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Soil organic carbon changes in particle-size fractions following cultivation of Black soils in China
Soil & Tillage Research 105 (2009) 21–26
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Soil organic carbon changes in particle-size fractions following cultivation of Black soils in China Aizhen Liang a, Xueming Yang b, Xiaoping Zhang a,*, Neil McLaughlin c, Yan Shen a, Wenfeng Li a a
Northeast Institute of Geography and Agroecology, Chinese Academy of Sciences, Changchun 130012, China Greenhouse and Processing Crops Research Centre, Agriculture and Agri-Food Canada, Harrow N0R1G0, Canada c Eastern Cereal and Oilseed Research Centre, Agriculture and Agri-Food Canada, Ottawa K1A0C6, Canada b
A R T I C L E I N F O
A B S T R A C T
Article history: Received 14 September 2008 Received in revised form 2 May 2009 Accepted 7 May 2009
Soil texture can be an important control on soil organic carbon (SOC) retention and dynamics. The (clay + silt)-sized SOC pool (SOC < 20 mm) in non-cultivated or grassland soils has been proposed to reach an equilibrium or maximum level named protective capacity. Proper knowledge of SOC in this size fraction in non-cultivated and cultivated Black soils is important to evaluate management-induced changes in SOC in NE China. Twenty-seven paired soil samples (non-cultivated vs. cultivated) were collected in the Black soil zone in Heilongjiang and Jilin provinces. Bulk soil was dispersed in water with an ultrasonic probe and then soil size fractions were collected using the pipette technique for SOC analyses. Soil organic carbon in bulk soil and size fractions was measured by dry combustion. Average content of SOC < 20 mm was 23.2 g C kg1 at the 0–30 cm depth for the non-cultivated soils, accounting for 75.1% of the total SOC at the same depth. There was significant positive relationship between soil clay plus silt content and SOC < 20 mm in non-cultivated soils. Accordingly, a model of the maximum SOC < 20 mm in 0–30 cm depth of non-cultivated Black soils was developed: y = 0.36x where y is the maximum SOC < 20 mm pool (g C kg1) and x is the percentage of clay + silt (<20 mm) content. The average content of SOC < 20 mm was 18.7 g C kg1 at 0–30 cm depth for cultivated soils, accounting for 81.5% of total SOC. This average value of SOC was 4.4 g C kg1 less than the maximum value (23.1 g C kg1) and accounted for 55.0% of the difference of SOC between non-cultivated and cultivated Black soils. Cultivation resulted in 45.0% loss of sand-sized (>20 mm) SOC concentration relative to SOC < 20 mm. This result indicates that SOC < 20 mm and sand-sized SOC both play important roles in SOC dynamics resulting from management practices. This model can be applied to calculate the actual potential to restore SOC for cultivated Black soils under conservation tillage in NE China. ß 2009 Elsevier B.V. All rights reserved.
Keywords: China Black soils Organic carbon Restoring potential Soil texture
1. Introduction Quantifying the potential of cropland soils to restore antecedent soil organic carbon (SOC) will help to evaluate the contribution of cropland soils as a C source or sink to the global C balance. However, there are many uncertainties in SOC dynamics of the soil system (Li, 2002), and these are probably some of the most limiting factors for correctly determining the potential of soil C sequestration (Smith, 2004). Some researchers found that particulate organic C (53–2000 mm) is more sensitive to management change than total SOC (Ellert and Gregorich, 1995; Chan et al., 2002; Six et al., 2002a), and others noted particulate organic C changes were limited before SOC in fine particles reached
* Corresponding author at: 3195 Weishan Road, Gaoxin District, Changchun, Jilin Province 130012, China. Tel.: +86 431 85542234; fax: +86 431 85542298. E-mail addresses: [email protected] (A. Liang), [email protected] (X. Zhang). 0167-1987/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.still.2009.05.002
saturation (Hassink, 1997; Carter et al., 2003). Hassink (1997) proposed that clay and silt (particles <20 mm) in soil physically retain SOC, and named this protective ability a ‘‘capacity factor’’ or the maximum SOC that could be stored in that combined size fraction. Furthermore, this ‘‘(clay + silt)-sized SOC’’ (SOC < 20 mm, for simplicity) could be modeled as a function of total clay + silt in soils as given in Eq. (1). SOC < 20 mm ¼ 4:09 þ 0:37ðclay þ siltÞ
(1)
in which SOC < 20 mm and clay + silt are in g kg1 soil. In NE China, the SOC < 20 mm in some cultivated Black soils was estimated as approximately 75% of total SOC (Zhao et al., 1993), but there is no information on how these values depart from ‘‘the maximum’’ observed prior to cultivation. Estimating this difference would be valuable for understanding both the potential of C sequestration in cultivated Black soils in NE China and the effect of cropping management on C dynamics. Accordingly, the objectives of this study were (1) to determine the maximum amount of
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SOC < 20 mm pool in non-cultivated Black soils and (2) to compare the difference of SOC < 20 mm between cultivated and noncultivated Black soils in NE China. 2. Materials and methods 2.1. Study site The study site was the bow-shaped Black soils (Udolls, US Soil Taxonomy) zone in Heilongjiang and Jilin provinces of NE China (Fig. 1). The topography in Black soil region is characterized by undulating plateau with slopes of 1–58. The Black soil zone is located in the temperate zone with a continental monsoon climate. The mean annual temperature varies between 0.5 8C and 6.0 8C, and the mean annual precipitation varies between 500 mm and 600 mm, with more than 80% occurring in June to September. The landscape was grassland in its natural state and steppe-meadow grasses are the predominant native species. Conversion to cultivated crops started early in the last century, and mass cultivation occurred in the 1940s and 1950s. Traditional cropping practices are continuous soybean in north, corn–soybean rotation in center, and continuous corn in south of the Black soil zone. Typical field management practices include removing all aboveground plant biomass after harvest, incorporating stubble and main roots into soils with two or three handhoeing operations, and forming ridges. More details about study site were given by Liang et al. (2009). 2.2. Soil sampling In 2004 and 2005, 27 pairs of cultivated/non-cultivated neighbor sites were sampled. In each paired site, the cultivated soil was approximately 50–100 m distant from its non-cultivated counterpart, which was never used for crop production. Several criteria were used to confirm sample sites as non-cultivated soils, including consultations with older residents in the local villages, observing vegetation and soil vertical profiles, and comparing
