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Storage and spatial patterns of organic carbon of soil profiles in Guangdong province, China.

Introduction

Global soil organic carbon (SOC) storage is within the range 1200-2500 Pg, which is twice the C in the atmosphere, and two- to threefold greater than that stored in vegetation (Intergovernmental Panel on Climate Change (IPCC) 1990; Schlesinger 1991; Lai 1999; Watson and Noble 2001). SOC plays a significant role in the global C cycle and global warming, and is an important determinant of the physical and chemical properties, as well as productivity, of agricultural soils (Singh et al. 2007). Many factors affect SOC content, such as parent materials, soil texture, climate, soil pH, land use, management, topography and drainage. Optimisation of some factors, especially land use, management and drainage, can increase C sequestration in soils and mitigate climate change (Smith et al. 2000; Lai 2004).

Although routine soil surveys collect C stock data to a depth of 1 m, some studies have focussed primarily on the A horizon, which usually contains the highest soil organic matter (SOM) content (Pan et al. 2003; Wang et al. 2003; Yan et al. 2007; Yu et al. 2007). SOC storage in the 30-100 cm soil horizon accounts for 46-63% of total SOC storage in the first 1 m of the world's soil (Batjes 1996). Tarnocai et al. (2009) observed that, in the northern circumpolar, at least 61 % of total soil C is stored below 30 cm depth. Subsoil C may be more important as a C source or sink than surface soil C (Rumpel and Kogel-Knabner 2011). Subsoil horizons with low C contents may not be saturated in organic C; therefore, they may have the potential to sequester organic C, for example through higher C input into subsoil by roots and dissolved organic carbon (DOC; Lorenz and Lai 2005). Recent studies have concentrated on the stabilisation of organic matter (OM) in subsoils, where subsoil C may become available for microbial decomposition bccause of new C input (Fontaine et al. 2007) or respond to land use or management changes (Guo and Gifford 2002; Wright et al. 2007; Wiesmeier et al. 2015).

SOC storage in China has been reported at the national level. Fang et al. (1996) estimated the national SOC storage at 185.7 Pg, whereas the estimate of Pan (1999) of 50 Pg C represents the lowest value among all the published data for China. Other estimated values are 92.4 Pg (Wang et al. 2001), 70.3 Pg (Wu et al. 2003), 89.14 Pg (Yu et al. 2007) and 83.8 Pg (Li et al. 2007). Few studies have investigated subsoil organic C of soil profiles on a large scale. Because of significant uncertainty induccd by the large variability in soil characteristics throughout the country, it is ncccssary to further estimate SOC at a regional scale. The present study was conducted to: (1) estimate the SOC storage for different soil types and land uses in Guangdong Province; (2) investigate movement trends of SOC from the surface soil to lower horizons in the study area; (3) evaluate the contribution of soil degradation and erosion as driving forces of SOC redistribution and export to the surrounding water. Such results would provide useful information for a rational estimate of SOC storage and environmentally sound management of SOC in the study area.

Materials and methods

Study area

Guangdong Province is located in the southern part of south China (Fig. lb; 20[degrees]10'-25[degrees]31'N, 109[degrees]41'-l 17[degrees]17'E). The area has a subtropical monsoon climate with an average annual precipitation of 1336 mm, average annual evaporation of 1100 mm, average annual temperature ranges between 17 and 27[degrees]C and average sunshine of 1828 h. From north to south, the landscape changes from mountain to plateau to flood plain. Hilly areas occupy approximately 60% of the total land area.

The soil area of Guangdong Province is 150 457 [km.sup.2]. The forest area is approximately 116 465 [km.sup.2], accounting for 77.4% of the total soil area, with a high diversity of forest types (Zhou et al. 2008). The area of paddy soil is 16 652 [km.sup.2], accounting for 11.0% of the total area. Pasture and vegetables occupy 10.4% of the total soil area (Fig. la).

Based on the Chinese Soil Taxonomy (National Soil Survey Office (NSSO) 1998), the soils in the study area are classified as Ferrallisols, Anthrosols, Amorphic soils and some other soil types (e.g. semi-hydromorphic soil, hydromorphic soil and saline-alkali soil). Ferrallisols are the most widely distributed soil type across the study area, covering 113 540 [km.sup.2] or approximately 75.5% of the total soil area. The Ferrallisols include four soil great groups: Latosols (main land uses forest and dryland), corresponding to Acrisols in the World Reference Base for Soil Resources by IUSS Working Group WRB (2014); Latosolic Red soils (main land uses forest and dryland), also corresponding to Acrisols (IUSS Working Group WRB 2014); Red soils (main land use forest), corresponding to Cambisols (IUSS Working Group WRB 2014); and Yellow soils (main land use forest), also corresponding to Cambisols (IUSS Working Group WRB 2014). These soils have obvious zonal features across the study area. The horizontal zonal feature consists of (from south to north) Latosols, Latosolic Red soils and Red soils. The vertical zonal feature is that soil groups change from Latosolic Red soils to Red soils to Yellow soils with increasing elevation (Fig. 1 d).

