Vertical distribution of soil organic and inorganic carbon pools in soils of northern China and their relationship under different land use types

Objectives The changes of soil inorganic carbon (SIC) storage strongly influence the sequestration of atmospheric CO₂ in semi-arid and arid regions. Investigation of the vertical distribution of SIC and its transformation along soil profiles under different land use types will improve the prediction of the response of soil carbon storage to global change and the mitigation of CO₂ emission.

Methods Soil organic carbon (SOC) and SIC data were extracted from published studies conducted in China from 1990 to 2018. Changes in SOC and SIC contents, and SOC/SIC along soil profile of 0–100 cm under deserted land, shrub, grassland, forest and cropland were quantified. The relationships between the SOC and SIC under different land use types were then investigated with correlation analyses.

Results SOC content decreased with the increase of soil depth, while the characteristics of SIC content changing with soil depth were significantly different among land use types. At 0–60 cm layer, crop and grass lands had higher SOC content, and shrub lands were characterized with higher SIC contents, whereas the contents of SOC and SIC were significantly lower ( P <0.05) in the deserted land. At 60–100 cm layer, the SOC and SIC contents of shrub and deserted lands were significantly lower than those of the other three land use types. At 0–20 cm soil layer, the SOC/SIC of cropland was the highest (0.80 ± 0.05), while those of deserted land (0.40 ± 0.02) and shrub land (0.50 ± 0.03) were significantly lower than the other land use types. At 20–60 cm soil layer, the SOC/SIC of grass and crop lands were significantly higher (P < 0.05) than those of deserted and shrub lands. At 60–100 cm soil layer, the SOC/SIC of grassland compared to the other four land use types was significantly higher compared to the other four land use types. SOC and SIC contents significantly and positively correlated with each other through the soil profiles under deserted, shrub, forest (except for 60–80 cm) and crop lands, where as they were negatively and correlated under grassland. By calculation, the estimated SIC stock in 0–100 cm soil accounted for 60%–80% of the total soil carbon pool (SOC+SIC). SOC that stored in 0–100 cm of grassland soil was C (56.65 ± 4.00) kg/m², which was 1.6–3.7 times of the other four land use types. The deserted land possessed the lowest SIC stock C (51.05 ± 5.11) kg/m², which was 51.1%–57.5% of those under the other land use types.

Conclusions In semi-arid and arid regions of China, input of organic carbon sources such as straw, root and litter debris under crop, grass, shrub and forest lands can stimulate the decomposition of soil microorganisms and the transformation of SOC into SIC, which promote the sequestration of atmospheric CO₂, whereas the deserted lands have negative effect on atmospheric CO₂ sequestration due to the less vegetation cover, weaker biochemical weathering processes and less resistant to disruption. Furthermore, management practices such as irrigation, cultivation and fertilization in cropland can induce the vertical transferring of carbonate into deeper soil layers, which lead to the accumulation of SIC in subsoil and increase the potential of CO₂ sequestration. By contrast, the transportation of water from deeper soil by root systems of shrub can drive the transferring of carbonate into surface layer, and result in an decrease of SIC storage and CO₂ sequestration in deep soil layers.