INTRODUCTION
Black soil, defined as Mollisol in the U.S. system of soil taxonomy or Chernozem in the Russian system (Liu et al., Reference Liu, Burras, Kravchenko, Duran, Huffman, Morras, Studdert, Zhang, Cruse and Yuan2012a), is often thought to be controlled by a semihumid or humid climate with low temperatures and lush meadow vegetation cover. The import of plant litter increases the level of biomass C in the soil organic horizon. Subsequently, biomass C migrates downward to the soil humus layer (Gong et al., Reference Gong, Zhang and Chen2007). A thick, humus-rich, and dark-coloured soil horizon is a discernible characteristic of black soil (FAO, 2022). Owing to its inherently high fertility (expressed by its richness in well-humified organic matter), black soil is known as the food basket of the world or the “giant panda in arable land” in Asia. These fertile soils have been widely cultivated in black soil belts such as the Northeast Plain, Ukrainian Plain, Pampas Plain, and Mississippi Plain, and have made substantial contributions to global food production (FAO, 2022).
In the Northeast Plain, the grain crop yield accounts for one-fifth of the total crop yield in China (Wei and Meng, Reference Wei and Meng2017). However, high-frequency tillage initially destroys the soil aggregate structure in the region, resulting in the direct loss of low-density particulate organic matter through erosion. Recently, a contour map depicting the erosion rate of black soil in northeast China revealed that the regional mean erosion rate is 2.22 mm/yr (Wang et al., Reference Wang, Yang, Wang, Gu, Xiong, Huang and Sun2022). Using an erosion rate of 2.22 mm/yr and an average black soil thickness of 25 cm as references, the black soil will be entirely eroded in approximately 113 yr without sustainable soil management. In 2021, the Ministry of Water Resources of China released a bulletin on soil and water conservation, revealing that black soils in northeast China face erosion in an area covering approximately 2.14 × 105 km2, which is close to 20% of the region's total land area. Under the pressures of maintaining a high grain yield and erosion, black soil is probably unable to maintain a stable level of sustainable food production.
Hence, an evaluation of soil erosion on a regional scale was conducted to protect black soil in northeast China from further degradation. In some watersheds and sloping farmlands, the soil erosion rate was quantified using 137Cs tracing, soil erodibility (the vulnerability of soil to erosion in the study; Song et al., Reference Song, Liu, Yan and Cao2005; Wang et al., Reference Wang, Zheng, Römkens and Darboux2013) was calculated using empirical equations, and the dominant agents of soil erosion were also determined. However, most existing documents from the region focus on exogenic erosive forces such as monsoons, precipitation, and human activities (e.g., Yang et al., Reference Yang, Zhang, Deng and Fang2003; Wang et al., Reference Wang, Yang and Liu2010; Xu et al., Reference Xu, Xu, Chen, Xu and Zhang2010; Liu et al., Reference Liu, Zhang, Zhang, Ding and Cruse2011; Wang et al., Reference Wang, Zheng, Römkens and Darboux2013; Kong et al., Reference Kong, Liu, Henderson, Zhou, Su, Wang, Wang and Wang2022; Wang et al., Reference Wang, Yang, Wang, Gu, Xiong, Huang and Sun2022). Furthermore, the physical properties of soils (e.g., structure, texture, and aggregate) are considered to be intrinsic to analysis soil erodibility (Song et al., Reference Song, Liu, Yan and Cao2005; Wang et al., Reference Wang, Zheng, Römkens and Darboux2013).
Notably, inheritance of the parent material plays a critical role in determining the main physical and chemical properties of the overlying soil (Chesworth, Reference Chesworth1973; Osher and Buol, Reference Osher and Buol1998; Rodrigo-Comino et al., Reference Rodrigo-Comino, Novara, Gyasi-Agyei, Terol and Cerdà2018). Measurements from soil plots in eastern Spain revealed that the parent material largely determines the regional erosion rate (Cerdà, Reference Cerdà1999, Reference Cerdà2002). For example, in the Mediterranean region, soil erosion rates in the marls and colluvial plots were 87.7 and 4.35 Mg/ha/yr, respectively, indicating that soils with the marl as their parent material have a higher erodibility (Rodrigo-Comino et al., Reference Rodrigo-Comino, Novara, Gyasi-Agyei, Terol and Cerdà2018). Thus, factors associated with the formation of black soil, especially the inheritance of parent materials, are required for the systematic evaluation of soil erodibility on a large scale.
