Introduction

There has been evidence that good land management such as organic amendments can increase soil organic carbon (SOC) in cropland1,2. Interestingly, a recent report showed that long-term straw incorporation and manure application not only led to SOC enhancement, but also resulted in more carbon sequestration in the form of carbonate in the arid cropland of northern China3. Similar finding was previously reported for the Saskatchewan soils in Canada4. In addition, there is also evidence of land use and climate change impacts on soil inorganic carbon (SIC) stocks. Particularly, land use has significantly affected SIC levels in cultivated soils in China, showing an increase in northwest China5. Similarly, Mikhailova and Post6 and Su et al.7 reported significantly higher SIC stocks in continuously cropped field relative to grassland. These results suggest that there may have been enhancements in SIC stock due to various reasons (e.g., fertilization and straw return into soils) and carbonate may be an important form for carbon sequestration in arid and semi-arid regions that cover about 35% of the Earth’s land surface, thus would play a big role in the global carbon cycle.

Soil inorganic carbon, primarily calcium carbonate, is the most common form of carbon in arid and semi-arid regions. The SIC pool consists of two major components: the lithogenic carbonate (LIC) and pedogenic carbonate (PIC). The former originates as detritus from parent materials, mainly limestone, whereas the latter is formed by dissolution and re-precipitation of LIC or through dissolution of carbon dioxide (CO2) into HCO3−, then precipitation with Ca2+ and/or Mg2+ originating from non-LIC minerals (e.g., silicate weathering, dust and fertilizers).

While there have been indications of PIC having potential for carbon sequestration and climate mitigation8,9,10, there have been limited studies to quantify carbon sequestration as PIC for the vast arid and semi-arid regions, where the SIC stock is 1–9 times higher than the SOC stock11,12. On the one hand, some studies indicated that PIC accumulation was extremely low (<3 g C m−2 y−1) in the arid and semi-arid regions of Canada, USA and New Zealand4,11. On the other hand, limited studies showed significant PIC accumulation (>10 g C m−2 y−1) in the arid and semi-arid lands of northern China3,13. Apparently, more studies are needed to evaluate the PIC dynamics under various land managements.

The News Report entitled “Have desert researchers discovered a hidden loop in the carbon cycle” (http://www.sciencemag.org/content/320/5882/1409.full.pdf), regarding the reports of significant CO2 uptake (>100 g C m−2 yr−1) in deserts14,15, prompted a great effort to better understand the carbon cycle in the arid region of northwest China. As a part of the effort, a survey was conducted in a typical arid area, the Yanqi Basin that is near the northeast edge of the Taklamakan Desert, China. More than 100 soil samples were collected from desert land, shrub land and cropland and SOC and SIC stocks were compared between land use types16.

In this study, we use data collected from the Yanqi Basin to investigate the impacts of land use change (conversion of shrub land to cropland) on PIC dynamics. In addition, we obtained 18 archived surface soil samples from a controlled long-term experiment (LTE) site in a typical semi-arid region, the Loess Plateau, which was established in 1979 with a mono-cropping system dominated by maize-wheat rotations. There were various fertilization treatments, which can be grouped into with and without organic material addition. Contents of SOC and SIC and their 13C compositions were measured for all soil samples. The objective of this work was to study the dynamics of both SOC and PIC in these arid and semi-arid regions and to evaluate impacts of land management on various soil carbon pools.

Results

Surface soil properties in the Yanqi Basin

In general, soil pH was high (7.8–9.1), showing no significant difference between the shrub land and cropland (Table 1). Soil electric conductivity (EC) was much greater in the shrub land than in the desert land and cropland. Overall, extractable Ca and Mg in surface soil were high (>11.7 and >0.22 g kg−1) except in the desert lands (<8 and <0.08 g kg−1). Surface soil showed a range of 3–16 g kg−1 for SOC and 10–30 g kg−1for SIC. Contents of extractable Ca and Mg, SOC and SIC followed the same order: cropland > shrub land > desert land. The δ13C value in root showed a wider range (from −23.4‰ to −28.3‰) for C3 plants but a smaller range (from −11.4‰ to −12.6‰) for C4 plant (i.e., maize). The δ13CSOC values in surface soil (−21.7‰ to −24.8‰) were close to those in C3 plants. Despite modest difference in the δ13C value of root between the shrub land (−25.38‰) and cropland (−20.55‰), the δ13CSOC value was similar (close to −24‰) between the two groups. Our estimated abundance of C4 plant varied much widely in the shrub land (from 0 to 29%) than in the cropland (from 2% to 20%). On average, relative contribution of C4 plant to SOC was 12% in the cropland and 10% in the shrub land.

