low hydrothermal conditions contributed to the preservation of TP. The low soil P density regions (i.e., south and southeast region) are located in the high temperature and precipitation area, which was also indicated by Zhang et al. (2005). The ratio between available P and total P indicates the availability of soil P to plants. Fig. 5 indicated that both temperature and rainfall had an impact on the ratios. The availability of P in paddy soils was significantly decreased from south to north and also slightly decreased from east to west. In south China under high annual temperature and precipitation, P availability in paddy soils was higher than

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Spatial distribution of available P (a and b) and total P densities (c and d) of main paddy soils in China.

Fig. 4 Phosphorus density variations with physiographic regions. Numbers 1 to 6 represent south, southeast, Yangtze River, central, southwest, and north regions of China, respectively. The box plot shows 5th/95th percentile of P density in paddy soils and bold black bar represents mean value of P density in paddy soils.

in north China. The coastal region had a slightly higher P availability than the inland southwest and central regions. In south, southeast, and Yangtze River regions P availability in 0–20 cm was 18% to 38% higher than that in 0–100 cm. Phosphorus in paddy soils, especially in the surface layer (0–20 cm), is easily weathered and released under high temperature and precipitation. P availability is increased because a moist environment can increase P release from rock and high temperatures promote the activity of soil microorganisms. Therefore, climate plays an important role in controlling soil P content. This is consistent with previous studies, which suggested that high temperature and precipitation in tropical and subtropical regions enhanced soil weathering and the P loss (Onthong et al., 1999; Neufeldt et al., 2000; Lehmann et al., 2001). Water stored in paddy soils often undergoes eutrophication, partly because of the input of exce-

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Fig. 5 Changes in the ratios of available P to total P, and mean annual temperature and precipitation among soil regions in China. Numbers 1 to 6 represent south, southeast, Yangtze River, central, north, and southwest regions of China, respectively. The box plot shows 5th/95th percentile of P density in paddy soils and bold black bar represents mean value of P density in paddy soils.

ssive P from paddy soils (Zhou et al., 2003; Wang et al., 2006). Thus, it is necessary to use P fertilizer in an appropriate way (such as rational fertilization) to reduce environmental problems caused by P loss in paddy soils. CONCLUSIONS Using the newly developed soil map of China and more soil profiles than the previous work, we estimated that the mean APD and TPD of paddy soils in 0–20 cm were 8.9 and 734.1 g m−3 , and in 0–100 cm were 6.7 and 698.5 g m−3 , respectively. The reliability of P storage provided by this research is higher than previous research in China. Waterlogged condition increased the APD for the surface layer and contributed to the homogeneous distribution of the APD in the whole profile for paddy soils. The availability of P in paddy soils increased with increasing temperature and precipitation. Therefore, we should pay close attention to rational fertilization in tropical area’s paddy soils. Although this GISbased analysis is a relatively coarse evaluation of the database, it does provide the first quantitative assessment of P storage in paddy soils across China using commonly available digital databases. Due to the variation of P in paddy soils affected by many factors, a further study is needed to determine the anthropogenic factor such as fertilizer use and land-use, which controls P content in paddy soils. REFERENCES Batjes, N. H. 2000. Effects of mapped variation in soil conditions on estimates of soil carbon and nitrogen stocks for South America. Geoderma. 97(1-2): 135–144. Bowatte, S., Tillman, R., Carran, A. and Gillingham, A. 2006. Can phosphorus fertilisers alone increase levels of soil nitrogen in New Zealand hill country pastures? Nutr. Cycl. Agroecosys. 75(1-3): 57–66. Editorial Board for Physical Geography of China (EBPGC). 1981. Physical Geography of China (in Chinese). Science Press, Beijing. Gardner, L. R. 1990. The role of rock weathering in the phosphorus budget of terrestrial watersheds. Biogeochemistry. 11(2): 97–110. He, Z. L., Zhang, M. K., Stoffella, P. J., Yang, X. E. and Banks, D. J. 2006. Phosphorus concentrations and loads in runoff water under crop production. Soil Sci. Soc. Am. J. 70(5): 1807–1816. Heredia, O. S. and Cirelli, A. F. 2007. Environmental risks of increasing phosphorus addition in relation to soil sorption capacity. Geoderma. 137(3-4): 426–431. Institute of Soil Science. 1978. Methods of Soil Analysis (in Chinese). Shanghai Science and Technology Press, Shanghai. Jiang, Y., Liang, W. J. and Zhang, Y. G. 2005. Spatial variability of soil phosphorus in field scale. Chinese J. Appl. Ecol. (in Chinese). 16(11): 2086–2091. Lehmann, J., G¨ unther, D., Socorro da Mota, M., Pereira de Almeida, M., Zech, W. and Kaiser, K. 2001. Inorganic and organic soil phosphorus and sulfur pools in an Amazonian multistrata agroforestry system. Agroforest. Syst. 53(2): 113–124.

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