To date, the effects of parent materials on soil erodibility have not been evaluated on a large scale owing to the absence of an effective indicator that truly mirrors the inheritance of parent materials. Ferromagnetic materials with fine particle sizes continuously accumulate during pedological processes, increasing soil magnetic susceptibility (Liu et al., Reference Liu, Roberts, Larrasoaña, Banerjee, Guyodo, Tauxe and Oldfield2012b). Once soils are eroded, the soil structure is destroyed, leading to fine particulate matter being washed away, further enhancing soil erodibility. This reveals that the eroded soils fail to protect the ferromagnetic materials absorbed by fine particulate matter from leaching. Considering the variations in ferromagnetic materials during pedological processes and excluding the effects of climatic factors, changes in frequency magnetic susceptibility (χfd, reflecting the amount of fine size ferromagnetic materials; Liu et al., Reference Liu, Roberts, Larrasoaña, Banerjee, Guyodo, Tauxe and Oldfield2012b) could indicate soil erodibility on a regional scale.
In this study, we analysed the χfd of samples from the black soil horizon of soil sections with loess parent materials and non-loess parent materials, covering an area of severe and slight erosion in northeast China. These data were used to compare variations in the ferromagnetic materials of black soil with different parent materials. In addition, the total organic carbon (TOC) content and stable carbon isotope composition of the bulk organic matter (δ13Corg) for selected samples were investigated to indirectly determine the relationship between changes in χfd and soil erosion. Our principal aim was to determine the effects of parent materials on the erodibility of black soil in northeast China.
METHODS
Study area
The area occupied by black soil in northeast China is geographically located in eastern Eurasia (Fig. 1a), especially in the Songnen, Sanjiang, Greater Khingan Range piedmont, and Liaohe Plains. The modern climate of the region is dominated by a continental monsoon climate that prevails in the cold temperate zone of the Northern Hemisphere. The land–sea thermal contrast in the region is weaker than that in the low latitudes of East Asia. The modern climate is marked by a longer interval of cold and dry conditions in winter and a shorter interval of warm and humid conditions in summer. In the region, the mean annual temperature is in the range of −1°C and 5°C, with the distributions showing a northward-decreasing trend; the mean annual precipitation is from 350 mm to 600 mm, with a decreasing trend from east to west. Precipitation from July to August contributes to 80–90% of the total annual precipitation (Ren et al., Reference Ren, Yang and Bao1985). In addition, black soil that developed during the Holocene covers a large area of the region (Cui et al., Reference Cui, Guo, Chen, Wang, Yang and Xiong2021). The slope angle of the region is from 0° to 5°. The main vegetation types in the region are meadow steppe (Gao et al., Reference Gao, Wang, Li, Li, Wang, Niu, Meng, Liu, Zhang and Jie2023), planted poplar, and crops (e.g., corn and soybean).
Figure 1. Black soil in northeast China (a), contour maps of the erosion rate with locations of the 20 soil sections (b), and lithology of black soil sections with loess and non-loess parent materials in the region (c). Distribution of black soil in northeast China is modified from ISRIC—World Soil Information (https://files.isric.org/public/soter). Detailed information about these soil sections is presented in Table 1.
Table 1. Detailed information about the 20 soil sections and the measured magnetic parameters of 142 samples in the study region.
Sampling
Twenty Holocene soil sections (QQHEA2, QQHEC2, QQHEC3, QQHEC4, QQHEC5, QQHEC7, QQHEC8, QQHEE1, QQHEE2, QQHEE5, NJA2, BAA3, BAB3, DQB1, HEBA1, SHE1, BQC2, NHB3, CCC6, and CCE1) in study areas were used to analyse the χfd for 142 samples (Table 1), which covered an area of severe and slight erosion (Fig. 1b). Thirty-two samples were selected from five soil profiles (QQHEE1, QQHEC2, BAB3, DQB1, and SHE1) to measure TOC content and δ13Corg. All samples were collected at 5–10 cm intervals from 20 sections. The parent materials of black soil from all sections were Quaternary unconsolidated sediments; for example, loess for 10 sections and non-loess materials for the others (Fig. 1c). The non-loess parent materials primarily included fluvial sediments and aeolian sands. The investigation showed that the thickness of the black soil layer for all sections was primarily within the range of 40–90 cm, whereas the minimum was approximately 10 cm (e.g., QQHEC2) and the maximum was approximately 130 cm (e.g., QQHEE1) (Fig. 1c). Additionally, black soil from 20 sections was characterised by a dark black colour (7.5YR5/3–10YR2/1) and relatively high TOC content. Furthermore, the black soils in the region are mature, because the principal soil horizons (O, A, E, B, C, and R) are distinguishable in these sections via a combination of characteristics from both the granular structure and texture of clay and/or silty clay in the A horizon.
