Changes in Carbon and Nitrogen of Chernozem Soil Along a Cultivation Chronosequence in a Semi-Arid Grassland

European Journal of Soil Science, December 2009, 60, 916–923 doi: 10.1111/j.1365-2389.2009.01174.x

Changes in carbon and nitrogen of Chernozem soil along a cultivation chronosequence in a semi-arid grassland

Q. W ANGa, L. Z HANGb, L. L Ia, Y. B AIa, J. C AOa & X. H ANa aState Key Laboratory of Vegetation and Environmental Change, Institute of Botany, the Chinese Academy of Sciences, 20 Nanxincun Xiangshan, Beijing 100093, China, and bInstitute of Microbiology, the Chinese Academy of Sciences, 1 West Beichen Road, Beijing 100101, China

Summary Changes in the carbon (C) stock of grassland soil in response to land use change will increase atmospheric CO2, and consequently affect the climate. In this study we investigated the effects of land use change on soil organic C (SOC) and nitrogen (N) along a cultivation chronosequence in the Xilin River Basin, China. The chronosequence consisted of an undisturbed meadow steppe, a 28-year-old cropland and a 42-year-old cropland (abbreviated as Steppe, Crop-28 Y and Crop-42Y, respectively). Crop-28Y and Crop-42Y were originally created on the meadow steppe in 1972 and 1958, respectively. The soil samples, in ten replications from three depth increments (0–10, 10–20 and 20–30 cm), were collected, respectively, in the two cropland fields and the adjacent undisturbed steppe. Bulk density, SOC, total N and 2 m KCl-extractable mineral N including ammonium and nitrate were measured. Our results showed that the greatest changes in the measurements occurred in the 0–10 cm soil depth. The SOC stock in the upper 30-cm soil decreased by 9.83 Mg C ha−1 in Crop-28Y and 21.87 Mg C ha−1 in Crop-42Y, which indicated that approximately 10 and 25% of the original SOC of the steppe had been emitted over 28 and 42 years, respectively. Similarly, the total N lost was 0.66 Mg Nha−1 and 1.18 Mg N ha−1, corresponding to approximately 9% and 16%, respectively, of the original N at the same depth and cropping duration as those noted for SOC. The mineral N concentration in the soil of both the two croplands was greater than that in the steppe soil, and the ammonium-N was less affected by cultivation than the nitrate-N. The extent of these changes depended on soil depth and cropland age. These effects of cultivation were much greater in the top 10 cm of soil than in deeper soil, and also greater in Crop-42Y than in Crop-28Y. The findings are significant for assessing the C and N sequestration potential of the land use changes associated with grassland conversion, and suggest that improved management practices are needed to sequester SOC and total N in the cropped soil in a semi-arid grassland.

Introduction versa (Kern & Johnson, 1993; Houghton, 2003), and consequently determine their future C storage potential. Grasslands cover over 40% of the world’s land area, and store Growing interest in the potential of soils to sequester atmo- more than one-third of the total terrestrial carbon (C), of which spheric CO2 has stimulated considerable efforts to monitor the more than 70% is stored in the upper 100 cm of soils (White et al., changes in soil organic C (SOC) upon conversion of grassland 2000). Moreover, diverse grassland soils can function as either a ecosystems (Tiessen et al., 1982; Cambardella & Elliott, 1992; source or a sink for atmospheric CO2 (Fisher et al., 1994; White Schuman et al., 1999; Conant et al., 2001; Dao et al., 2002; et al., 2000; Follett et al., 2001). Therefore, grassland soils are a critical component of the global C cycle. Carbon dynamics in these Schwendenmann & Pendall, 2006; Don et al., 2007; Maquere ecosystems are controlled by various ecosystem processes, such et al., 2008). The introduction of crop production on grasslands is as photosynthesis and decomposition, both of which are affected one of the most prevailing land-use changes worldwide, because by land-use practices (Ganjegunte et al., 2005). Thus, the land- of rapid population growth (Whitbread et al., 1998; Bronson et al., use changes in grasslands are the driving forces that can largely 2004; Hart, 2008; Zhang et al., 2008). White et al. (2000) reported determine their transition from a C sink to a C source or vice that approximately 20% of original grasslands had been converted to croplands in Mongolia, Russia and China, and continuous

