Shifts in soil organic carbon and nitrogen dynamics for afforestation in central China

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 δ 13C and δ 15N values. Afforestation enhanced the soil C and N storage, primarily due to increases in soil C and N storage of macroaggregates (>2000 μm) with the largest (65-87%) fraction in iPOM. The C: N ratios generally increased from the cropland to shrubland to woodland across all fractions. The 13C values indicated that the fastest decay rates (k = 0.024 yr−1) in LF of macroaggregates (>2000 μm) was in woodland, but the fastest decay rates (0.114-0.137 yr−1) in iPOM of all aggregates were observed in cropland. Meanwhile, the most enriched δ15N values (0.87-3.81) were found in cropland soil. Our results suggest that afforestation could alter weight distribution of soil fractions and increase the soil organic C and N storage, thereby affecting the turnover and stability of SOM.