results from the laboratory chemical and physical analysis with published results. The land was converted from natural grassland to cultivated crops less than 100 years ago, and the elders in the local villages knew both where the soil had never been disturbed by cultivation and the reasons why. For some sites, the reasons are based on local protection policy, such as retaining non-cultivated land to provide biomass for use as fuel for heating and cooking in houses, or establishing ecological sites where removal of natural vegetation is prohibited. Also, position on the landscape resulted in some sites not being cultivated, such as a non-cultivated site in a small triangular zone on the edge of a deep natural waterway eroded in the landscape (Fig. 2). Natural vegetations on Black soils are steppe-meadow species and most are the herbage community including Bupleurum scorzonerifolium, Sanguisorba officinalis, Potentilla chinensis, Salix rosmarinifolia, Platycodon grandiflorus (Fu, 1995). Presence of these species is a good indicator of a non-cultivated site. As a final check profiles of non-cultivated soils in the Black soil zone were examined, and laboratory chemical and physical analyses were compared to published data on non-cultivated Black soils. Soil core samples were taken to a depth of 30 cm using a manual soil probe with 2.64 cm internal diameter (Jia et al., 1995) which allowed separation of each soil core into 4 segments including 0– 5 cm, 5–10 cm, 10–20 cm, and 20–30 cm. The diameter of the cutting edge of the probe was 4 mm less than the inside diameter of the barrel which eliminated core compaction resulting from friction between the core and barrel. Five to seven sub-samples were taken at each site and combined into a single composite sample. The soil samples were gently broken to pass a 7 mm sieve and air-dried. Visibly identifiable crop residues were manually removed and discarded. 2.3. Soil physical analysis The bulk density of soil samples was calculated using the inner diameter of the core sampler cutting edge, segment depth and
Fig. 1. Distribution of sampling sites in Jilin and Heilongjiang provinces.
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ground for SOC analyses. All soil samples were free of carbonate, hence SOC content was assumed equal to total C. Soil organic carbon in the soil particle fractions was determined using the FlashEA1112 elemental analyzer (ThermoFinnigan, Milan, Italy). The ANOVA followed by the LSD test was used to examine the effects of tillage on SOC in each size fraction. Pearson correlation was used to evaluate the relationship between the percentages of soil particles and both total SOC and C fractions. These procedures were performed using SAS (SAS, 2004). Statistical significance was determined at the P < 0.05 level except if indicated differently. 3. Results 3.1. Selected properties of non-cultivated and cultivated soils
Fig. 2. An example of a particular non-cultivated site in a triangular zone on the edge of a deep natural waterway eroded in the landscape in Heilongjiang province. A 1.2-m wide trail ran north toward the waterway, and then turned west parallel to the waterway. The triangular zone on the outside of the turn in the trail was a ‘‘waste’’ zone too short for cropped ridges.
oven-dried soil weight (105 8C). Soil water suspensions (soil:water = 1:5) were prepared, allowed to settle for one half hour, and soil pH was measured using an acidity meter (pHS-3B, Leici, Shanghai). Samples were dispersed in 0.1 M NaOH (for acidic soils) or 0.1 M Sodium oxalate (for neutral soils), organic matter was oxidized with H2O2, and soil texture determined by the pipette method (Soil Science Society of China, 2000).
Both non-cultivated and cultivated Black soils in our study were typically clay loams, and soil texture varied little with land use and depth (Table 1). Both non-cultivated and cultivated soils were neutral or slightly acidic with average pH of 7.0 and 6.5, respectively (Table 1). Soil bulk densities at 0–5 cm, 5–10 cm and 10–20 cm depths were lower for the non-cultivated soils than corresponding depths in the cultivated soils, and almost identical for both soils at the 20–30 cm depth (Table 1). Bulk density increased with increasing depth for non-cultivated soils, but not much for cultivated soils (Table 1). Average SOC contents in bulk soils for 0–30 cm depth were lower in cultivated (22.9 g C kg1) than in non-cultivated soils (30.9 g C kg1). Although SOC contents decreased with soil depth for both land uses, the decrease was more pronounced in noncultivated sites (Table 1). 3.2. The model of the maximum SOC < 20 mm
2.4. Soil organic carbon analysis Twenty-five grams of air-dried soil were placed in a 250 ml beaker and 125 ml distilled water was added. The soil suspension was mixed and allowed to settle over night. The suspensions were dispersed by treating with an ultrasonic probe (JY92, Xinzhi, Ningbo, China) for 10 min at 24 kHz (Hassink, 1997; Oorts et al., 2005; Zhao et al., 2006). The ultrasonic energy needed for complete dispersion was 480 J ml1 (Schmidt et al., 1999a,b); suspension temperature was kept <32 8C by using a 50% duty cycle (1 s on/1 s off) and by placing the beaker in a running tap water bath. The dispersed soil suspension was transferred to a 1 l glass cylinder, and the cylinder was capped and shaken end over end to thoroughly homogenize the soil water suspension. Silt plus clay (<20 mm) and clay (<2 mm) fractions were collected by siphoning the suspension at the appropriate depth and time based on Stokes’ law (Soil Science Society of China, 2000), oven-dried at 60 8C, and
Unless otherwise stated, SOC contents of the various size fractions are reported here as g SOC kg1 of bulk soil. Size fractions used here are clay <2 mm, (clay + silt) <20 mm, and sand >20 mm. Sand-sized SOC was calculated by subtracting SOC < 20 mm from total SOC. The SOC < 20 mm obtained here includes water-soluble carbon, which has been shown by an independent work (Fang, 2005) to account for only 0.15–0.20% of total SOC. The average SOC < 20 mm was 23.2 g C kg1 in a 0–30 cm depth of noncultivated soils (Table 2), accounting for 75.1% of total SOC. Claysized SOC was 8.96 g C kg1, accounting for 38.6% of SOC < 20 mm. Subsurface soil (10–30 cm) had lower SOC than the surface layer (0–10 cm depth), which did not occur for the SOC < 20 mm (Fig. 3). The amount of sand-sized SOC accounted for 24.9% of the total SOC for the non-cultivated soil, and decreased substantially with depth. For cultivated soils, the average content of SOC < 20 mm was 18.7 g C kg1 in 0–30 cm depth (Table 2), accounting for 81.5% of
Table 1 Selected soil properties in a depth of 0–30 cm in non-cultivated and cultivated soils. Soil depth (cm)
Clay (%)
Silt (%)
Sand (%)
pH
Bulk density (g cm3)
Total C (g kg1)
Non-cultivated soil 0–5 5–10 10–20 20–30 Weighted mean
35.5 1.44a 36.2 1.17 37.1 1.19 36.1 1.24 36.4
29.2 0.85 27.4 0.84 27.1 0.77 27.0 0.81 27.5
35.3 1.46 36.4 1.27 35.8 1.44 36.9 1.24 36.2
6.91 0.15 6.90 0.16 7.00 0.16 7.09 0.16 7.0
0.94 0.04 1.08 0.04 1.08 0.03 1.17 0.03 1.09
44.2 2.21 34.3 2.33 30.0 1.85 24.5 1.87 31.8
Cultivated soil 0–5 5–10 10–20 20–30 Weighted mean
31.8 1.41 32.1 1.56 35.7 1.56 36.3 1.64 34.7
30.2 0.83 30.2 1.07 28.8 0.95 29.7 1.30 29.6
38.0 1.49 37.7 1.41 35.5 1.44 33.9 1.33 35.8
6.37 0.16 6.36 0.16 6.52 0.14 6.71 0.13 6.53
1.04 0.02 1.19 0.03 1.20 0.03 1.18 0.03 1.17
24.2 1.39 24.1 1.40 23.1 1.64 21.4 1.92 22.9
a
Means plus or minus standard error.