Paddy soils are the major soil group of the Anthrosol class in the study area, and are a unique soil type in China's taxonomy, with a long history of rice cultivation under irrigation (Gong 1999). Generally, a paddy soil profile consists of a cultivated horizon overlying a plough pan. The cultivated horizon shows sorted soil aggregates, vesicular pores and a specific Munsell colour. The plough pan (P-horizon) has a platy structure and a bulk density that is at least 10% higher than that of the cultivated horizon (Huang et al. 2015). Ploughing and puddling are the most common practices of land preparation for rice cultivation. Repeated ploughing and puddling by water buffalos or machines over long periods of time lead to the formation of a compacted P-pan, which reduces percolation and improves water and nutrient efficiency in the paddy fields. Fertiliser application is another important management practice for paddy cultivation to improve soil fertility and increase ricc yield.

The Amorphic soils are the third soil type in the study area, covering an area of 16 502 [km.sup.2] or approximately 10.9% of the total soil area. These soils correspond to the IUSS Working Group WRB's (2014) Cambisols. The other soil classes (semihydromorphic soil, hydromorphic soil and saline-alkali soil) occupy only approximately 3.2% of the total soil area (Fig. 1 d).

Field sampling

Soil samples used in the present study were collected from locations shown in Fig. 1 c. All 211 sample sites were located far from contaminated or urbanised areas. Each soil profile was divided into A, B and C horizons, according to the pedogenic process. All soil samples, free of plant roots, were air dried and crushed by a wooden stick, and then one portion, used for analysis of SOM and total N, was sieved through a 20-mesh nylon screen (aperture size 1 mm) and stored at room temperature (25[degrees]C). A second portion was sieved through a 2-mm nylon screen for analysis of grain size fraction.

Chemical analyses

The SOM content was measured using wet combustion with [Cr.sub.2][O.sub.7.sup.2-] (Walkley and Black 1934; Wen 1984). Total N was measured using the Kjeldahl method (Liu 1996). The pH of soil was measured by placing 10 g sample in 25 mL deionised water (Chinese National Standard Agency 1998). Soil bulk density was measured using the cylindrical core method (Blake and Hartge 1986). Soil particlc diameters (sand, silt and clay) were measured based on wet and dry sieving techniques and the pipette method (Gee and Bauder 1986).

Calculation of SOC density and SOC storage

SOC density (SOCD) was calculated using the following equation:

[SOCD.sub.ij] = [(1 - [[delta].sub.ij]%) x [[rho].sub.ij] x [C.sub.ij] x [T.sub.ij]]/100

where [SOCD.sub.ij] is the SOC density in the j horizon of the i soil profile, [[delta].sub.ij]% is the volumetric percentage of the >2 mm grain size fraction in the j horizon of the i soil profile, [[rho].sub.ij] is the bulk density in the j horizon of the i soil profile, [C.sub.ij] is the organic C content in the j horizon of the i soil profile and [T.sub.ij] is the thickness of the j horizon in the i soil profile. The organic C content was calculated by multiplying the SOM content by 0.58 (Hollis et al. 2012).

SOC storage in the i soil profile (SOCS,) and total SOC storage in the study area ([SOCS.sub.Total]) were calculated using the following equations:

[SOCS.sub.i] = [3.summation over (j=1)] [SOCD.sub.ij] x [Area.sub.i]

[SOCS.sub.TOTAL] = [N.summation over (i=1)][3.summation over (j=1)][SOCD.sub.ij] X [Area.sub.i]

where [Area.sub.i] represents the area of the i soil profile.

Data processing and statistical analyses

Because all data fit a log normal distribution with a positive skew value (Table 1; Fig. 2), SOC and soil total N contents were log transformed, and the geometric mean was used to represent the central tendency and variation of the data. Minitab version 14 (Minitab Inc, State College, Pennsylvania, USA) was used for all probability and statistical analyses of soil samples.

Results and discussion

SOC concentration

SOC concentrations in A-horizon soils ranged from 0.04% to 5.03%, with a geometric mean of 1.18% and an arithmetic mean of 1.45%. In soils from the B horizon, the SOC concentration ranged from 0.02% to 5.24%, with a geometric mean of 0.71% and an arithmetic mean of 0.94%. The SOC concentrations in the C horizon ranged from 0.01% to 4.85%, with a geometric mean of 0.45% and an arithmetic mean of 0.66%. The highest SOC concentration was 5.24%, which was observed in the P-pan of paddy soil. All data had a high positive skew in each horizon soil (Table 1).

Soil properties

From the A to C horizons, soil density increased slightly from 1.16 to 1.17 to 1.20gem 3 respectively (Table 1). The highest soil density was in the P-pan of paddy soil (1.8 g [cm.sup.-3]). Yellow soils had the lowest soil density (arithmetic mean 0.74g [cm.sup.-3]) and highest SOC content (arithmetic mean 3.15%), whereas the Latosols had the highest soil density (arithmetic mean 1.26g [cm.sup.-3]) and the lowest SOC content (arithmetic mean 0.94%).

The mean soil clay content in the A, B and C horizons was 20.45%, 25.27% and 25.93% respectively (Table 1). The highest clay content was 85.22% in the Latosolic Red soils. Soil samples with a higher SOC content generally had a clay content in the range 10-30%. Moreover, in Latosols soils and Latosolic Red soils (WRB Acrisols), the SOC content had a better relationship with a soil clay content than in the other soil types. The possible reason for this phenomenon is that soil organic matter tended to be adsorbed by soil clay under conditions of intensive eluviation.