Correspondence: Q. Wang. E-mail: [email protected] expansion of agricultural land into grasslands was threatening the Received 16 April 2008; revised version accepted 16 June 2009 capacity of soil as a C sink and its potential to sequester C. When

© 2009 The Authors 916 Journal compilation © 2009 British Society of Soil Science Changes in soil C and N along a cultivation chronosequence 917 grasslands are converted to croplands, the removal of native veg- pools by comparing SOC and N concentrations and stocks along etation and cultivation reduce surface cover and destabilize soils, a cultivation chronosequence in this area of Inner Mongolia. and thereby enhance microbial decomposition of SOC, leading to the loss of organic carbon (Tiessen et al., 1982; Dumanski et al., Materials and methods 1986; Angers et al., 1992; Whitbread et al., 1998). The extent of C loss depends on grassland types, climatic and environmental Study area conditions, initial soil properties and the period of time follow- The study area is located in the southeast of the Xilin River ◦ ◦ ing the establishment of cropland (Mann, 1986; Whitbread et al., Basin, Inner Mongolia, China (Figure 1, 43 30N, 116 49E, 1998; White et al., 2000). Mann (1986) found that the soil with a 1350–1400 m above sea level). The area has a strong continen- large C content lost at least 20% of its initial organic C after cul- tal semi–arid climate with a warm summer (daily temperature ◦ tivation; in contrast, in the soil with very small initial C contents, ranges from 11.5 to 19.0 C) and a very cold winter (daily tem- ◦ the organic C increased slightly. Whitbread et al. (1998) reported perature ranges from–23 to 1.5 C). The growing season is from that a Grey Clay soil that had been cropped for more than 40 years May to September. The mean annual precipitation is approxi- lost 51% of its original total C, while an adjacent Grey Clay that mately 350 mm, more than 80% of which falls during June to had been cropped for only 2 years lost 26% of its total C. Guo September (Chen, 1988; Wang et al., 2000). The soil is classified & Gifford (2002) conducted a meta-analysis of worldwide studies as a Chernozem (loam: 52% sand, 27% silt and 21% clay in the of SOC upon land-use changes and concluded that the largest soil 0–10 cm-depth layer) in the Chinese soil taxonomic system, or C loss occurred after the land-use change from grassland to crop- as a Ustoll in the US Soil Taxonomy. The natural vegetation is land. Significant C loss under long-term cultivation might be up to meadow steppe, which originally covered the entire study area. It 60–70% of original organic C (Dumanski et al., 1986). However, has 29–39 plant species per square metre, dominated by Filifolium Breuning-Madsen et al. (2009) recently found that the generaliza- sibirium, Stipa baicalensis, Leymus chinensis and Carex pedi- tion about the detrimental effects of modern farming with respect formis. The vegetation coverage is 50–90%, with 150–210 g m−2 to SOC was not universally valid. For instance, a change in land- standing biomass. Human activities rarely occurred in the basin use system from small to large inputs of manure will give rise to prior to the 1950s. However, in order to meet the increasing food C sequestration. demands from a fast growing population, a large area of grass- It has been well documented that over 90% of the N in most lands was converted to croplands in the 1950s and the early 1970s surface soils occurs in organic forms coupled with SOC (Nieder in this area. & Benbi, 2008). Thus, changes in SOC will probably result in A cropping system, which consists of a wheat (Triticum spp.) concomitant changes in soil N, and grassland-use conversion may or rapeseed (Brassica napus)-fallow rotation, has been commonly have significant implications for global C and N cycling (Conant used on these croplands. Wheat or rapeseed is usually sowed in et al., 2005). Soil N is important for understanding the long-term early May, and harvested in late July to early August. In the C source or sink because it is often a limited nutrient in semi-arid areas (Vitousek et al., 1997). Despite the availability of a large body of data, changes in SOC and N over a cropland chronosequence have rarely been measured to determine temporal changes in SOC and N after the establishment of cropland on previously native grassland ecosystems. Information on SOC and N concentrations/pools in a long-term cropping chronosequence is very important to identify the strategies for sustainable management of cropped soils on former grasslands. The Xilin River Basin is located in the northeast of China, approximately 450 km north of Beijing. It covers an area of approximately 10 000 km2 with natural vegetation of temperate grasslands. Meadow steppe is one of the dominant vegetation types (Li et al., 1988). Because the meadow steppe is rich in SOC and of high fertility in this area, the grassland conversions mainly occurred on the steppe. Because of demographic pressure, most of the changes occurred in the 1950s and the 1970s. An investigation in 2000 showed that a total of 332 km2 of grasslands had been converted to croplands in this basin (Chen et al., 2003). However, little information is available regarding the impact of grassland conversion on SOC and total N stocks in this area over time. The objectives of our study were to quantify the effects of the land use change on the SOC and N Figure 1 Location of the study area.

