Spatial and vertical characteristics of soil organic carbon fractions under the context of land consolidation

Background Soil is one of the largest carbon pools. It plays a crucial role in the global carbon cycle and has a profound impact on climate change. Since 2010, Shanghai have gradually initiated land consolidation, integrating farmland improvement, construction land reorganization, and ecological protection and restoration. Changes in land use types are one of the key factors influencing the carbon cycle in terrestrial ecosystems. During the land consolidation in Shanghai, it is crucial to clarify the variations in land use types and their impact on soil organic carbon (SOC), as well as the structure and content of its components.

Methods Based on the context of comprehensive land consolidation in Shanghai, we selected 10 towns undergoing land consolidation as experimental sites. In August and September 2023, samples were collected from five single land use types: woodland, cropland, grassland, building land, and water, as well as from composite land use types such as combination of planting and farming and forest-wetland composite. For each land use type, soil profiles were established at two depths: the upper layer (0–20 cm) and the lower layer (20–40 cm), resulting in the collection of 61 sampling sites and 122 soil samples. Soil bulk density was measured with the cutting ring method, soil pH with the electrode method, and soil moisture content with the drying method. SOC was determined by potassium dichromate oxidation external heating method. Soil dissolved organic carbon (DOC) was extracted using K₂SO₄ as the extractant and measured with the total organic carbon analyzer, while soil microbial biomass carbon (MBC) was measured using the chloroform fumigation-extraction method.

Results 1) The soil moisture content in samples from different land use types in Shanghai ranged from 19.88% to 34.45%, while soil pH values varied between 7.44 and 8.21, indicating a generally weakly alkaline condition. Soil bulk density ranged from 1.29 to 1.54 g/cm³, with land consolidation having a minimal impact on the basic physicochemical parameters of the soil. However, the geographical variation in SOC content after land consolidation is significant. 2) SOC content in composite land use types, such as the combination of planting and farming and forest-wetland composite, was 15.81 and 15.50 g/cm³, respectively, significantly higher than that in single land use types. The trends in the content of soil DOC and soil MBC were similar to those of SOC. 3) SOC content in the upper layer was significantly higher than that in the lower layer, displaying a stratified pattern from top to bottom. 4) In samples with land consolidation duration of less than 36 months, the differences in the content of SOC and soil DOC between the upper and lower soil layers were significantly greater than in other samples.