The mean thickness of the soil horizons increased from 17.0 cm in the A horizon to 29.5 and 48.9 cm in the B and C horizons respectively (Table 1). The highest value was 137 cm in the B horizon for Ferrallisols and Red soil great groups located in the north of the study area. Red soils had the highest soil thickness (arithmetic mean 72 cm). Thus, although Red soils had a low SOC content (arithmetic mean 0.59%), their SOCD was as high as 4.83 kg C [m.sup.-2].

Soil pH values in the A, B and C horizons were 5.77, 5.92 and 6.05 respectively (Table 1). Paddy soils had the lowest soil pH values in the cultivated horizon (3.4), P-pan (3.0) and C-horizon (2.8) in the study area.

Concentration, density and storage of SOC in soil profiles

In the study area, the geometric mean concentration of SOC in A-horizon soils was 1.18%, ranging from 0.04% to 5.03%. The geometric mean SOC concentrations in the B and C horizons were 0.71% and 0.44% respectively (Table 1). SOC storage in the total soil profiles was 1.25 Pg, of which approximately 0.41 Pg SOC was in A-horizon soils, 0.51 Pg SOC was in B-horizon soils and 0.33 Pg SOC was in C-horizon soils. The subsoil was the primary location for total SOC storage in the study area. The mean SOC density of 8.31 kg C [m.sup.-2] was close to that reported by Wu et al. (2003) in China (8.01 kg C [m.sup.-2]), but lower than that reported by Wang et al. (2001), i.e. 10.53 kg C [m.sup.-2].

In the study area, soil samples with high SOC density, primarily forest soils and paddy soils, were distributed in the north of Guangdong Province and soil samples with a low SOC density were distributed in the southern region of the study area (Fig. 3). This regional difference in SOC density was closely related to regional hydrothermal changes from south to north in the study area. The average annual temperature in the north of Guangdong Province is approximately 18.8[degrees]C, but in the Leizhou Peninsula (in the south of Guangdong Province) the average annual temperature is 23.2[degrees]C. Thus, the regional difference in SOC density observed in the present study may be the result of differences in the mineralisation rate of SOC controlled by regional temperature (Lai 2004). As such, this further confirms that SOC content is dependent on biomass production and the mineralisation intensity of organic matter in different soil types, which are controlled primarily by hydrothermal conditions (NSSO 1998; Wu et al. 2003). Although soil clay should have some effect on changes in SOC, only weak positive correlations were found in the present study between soil clay content and SOC (Table 2).Thesc weaker correlations may be the result of the very rapid weathering of parent rock in the study area. Because of the short soil formation process and intensive weathering, the soil clay content was predominantly controlled by the mineral components and texture of the parent rocks (GSGIO 1993; Zhang et al. 2008). Moreover, for paddy soils with a high SOC density in the northern region of the study area, the classical cultivation method consists of rice field-upland field rotation. A considerable proportion of rice straw is removed from soils after harvesting to make it easier to plant vegetables. The input of organic matter to soils is generally dependent on the use of fertilisers. Thus, agricultural activity, such as cultivation and fertilisation, are also important causes of differences in SOC density.

SOC content, density and storage by soil type Ferrallisols

The Ferrallisol area was 113 540 [km.sup.2], approximately 75.5% of the total soil area in the study area. Ferrallisols include four soil groups (Latosols, Latosolic Red soils, Red soils and Yellow soils), as described in the Chinese soil taxonomy (NSSO 1998).

Hydrothermal conditions have an obvious influence on changes in SOC content in the study area. From south to north, the temperature and rainfall in the study area decreased gradually. As the leaching and mineralisation intensity weakened along this gradient, the mean SOC content and mean SOC density of A-horizon soils increased; from Latosols to Latosolic Red soils to Ted soils, SOC content increased from 0.94% to 1.08% to 1.52% respectively, whereas SOCD increased from 0.91 to 2.15 to 2.53 kg C[m.sup.2] (Table 3). The Yellow soils in the study area generally developed in areas at high elevation (>800 m) because of the cooler conditions there that are favourable to the accumulation of soil organic matter. The mean A-horizon SOC content in the yellow soils was 3.15% and the mean SOC density and profile SOC density were 4.45 kg C[m.sup.-2] and 11.23 kg C[m.sup.-2], respectively.

In the study area, the SOC densities of the subsoil were significantly higher than in A-horizon soils, except for Yellow soils (Table 3). From Latosols, Latosolic Red soils to Red soils, the SOC storage of subsoils was approximately 3.4-, 2.7- and 1.9-fold higher than in corresponding A-horizon soils respectively. However, in the case of Yellow soils, the subsoil organic carbon storage was lower than in the A-horizon soil because of the higher SOC content of A-horizon soils, thinner soil thickness of B-horizon soils and weaker leaching compared with other soils. The profile SOC density increased with soil type from Latosols to Latosolic Red soils to Red soils and to Yellow soils as follows: 4.74, 8.11, 9.66 and 11.23 kg C[m.sup.-2] respectively. The profile SOC density should be closely related to zonal hydrothermal conditions. With a decrease in both rainfall and temperature, the leaching and mineralisation intensity weakened, and so SOC content and SOC density in the profiles increased along this gradient. As a result, SOC was primarily controlled by hydrothermal conditions in the study area.