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Table 2 Clay + silt contents in soils, and SOC < 20 mm in non-cultivated and cultivated Black soils (0–30 cm depth). Sample ID
(clay + silt) contents in non-cultivated soils (%)
SOC < 20 mm in non-cultivated soils (g kg1)
SOC < 20 mm in cultivated soils (g kg1)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27
70.8 70.0 67.1 71.4 71.8 69.1 68.1 79.3 63.6 64.3 71.6 67.3 71.9 60.0 61.6 57.1 58.7 58.2 58.2 60.6 53.8 57.1 61.6 52.5 60.2 58.3 59.7
23.2 21.9 20.8 25.4 21.5 28.5 21.1 29.2 26.4 21.7 26.5 28.0 30.7 29.7 29.4 25.7 22.0 27.8 15.9 29.2 16.0 14.5 26.2 12.9 18.3 16.8 18.3
18.9 19.0 16.9 18.3 17.1 23.0 17.9 21.5 25.6 15.5 26.0 27.8 25.9 18.0 24.6 22.7 21.7 21.7 11.9 21.0 13.3 12.9 13.4 8.4 12.7 12.7 15.2
Average
63.8
23.2
18.7
the total SOC while the amount of sand-sized SOC accounted for only 18.5% of total SOC. The amounts of SOC < 20 mm and sandsized SOC at 0–5 cm and 5–10 cm were both lower in cultivated than non-cultivated soils. The clay plus silt content in non-cultivated soils ranged from 51% to 83%. There were linear positive relationships between the
clay plus silt content, and the SOC < 20 mm in all depths, 0–5 cm, 5–10 cm, 10–20 cm and 20–30 cm (data not shown). We did an analysis of covariance with clay + silt content as the covariate, and depth as a factor. The covariate (clay + silt content) was significant (P < 0.05) but depth was not significant (P < 0.05). Consequently, the data for the entire 0–30 cm depth were pooled and a simple linear regression was done with SOC < 20 mm as the dependent variable and clay + silt content of the soil as the independent variable. The intercept for this regression was not significant (P > 0.05) and so a reduced model without intercept was used in the final regression model given in Eq. (2). y ¼ 0:36x
ðr ¼ 0:510; P < 0:01; n ¼ 27Þ
(2)
where y = the maximum SOC < 20 mm (g kg1 soil), x = the percentage of clay plus silt particles (%). Observed and predicted SOC < 20 mm are plotted in Fig. 4. The SOC < 20 mm predicted with the model developed by Hassink (1997) is plotted on the same graph for comparison. 3.3. The theoretical potential to restore SOC for cultivated Black soils We applied our model (y = 0.36x) to cultivated Black soils to estimate the potential restoring capacity of SOC < 20 mm (Table 3). Average estimated maximum of SOC < 20 mm for the 27 cultivated sites was equal to 23.1 g kg1 in the top 30 cm of soil, which was as expected nearly identical to the observed average value (23.2 g kg1) for the 27 non-cultivated soils. Observed SOC < 20 mm (18.7 g kg1) was 4.4 g kg1 less than the maximum in cultivated soil, accounting for a 55.0% difference of total SOC between non-cultivated and cultivated soil. Therefore, average estimated potential to restore SOC < 20 mm of the 27 cultivated sites was 4.4 g kg1 in the top 30 cm of soil. Sand-sized SOC was 3.6 g kg1 less in cultivated soil than the non-cultivated soil, accounting for 45.0% of the difference of total SOC between these two soils.
Fig. 3. Contents of total SOC and SOC in different particles. Clay-sized SOC (<2 mm) is also included in the SOC < 20 mm, and consequently, the sand-sized (>20 mm) SOC, SOC < 20 mm and clay-sized SOC pools do not add up to the total SOC (error bar is standard error, n = 27).
A. Liang et al. / Soil & Tillage Research 105 (2009) 21–26
Fig. 4. Plot of observed SOC < 20 mm vs. percent clay + silt. Line a is the maximum SOC < 20 mm predicted by Hassink’s model, line b is maximum SOC < 20 mm predicted by our regression model: y = 0.36x (r = 0.510, n = 27), and lines c and d are the 95% confidence intervals for our model.
Table 3 Comparison of total SOC, SOC < 20 mm and sand-sized SOC in non-cultivated and cultivated Black soils (0–30 cm).
Total SOC SOC < 20 mm Sand-sized SOCa
Non-cultivated soil (g kg1)
Cultivated soil (g kg1)
Difference (g kg1)
30.9 23.1 7.8
22.9 18.7 4.2
8.0 4.4 3.6
SOC loss in size fraction/ total SOC loss
55.0% 45.0%
a Sand-sized SOC was calculated as the difference between total SOC and the SOC < 20 mm.