© 2009 The Authors Journal compilation © 2009 British Society of Soil Science, European Journal of Soil Science, 60, 916–923 918 Q. Wang et al. fallow year, all of the weeds are incorporated into the soil as cultivated field. All cores were transported to the laboratory, and green manure by ploughing. Fertilizers containing 24–72 kg N carefully washed on a 60-mesh sieve to separate the roots from ha−1 and 41–123 kg P ha−1 have been applied with sowing once the soil as soon as possible. These washed roots were weighed ◦ every cropping year since the early 1980s. The nearby meadow after oven-drying at 65 C for 48 hours. steppe is mown for hay production only once a year, usually at the end of August corresponding to the time of annual peak of Calculations and data analysis production of above-ground biomass. After a careful examination of the land-use history including This study did not involve replicated fields of the systems of years of cultivation, fertilization, crop rotation, crop yields and Steppe, Crop-28Y and Crop-42Y because there were no similar fallow, we identified three sampling fields including two croplands croplands available in the area. Rather, we randomly identified 10 and one undisturbed meadow steppe. The two croplands were sampling sites with similar positions in each field for sampling. created in 1972 (Crop-28Y) and in 1958 (Crop-42Y), respectively, Total organic C and N stocks at 0–30 cm soil depth were and both were planted with spring wheat in the sampling year. obtained by the sum of organic C and N stocks of the three depths. Crop-42Y is approximately 1.5 km southwest of Crop-28Y. The For each depth interval, SOC and total N stocks were calculated undisturbed meadow steppe (Steppe) is immediately adjacent to with the following equation: the east of Crop-28Y. The sizes of the three sampling fields are 1100 × 1800 m for Crop-42Y and 600 × 3000 m for Crop-28Y = × × × × −6 and Steppe. All the three fields are well-drained, and share similar S BD EC T k 10 , (1) topography and soil texture (sandy clay loam soil). where S is the element stocks (kg m−2), BD is the bulk density − − Soil sampling and laboratory analysis (g cm 3), EC is the element concentration (g kg 1), T is the thickness of the soil layers (10 cm) and k is the area multiplier The soil samples were collected to a depth of 30 cm at three (104). intervals of 0–10, 10–20 and 20–30 cm with a 6-cm diameter Data were analysed to provide mean and standard error for each soil core. Within each field, we identified 10 random sampling variable measured at every depth in a land-use system. We used sites to provide replicates. The sites were at least 50 m apart from anova to test the treatment effects (i.e. land use types, soil depth) each other and 30 m away from the boundary. At each site, three on bulk density, SOC, total N, mineral N and biomass. Means of cores were collected and combined from each soil depth. A total of the main effects were separated using Duncan’s Multiple Test at 90 soil samples were obtained. The samples were sealed in plastic P 0.05. All statistical analyses were carried out with the SAS bags and transported to the laboratory. Soil bulk density (BD) of software package (SAS Institute Inc, 2000). each depth increment for every sampling site was measured using the core method (Blake & Hartage, 1986). After visible plant materials were removed, all soil samples Results and discussion were air-dried, and passed through a 0.5-mm sieve for the Root biomass and above-ground residue following measurements. Total SOC was determined by loss on ◦ ignition at 500 C (Storer, 1984). Total N (TN) was measured The data in Table 1 clearly show that (i) more than 50% of the by the Kjeldahl method (Bremner, 1996). Ammonium and nitrate roots are found in the top 10 cm of soil regardless of the land were extracted with 2 m KCl using a 1:10 soil-to-extractant (w/v) use types and the duration of cropping (cropland age), and (ii) ratio (Bronson et al., 2004), and their concentrations in the filtered the cultivation leads to the significant differences in both above- extracts were determined on a Bran-Luebbe AA3 autoanalyser ground residues and root biomass between the croplands and (Bran & Luebbe, Hamburg, Germany). Steppe. In the 30-cm topsoil, Steppe maintained the highest root biomass (2504 g m−2), more than four times that in either of the two croplands. Above-ground residue and root biomass measurements The land-use change profoundly altered the above-ground In each field, we randomly selected three quadrats (1 × 1m) residues, which were 172 g m−2 for Steppe, 248 g m−2 for from 10 soil sampling sites for measuring plant residue and root Crop-28Y and 232g m−2 for Crop-42 Y. However, the residues biomass. All plant residues (both standing and scattered on the amounted to only approximately one-third of the root biomass for soil) within each quadrat were collected, and weighed after oven- the croplands and one-fifteenth for the steppe (Table 1). Therefore, ◦ drying at 65 C for 48 hours. the roots were the dominant source for the soil C input in these After above-ground plant residues were collected, four soil ecosystems. cores (7-cm diameter) were collected from each of the three soil The significant decrease in total C input (above-ground residue layers (0–10, 10–20 and 20–30 cm) to measure root biomass in and root biomass) in the cropped land use indicates that the each quadrat. For the croplands, two cores were in the row and replacement of perennial vegetation with annual crops alters the two were in the inter-row to represent the spatial variation of a quantity of the residue added to soil. The above-ground residue