For the Ferrallisols, the profile SOC density was approximately 8.60 kg C[m.sup.-2], slightly higher than the average level of Chinese soil (-8.0 kg C [m.sup.-2]; Wu et al. 2003). SOC storage in Ferrallisols was approximately 0.976 Pg for the total soil profile, accounting for 78.1% of total SOC storage, of which 0.313 Pg SOC was in A-horizon soils. The SOC storage of the subsoil was 2.12-fold higher than that of A-horizon soils in the study area.

Anthrosols

Paddy soil is the major soil group of the Anthrosol class in the study area. The area of paddy soils accounted for approximately 11.0% of the total soil area. The SOC storage in paddy soils was 0.17 Pg, accounting for 13.47% of total SOC storage (Table 3), of which 0.054 Pg SOC was in the upper plough horizon, 0.044 Pg SOC was in the P-pan horizon and 0.07 Pg SOC was in the C-horizon.

Previous studies have indicated that both a high input of organic material (Yang et al. 2005) and reduccd decomposition rates under anaerobic conditions (Sahrawat 2003) contribute to the accumulation of organic carbon in paddy soils. In addition, some studies have shown that long-term rice cultivation could result in a downward transport of organic carbon from the surface horizon and its accumulation in subsoils (Brauer et al. 2013, 2012; Ci and Yang 2013). In the study area, the mean SOC content in C-horizon soils (0.81%) and in the P-pan horizon (1.14%) was much higher than in other soil types. This confirms that, at the regional scale, the SOC of surface soils in paddy soils can be transferred to the subsoil horizons more easily due to long-term ricc cultivation. As a result, the mean SOC density in the C-horizon was up to 4.74 kg C[m.sup.-2], much higher than that in the A-horizon soils (3.09 kg C[m.sup.-2]) and P-pan soils (2.73 kg C[m.sup.-2]). The profile SOC density was 10.21kgC[m.sup.-2], higher than that of the Ferrallisols (8.60 kg C[m.sup.-2]). The SOC storage of subsoils was approximately 2.1-fold higher than in the A-horizon soils in the paddy soil area.

Amorphic soils

The Amorphic soil area was 16 502 [km.sup.2], accounting for 10.9% of the total soil area. These soils are generally part of the Inceptisols in the soil classification system (Soil Survey Staff 1999). The SOC storage in Amorphic soils was 0.083 Pg (Table 3). The thickness of the A-horizon soil was approximately 14.5 cm, thinner than that of the other soil types because of the short time for soil formation and soil loss in the study area. The profile SOC density of Amorphic soils was 5.03 kg C [m.sup.-2], lower than that of other soil types. SOC storage in B-horizon soils was much higher than in A-horizon soils (0.040 vs 0.026 kg C [m.sup.-2] respectively). The subsoil was still the primary location of SOC storage in the Primarosol soil profile.

Effects of land use on SOC storage and sequestration Forest soil

Forest soil is the most important SOC pool. Dixon et al. (1994) noted that as much as two-thirds of terrestrial C in forest ecosystems is contained in soils. In Guangdong Province, the forest area is 116 465 [km.sup.2], accounting for 77.4% of the total soil area. Forest SOC storage was approximately 1.0 Pg, accounting for 80.27% of total storage. The B-horizon soils had the highest SOC storage (0.425 Pg), followed by the A- and C-horizon soils (0.33 and 0.246 Pg respectively; Table 4). The mean SOC density of forest soil profiles was 8.58 kg C [m.sup.-2] in the study area, much lower than the previously published value (14.3 kg C [m.sup.-2]) in China (Yu et al. 2007). Zhou et al. (2008) reported that the carbon storage of forest vegetation in Guangdong Province has increased steadily from 0.17 x 1012 kg in 1994 to 0.21 x 1012 kg in 2008, and that the ratio of soil C : plant C is approximately 5. This value is much higher than the ratio of 1.2 reported in China by Dixon et al. (1994) and Lai (2005), suggesting the forest vegetation still has a higher potential for C sequestration in Guangdong Province.

Dryland soils

In the study area, pasture and vegetables occupy 10.4% of the total area. The SOC storage in drylands was only 0.073 Pg, accounting for 5.81% of total SOC storage. The profile SOC density was low, at 4.65 kg Cm 2. The SOC storage in the subsoil horizon was approximately 3.3-fold higher than in Ahorizon soils (Table 4). Lai (2005) noted that agricultural soils, and especially eroded agricultural soils, usually contain store less SOC than their potential capacity. Afforestation of these degraded soils can mitigate some of the degradation processes and lead to C sequestration and SOC storage (Ross et al. 2002).

Paddy soils

Compared with dryland cultivation, irrigation-based rice cultivation in Guangdong has led to significant enrichment of SOC storage (0.17 Pg) in paddy soils (Table 4). In Guangdong Province, approximately 57% of SOC storage (0.3 Pg) is in paddy soils (Pan et al. 2003). Pan et al. (2003) estimated that the current C sequestration rate in the plough horizon was 0.022 kg C [m.sup.-2] annually in Guangdong Province based on data from national soil monitoring sites of paddy soils and some long-term pilot experiments. According to this rate, the SOC sequestration potential in the paddy area in Guangdong Province is approximately 3.56 Tg C annually.