4. Discussion The total SOC content in the non-cultivated soils was the highest near the surface in the top 5 cm and steadily decreased with increasing depth, but in the cultivated soils, the soil organic carbon was much lower and relatively constant in the 0–20 cm depth (Table 1). Cultivation both mixed the soil in the plow layer and promoted mineralization of the SOC resulting in nearly constant SOC in the 0–20 cm depth (Liang et al., 2008). The current study was based on an assumption that the amount of SOC < 20 mm had a limit and that C in non-cultivated Black soils was saturated. The non-cultivated soils have not been disturbed over thousands of years, and consequently, it is reasonable to expect that SOC has reached an equilibrium where annual input of C from the natural vegetation biomass is equal to the annual output of C via mineralization or consumption by soil micro flora and fauna. Hassink’s model was developed from published SOC data for soils from around the world with a broad range of both texture and mineralogy while our model is based on a much smaller data set from the Black soils in NE China. The slopes for the two models are nearly identical, 0.36 g C kg1 clay + silt for our model compared to 0.37 g kg1 clay + silt for the Hassink model. Our model does not have an intercept while Hassink’s model has an intercept of 4.09. Different parameters are expected, because the maximum of SOC associated with the fine size fraction is influenced by many factors, including clay mineralogy and climate (Li, 2001; Six et al., 2002b; Haider and Guggenberger, 2002). Clay minerals in soils used in this study were dominated by 2:1 type minerals, and so we did not have variations in clay mineralogy to influence SOC < 20 mm. Australian soils investigated in Hassink’s study (1997) had substantially lower SOC, likely a result of high temperatures, low rainfall and limited input of plant residues, and were excluded
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from his model. All of these factors provide an indication of the complexity of SOC < 20 mm. Hassink’s model over-predicted SOC < 20 mm measured in our study by an average 19.2% (Fig. 4). A model developed for the local data generally fits the local data much better than a more general model such as the Hassink model which is based on published data for a wide range of soils around the world. Zinn et al. (2005) also proposed that simple linear functions relating SOC to clay + silt may not be the best descriptors for every region and different functions must be tested for each region. The SOC capacity models of soils such as Eq. (2) and the Hassink (1997) model (Eq. (1)) provide a means to estimate SOC change under cultivation, and the mechanism and potential for soil C sequestration. The SOC < 20 mm pool increases with increasing clay + silt contents, which is the main mechanism of the textural control on SOC retention (Zinn et al., 2007). In a study applying this function to Baijiang (Albolls, US Soil Taxonomy) cultivated soils in Jilin Province, China, Zhao et al. (2006) found that the actual content of SOC < 20 mm was greater than the maximum calculated from Hassink’s model. The agricultural use of Black soils led to great SOC loss from sand-sized C. This result was different from the finding that the change of sand-sized C was limited when SOC in fine particles reached saturation (Hassink, 1997; Carter et al., 2003). Although SOC < 20 mm was believed to be relatively stable and played an important role in maintaining C levels, the share of 55.0% total SOC loss from this fraction in cultivated Black soils suggests that attention needs to be paid to the fate of C both in fine and coarse particles when studying effect of agriculture on C dynamics. However, Jolivet et al. (2003) found that SOC < 20 mm decreased by only 20% after 30 years of cultivation, so long-term C accumulation was attributed mainly to the clay + silt fraction, which protected complex molecules supplied by decomposition of sand-sized organic matter. The soil’s capacity to accumulate newly added carbon was determined by the degree of saturation of SOC < 20 mm (Hassink, 1995; Hassink and Whitmore, 1997; Jolivet et al., 2003). Six et al. (2002b) suggested that with increased C input, the increase in SOC content was smaller when the protective capacity approached saturation. Our present studies are focused on the potential of conservation tillage as a management practice to prevent soil erosion and to restore SOC in the Black soils in NE China. The model will play a key role in understanding SOC dynamics in these studies. Certainly, clay + silt pool is not homogeneous and C in each particle size is probably retained by different mechanisms and has different C:N ratios (Zinn et al., 2007). While the clay + silt pool concept appears useful, it is a simplification and clearly, further study is needed. 5. Conclusion Paired soil samples were obtained from 27 non-cultivated and adjacent cultivated soil sites in the Black soil zone in NE China, and SOC determinations were made in different size fractions. In noncultivated Black soils, significant positive relations were found between clay + silt contents and both total SOC and SOC < 20 mm. Based on those relations and on the assumption that SOC < 20 mm was saturated, a model of the maximum SOC < 20 mm in noncultivated Black soils was developed. In cultivated Black soils the mean difference between present SOC < 20 mm and estimated potential maxima was 4.4 g C kg1, or 55.0% of total SOC difference between non-cultivated and cultivated soils. The other 45.0% of total C lost was from the sand fraction. The model provides a tool to estimate the potential of cultivated Black soils restoring SOC < 20 mm, but more work is needed to validate and refine this model.