Data analysis
A total of 142 samples were collected from 20 soil sections to measure magnetic parameters (Table 1), including low-frequency magnetic susceptibility (χlf) and high-frequency magnetic susceptibility (χhf), at the Institute of Geology and Geophysics, Chinese Academy of Sciences. After removal of modern rootlets, χlf and χhf were measured on air-dried samples (~10 g) using a Bartington Instruments MS3 magnetic susceptibility meter at frequencies of 0.47 and 4.7 kHz, respectively. Moreover, χfd was calculated using the formula: χfd = [(χlf − χhf)/χlf] × 100%.
Among these sections, 32 samples were selected from five soil profiles (QQHEE1, BAB3, DQB1, SHE1, and QQHEC2) to measure TOC content and δ13Corg (Guo et al., Reference Guo, Xiong, Chen, Cui, Yang, Wang, Wang and Ding2023). The measurements were conducted at the Institute of Geology and Geophysics, Chinese Academy of Sciences. Modern rootlets were removed from samples (approx. 3 g) and treated with 10% HCl at approximately 25°C for 24 h to eliminate inorganic carbonate. Thereafter, the residues were washed to a near-neutral pH with distilled water and dried at 45°C. Approximately 500 mg of dried sample was combusted for >4 h at 850°C in evacuated sealed quartz tubes in the presence of silver foil and cupric oxide. The carbon isotopic composition of evolved CO2 was measured using a MAT-253 gas mass spectrometer with a dual-inlet system, and the TOC content of the samples was determined simultaneously.
In addition, Wang et al. (Reference Wang, Yang, Wang, Gu, Xiong, Huang and Sun2022) compiled 24 soil sections to quantify the erosion rate of black soil using 137Cs tracing across the occupied area in northeast China. A kriging interpolation method was subsequently used to generate an estimated surface from the measured erosion rates in the region, which provided the extraction erosion rates for the 20 soil sections (Table 1).
RESULTS
Magnetic characteristics of black soil with loess and non-loess parent materials
The χlf and χfd are always used to depict the variations in ferromagnetic materials in multiple disciplines (Dearing, Reference Dearing1999; Liu et al., Reference Liu, Roberts, Larrasoaña, Banerjee, Guyodo, Tauxe and Oldfield2012b). Both proxies were used to depict the magnetic characteristics of black soil with loess and non-loess parent materials in northeast China. Our data showed that the values of χlf (χfd) of the black soil layer for 20 sections in northeast China (142 samples) are 9.04 × 10−8 to 124.36 × 10−8 m3/kg (0–11.07%) (Figs. 2 and 3). The average values of χlf and χfd of the black soil layer for a single section in northeast China (20 sections in total), presented in Table 1, reveal that the average χlf values of the black soil layer are 14.03 × 10−8 to 113.08 × 10−8 m3/kg, with clustered values of 30 × 10−8 to 60 × 10−8 m3/kg, and the average values of χfd of the black soil layer are 2.33–10.62%.
Figure 2. Variations in low-frequency magnetic susceptibility (χlf) of black soils with (a) loess (group A) and (b) non-loess (group B) parent materials with increasing erosion rate in northeast China, and (c) box plots of χlf for both groups with low (≤3 mm/yr) and high (>3 mm/yr) erosion rates. The raw data were curve fitted with a locally weighted scatter plot smoothing model. The solid line is the best-fit line, and the colour-filled area shows the 95% confidence level.
Figure 3. Variations in frequency magnetic susceptibility (χfd) of black soil with (a) loess (group A) and (b) non-loess (group B) parent materials with increasing erosion rate in northeast China, and (c) box plots of χfd for both groups with low (≤3 mm/yr) and high (>3 mm/yr) erosion rates. The raw data were curve fitted with a locally weighted scatter plot smoothing model. The solid line is the best-fit line, and the colour-filled area shows the 95% confidence level.