Generally, subsoil is important for SOC storage regardless of land use type. Soil cultivation is not generally associated with a strong decline in SOC because tillage can promote the formation of organic-mineral associations, and dilution of SOC caused by a deepening of the topsoil probably decreases SOM decomposition (Don et al. 2013; Wicsmeicr et al. 2015).

Correlations between SOC, soil total nitrogen and other soil properties

Pearson correlation coefficients for correlations among SOC, total nitrogen (TN) and soil properties are given in Table 2. Linear trends between soil properties (soil density, clay content and soil pH) and SOC concentrations were mostly nonsignificant (Table 2). Pearson correlation coefficients between SOC and soil TN in the A, B and C horizons were 0.86, 0.87 and 0.62 respectively (Table 2). These correlations indicate that, in the study area, soil samples with high soil TN have a higher capacity for SOC sequestration, whereas soil samples with low soil TN have a limited capacity for SOC sequestration. Kirkby et al. (2011, 2014) pointed out that there was a constant stoichiometric ratio of C: N in SOM across a wide range of global soils. Such a ratio may imply that SOC levels could be limited by the supply of N and not just by C input. In subsoils, the C: N ratio is generally characterised by very low organic matter contcnt, and high N content may be related to the presence of mineral N sorbed to clay surfaces (Jenkinson and Coleman 2008), which could suggest subsoils still have a large potential for C sequestration.

There were significant correlations between SOC content in different horizons, with correlation coefficients of 0.767 between the A and B horizons and 0.774 between the B and C horizons. There were also significant correlations between TN in different horizons with r-values of 0.731 between the A and B horizons and 0.632 between the B and C horizons (Fig. 4). These correlations could suggest that there is movement of SOC and soil TN from A-horizon soils to much lower horizons because of farming practices and intensive eluviation in the study area. Fontaine et al. (2007) reported that the movement of SOC from the surface soil to the lower horizon increases the distribution of fresh C along the soil profile; the delivery of fresh C to the subsoil could stimulate the loss of ancient buried C. Thus, SOC storage in the subsoil at the large regional scale may be decided by both the potential for C sequestration and the loss of ancient buried C.

SOC export and redistribution induced by soil erosion and degradation

Soil erosion is a global issue that is responsible for the removal of considerable quantities of topsoil (Lai 2001), which may continue and perhaps be exacerbated by projected extremes. Many studies have indicated that water, tillage and wind erosion contribute significantly to the loss and redistribution of SOC across the landscape, with both soil and SOC being redeposited within the field, as well as being moved off the field (Smith et al. 2001; McCarty and Ritchie 2002; Ritchie et al. 2007). Most degraded soil, which has lost a large fraction of the SOC pool, can be restored by adopting judicious land use practices (Lai 2004). Moreover, Chappell et al. (2016) indicated that SOC erosion was an important factor in estimating global SOC cycling schemes.

In the study area, the area of soil erosion and degradation is approximately 14 200 [km.sup.2] and is primarily distributed along the Hanjiang, Bcijiang, Dongjiang and Jianjiang watersheds (Wan 2005). The median value of sediment delivery ratio is 0.39 in the Pearl River Watershed (Li and Liu 2006). The geometric mean of SOC density in A-horizon soils is 2.13 kg C[m.sup-2]. According to these data, as a rough estimate, approximately 1.18 x 1010 kg SOC enters the river water annually because of soil degradation and erosion, accounting for 2.9% of the SOC storage of A-horizon soils. Approximately 1.85 x [10.sup.10]kg SOC was redistributed in surfacc soils of the study area each year. Recent studies indicate that soil erosion and subsequent redistribution within fields can stimulate C sequestration in agricultural ecosystems (Ritchie et al. 2007; VandenBygaart 2001; Hao et al. 2001). Therefore, restoring degraded soils and ecosystems could have high potential for sequestering soil C in these areas.

Conclusion

Based on the data of soil profiles investigated, we have comparatively examined the distribution and storage of SOC among different soil and land use types. The results reveal that the forest has the largest SOC storage, and paddy soils have the highest SOC density in Guangdong Province. Because of intensive eluviation, the SOC of surfacc soils can enter subsoils along a soil profile. Subsoil is an important SOC storage, with SOC storage in subsoils approximately twofold higher than that in A-horizon soils. Soil degradation and erosion have caused significant SOC loss in A-horizon soils in the study area, with the amount of SOC loss estimated to be 11.8 Tg annually. Improved land management, restoring degraded soil and controlling soil erosion in the study area may contribute significantly to the sequestration of C from the atmosphere.

http://dx.doi.org/10.1071/SR16174

Acknowledgements

This work was funded by grants from the National Natural Science Foundation of China (31270516, 41171387, 41171446), the Natural Science Foundation of Guangdong Province, China (S2012030006144), the Science and Technology Planning Project of Guangdong Province, China (2012A020100003 and 2011B030900013), the Science and Technology Planning Project of Guangzhou (2013J2200003) and the National Key Technologies R&D Program in the 12th Five year Plan of China (2012BAH32B03-5).

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Huihua Zhang (A,D), Junjian Chen (A), Zhifeng Wu (A,B,D), Dingqiang Li (A), and Li Zhu (C)

(A) Guangdong Key Laboratory of Agricultural Environment Pollution Integrated Control, Guangdong Institute of Eco-Environmental and Soil Sciences, Guangzhou 510650, China.

