The afforestation has been proposed as a primary means of sequestering carbon (C) from the atmosphere, thereby mitigating climate change. However, consequences of afforestation on soil organic C and nitrogen (N) dynamics due to spatial heterogeneity are not fully understood. The objectives of this study were to identify the consequences of 18 years of afforestation on C and N dynamics in the Danjiangkou Reservoir area of central China. Soil samples from the woodland, shrubland, cropland and adjacent open area soils (i.e. the control) were separated into four aggregate sizes (> 2000 μm, 250 - 2000 μm, 53 - 250 μm and < 53 μm), and three density fractions [free light fraction (LF), intra-aggregate particulate organic matter (iPOM) and mineral-associated organic matter (mSOM)]. All fractions were analyzed for their C and N content, and δ13C and δ15N values.
The conversion of uncultivated area to afforested areas (woodland and shrubland) increased the soil organic C and N by enhancing the soil C and N of the macroaggregates (> 2000 μm) with most of the C and N stored in the iPOM (65-87%). However, the soil organic N of the aggregates (< 2000 μm) in the woodland was lower than in the shrubland and cropland, due to decreases in N concentrations (i.e. higher C:N ratios) in litter likely overshadowed any enhancements in litter mass. The C:N ratios in the soil fractions reflected larger litter inputs into afforested soils than into cropped soils. The δ15N values of the soil fractions indicated a rapid degree of decomposition of the SOM in the cropland, and further confirmed that afforestation indeed enhanced the protection of SOM against decomposition. The δ13C values of the LF of the macroaggregates (> 2000 μm) in the woodland indicated that the largest new C input and the fastest decay rate was old C. Our results suggest that a shift in vegetation following land use conversion could alter soil aggregate weight distribution and/or C and N storage, consequently affecting soil C and N pools dynamics in the terrestrial ecosystem. The study is helpful for long-term carbon and ecosystem services management.