Table 1 Surface soil pH, electric conductivity (EC), exchangeable Ca and Mg, organic carbon (SOC), inorganic carbon (SIC) and stable13C in root and SOC and estimated abundance of C4 plant for each site.
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Soil carbon variations in the Yanqi Basin

Our data showed large ranges in both SOC (1–12 kg C m−2) and SIC (6–45 kg C m−2) stocks over the 0–100 cm (Fig. 1). Both the SOC and SIC stocks were significantly higher in the cropland than in the desert land. Although there was no significant relationship between the SOC and SIC stocks in soils collected from the cropland, the whole dataset showed a strong positive correlation (r = 0.80, P < 0.001) between the SOC stock and SIC stock.

Figure 1
figure 1

Relationship between soil organic carbon (SOC) and inorganic carbon (SIC) stocks (0–100 cm) for the Yanqi Basin:

2 profiles from desert land (stars), 1 from semi-desert land (diamonds), 9 from shrub land (triangles) and 13 from cropland (circles). Significance of the linear regression was marked with three (P < 0.001) asterisks.

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Figure 2 presented the relationship between SIC content and δ13CSIC value using all soil samples, including subsoils. There was a relatively wider range in δ13CSIC value, from −6.7‰ to 0.58‰, with the lowest in the cropland and highest in the desert land. Our data exhibited a profound linear relationship (r = 0.72, P < 0.001) between the δ13CSIC value and SIC content. Higher SIC contents (>30 g C kg−1) were found in those with significant 13C depletion (i.e., δ13CSIC < −4‰).

Figure 2
figure 2

Relationship between SIC content and stable C isotopic composition in SIC (δ13CSIC).

Significance of the linear regression was marked with three (P < 0.001) asterisks.

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We estimated that PIC stock ranged from 0 to 34 kg C m−2 over the 0–100 cm. Our data showed a strong correlation (r = 0.75, P < 0.001) between PIC stock and SOC stock (Fig. 3). Overall, PIC stock was higher in the cropland than in the shrub land (also see Table 2). Our analyses suggested that on average, an increase of 1 kg C m−2 in the SOC stock was associated with an increase of 1.9 kg C m−2 in the PIC stock in this region.

Table 2 Means and standard deviations (in parentheses) of SOC, SIC, PIC and LIC stocks (kg C m−2) for cropland (n = 9) and shrub land (n = 9).
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Figure 3
figure 3

Relationship between pedogenic carbonate (PIC) and SOC stocks (0–100 cm).

Significance of the linear regression was marked with three (P < 0.001) asterisks. Symbols are the same as in Figure 1.

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Table 2 illustrated that average SOC stock in the top 0–30 cm was significantly higher in the cropland (4.6 kg C m−2) than in the shrub land (3.2 kg C m−2), but SOC stock in the subsoil (30–100 cm) was the same (4.6 kg C m−2). However, SIC stock showed a much bigger difference in the subsoil (3.9 kg C m−2) than in the topsoil (1.2 kg C m−2) between the cropland and shrub land. Similarly, difference in PIC was bigger in the subsoil than in the topsoil. Statistical analyses indicted a significant increase of SOC in the topsoil (P < 0.01), but SIC and PIC in the subsoil (P < 0.05) in the cropland. On average, the SOC, SIC and PIC stocks over the 0–100 cm increased by 1.4, 5.1 and 5.2 kg C m−2, respectively, as a result of land use change.