Table 1 Above-ground plant residue and root biomass in Steppe, Crop-28Y and Crop-42Ysystems. Values are mean ± SE (n = 3 for above-ground plant residue, and n = 4 for root biomass). Signiﬁcant differences between the land-uses at the same depth are indicated by different letters at P = 0.05

Root biomass / g m−2

Land-use Above-ground residue / g m−2 0–10 cm 10–20 cm 20–30 cm Total

Steppe 172.0 ± 20.7b 1642.2 ± 47.3a 521.3 ± 35.6a 341.0 ± 3.9a 2504.4 ± 33.1a Crop-28Y 248.3 ± 34.5a 325.8 ± 28.0b 199.7 ± 21.7b 90.1 ± 7.5b 615.7 ± 51.1b Crop-42Y 232.3 ± 17.4a 326.7 ± 10.1b 214.5 ± 8.5b 107.0 ± 7.6b 648.2 ± 3.3b and root biomass are the major input of SOC and N. Therefore, Soil organic C and total N concentrations the reduction in SOC and N input will subsequently result in the decreases in SOC and N. Given that the grassland root turnover Soil C and N cycles are complex processes controlled by various rate is 53% on average, and that some of the dead roots eventually biological, chemical and physical factors. In order to compare form soil organic mater (Gill & Jackson, 2000), the C and N input soil C and N changes among the land-uses in the study area, into soil should be mainly from the root biomass in the three fields, we assumed that any differences in the examined soil properties especially in the steppe. were not because of differences in inherent soil properties, but because of the land-use effects as the soils of the three fields are Soil bulk density classified as the same soil type (Mikhailova et al., 2000; Martens et al., 2003). For the three fields, the SOC and total N concentrations Table 2 shows that the BD values are significantly different peaked in the 0–10 cm layer, and declined with depth, as they between the land use types. The steppe had a smaller BD com- did with the increase in the cultivation age, except for total N pared with either of the two croplands (P = 0.05) in each of the − − − in the 20–30 cm soil depth of Crop-42Y. Generally, the SOC three soil depths (i.e. 1.05 g cm 3, 1.08 g cm 3 and 1.17 g cm 3 and total N concentrations were markedly altered by cultivation in the 0–10, 10–20 and 20–30 cm depths, respectively). The (Figures 2A, 3A). mean BDs increased in the order Steppe, Crop-28Y and Crop- The surface soil stored the largest amount of SOC in the three 42Y. The increase in the BD of the croplands may result from the fields. In the top 10 cm, the SOC was significantly (P = 0.05) compaction of topsoil by agricultural practices. Our finding gen- greater in the steppe (35.3 g C kg−1) than in Crop-28Y (24.1 g C erally provides some support for the result of Mikhailova et al. kg−1) and Crop-42Y (19.8 g C kg−1) (Figure 2A). For the steppe, (2000) that the larger soil BD in the croplands is attributed to the the SOC