Further investigation revealed that values of χlf of black soil with loess parent materials (group A) showed a marginal decreasing trend as erosion rate increased (Fig. 2a), whereas the values of χlf of black soil with non-loess parent materials (group B) exhibited fluctuations (Fig. 2b). Although average values of χlf for groups A and B are 43.42 × 10−8 m3/kg and 45.85 × 10−8 m3/kg, respectively, they fluctuated at an approximate χlf value of 40 × 10−8 m3/kg (Fig. 2c). For variations in χfd with an increasing erosion rate, both groups showed an overall decreasing trend, whereas the decreased range of group B was larger than that of group A (Fig. 3a and b). Compared with χfd values for group A, the χ fd values for group B were higher at low erosion rates (≤3 mm/yr) and lower at high erosion rates (>3 mm/yr) (Fig. 3c). Overall, the magnetic characteristics of the soil sections with loess and non-loess parent materials were clearly distinguishable through a comparison of χfd of both groups (Fig. 3).
Variations in TOC content and δ13Corg for selected samples
The values for TOC content and δ13Corg of 32 samples selected from five soil profiles (QQHEE1 [erosion rate = 3.75 mm/yr], QQHEC2 [erosion rate = 3.67 mm/yr], BAB3 [erosion rate = 2.47 mm/yr], DQB1 [erosion rate = 0.40 mm/yr], and SHE1 [erosion rate = 2.78 mm/yr] sections) are presented in Figure 4. The range of δ13Corg values was −24.32‰ to −20.92‰ (Fig. 4a and b). The TOC content was <1%, excluding three samples from SHE1 (Fig. 4c and d). There was a positive correlation between the magnetic parameters (χfd and χlf) and TOC content or δ13Corg for the DQB1 (erosion rate = 0.40 mm/yr) section, and weak correlations were observed in other sections (Fig. 4).
Figure 4. Relationship of (a and b) δ13Corg and (c and d) total organic carbon (TOC) content with magnetic parameters (frequency magnetic susceptibility, χfd; and low-frequency magnetic susceptibility, χlf) for selected sections in northeast China. The values marked in the figure are the erosion rates of the study site (as shown in Table 1). Data from the QQHEE1, BAB3, DQB1, and QQHEC2 sections are provided in Guo et al. (Reference Guo, Xiong, Chen, Cui, Yang, Wang, Wang and Ding2023).
DISCUSSION
Contribution of parent materials to erodibility of black soil in northeast China
Soil erodibility is generally estimated using empirical equations based on indicators related to intrinsic soil properties (e.g., soil chemical composition, soil structure, texture, and aggregates) or exogenic erosive forces (e.g., runoff, vegetation, precipitation, and human activities) (Song et al., Reference Song, Liu, Yan and Cao2005; Wang et al., Reference Wang, Zheng, Römkens and Darboux2013). These empirical equations can successfully evaluate soil erodibility within plots under highly controlled conditions based on an experimental design. The effects of parent materials on soil erodibility have been discussed by comparing soil loss rates (Cerdà, Reference Cerdà1999, Reference Cerdà2002; Rodrigo-Comino et al., Reference Rodrigo-Comino, Novara, Gyasi-Agyei, Terol and Cerdà2018). However, the soil loss rate is indistinguishably controlled by parent materials and exogenic erosive forces. Therefore, quantitatively evaluating the contribution of parent materials to soil erodibility on a larger scale has proven challenging.
Existing studies suggest that χlf is an effective indicator of pedogenesis (Zhou et al., Reference Zhou, Oldfield, Wintle, Robinson and Wang1990), because higher values indicate the presence of abundant ferromagnetic minerals with continuously accumulated fine size fractions (Dearing, Reference Dearing1999). As eroded soils have less fine particulate matter, low χlf values occur in some typically eroded locations (Dearing et al., Reference Dearing, Morton, Price and Foster1986). Additionally, enhanced pedogenesis can result in a quantitative increase in superparamagnetic particles of fine size when the value of χfd synchronously increases (Liu and Deng, Reference Liu and Deng2009; Liu et al., Reference Liu, Roberts, Larrasoaña, Banerjee, Guyodo, Tauxe and Oldfield2012b). Moreover, the formula χfd = [(χlf − χhf)/χlf] × 100% reveals that χfd can be used to trace variations in fine size fractions adsorbed in abundant ferromagnetic minerals and superparamagnetic particles for the soil erosion process. Thus, considering the changes in ferromagnetic materials absorbed in fine size fractions in the pedogenic and erosion processes, changes in χlf and χfd can serve as reliable indicators of soil erodibility on a regional scale with a close climatic background.