Soil carbon contents and isotopic compositions at the LTE site

Surface SOC content was significantly higher with addition of organic materials (8.1–10.1 g kg−1) than without (6.7–7.1 g kg−1) at the LTE site (Fig. 4). Our data revealed an increasing trend in SOC over time in the former but little change in the latter. Surface SIC content was <10 g C kg−1 in the Loess, which was much lower than those in the Desert Soil of the Yanqi Basin. There seemed to be no significant difference in surface SIC between the two groups. However, surface SIC showed an increasing trend over time with the addition of organic materials. Overall, there was a positive relationship (r = 0.535, P < 0.05) between the SIC and SOC contents at the LTE site.

Figure 4
figure 4

Values and error bars of SOC, SIC and δ13C of SIC in soils (0–20 cm) collected in 1996, 2002 and 2009 from the LTE site.

(a) SOC content, (b) SIC and (c) δ13C of SIC with (empty bars) and without (solid bars) addition of organic materials.

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The δ13CSIC value varied from −6.2‰ to −7.1‰ at the LTE site (Fig. 4), which was close to the most negative values observed in the Yanqi Basin (see Fig. 3). In general, the δ13CSIC value was more negative with the addition of organic materials relative to those without. The greatest difference was found in soils collected in 2009. As a result, the difference in δ13CSIC value between 2009 and 1996 was much larger with organic material addition than without.

Carbon stocks and accumulation rates at the LTE site

Surface SOC stock at the LTE was significantly lower than SIC without the addition of organic materials (Table 3). For example, SOC stock in the 0–20 cm remained at ~1.8 kg C m−2 during the period of 1996–2009 whereas SIC stock increased from 1.92 kg C m−2 in 1996 to 2.21 kg C m−2 in 2009. We estimated that PIC stock was in a range of 1.68–1.91 kg C m−2. Our analyses suggested that the accumulation rates of SOC, SIC and PIC in the 0–20 cm were 3.5, 21.8 and 9.3 g C m−2 y−1, respectively.

Table 3 Means and standard deviations (in parentheses) of SOC, SIC, PIC and LIC stocks (kg C m−2) and their change rates (g C m−2 y−1) and regression coefficients (in parentheses) in 0-20 cm at the LTE.
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Long-term application of organic materials resulted in a greater increase in SOC stock relative to SIC stock in the top 20 cm. In particular, SOC stock with addition of organic materials varied from 2.17 kg C m−2 in 1996 to 2.63 kg C m−2 in 2009, which was significantly higher than those (1.74–1.85 kg C m−2) without organic materials addition. While SIC stock was lower than SOC stock, SIC stock showed a greater increasing trend over time, i.e., from 1.98 kg C m−2 in 1996 to 2.59 kg C m−2 in 2009. Similarly, estimated PIC enhanced from 1.6 kg C m−2 in 1996 to 2.15 kg C m−2 in 2009. Our analyses showed that long-term application of organic materials led to much greater accumulation rates of SOC, SIC and PIC, i.e., 35.4, 46.6 and 42 g C m−2 y−1, respectively.

Discussion

Relationship between SOC and SIC stocks

Our estimated stocks (0–100 cm) of SOC and SIC are 7.9 and 37 kg C m−2 in the shrub land and 9.2 and 42 kg C m−2 in the cropland of the Yanqi Basin. The SOC stock (0–20 cm) in the Loess is comparable to that in the shrub land whereas SIC stock is much lower in the former than in the latter. Interestingly, both datasets reveal a strong positive correlation between the SOC stock and SIC stock, which is in disagreement with an earlier report13 that showed a negative relationship between SOC and SIC for semi-arid regions in northwest China. However, our finding of positive correlation between the SOC and SIC stocks is consistent with recent reports in soils of China’s arid regions, i.e., near the eastern boundary of Tengger Desert, Inner Mongolia17 and at the edge of Badan Jaran Desert, Gansu7. These inconsistent findings imply the complex relationship between SOC and SIC because of various processes involved with their accumulations and transformations and decoupling of these processes over time and space18.

The dissolution and precipitation of carbonate involve two main reactions:

In general, an increase in soil CO2 concentration, as a result of SOC decomposition under high SOC level, would lead to production of both and H+. When there is enough H+ to create acidic conditions, carbonate dissolution may occur, in which a negative relationship between SOC and SIC is established. On the other hand, the production of can drive the reaction (2) to the right when there is no Ca limitation, resulting in precipitation of carbonate, which could lead to a positive relationship between SOC and SIC. Soils in arid regions usually have a pH greater than 7.5 and are rich in available Ca2+ and/or Mg2+. Thus, we postulate that a positive relationship would be more common, particularly in higher pH soils such as in the Yanqi Basin.