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Acknowledgements This research was supported by the projects from National Natural Science Foundation of China (40801071), National Scientific and Technical Supporting Programs (2006BAD15B01), Program for Advanced Science Field (KZCX3-SW-NA3-31) and Doctoral Research Foundation (O8H2041) in Northeast Institute of Geography and Agroecology, Chinese Academy of Sciences. References Carter, M.R., Angers, D.A., Gregorich, E.G., Bolinde, M.A., 2003. Characterizing organic matter retention for surface soils in eastern Canada using density and particle size fractions. Canadian Journal of Soil Science 83, 11–23. Chan, K.Y., Heenan, D.P., Oates, A., 2002. Soil carbon fractions and relationship to soil quality under different tillage and stubble management. Soil & Tillage Research 63, 133–139. Ellert, B.H., Gregorich, E.G., 1995. Management-induced changes in the actively cycling fractions of soil organic matter. In: McFee, W.W., Kelly, J.M. (Eds.), Carbon Forms and Functions in Forest Soils. Soil Science Society of America, Madison, Wisconsin, pp. 119–138. Fang, H.J., 2005. Study on redistribution of organic carbon of black soil in a sloping farmland and its dynamics. Ph.D. dissertation. Northeast Institute of Geography and Agroecology, Chinese Academy of Sciences, Changchun, China (in Chinese). Fu, P.Y., 1995. Botany Key in Northeast China, 2nd ed. Science Press, Beijing, pp. 92, 302, 312, 465, 636 (in Chinese). Haider, K., Guggenberger, G., 2002. Soil minerals and organic components: impact on biological processes, human welfare and nutrition. In: Proceedings of the 17th World Congress of Soil Science, 14–20 August, 2002, Bangkok, Thailand. Hassink, J., 1995. Organic matter dynamics and N mineralization in grassland soils. Ph.D. dissertation. Wageningen Agricultural University, Wageningen, The Netherlands. Hassink, J., 1997. The capacity of soils to preserve organic C and N by their association with clay and silt particles. Plant and Soil 191, 77–87. Hassink, J., Whitmore, A.P., 1997. A model of the physical protection of organic matter in soils. Soil Science Society of America Journal 61, 131–139. Jia, S.G., Yang, X.M., Wang, S.P., 1995. New series of soil samplers and their application. Pedosphere 5, 179–182. Jolivet, C., Arrouays, D., Le´ve`que, J., Andreux, F., Chenu, C., 2003. Organic carbon dynamics in soil particle-size separates of sandy Spodosols when forest is cleared for maize cropping. European Journal of Soil Science 54, 257–268.
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Soil & Tillage Research journal homepage: www.elsevier.com/locate/still
Soil organic carbon changes in particle-size fractions following cultivation of Black soils in China Aizhen Liang a, Xueming Yang b, Xiaoping Zhang a,*, Neil McLaughlin c, Yan Shen a, Wenfeng Li a a
Northeast Institute of Geography and Agroecology, Chinese Academy of Sciences, Changchun 130012, China Greenhouse and Processing Crops Research Centre, Agriculture and Agri-Food Canada, Harrow N0R1G0, Canada c Eastern Cereal and Oilseed Research Centre, Agriculture and Agri-Food Canada, Ottawa K1A0C6, Canada b
A R T I C L E I N F O
A B S T R A C T
Article history: Received 14 September 2008 Received in revised form 2 May 2009 Accepted 7 May 2009
Soil texture can be an important control on soil organic carbon (SOC) retention and dynamics. The (clay + silt)-sized SOC pool (SOC < 20 mm) in non-cultivated or grassland soils has been proposed to reach an equilibrium or maximum level named protective capacity. Proper knowledge of SOC in this size fraction in non-cultivated and cultivated Black soils is important to evaluate management-induced changes in SOC in NE China. Twenty-seven paired soil samples (non-cultivated vs. cultivated) were collected in the Black soil zone in Heilongjiang and Jilin provinces. Bulk soil was dispersed in water with an ultrasonic probe and then soil size fractions were collected using the pipette technique for SOC analyses. Soil organic carbon in bulk soil and size fractions was measured by dry combustion. Average content of SOC < 20 mm was 23.2 g C kg1 at the 0–30 cm depth for the non-cultivated soils, accounting for 75.1% of the total SOC at the same depth. There was significant positive relationship between soil clay plus silt content and SOC < 20 mm in non-cultivated soils. Accordingly, a model of the maximum SOC < 20 mm in 0–30 cm depth of non-cultivated Black soils was developed: y = 0.36x where y is the maximum SOC < 20 mm pool (g C kg1) and x is the percentage of clay + silt (<20 mm) content. The average content of SOC < 20 mm was 18.7 g C kg1 at 0–30 cm depth for cultivated soils, accounting for 81.5% of total SOC. This average value of SOC was 4.4 g C kg1 less than the maximum value (23.1 g C kg1) and accounted for 55.0% of the difference of SOC between non-cultivated and cultivated Black soils. Cultivation resulted in 45.0% loss of sand-sized (>20 mm) SOC concentration relative to SOC < 20 mm. This result indicates that SOC < 20 mm and sand-sized SOC both play important roles in SOC dynamics resulting from management practices. This model can be applied to calculate the actual potential to restore SOC for cultivated Black soils under conservation tillage in NE China. ß 2009 Elsevier B.V. All rights reserved.
Keywords: China Black soils Organic carbon Restoring potential Soil texture
1. Introduction Quantifying the potential of cropland soils to restore antecedent soil organic carbon (SOC) will help to evaluate the contribution of cropland soils as a C source or sink to the global C balance. However, there are many uncertainties in SOC dynamics of the soil system (Li, 2002), and these are probably some of the most limiting factors for correctly determining the potential of soil C sequestration (Smith, 2004). Some researchers found that particulate organic C (53–2000 mm) is more sensitive to management change than total SOC (Ellert and Gregorich, 1995; Chan et al., 2002; Six et al., 2002a), and others noted particulate organic C changes were limited before SOC in fine particles reached
* Corresponding author at: 3195 Weishan Road, Gaoxin District, Changchun, Jilin Province 130012, China. Tel.: +86 431 85542234; fax: +86 431 85542298. E-mail addresses: [email protected] (A. Liang), [email protected] (X. Zhang). 0167-1987/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.still.2009.05.002
