Changes in Soil Organic Carbon and Carbon Fractions Under Different Land Use and Management Practices After Development From Parent Material of Mollisols

Soil organic carbon (SOC) is important to soil nutrient status in agroecosystems. Some of the soils of the Northeast of China, noted for their high SOC content, suffer from serious soil erosion to the point of having the parent material exposed or near the surface, which has raised concerns for food security. The Chinese Mollisols were derived from loamy Quaternary loess that developed from parent material. To effectively restore parent material to productive soils, information on the effects of land use/management practices on SOC concentration and C fractions in loess parent material of Chinese Mollisols is needed. The main objective of this study was to investigate the changes in C sequestration and C density fractions by physical and chemical fractionation (humic substances) occurring in the process of soil development from parent material under different management practices and land use. Six treatments were imposed in plots of loess parent material in a 5-year experiment: (1) natural fallow without weed control; (2) alfalfa; (3) soybean-maize rotation (S-M), straw of unfertilized maize removed; (4) S-M, straw of chemically fertilized maize removed; (5) S-M, straw of chemically fertilized maize and dried soybean powder incorporated; (6) S-M, biomass, including grain, of chemically fertilized maize incorporated. The SOC content increased by 15% to 77% depending on treatments. In the process of soil development, the C fractions of the parent material changed rapidly. The heavy fraction C pool accounted for a larger proportion of total SOC (78%-89%) than both the free light fraction (2.1%-10.2%) and the occluded light fraction (1.3%-12.9%) pools. The occluded light fraction was more sensitive than the free light fraction as indicator of soil C changes because of different land use and management practices. Humin accounted for a larger proportion (29.9%-54.7%) of SOC than fulvic acid (18.0%-34.4%), which was larger than the humic acid fraction (11.8%-14.8%). Our results indicate that SOC increase in loess parent material depends on types and amounts of organic matter inputs. The treatments, in which aboveground crop biomass and grain were incorporated, contributed more to C sequestration, distributions of density fraction, and humic substances than the treatments without organic matter. Management practices maximizing biomass inputs are recommended to restore SOC in degraded Chinese Mollisols in order to restore their fertility.