Soil carbonate accumulation rate

There have been a number of studies of the magnitudes and spatial distributions of SIC at regional scales in China5,19. However, detailed analyses of SIC accumulation are limited. A study of China’s grassland showed that SIC stock in the top 10 cm had declined (at a rate of 26.8 g C m−2 year−1)20 whereas the data from the cropland at the LTE show an increasing trend (32–55 g C m−2 year−1) for the SIC stock (0–20 cm). There was also evidence of significant accumulation of SIC in other parts of the cropland in northern China3. The larger decrease in the SIC stock of the grassland was observed in those locations with stronger soil acidification20 whereas the larger increase in the SIC stock of the cropland was found under long-term application of organic materials (see Table 3).

Our estimated PIC accumulation rates in the cropland (>20 g C m−2 year−1) are significantly higher than previously reported values (<3 g C m−2 y−1) for Canada, USA and New Zealand4,11, but comparable to the earlier reported rates (10–40 g C m−2 y−1) for the Aridisols in the northwest China13 and the recent reported rate for the cropland of northern China3. The large discrepancy is probably attributed to the differences in various factors between these regions, e.g., Ca2+ and Mg2+ availability11,21. Soils in the Mojave Desert and semi-arid region of Canada may be limited by calcium4,21,22 whereas the croplands in northern China may have various sources of Ca and/or Mg, including fertilizers, dust, irrigation water and weathering of calcium/magnesium silicate minerals3. The extractable Ca in the Yanqi Basin is significantly higher in the cropland and shrub land than in the desert land (Table 1), indicating that groundwater (accessible in the cropland and shrub land) may play a role in supplying Ca.

Implications of intensive cropping for soil carbon dynamics

There have been inconsistent finding on the impacts of land use changes for soil carbon dynamics. On the one hand, there is evidence that tillage during farming may cause a decline in SOC in many regions, e.g., temperate and tropical regions23,24,25. On the other hand, recent studies based on some LTEs revealed an increasing trend in SOC of the topsoil in the cropland of arid and semi-arid regions in northwest China2,26. The decline of SOC in the temperate and tropical regions is due to enhanced decomposition as a result of tillage whereas the increase of SOC in the arid and semi-arid regions is associated with fertilization and irrigation that lead to enhanced plant growth and subsequent increased organic carbon inputs into the topsoil1,27,28.

Our data demonstrate a significant increase of SOC in the topsoil and an increase of PIC in the subsoil following the conversion of shrub land to cropland in the arid region. Given that the cropland in the Yanqi Basin was converted from shrub land 60 years ago, one could estimate the PIC accumulation rate in the cropland by assuming that the difference between the cropland and shrub land was representative of the carbonate accumulation in the cropland over the past 60 years. This approach yielded an accumulation rate of 87 g C m−2 year−1 for PIC over the 0–100 cm for the cropland in the Yanqi Basin.

There may be some uncertainties in our estimates of PIC stock and accumulation due to the assumptions made the relative distributions of SOC decomposition, root respiration and atmospheric CO2 mixing. To assess these uncertainties, we conducted a sensitivity analysis. As demonstrated by Table 4, changing the contribution of SOC decomposition (thus root respiration) to CO2 production showed little effect on PIC estimation; increasing the contribution of the atmospheric CO2 yielded a higher PIC fraction in the soils of the shrub land (thus a lower accumulation rate of PIC in the cropland). However, the assumption of 30–90% of soil CO2 originating from the atmosphere implies that the arid land is a considerable CO2 sink, which has been questioned29. Nevertheless, despite the uncertainties, our analyses from this study and Wang, et al.30 indicate that intensive cropping with sound agricultural practice (particularly applications of organic materials) can lead to enhancements of both SOC and carbonate in arid regions. Yet, more studies are still needed to show and quantify carbonate accumulation and to understand the regulating mechanisms under various environments and time scales.