(B) School of Geographical Sciences, Guangzhou University, Guangzhou 510006, China.

(C) Management School, Jinan University, Guangzhou 510632, China.

(D) Corresponding author. Email: hhzhang@soil.gd.cn

Caption: Fig.1. (a) Land use types, (b) location of Guangdong Province China, (c) sampling locations and (d) soil types.

Caption: Fig. 2. Distribution frequency of soil organic carbon (SOC) content in (a, b) A-, (c, d) B- and (e, f) C-horizon soils in Guangdong Province, (a,c,e) Original data; (b,d,f) In-transformed data.

Caption: Fig. 3. Spatial distribution of soil organic carbon (SOC) density in soil profiles in Guangdong Province.

Caption: Fig. 4. Fitted line plots showing correlations of soil organic carbon (SOC) and soil total nitrogen (TN) in the A, B and C horizons.
Table 1. Summary statistics of the data for 211 soil profiles in
Guangdong Province, China

GM, geometric mean; OC, organic carbon; TN, total nitrogen;
[pH.sub.w], pH as measured in water

                                   No. of    Minimum   Median    GM
                                   samples

A horizon
  OC (%)                             211      0.04      1.29    1.18
  TN (%)                             211      0.00      0.11    0.11
  Soil density (g [cm.sup.3])         211      0.52      1.19
  Clay (%)                           211      0.00     18.69
  Thickness (cm)                     211       3.0      15.0
  [pH.sub.w]                         211       3.4      5.5
B horizon
  OC (%)                             203      0.02      0.75    0.71
  TN (%)                             203      0.00      0.07    0.07
  Soil density (g [cm.sup.3])         203      0.46     1.165
  Clay (%)                           203      0.00     23.51
  Thickness (cm)                     203       3.0      20.0
  [pH.sub.w]                         203       3.0      5.7
C horizon
  OC (%)                             185      0.01      0.44    0.45
  TN (%)                             185      0.01      0.05    0.05
  Soil density (g [cm.sup.3])         185      0.40      1.25
  Clay (%)                           185      0.00     23.93
  Thickness (cm)                     185       5.0      46.0
  [pH.sub.w]                         185       2.8      5.8

                                   Mean    Maximum     Skew (A)

A horizon
  OC (%)                           1.45     5.03     1.29 (0.91)
  TN (%)                           0.12     0.41     1.18 (0.79)
  Soil density (g [cm.sup.3])       1.16     1.66         0.37
  Clay (%)                         20.45    64.4         0.62
  Thickness (cm)                   17.0      40          0.97
  [pH.sub.w]                       5.77      9.1         0.87
B horizon
  OC (%)                           0.94     5.24     2.51 (0.83)
  TN (%)                           0.08     0.39     1.91 (-0.62)
  Soil density (g [cm.sup.3])       1.17      1.8        -0.30
  Clay (%)                         25.27    85.22        0.96
  Thickness (cm)                   29.5     137.0        1.43
  [pH.sub.w]                       5.92     11.9         1.03
C horizon
  OC (%)                           0.66     4.85     2.87 (-0.77)
  TN (%)                           0.06     0.56     4.65 (-0.02)
  Soil density (g [cm.sup.3])       1.20     1.77        -0.56
  Clay (%)                         25.93    74.31        0.70
  Thickness (cm)                   48.9     115.0        0.19
  [pH.sub.w]                       6.05      9.1         0.35

(A) Where two values are given, the data show untransformcd data with
ln-transformed data in parentheses.

Table 2. Pearson correlation coefficients (r) between soil
organic-carbon (SOC) content and selected soil properties

TN, total nitrogen

             SOC     TN     Density   Clay   pH

A horizon
  SOC         1
  TN        0.86      1
  Density   0.15    -0.12      1
  Clay      0.12     0.15    -0.14       1
  PH        0.07     0.09     0.13     0.10   1
B horizon
  SOC         1
  TN        0.87      1
  Density   0.07    0.07       1
  Clay      0.14    0.24     0.14        1
  PH        0.02    0.08     0.16     0.02    1
C horizon
  SOC         1
  TN        0.62      1
  Density   -0.06   0.03       1
  Clay      0.15    0.13     0.08        1
  PH        0.15    0.07     0.15     0.03    1

Table 3. Soil organic carbon content, density and storage by soil
type in Guangdong soil profiles

SOC, soil organic carbon; SOCD, SOC density; SOCS, SOC storage

Soil type                Area (A)      Profile   Clay (%)
                       ([km.sup.2])
Ferrallisol
Latosols               6530 (4.3%)        A       19.72
                                          B       26.70
                                          C       31.56
Latosolic Red soils   65 870 (43.8%)      A       25.30
                                          B       34.66
                                          C       33.86
Red soils             32 410 (21.5%)      A       18.97
                                          B       28.85
                                          C       21.33
Yellow soils           8730 (5.8%)        A        9.99
                                          B       16.46
                                          C       18.26
Anthrosols
Paddy soil            16 652 (11.0%)      A       21.35
                                          P       23.67
                                          C       26.16
Amorphic soils        16 503 (10.9%)      A       18.62
                                          B       24.31
                                          C       20.49
Others                 3762 (3.2%)        A       17.43
                                          B       19.55
                                          C       16.82
Total                     150457          A       20.50
                                          B       25.27
                                          C       25.93