concentration (21.5 g C kg−1) in the 20–30 cm depth duration and intensity of cultivation. was less than two-thirds of that (35.3 g C kg−1)inthetop It is also widely thought that soil BD declines with an increase 10 cm layer. For Crop-42Y with the longest cultivation, the in soil organic matter because of the increase in volume of porosity SOC concentration was relatively consistent down the profile. (Whalen et al., 2003). Therefore, the linear relationship between In the 0–30 cm depth, the steppe also maintained the largest BD and SOC was established in various ecosystems, such as agro- SOC concentration (27.6 g C kg−1), while Crop-42Y stored the ecosystems and grazed, modified and relict grasslands in North least SOC (19.2 g C kg−1) (Figure 2A). Crop-28Y and Crop–42Y America (Bauer & Black, 1992; Whalen et al., 2003). In our retained smaller mean SOC concentrations in the 0–30 cm depth study, the magnitude of SOC decreased with the increase of BD (16% and 31%, respectively) compared with the steppe. These in the three fields (Table 2, Figure 2), but only for Steppe was decreases in SOC are of similar magnitude to field observations there a linear relationship that can be described by the following made elsewhere (Tiessen et al., 1982; Chan, 1997; Rosell & equation: BD = 1.116 − 0.0173 SOC (R2 = 0.693,P <0.05). Galantini, 1998; Franzluebbers et al., 2000; Mikhailova et al., 2000; Whalen et al., 2003). A meta-analysis showed that up to 78% of the original SOC might be lost because of the cultivation Table 2 Soil bulk density (g cm−3) in Steppe, Crop-28Y and Crop-42Y of grassland (Guo & Gifford, 2002). at different soil depths. Values are mean ± SE (n = 10). Significant The SOC concentration in Crop-28Y was approximately 17% differences between the land-uses at the same depth are indicated by greater than that in Crop-42Y (23.2 g C kg−1 compared with = different letters at P 0.05 19.2 g C kg−1, P = 0.05). This significant inverse relationship Land-use 0–10 cm 10–20 cm 20–30 cm of SOC and cropland age indicates that the duration of cropping has a profound negative effect on SOC. Steppe 1.05 ± 0.05b 1.08 ± 0.05b 1.17 ± 0.03b The patterns for soil total N were not substantially different to Crop-28Y 1.16 ± 0.06a 1.13 ± 0.05a 1.22 ± 0.03a those for soil C (Figures 2A, 3A); thus the total N (Figure 3A) Crop-42Y 1.17 ± 0.03a 1.16 ± 0.02a 1.21 ± 0.01a and the C/N ratio (data not shown) followed similar patterns to

Soil organic C concentration / g C kg–1 Soil organic C stock / Mg C ha–1 05010 20 30 40 05010 20 30 40 0 A B

10 Depth / cm 20 Steppe Crop-28Y Crop-42Y

Figure 2 Variations in soil organic carbon concentration (A) and soil organic carbon stock (B) at three soil depths in Steppe, Crop-28Y and Crop-42Y systems. Values are means ± SE (n = 10).