Table 4 Estimated PIC percentage and stock over 0–100 cm for cropland and shrub land in the Yanqi Basin, using different values for relative contributions (%) of SOC decomposition (ASOC, i.e., 1-a-β) and atmospheric CO2 (λ).
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Methods

The Yanqi Basin is located on the southeastern flank of the Tianshan Mountain, with a small variation in altitude (from 1030 to 1160 m). Parent material is primarily alluvium, composed of calcareous silt and sand, originating from weathered limestone from the mountain. Main soil types are Brown Desert Soil and Alluvial Soil that are classified as a Haplic Calcisol and Calcaric Cambisol, respectively31. Soils are characterized with high pH (from 8 to 9.4) and high sand/silt contents32. The sampling area spans both sides of the Kaidu River (Fig. 1), which has various land uses, including desert land, shrub land and cropland. The shrub land and cropland cover approximately 1000 km2 with 75% as cropland that was converted from shrub land 60 years ago and had been used for farming with regular applications of mineral fertilizers (i.e., urea and calcium superphosphate) and often combined with organic materials (e.g., farmyard manure). The area has an annual precipitation of about 80 mm, annual evaporation of >2000 mm, but abundant groundwater 1–2 m below the land surface. The lands except the desert land have access to groundwater. The croplands have irrigation systems that extract water from the Kaidu River whereas the other lands rely on rainfall and groundwater. Both ground water and river water are rich in various salts, including Ca and Mg ions.

The LTE site has annual precipitation of approximately 540 mm, with 60% from July through September. The soil with a texture of silt loam is classified as Calcarid Regosols31. The surface soil had a pH of 8.2, SOC of 6.2 g kg−1 and total nitrogen of 0.95 g kg−1 prior to the LTE. There are six fertilization treatments: no fertilization, nitrogen fertilization, nitrogen-phosphorus fertilization (NP), manure fertilization, nitrogen fertilization with straw return and NP fertilization with manure. We group these into two: with (the last three treatments) and without (the first three treatments) organic material addition. Each plot is 16.7 m by 13.3 m with a buffer zone of 1.0 m between plots. Detailed description of the experiment was reported by Fan, et al.26.

Soil sampling and analyses

The survey was conducted during August and November, 2010 in the Yanqi Basin, with soil samples and belowground roots collected from various land uses. We randomly selected 25 sites (3 in desert land, 9 in shrub land and 13 in cropland) and sampled one pit (70 cm wide, 100 cm deep) at each site. We collected soil samples from five layers for most soil profiles, i.e., 0–5, 5–15, 15–30, 30–50 and 50–100 cm. Each layer was sampled evenly across the 70 cm width to obtain 1000–2000 g of soil that was then air-dried, thoroughly mixed and sieved to pass a 2-mm screen. We measured soil physical properties, i.e., water content, bulk density and rock content. Soil pH and EC were measured using a soil:water (1:5) mixture. Exchangeable Ca and Mg were measured by using a flame-atomic absorption spectroscopy with a modified procedure of Mômmik33, which included pretreating soils with NH4Cl, removing the liquids to eliminate the possible overestimation due to the dissolution of CaCO3 and extracting the pretreated soils with 1 M NH4Ac. Representative soil and root samples were ground to pass a 0.25 mm screen for analyses of SOC and SIC contents and the stable isotopic compositions in SOC, SIC and root. Detailed procedures were reported by Wang, et al.16 who presented data from three desert sites and nine sites each for shrub land and cropland. In this study, we also included SOC and SIC data of four extra profiles from cropland.

The LTE site had archived surface soil (0–20 cm) samples that were collected in fall each year by mixing 6 cores (in 5-cm-diam) of soil in each plot. We obtained samples collected in 1996, 2002 and 2009 with three replicates. SOC and SIC were determined using a CNHS-O analyzer because of small sample size. For SOC measurement, 1 g soil was pretreated with 10 ml 1 M HCl for 12 hours to remove carbonate, then combusted at 1020°C with a constant helium flow carrying pure oxygen to ensure completed oxidation of organic materials. Production of CO2 was determined by a thermal conductivity detector. Soil total carbon (STC) was measured using the same procedure without pretreatment of HCl. SIC was calculated as the difference between STC and SOC. Stable carbon isotope was measured using an isotope ratio mass spectrometer (Delta Plus XP, Thermo Finnigan MAT, Germany), with SOC analyzed at the State Key Laboratory of Lake Science and Environment (SKLLSE), Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences (CAS) and SIC at the Nanjing Institute of Geology and Paleontology, CAS34. For 13C in SOC, CO2 was collected in the same way as that for SOC. For 13C in SIC, CO2 was collected during the reaction of soil with concentrated H3PO4.