saturation (Hassink, 1997; Carter et al., 2003). Hassink (1997) proposed that clay and silt (particles <20 mm) in soil physically retain SOC, and named this protective ability a ‘‘capacity factor’’ or the maximum SOC that could be stored in that combined size fraction. Furthermore, this ‘‘(clay + silt)-sized SOC’’ (SOC < 20 mm, for simplicity) could be modeled as a function of total clay + silt in soils as given in Eq. (1). SOC < 20 mm ¼ 4:09 þ 0:37ðclay þ siltÞ
(1)
in which SOC < 20 mm and clay + silt are in g kg1 soil. In NE China, the SOC < 20 mm in some cultivated Black soils was estimated as approximately 75% of total SOC (Zhao et al., 1993), but there is no information on how these values depart from ‘‘the maximum’’ observed prior to cultivation. Estimating this difference would be valuable for understanding both the potential of C sequestration in cultivated Black soils in NE China and the effect of cropping management on C dynamics. Accordingly, the objectives of this study were (1) to determine the maximum amount of
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SOC < 20 mm pool in non-cultivated Black soils and (2) to compare the difference of SOC < 20 mm between cultivated and noncultivated Black soils in NE China. 2. Materials and methods 2.1. Study site The study site was the bow-shaped Black soils (Udolls, US Soil Taxonomy) zone in Heilongjiang and Jilin provinces of NE China (Fig. 1). The topography in Black soil region is characterized by undulating plateau with slopes of 1–58. The Black soil zone is located in the temperate zone with a continental monsoon climate. The mean annual temperature varies between 0.5 8C and 6.0 8C, and the mean annual precipitation varies between 500 mm and 600 mm, with more than 80% occurring in June to September. The landscape was grassland in its natural state and steppe-meadow grasses are the predominant native species. Conversion to cultivated crops started early in the last century, and mass cultivation occurred in the 1940s and 1950s. Traditional cropping practices are continuous soybean in north, corn–soybean rotation in center, and continuous corn in south of the Black soil zone. Typical field management practices include removing all aboveground plant biomass after harvest, incorporating stubble and main roots into soils with two or three handhoeing operations, and forming ridges. More details about study site were given by Liang et al. (2009). 2.2. Soil sampling In 2004 and 2005, 27 pairs of cultivated/non-cultivated neighbor sites were sampled. In each paired site, the cultivated soil was approximately 50–100 m distant from its non-cultivated counterpart, which was never used for crop production. Several criteria were used to confirm sample sites as non-cultivated soils, including consultations with older residents in the local villages, observing vegetation and soil vertical profiles, and comparing
results from the laboratory chemical and physical analysis with published results. The land was converted from natural grassland to cultivated crops less than 100 years ago, and the elders in the local villages knew both where the soil had never been disturbed by cultivation and the reasons why. For some sites, the reasons are based on local protection policy, such as retaining non-cultivated land to provide biomass for use as fuel for heating and cooking in houses, or establishing ecological sites where removal of natural vegetation is prohibited. Also, position on the landscape resulted in some sites not being cultivated, such as a non-cultivated site in a small triangular zone on the edge of a deep natural waterway eroded in the landscape (Fig. 2). Natural vegetations on Black soils are steppe-meadow species and most are the herbage community including Bupleurum scorzonerifolium, Sanguisorba officinalis, Potentilla chinensis, Salix rosmarinifolia, Platycodon grandiflorus (Fu, 1995). Presence of these species is a good indicator of a non-cultivated site. As a final check profiles of non-cultivated soils in the Black soil zone were examined, and laboratory chemical and physical analyses were compared to published data on non-cultivated Black soils. Soil core samples were taken to a depth of 30 cm using a manual soil probe with 2.64 cm internal diameter (Jia et al., 1995) which allowed separation of each soil core into 4 segments including 0– 5 cm, 5–10 cm, 10–20 cm, and 20–30 cm. The diameter of the cutting edge of the probe was 4 mm less than the inside diameter of the barrel which eliminated core compaction resulting from friction between the core and barrel. Five to seven sub-samples were taken at each site and combined into a single composite sample. The soil samples were gently broken to pass a 7 mm sieve and air-dried. Visibly identifiable crop residues were manually removed and discarded. 2.3. Soil physical analysis The bulk density of soil samples was calculated using the inner diameter of the core sampler cutting edge, segment depth and
Fig. 1. Distribution of sampling sites in Jilin and Heilongjiang provinces.
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ground for SOC analyses. All soil samples were free of carbonate, hence SOC content was assumed equal to total C. Soil organic carbon in the soil particle fractions was determined using the FlashEA1112 elemental analyzer (ThermoFinnigan, Milan, Italy). The ANOVA followed by the LSD test was used to examine the effects of tillage on SOC in each size fraction. Pearson correlation was used to evaluate the relationship between the percentages of soil particles and both total SOC and C fractions. These procedures were performed using SAS (SAS, 2004). Statistical significance was determined at the P < 0.05 level except if indicated differently. 3. Results 3.1. Selected properties of non-cultivated and cultivated soils
Fig. 2. An example of a particular non-cultivated site in a triangular zone on the edge of a deep natural waterway eroded in the landscape in Heilongjiang province. A 1.2-m wide trail ran north toward the waterway, and then turned west parallel to the waterway. The triangular zone on the outside of the turn in the trail was a ‘‘waste’’ zone too short for cropped ridges.
oven-dried soil weight (105 8C). Soil water suspensions (soil:water = 1:5) were prepared, allowed to settle for one half hour, and soil pH was measured using an acidity meter (pHS-3B, Leici, Shanghai). Samples were dispersed in 0.1 M NaOH (for acidic soils) or 0.1 M Sodium oxalate (for neutral soils), organic matter was oxidized with H2O2, and soil texture determined by the pipette method (Soil Science Society of China, 2000).
