Labile organic matter content and distribution as affected by six-year soil amendments to eroded Chinese mollisols

Labile organic carbon (LOC) is a fraction of soil organic carbon (SOC) with rapid turnover time and is affected by soil fertilization. This investigation characterized the SOC content, LOC content and LOC distribution in the treatment plots of surface soil erosion at five levels (0-, 5-, 10-, 20- and 30-cm erosion). The soil had received contrasting fertilizer treatments (i.e., chemical fertilizer or chemical fertilizer + manure) for 6 years. This study demonstrated that both SOC and various LOC fractions contents were higher in the plots with fertilizer + manure than in those with fertilizer alone under the same erosion conditions. The SOC and LOC contents decreased as the erosion depth increased. Light fraction organic carbon, particulate organic carbon, easily oxidizable organic carbon (KMnO₄-oxydizable organic carbon), and microbial biomass carbon were 27%–57%, 37%–47%, 20%–25%, and 29%–33% higher respectively in the fertilizer + manure plots, than in the fertilizer alone plots. Positive correlations (p < 0.05) between SOC content and different fractions contents were observed in all plots except the correlation between total SOC content and water-soluble organic carbon content in the different fertilization treatments. Obviously, fertilizer + manure treatments would be conducive to the accumulation of LOC and SOC in the Black soil of Northeast China.