In addition, for eroded soil with fewer fine particles, soil organic matter decomposes to a lower degree, because the higher degree of decomposition requires longer pedogenesis time, and the resultant fractionation would lead to a more positive value of δ13Corg. In the study region, δ13Corg increased with the χfd and χlf values in the DQB1 section (erosion rate = 0.40 mm/yr), while weak correlations were observed in other sections (erosion rate = 2–4 mm/yr) (Fig. 4a and b). This suggested that the increases in ferromagnetic minerals and superparamagnetic particles correspond to a high degree of decomposition of soil organic matter, with a positive tendency for δ13Corg, supporting the idea that variations in magnetic parameters could depict soil erodibility. Moreover, soil erosion in the Chinese Loess Plateau over the last 60 yr has been effectively monitored by changes in χlf (Dong et al., Reference Dong, Song, Chen, Liu, Fu. and Xie2022).
Climatic and nonclimatic (e.g., inheritance of parent materials) factors can operate together to control changes in ferromagnetic materials during the pedogenic process, further altering the values of soil χlf and χfd. Contour maps depicting climatic factors showed that modern mean annual precipitation and temperature of the area covered by the 20 soil sections are from 400 mm to 600 mm and from 0°C to 6°C, respectively (Fig. 5a and b). Although latitudinal variations in temperature were observed, changes in χlf and χfd for black soil with latitude were not pronounced (Fig. 5c and d). Thus, the relatively narrow range of climatic factors plays a secondary role in the variations in χlf and χfd of the 20 soil sections in the region.
Figure 5. (a) Modern mean annual precipitation (MAP) and (b) mean annual temperature (MAT) over 30 years (1981–2010) in northeast China, together with variations in average values of (c) low-frequency magnetic susceptibility (χlf) and (d) frequency magnetic susceptibility (χfd) for black soil with latitude. Contour maps of meteorological factors were generated using ArcGIS 10.2, based on long-term climate data provided by the National Meteorological Information Centre of the China Meteorological Administration (http://data.cma.cn).
Data analysis showed that χlf and χfd is proportional to decreases in erosion rate in the region (Fig. 2 and 3), especially the correlation of χfd and erosion rate (Fig. 3). This is because fine particulate matter was washed away at a high soil erosion rate, which increased soil erodibility. This suggested that eroded soil fails to protect ferromagnetic materials from runoff. Comparison of black soil with loess (group A) and non-loess (group B) parent materials showed that χlf fluctuated at approximately 40 × 10−8 m3/kg (Fig. 2c), and the decreased range of χfd in group B was larger than that of group A (Fig. 3), revealing that superparamagnetic particles with fine size are more likely eroded in soils formed on non-loess. Consequently, the parent material can be a vital reason for the spatial heterogeneity of the erodibility of black soil in northeast China. In addition, the change in χfd is narrow in range for black soil with loess parent materials (Fig. 3), suggesting that the erodibility of black soil in the region was relatively low. This conclusion was also supported by the relatively low erodibility (expressed by the k-value) of loess soil from China compared with data from the United States (Fig. 6a).
Figure 6. (a) Erodibility (expressed by k-value) of soils from the United States and China and (b and c) variations in soil loss (or soil loss rate) with the corresponding total organic carbon (TOC) content. (a) The k-value is calculated using a soil erodibility nomograph, as proposed by Wischmeier et al. (Reference Wischmeier, Johnson and Cross1971). (b) soil loss data are from the Ntabelanga area, Eastern Cape Province, South Africa (Parwada and Tol, Reference Parwada and Tol2017). (c) Soil erosion rates from Keshan County, Heilongjiang Province, China (He et al., Reference He, Zhang and Yang2021). The solid line is the best-fit line, and the dotted lines show the upper and lower 95% confidence limits.
Soil organic carbon induced a decrease in erodibility of black soil with loess parent materials in northeast China
Modern geomorphic regimes have revealed that some fragmented landforms and ecological problems promote erosion in regions covered by loess (Fu, Reference Fu1989; Fu et al., Reference Fu, Zhang, Chen, Zhao, Gulinck, Liu, Yang and Zhu2006; Wu et al., Reference Wu, Wei, Fu, Wang, Zhao and Moran2020). The most important environmental features of the Loess Plateau are the crossbar gullies on the surface and vulnerable ecological conditions. In 2021, the area of soil erosion in the Loess Plateau was up to ~2.06 × 105 km2, as reported by the Bulletin of China on Soil and Water Conservation released by the Ministry of Water Resources of the People's Republic of China. Moreover, the modern riverbed downstream of the Yellow River is above the land, which is the result of a large amount of sediment from the Loess Plateau in the lower reaches with small flow rates. Sediments with relatively coarse particles (e.g., loess) are highly susceptible to erosion. The high erodibility of loess is mismatched with the relatively low erodibility of black soil with loess parent materials in northeast China.