Both non-cultivated and cultivated Black soils in our study were typically clay loams, and soil texture varied little with land use and depth (Table 1). Both non-cultivated and cultivated soils were neutral or slightly acidic with average pH of 7.0 and 6.5, respectively (Table 1). Soil bulk densities at 0–5 cm, 5–10 cm and 10–20 cm depths were lower for the non-cultivated soils than corresponding depths in the cultivated soils, and almost identical for both soils at the 20–30 cm depth (Table 1). Bulk density increased with increasing depth for non-cultivated soils, but not much for cultivated soils (Table 1). Average SOC contents in bulk soils for 0–30 cm depth were lower in cultivated (22.9 g C kg1) than in non-cultivated soils (30.9 g C kg1). Although SOC contents decreased with soil depth for both land uses, the decrease was more pronounced in noncultivated sites (Table 1). 3.2. The model of the maximum SOC < 20 mm
2.4. Soil organic carbon analysis Twenty-five grams of air-dried soil were placed in a 250 ml beaker and 125 ml distilled water was added. The soil suspension was mixed and allowed to settle over night. The suspensions were dispersed by treating with an ultrasonic probe (JY92, Xinzhi, Ningbo, China) for 10 min at 24 kHz (Hassink, 1997; Oorts et al., 2005; Zhao et al., 2006). The ultrasonic energy needed for complete dispersion was 480 J ml1 (Schmidt et al., 1999a,b); suspension temperature was kept <32 8C by using a 50% duty cycle (1 s on/1 s off) and by placing the beaker in a running tap water bath. The dispersed soil suspension was transferred to a 1 l glass cylinder, and the cylinder was capped and shaken end over end to thoroughly homogenize the soil water suspension. Silt plus clay (<20 mm) and clay (<2 mm) fractions were collected by siphoning the suspension at the appropriate depth and time based on Stokes’ law (Soil Science Society of China, 2000), oven-dried at 60 8C, and
Unless otherwise stated, SOC contents of the various size fractions are reported here as g SOC kg1 of bulk soil. Size fractions used here are clay <2 mm, (clay + silt) <20 mm, and sand >20 mm. Sand-sized SOC was calculated by subtracting SOC < 20 mm from total SOC. The SOC < 20 mm obtained here includes water-soluble carbon, which has been shown by an independent work (Fang, 2005) to account for only 0.15–0.20% of total SOC. The average SOC < 20 mm was 23.2 g C kg1 in a 0–30 cm depth of noncultivated soils (Table 2), accounting for 75.1% of total SOC. Claysized SOC was 8.96 g C kg1, accounting for 38.6% of SOC < 20 mm. Subsurface soil (10–30 cm) had lower SOC than the surface layer (0–10 cm depth), which did not occur for the SOC < 20 mm (Fig. 3). The amount of sand-sized SOC accounted for 24.9% of the total SOC for the non-cultivated soil, and decreased substantially with depth. For cultivated soils, the average content of SOC < 20 mm was 18.7 g C kg1 in 0–30 cm depth (Table 2), accounting for 81.5% of
Table 1 Selected soil properties in a depth of 0–30 cm in non-cultivated and cultivated soils. Soil depth (cm)
Clay (%)
Silt (%)
Sand (%)
pH
Bulk density (g cm3)
Total C (g kg1)
Non-cultivated soil 0–5 5–10 10–20 20–30 Weighted mean
35.5 1.44a 36.2 1.17 37.1 1.19 36.1 1.24 36.4
29.2 0.85 27.4 0.84 27.1 0.77 27.0 0.81 27.5
35.3 1.46 36.4 1.27 35.8 1.44 36.9 1.24 36.2
6.91 0.15 6.90 0.16 7.00 0.16 7.09 0.16 7.0
0.94 0.04 1.08 0.04 1.08 0.03 1.17 0.03 1.09
44.2 2.21 34.3 2.33 30.0 1.85 24.5 1.87 31.8
Cultivated soil 0–5 5–10 10–20 20–30 Weighted mean
31.8 1.41 32.1 1.56 35.7 1.56 36.3 1.64 34.7
30.2 0.83 30.2 1.07 28.8 0.95 29.7 1.30 29.6
38.0 1.49 37.7 1.41 35.5 1.44 33.9 1.33 35.8
6.37 0.16 6.36 0.16 6.52 0.14 6.71 0.13 6.53
1.04 0.02 1.19 0.03 1.20 0.03 1.18 0.03 1.17
24.2 1.39 24.1 1.40 23.1 1.64 21.4 1.92 22.9
a
Means plus or minus standard error.
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Table 2 Clay + silt contents in soils, and SOC < 20 mm in non-cultivated and cultivated Black soils (0–30 cm depth). Sample ID
(clay + silt) contents in non-cultivated soils (%)
SOC < 20 mm in non-cultivated soils (g kg1)
SOC < 20 mm in cultivated soils (g kg1)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27
70.8 70.0 67.1 71.4 71.8 69.1 68.1 79.3 63.6 64.3 71.6 67.3 71.9 60.0 61.6 57.1 58.7 58.2 58.2 60.6 53.8 57.1 61.6 52.5 60.2 58.3 59.7
23.2 21.9 20.8 25.4 21.5 28.5 21.1 29.2 26.4 21.7 26.5 28.0 30.7 29.7 29.4 25.7 22.0 27.8 15.9 29.2 16.0 14.5 26.2 12.9 18.3 16.8 18.3
18.9 19.0 16.9 18.3 17.1 23.0 17.9 21.5 25.6 15.5 26.0 27.8 25.9 18.0 24.6 22.7 21.7 21.7 11.9 21.0 13.3 12.9 13.4 8.4 12.7 12.7 15.2
Average
63.8
23.2
18.7
the total SOC while the amount of sand-sized SOC accounted for only 18.5% of total SOC. The amounts of SOC < 20 mm and sandsized SOC at 0–5 cm and 5–10 cm were both lower in cultivated than non-cultivated soils. The clay plus silt content in non-cultivated soils ranged from 51% to 83%. There were linear positive relationships between the
clay plus silt content, and the SOC < 20 mm in all depths, 0–5 cm, 5–10 cm, 10–20 cm and 20–30 cm (data not shown). We did an analysis of covariance with clay + silt content as the covariate, and depth as a factor. The covariate (clay + silt content) was significant (P < 0.05) but depth was not significant (P < 0.05). Consequently, the data for the entire 0–30 cm depth were pooled and a simple linear regression was done with SOC < 20 mm as the dependent variable and clay + silt content of the soil as the independent variable. The intercept for this regression was not significant (P > 0.05) and so a reduced model without intercept was used in the final regression model given in Eq. (2). y ¼ 0:36x
ðr ¼ 0:510; P < 0:01; n ¼ 27Þ
(2)
where y = the maximum SOC < 20 mm (g kg1 soil), x = the percentage of clay plus silt particles (%). Observed and predicted SOC < 20 mm are plotted in Fig. 4. The SOC < 20 mm predicted with the model developed by Hassink (1997) is plotted on the same graph for comparison. 3.3. The theoretical potential to restore SOC for cultivated Black soils We applied our model (y = 0.36x) to cultivated Black soils to estimate the potential restoring capacity of SOC < 20 mm (Table 3). Average estimated maximum of SOC < 20 mm for the 27 cultivated sites was equal to 23.1 g kg1 in the top 30 cm of soil, which was as expected nearly identical to the observed average value (23.2 g kg1) for the 27 non-cultivated soils. Observed SOC < 20 mm (18.7 g kg1) was 4.4 g kg1 less than the maximum in cultivated soil, accounting for a 55.0% difference of total SOC between non-cultivated and cultivated soil. Therefore, average estimated potential to restore SOC < 20 mm of the 27 cultivated sites was 4.4 g kg1 in the top 30 cm of soil. Sand-sized SOC was 3.6 g kg1 less in cultivated soil than the non-cultivated soil, accounting for 45.0% of the difference of total SOC between these two soils.
