Soil organic matter turnover depending on land use change: Coupling C/N ratios, δ13C, and lignin biomarkers

Soil organic carbon (SOC) stocks have been greatly depleted across the globe by conversion of wetlands to croplands, agroforestry, and urban areas. Here, we investigated SOC distribution and turnover in four land use types: a wetland, cropland, forestland, and construction land in the Baiyangdian Wetland, Northern China. The C : N ratios were up to 1.70-times larger in cropland, forestland, and construction land than in the original wetland because of faster N losses compared to C following wetland conversion. The delta C-13 values of SOC increased with depth in wetland, and showed an overall depletion compared with the other three land use types. Acid-to-Aldehyde ratios of syringyl in wetland were 0.72-1.14-, 0.72-1.72-, and 1.18-1.43-times, and cinnamyl/vanillyl ratios were 0.56-1.05-, 0.22-0.48-, and 0.40-0.76-times those of cropland, forestland, and construction land, which reflects faster lignin decomposition rate in wetlands. The beta value was defined by the slope of the linear regression between the logarithm of SOC and delta(SOC)-S-13 values, and decreased from cropland over construction land and forestland to wetland, reflecting the faster SOC turnover with the lower beta values. However, SOC content and storage were up to 2.29- and 2.07-times higher in wetlands than in soils of other land use types. The combination of C : N ratios, delta C-13, and lignin monomer composition can explain the decrease of beta value (corresponding to faster SOC turnover), and can be used as effective proxies to evaluate the sources and turnover of SOC in response to land use changes.