One of the main reasons for the baffling erodibility of loess and black soil with loess parent materials is the difference in soil organic carbon. For carbon migration during the pedogenic process, soil organic carbon is concentrated in the fine-sized fraction (Roose et al., Reference Roose, Lal, Feller, Barthès and Stewart2006) because of the greater surface area provided by fine particles. Once soils are eroded, the soil aggregate structure is destroyed, resulting in a direct loss of low-density particulate organic carbon (He et al., Reference He, Zhang and Yang2021). Erosion induces the decrease in fine particulate matter (Meyer and Harmon, Reference Meyer and Harmon1984; Lee and Gill, Reference Lee and Gill2015), including adsorbed soil organic carbon. Therefore, TOC can be used as a large-scale robust early warning indicator of soil degradation (Guo et al., Reference Guo, Xiong, Chen, Cui, Yang, Wang, Wang and Ding2023). Variations in soil loss (or soil loss rate) with the corresponding TOC content from South Africa and northeast China show that a high TOC content corresponds to weak soil erosion (Fig. 6b and c), revealing that the addition of soil organic matter can stabilise the soil against erosion on a global scale (Parwada and Tol, Reference Parwada and Tol2017). Although the TOC content of section SHE1 (erosion rate = 2.78 mm/yr) was higher than that of the other sections with high and low erosion rates (Fig. 4c and d), the high values were only from one site. Consequently, we speculated that more soil organic carbon could induce a decrease in the erodibility of black soil with loess parent materials.
In practice, soil organic carbon decreases under the pressure of maintaining high grain yields and erosion (as mentioned in the “Introduction”), necessitating sustainable soil management policies to protect black soil in northeast China to maintain a stable level of sustainable food production. Black soil sustains grain growth with abundant precipitation in summer and autumn when excessive deep ploughing destroys the soil aggregate structure and accelerates soil erosion. Excessive deep ploughing should be avoided during the ploughing season. For winter and spring, both freeze–thaw action and strong winds result in sparse vegetation cover in the region failing to defend wind erosion for surface soil in northeast China, supported by the works of Chepil (Reference Chepil1954) and Coote et al. (Reference Coote, Malcolm, Wall, Dickinson and Rudra1988). Consequently, laying straw over the field to protect surface soil from exposure to strong winds is practical during the fallow season.
CONCLUSIONS
A compilation of values of χlf and χfd of 142 samples from the black soil horizon of 10 soil sections with loess parent materials and 10 sections with non-loess parent materials was used to investigate the contributions of parent materials to the erodibility of black soil in northeast China. Moreover, the TOC content and δ13Corg values of 32 samples from five selected sections were also measured. The data revealed that the magnetic parameters (χlf and χfd) are proportional to the decrease in the erosion rate, primarily owing to the erosion-induced leaching of ferromagnetic materials and superparamagnetic particles absorbed in fine size fractions. Further, χlf of black soil with loess (group A) and non-loess (group B) parent materials fluctuated at approximately 40 × 10−8 m3/kg, and the decreased range of χfd of group B was larger than that of group A. Considering the changes in ferromagnetic materials absorbed in fine size fractions in the pedogenic and erosion processes, the results suggested that superparamagnetic particles with fine sizes are more likely to be eroded in soils formed on non-loess, indicating that the parent material meaningfully contributes to the spatial heterogeneity of the erodibility of black soil in northeast China. A narrow change in χfd for group A also indicated the relatively low erodibility of black soil with loess parent materials in the region. Compared with loess marked by less soil organic carbon, more soil organic carbon could induce a decrease in the erodibility of black soil with loess parent materials in northeast China. In addition, we suggest that avoiding excessive deep ploughing in summer and autumn and laying straw over the field in winter and spring should be executed promptly to protect black soil from further erosion, thereby ensuring future food security.
Acknowledgments
This study was supported by the National Natural Science Foundation of China (42007282 and 42077409) and the Key Research Program of the Institute of Geology & Geophysics, Chinese Academy of Sciences (IGGCAS-201905).