Fig. 3. Contents of total SOC and SOC in different particles. Clay-sized SOC (<2 mm) is also included in the SOC < 20 mm, and consequently, the sand-sized (>20 mm) SOC, SOC < 20 mm and clay-sized SOC pools do not add up to the total SOC (error bar is standard error, n = 27).
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Fig. 4. Plot of observed SOC < 20 mm vs. percent clay + silt. Line a is the maximum SOC < 20 mm predicted by Hassink’s model, line b is maximum SOC < 20 mm predicted by our regression model: y = 0.36x (r = 0.510, n = 27), and lines c and d are the 95% confidence intervals for our model.
Table 3 Comparison of total SOC, SOC < 20 mm and sand-sized SOC in non-cultivated and cultivated Black soils (0–30 cm).
Total SOC SOC < 20 mm Sand-sized SOCa
Non-cultivated soil (g kg1)
Cultivated soil (g kg1)
Difference (g kg1)
30.9 23.1 7.8
22.9 18.7 4.2
8.0 4.4 3.6
SOC loss in size fraction/ total SOC loss
55.0% 45.0%
a Sand-sized SOC was calculated as the difference between total SOC and the SOC < 20 mm.
4. Discussion The total SOC content in the non-cultivated soils was the highest near the surface in the top 5 cm and steadily decreased with increasing depth, but in the cultivated soils, the soil organic carbon was much lower and relatively constant in the 0–20 cm depth (Table 1). Cultivation both mixed the soil in the plow layer and promoted mineralization of the SOC resulting in nearly constant SOC in the 0–20 cm depth (Liang et al., 2008). The current study was based on an assumption that the amount of SOC < 20 mm had a limit and that C in non-cultivated Black soils was saturated. The non-cultivated soils have not been disturbed over thousands of years, and consequently, it is reasonable to expect that SOC has reached an equilibrium where annual input of C from the natural vegetation biomass is equal to the annual output of C via mineralization or consumption by soil micro flora and fauna. Hassink’s model was developed from published SOC data for soils from around the world with a broad range of both texture and mineralogy while our model is based on a much smaller data set from the Black soils in NE China. The slopes for the two models are nearly identical, 0.36 g C kg1 clay + silt for our model compared to 0.37 g kg1 clay + silt for the Hassink model. Our model does not have an intercept while Hassink’s model has an intercept of 4.09. Different parameters are expected, because the maximum of SOC associated with the fine size fraction is influenced by many factors, including clay mineralogy and climate (Li, 2001; Six et al., 2002b; Haider and Guggenberger, 2002). Clay minerals in soils used in this study were dominated by 2:1 type minerals, and so we did not have variations in clay mineralogy to influence SOC < 20 mm. Australian soils investigated in Hassink’s study (1997) had substantially lower SOC, likely a result of high temperatures, low rainfall and limited input of plant residues, and were excluded
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from his model. All of these factors provide an indication of the complexity of SOC < 20 mm. Hassink’s model over-predicted SOC < 20 mm measured in our study by an average 19.2% (Fig. 4). A model developed for the local data generally fits the local data much better than a more general model such as the Hassink model which is based on published data for a wide range of soils around the world. Zinn et al. (2005) also proposed that simple linear functions relating SOC to clay + silt may not be the best descriptors for every region and different functions must be tested for each region. The SOC capacity models of soils such as Eq. (2) and the Hassink (1997) model (Eq. (1)) provide a means to estimate SOC change under cultivation, and the mechanism and potential for soil C sequestration. The SOC < 20 mm pool increases with increasing clay + silt contents, which is the main mechanism of the textural control on SOC retention (Zinn et al., 2007). In a study applying this function to Baijiang (Albolls, US Soil Taxonomy) cultivated soils in Jilin Province, China, Zhao et al. (2006) found that the actual content of SOC < 20 mm was greater than the maximum calculated from Hassink’s model. The agricultural use of Black soils led to great SOC loss from sand-sized C. This result was different from the finding that the change of sand-sized C was limited when SOC in fine particles reached saturation (Hassink, 1997; Carter et al., 2003). Although SOC < 20 mm was believed to be relatively stable and played an important role in maintaining C levels, the share of 55.0% total SOC loss from this fraction in cultivated Black soils suggests that attention needs to be paid to the fate of C both in fine and coarse particles when studying effect of agriculture on C dynamics. However, Jolivet et al. (2003) found that SOC < 20 mm decreased by only 20% after 30 years of cultivation, so long-term C accumulation was attributed mainly to the clay + silt fraction, which protected complex molecules supplied by decomposition of sand-sized organic matter. The soil’s capacity to accumulate newly added carbon was determined by the degree of saturation of SOC < 20 mm (Hassink, 1995; Hassink and Whitmore, 1997; Jolivet et al., 2003). Six et al. (2002b) suggested that with increased C input, the increase in SOC content was smaller when the protective capacity approached saturation. Our present studies are focused on the potential of conservation tillage as a management practice to prevent soil erosion and to restore SOC in the Black soils in NE China. The model will play a key role in understanding SOC dynamics in these studies. Certainly, clay + silt pool is not homogeneous and C in each particle size is probably retained by different mechanisms and has different C:N ratios (Zinn et al., 2007). While the clay + silt pool concept appears useful, it is a simplification and clearly, further study is needed. 5. Conclusion Paired soil samples were obtained from 27 non-cultivated and adjacent cultivated soil sites in the Black soil zone in NE China, and SOC determinations were made in different size fractions. In noncultivated Black soils, significant positive relations were found between clay + silt contents and both total SOC and SOC < 20 mm. Based on those relations and on the assumption that SOC < 20 mm was saturated, a model of the maximum SOC < 20 mm in noncultivated Black soils was developed. In cultivated Black soils the mean difference between present SOC < 20 mm and estimated potential maxima was 4.4 g C kg1, or 55.0% of total SOC difference between non-cultivated and cultivated soils. The other 45.0% of total C lost was from the sand fraction. The model provides a tool to estimate the potential of cultivated Black soils restoring SOC < 20 mm, but more work is needed to validate and refine this model.
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