Total N content / g N kg–1 Total N stock / Mg N ha–1 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 0.0 1.0 2.0 3.0 4.0 0 A B

10 Depth / cm

20 Steppe Crop-28Y Crop-42Y

Figure 3 Variations in total N concentration (A) and total N stock (B) at three soil depths in Steppe, Crop-28Y and Crop-42Y systems. Values are means ± SE (n = 10). that of SOC under the three ﬁelds. This indicated that the change existence of a signiﬁcant linear relationship between SOC con- in soil N was consistent with that of SOC. centrations and C input from residues (Duiker & Lal, 1999) may The larger amounts of SOC and total N in the surface soil can be the best explanation for the declines in the SOC and N con- be explained by a root-decay process. Most of the plant roots are centrations under the croplands compared with those under the steppe. Long-term soil C and N changes can be attributed to the located in the surface soil (Table 2). Soil N is usually moved by degradation of soil structure and the exposure of organic matter roots from subsurface to surface during plant growth, and the soil to mineralization and decomposition processes. Moreover, recent C and N are retained in the surface soil when roots die, which addition of fertilizer increased the amount of mineral N available results in the increases in C and N concentrations in the surface to microbes, and subsequently promotes the soil organic matter soil. decomposition that results in C loss. Further studies are needed The removal of above-ground and even below-ground biomass for a better understanding of the mechanisms of changes in SOC was a common agricultural practice in this area, therefore the and N of the Chernozem soil under different land uses.

Mineral N Mineral N /mg N kg–1 It is evident from Figure 4c that the cropped land-use significantly 0305 10 15 20 25 0 increases soil mineral N. The average soil mineral N concentra- − A tions in the 0–30 cm layer were 15.4 mg N kg 1 for Steppe, 19.2 mg N kg−1 for Crop-28Y and 19.6 mg N kg−1 for Crop-42Y. This finding is consistent with the findings in several similar stud- 10 ies that soil mineral N concentrations were considerably greater in cropped soils than in undisturbed soils (Dowdell et al., 1983; Egh- Depth / cm ball et al., 1994; Rasmussen et al., 1998; Whalen et al., 2003), 20 Steppe and it supports the hypothesis that cultivation enhances soil aer- Crop-28Y Crop-42Y ation and thereby stimulates N mineralization. In addition, the two possible mechanisms proposed by Whalen et al. (2003) can 30 also be applicable to the present study. One possibility is that the 0 plant community of the steppe absorbs more mineral N through B the plant fibrous root systems from the soil solution than that of the cropland. The other possibility is that the N immobilization 10 by microbial biomass is greater in the steppe than in the cropland. Steenwerth et al. (2002) found that microbial biomass was greater in the grassland soil than in the cropped soils. Depth / cm 20 Further analysis of the data revealed that the cultivation had greater influence on the distribution of nitrate than on that of ammonium (Figure 4A, 4B). Along the soil profile, the nitrate 30 −1 concentrations ranged from 1.1 to 1.8 mg N kg in the steppe, 0 − while they ranged from 3.6 to 6.5 mg N kg 1 in the cropped C soils. A large soil nitrate level may lead to a greater risk of surface and ground water contamination because of nitrate runoff 10 and leaching, especially during the rainy season. Ammonium is relatively immobile in soil, therefore its distribution along the soil profile may be determined by agricultural practices such as Depth / cm 20 ploughing.