Soil type                Area (A)      Profile   Thickness
                       ([km.sup.2])                (cm)      Median
Ferrallisol
Latosols               6530 (4.3%)        A        16.9       0.73
                                          B        34.4       0.45
                                          C        40.6       0.39
Latosolic Red soils   65 870 (43.8%)      A        16.4       1.03
                                          B        35.0       0.57
                                          C        56.8       0.42
Red soils             32 410 (21.5%)      A        16.9       1.56
                                          B        73.3       0.59
                                          C        32.8       0.42
Yellow soils           8730 (5.8%)        A        19.1       3.16
                                          B        46.8       1.04
                                          C        44.0       0.21
Anthrosols
Paddy soil            16 652 (11.0%)      A        16.2       1.44
                                          P        17.5       0.86
                                          C        51.6       0.49
Amorphic soils        16 503 (10.9%)      A        14.5       0.96
                                          B        32.1       0.51
                                          C        46.3       0.38
Others                 3762 (3.2%)        A        23.0       1.31
                                          B        33.0       0.81
                                          C        38.9       0.59
Total                     150457          A        17.0       1.29
                                          B        29.5       0.75
                                          C        48.9       0.44

Soil type           Area (A)      Profile   SOC (%)
                  ([km.sup.2])               Mean       Range
Ferrallisol
Latosols          6530 (4.3%)        A       0.94     0.26-3.18
                                     B       0.64     0.26-2.13
                                     C       0.47     0.13-1.60
Latosolic        65 870 (43.8%)      A       1.08     0.31-2.23
Red soils                            B       0.62     0.19-1.51
                                     C       0.42     0.09-0.88
Red soils        32 410 (21.5%)      A       1.52     0.04-3.05
                                     B       0.59     0.02-0.99
                                     C       0.42     0.01-0.73
Yellow soils      8730 (5.8%)        A       3.15     2.05-4.34
                                     B       0.95     0.37-1.21
                                     C       0.34     0.16-0.66
Anthrosols
Paddy soil       16 652 (11.0%)      A       1.53     0.41-4.42
                                     P       1.14     0.16-5.24
                                     C       0.81     0.01-4.85
Amorphic soils   16 503 (10.9%)      A       1.19     0.10-2.82
                                     B       0.83     0.05-2.70
                                     C       0.48     0.01-1.41
Others            3762 (3.2%)        A       1.55     0.064-5.03
                                     B       1.03     0.04-2.87
                                     C       0.83     0.03-1.89
Total                150457          A       1.45     0.04-5.03
                                     B       0.94     0.02-5.24
                                     C       0.66     0.01-4.85

Soil type         Area (A)      Profile   SOCD (kg C [m.sup.-2])
                ([km.sup.2])                 Median     Mean
Ferrallisol
Latosols        6530 (4.3%)        A          1.15      0.91
                                   B          1.92      2.58
                                   C          1.84      2.47
Latosolic      65 870 (43.8%)      A          1.80      2.15
Red soils                          B          1.93      2.55
                                   C          3.00      2.93
Red soils      32 410 (21.5%)      A          2.20      2.53
                                   B          4.47      4.83
                                   C          1.08      1.48
Yellow soils    8730 (5.8%)        A          3.75      4.45
                                   B          2.81      3.61
                                   C          1.29      1.07
Anthrosols
Paddy soil     16 652 (11.0%)      A          2.51      3.09
                                   P          1.09      2.73
                                   C          2.94      4.74
Amorphic       16 503 (10.9%)      A          1.18      2.02
soils                              B          1.68      2.58
                                   C          1.82      2.61
Others          3762 (3.2%)        A          2.70      4.06
                                   B          2.86      3.85
                                   C          2.72      3.74
Total              150457          A          2.29      2.87
                                   B          1.68      3.00
                                   C          2.45      3.75

Soil type         Area (A)      Profile      SOCD       SOCS
                ([km.sup.2])                (kg C
                                          [m.sup.-2])
                                             Range      (Pg)
Ferrallisol
Latosols        6530 (4.3%)        A       0.48-5.78    0.007
                                   B      0.55-10.39    0.014
                                   C       0.32-9.73    0.01
Latosolic      65 870 (43.8%)      A       0.42-6.82    0.146
Red soils                          B       0.20-8.12    0.174
                                   C       0.43-6.97    0.214
Red soils      32 410 (21.5%)      A       0.05-5.31    0.107
                                   B       0.23-12.6    0.195
                                   C       0.06-4.81    0.011
Yellow soils    8730 (5.8%)        A       2.47-7.67    0.053
                                   B       0.85-8.27    0.041
                                   C       0.37-1.64    0.004
Anthrosols
Paddy soil     16 652 (11.0%)      A      0.29-10.68    0.055
                                   P       0.13-11.9    0.044
                                   C      0.24-31.28    0.071
Amorphic       16 503 (10.9%)      A       0.07-7.92    0.026
soils                              B       0.15-8.02    0.040
                                   C       0.05-8.11    0.017
Others          3762 (3.2%)        A      0.13-15.50    0.010
                                   B      0.09-13.34    0.007
                                   C      0.17-14.58    0.004
Total              150457          A      0.05-15.51    0.41
                                   B      0.09-41.92    0.51
                                   C      0.05-31.28    0.33