Estimation of pedogenic carbonate

Following Landi, et al.4 and Wang, et al.30 , PIC was calculated as:

where δ13CSIC, δ13CPM and δ13CPIC were the stable 13C in carbonate for the bulk SIC, parent material and pure PIC, respectively. The value of δ13CPM was set differently for the two areas. For the Yanqi Basin, we used the largest δ13C value (0.58‰) that was obtained from a sample on the desert land to the south of the Kaidu River (see Fig. 5) and similar to these δ13C values reported for the surface soils in the Taklamakan Desert35. For the LTE site, we set δ13CPM as −1‰ that was the highest found in the Loess36. Because it was not possible to sample PIC in both areas, we calculated δ13CPIC using the following approach.

Figure 5
figure 5

Map and sampling area in the Yanqi Basin and the location of the long-term experiment (LTE) in Pingliang, Gansu.

The maps were created using the ARC GIS 9.3. Symbols for the Yanqi Basin are the same as in Figure 1. Soil carbon contents were measured for all profiles, but δ13C only determined in those profiles marked with closed symbols.

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For the LTE site that is under long-term maize-wheat rotation, δ13CPIC was calculated from the stable 13C in SOC (δ13CSOC), according to Wang et al.30:

where 14.9 represented the sum of two isotopic fractionation, i.e., diffusion (4.4) and carbonate precipitation (10.5)37,38,39.

For the Yanqi Basin with unknown history of cropping and vegetation, δ13CPIC was calculated as:

where δ13CSOC,δ13CC3 and δ13CC4 were theδ13C in SOC, C3 and C4 plants and α and β the relative contributions of C3 and C4 plants to total CO2 production, respectively.

There was evidence that during the growing season, root respiration might exceed SOC decomposition in arid and semi-arid ecosystems40,41, which was due to low SOC contents. Taking into account that there was no root respiration during non-growing season, it was reasonable to assume that on an annual base, SOC decomposition and root respiration would make equal contribution to CO2 production, i.e., α + β = 0.5. Despite uncertainties in this assumption, the δ13CRoot and δ13CSOC values were very close except at three maize sites (see Table 1). Thus, we consider that the assumption would not affect the main results/conclusions.

It is well known that δ13C value in SOC provides a means to trace abundance of C3/C4 vegetation42,43. Thus, the relative contributions of C3 and C4 plants were calculated as:

For those sites without data, δ13CC3 or δ13CC4 were set as the average values for the Yanqi Basin, i.e., −25.4‰ and −12‰, respectively (see Table 1).

Previous studies indicated that atmospheric CO2 might be transferred into soil pores under low rates of soil respiration37,44,45, which would alter the isotopic composition of soil CO2. Therefore, for those sites with surface (0–15 cm) SOC less than 7 g kg−1 (only six sites: three in the desert land and three in the shrub land), we calculated δ13CPIC as:

where δ13CAir was the stable 13C composition in the atmospheric CO2 and λ the proportion of CO2 originating from the atmosphere. We set δ13CAir to −8‰ and λ value to 0.7, 0.5, 0.3 and 0.1 for the 0–5, 5–15, 15–30 and 30–50 cm, respectively.

Statistical analyses

Linear regression analyses were carried out to evaluate the relationships between various carbon variables (i.e., SOC vs. SIC, SOC vs. PIC and SIC vs. δ13CSIC). Student’s t-test was used to determine the significance in the differences in carbon stocks between the cropland and shrub land. All statistical analyses were performed using the SPSS 16.0 software.

Additional Information

How to cite this article: Wang, X.J. et al. Carbon accumulation in arid croplands of northwest China: pedogenic carbonate exceeding organic carbon. Sci. Rep. 5, 11439; doi: 10.1038/srep11439 (2015).