Soil organic C and N losses and gains in the cropped soils 30

Mikhailova et al. (2000) found that the SOC and N contents in Figure 4 Variation in nitrate-N (A), ammonium-N (B) and total mineral the Chernozem soil under annually-mown grassland were similar N (C) at three soil depths in Steppe, Crop-28Y and Crop-42Y systems. ± = to, or slightly greater than, those in the same type of soil under the Values are means SE (n 10). corresponding natural grassland. In order to estimate the losses or gains of SOC and N in the cropped soils, we assumed that the in which the surface soil C and N are leached to the subsurface ﬁnding above was applicable to our research area. soil, and deposited there. Table 3 presents the losses or gains of the SOC and the total N Mann (1986) found that the greatest rate of change in SOC in the cultivated soils relative to the meadow steppe soil, based occurred in the ﬁrst 20 years after native vegetation was converted on calculations in which the variability in BD, organic C and to permanent cropland. However, our results show that the average N contents and depth were taken into account. Our results show SOC loss rates are 0.31 Mg C ha−1 year−1 for Crop-28Y and that Crop-28Y loses 9.83 Mg C ha−1 and 0.66 Mg N ha−1 in 0.57 Mg C ha−1 year−1 for Crop-42Y (i.e. the smaller SOC rate the 0–30-cm soil layer, accounting for 10.8% of the original total of loss was found in the younger cropland, Crop-28Y). This is organic C and 8.8% of the original total N, respectively, and Crop- probably because of the more intense fertilization soon after the 42Y loses 21.87 Mg C ha−1 and 1.18 Mg N ha−1 from the same conversion for Crop-28Y than for Crop-42Y as both the croplands soil depth, accounting for 24.1% of the original organic C and have been fertilized only from the early 1980s. Brye et al. (2002) 15.7% of the original total N, respectively. The losses of SOC suggested that fertilizer addition could increase the amount of and N were observed mainly in the top 10 cm of soil, and the the plant biomass and residue that returned to the soil, and thus gains of SOC and N occurred in the 20–30 cm layer except for increase the SOC and to some extent offset the organic C loss Crop-42Y for SOC gain (Figures 2B, 3B). The soil C and N gains caused by cultivation. Conant et al. (2001) also found that the in the 20–30 cm layer can be best explained by a leaching process substantial contribution of fertilizer inputs to SOC sequestration

Table 3 Losses (negative) and gains (positive) of soil organic C and total to maintain SOC and total N under the current crop system, N at different depths in the cultivated soils relative to the soil of Steppe compared with the biomass inputs in the perennial steppe. To some extent, our results may provide the basis for the prediction Crop-28Y Crop-42Y of SOC and total N response to cropping in a semi-arid grassland Depth (cm) Mass (Mg ha−1)%Mass(Mgha−1)% area. One implication of this study is that the conversion of native meadow steppe to cropland may have eliminated an important C Soil organic C storage area. Given the increase in grassland conversion as a result 0–10 –9.40∗ –25.2∗ –14.41∗ –38.6∗ 10–20 –1.23 –4.4–4.63∗ –16.4∗ of the demand for food crops, further research on the mechanisms 20–30 0.80 3.2–2.83 –11.3 of C loss and the management practices to control the loss in this 0–30 –9.83∗ –10.8∗ –21.87∗ –24.1∗ ecosystem are warranted. Total N 0–10 –0.76∗ –24.4∗ –1.02∗ –32.7∗ ∗ ∗ 10–20 –0.05 –2.2–0.17 –7.3 Acknowledgements 20–30 0.15 7.30.01 0.5 0–30 –0.66∗ –8.8∗ –1.18∗ –15.7∗ We thank the two anonymous reviewers for their comments, which significantly improved the manuscript. This study was supported ∗ Indicates significant difference at P = 0.05. by the Knowledge Innovation Program of the Chinese Academy of Sciences (KZCX2-YW-432), the National Basic Research could be as much as 39%. However, the average SOC rate of Program (2007CB106800, 2004CB41850x) and the State Key loss does not imply a constant rate over time. Our estimate of Laboratory of Vegetation and Environmental Change, Institute of Botany, the Chinese Academy of Sciences. the organic C loss because of cultivation shown in Table 3 is in general agreement with the finding that in semi-arid Senegal, West Africa, approximately 24% of the SOC was lost in a 40- References year-old cropland (Elberling et al., 2003). 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