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 toidentify the consequences of 18 years of afforestation on C and N dynamics in the Danjiangkou Reservoirarea 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 m, 250-2000 m, 53-250 m and<53 m), 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 Ncontent, and ?3C and ?5N values. Afforestation enhanced the soil C and N storage, primarily due toincreases in soil C and N storage of macroaggregates (>2000 m) with the largest (65-87%) fraction iniPOM. The C: N ratios generally increased from the cropland to shrubland to woodland across all fractions.The13C values indicated that the fastest decay rates (k = 0.024 yr-1) in LF of macroaggregates (>2000 m)was in woodland, but the fastest decay rates (0.114-0.137 yr-1) in iPOM of all aggregates were observedin cropland. Meanwhile, the most enriched 15N values (0.87-3.81) were found in cropland soil. Ourresults suggest that afforestation could alter weight distribution of soil fractions and increase the soilorganic C and N storage, thereby affecting the turnover and stability of SOM.
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