Soil type         Area (A)      Profile   Profile SOCD    Profile
                ([km.sup.2])                  (kg        SOCS (Pg)
Ferrallisol                               C[m.sup.-2])

Latosols        6530 (4.3%)        A          4.74         0.031
                                   B
                                   C
Latosolic      65 870 (43.8%)      A          8.11         0.534
Red soils                          B
                                   C
Red soils      32 410 (21.5%)      A          9.66         0.313
                                   B
                                   C
Yellow soils    8730 (5.8%)        A         11.23         0.098
                                   B
                                   C
Anthrosols
Paddy soil     16 652 (11.0%)      A         10.21         0.17
                                   P
                                   C
Amorphic       16 503 (10.9%)      A          5.03         0.083
soils                              B
                                   C
Others          3762 (3.2%)        A          5.58         0.021
                                   B
                                   C
Total              150457          A          8.31         1.25
                                   B
                                   C

(A) Values in parentheses indicate the percentage of total soil area.

Table 4. Soil organic carbon content and storage in four land use
types in Guangdong soil profiles

SOC, soil organic carbon; SOCD, SOC density; SOCS, SOC storage

Land use   Area (A) ([km.sup.2])   Profile   Clay (%)   Thickness
                                                          (cm)

Forest        116465 (77.4%)          A       21.50       17.5
                                      B       28.50       45.5
                                      C       27.19       44.1
Paddy         16 651 (10.8%)          A       21.92       16.2
                                      B       24.43       17.5
                                      C       26.72       51.6
Dryland       15 707 (11.0%)          A       14.23       16.4
                                      B       21.27       28.7
                                      C       22.47       52.7
Total             150457              A       20.50       17.0
                                      B       25.27       29.5
                                      C       25.93       48.9

Land use   Area (A) ([km.sup.2])   Profile    Soil density
                                             (g [cm.sup.-3])   Median

Forest        116465 (77.4%)          A           1.09         1.29
                                      B           1.14         0.66
                                      C           1.15         0.40
Paddy         16 651 (10.8%)          A           1.20         1.44
                                      B           1.18         0.86
                                      C           1.22         0.49
Dryland       15 707 (11.0%)          A           1.25         0.86
                                      B           1.28         0.50
                                      C           1.30         0.38
Total             150457              A           1.16         1.29
                                      B           1.17         0.75
                                      C           1.20         0.44

Land use   Area (A) ([km.sup.2])   Profile   SOC (%)
                                              Mean       Range

Forest        116465 (77.4%)          A       1.59     0.04-5.03
                                      B       0.78     0.02-2.62
                                      C       0.51     0.01-1.87
Paddy         16 651 (10.8%)          A       1.53     0.41-4.42
                                      B       1.14     0.16-5.24
                                      C       0.81     0.01-4.85
Dryland       15 707 (11.0%)          A       0.94     0.24-2.59
                                      B       0.61     0.04-2.70
                                      C       0.40     0.03-1.15
Total             150457              A       1.45     0.04-5.03
                                      B       0.94     0.02-5.24
                                      C       0.66     0.01--4.85

Land use      Area (A)      Profile        SOCD (kgC[m.sup.-2])
            ([km.sup.2])              Median   Mean     Range

Forest     116465 (77.4%)      A       2.35    2.96   0.05-15.51
                               B       2.61    3.64   0.19-13.3
                               C       1.61    2.67   0.05-14.58
Paddy      16 651 (10.8%)      A       2.51    3.09   0.29-10.68
                               B       1.09    2.73   0.13-41.9
                               C       2.94    4.74   0.24-31.28
Dryland    15 707 (11.0%)      A       1.28    1.98   0.42-8.41
                               B       1.06    1.98   0.09-7.65
                               C       2.21    2.80   0.17-12.34
Total          150457          A       2.29    2.87   0.05-15.51
                               B       1.68    3.00   0.09-4.92
                               C       2.45    3.75   0.05-31.28

Land use      Area (A)      Profile   SOCS     Profile SOCD
            ([km.sup.2])              (Pg)    (kgC[m.sup.-2])

Forest     116465 (77.4%)      A      0.33         8.58
                               B      0.43
                               C      0.25
Paddy      16 651 (10.8%)      A      0.055        10.21
                               B      0.044
                               C      0.071
Dryland    15 707 (11.0%)      A      0.017        4.65
                               B      0.042
                               C      0.014
Total          150457          A      0.41         8.31
                               B      0.51
                               C      0.33

Land use   Area (A) ([km.sup.2])   Profile      Profile
                                             SOCS (B) (Pg)

Forest        116465 (77.4%)          A      1.01 (80.27%)
                                      B
                                      C
Paddy         16 651 (10.8%)          A      0.17 (13.47%)
                                      B
                                      C
Dryland       15 707 (11.0%)          A      0.073 (5.81%)
                                      B
                                      C
Total             150457              A          1.25
                                      B
                                      C

(A) Values in parentheses indicate the percentage of total soil area.

(B) Values in parentheses indicate the percentage of total SOCS.
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Author:Zhang, Huihua; Chen, Junjian; Wu, Zhifeng; Li, Dingqiang; Zhu, Li
Publication:Soil Research
Article Type:Report
Geographic Code:9CHIN
Date:Jul 1, 2017
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