Dynamics of soil and root C stocks following afforestation of croplands with poplars in a semi-arid region in northeast China

Background and aims Afforestation on croplands can help sequester atmospheric CO2 through increased carbon (C) storage in the soil and vegetation. However, the dynamics of soil organic C (SOC) and root C stocks, particularly those in the deeper soil layers, following afforestation are not well documented for semi-arid regions. The aim of this study was to investigate the dynamics of soil and root C stocks to 1 m depth following afforestation with poplar (Populus x xiaozhuanica W. Y. Hsu & Liang) on croplands at the Keerqin Sandy Lands in northeast China. Methods Forest floor, root and mineral soil samples were collected from 23 paired plots of poplar plantations with different stand basal areas (SBA, ranging from 0.2 m(2)ha(-1) to 32.6 m(2)ha(-1)) and reference croplands using a paired-site design. Changes of SOC concentration and content, and root C content were analyzed using paired t tests, and the relationships between forest floor C content, soil and root Delta(C) (Delta(C) refers to the difference in C stocks between a poplar plantation and the paired cropland) and SBA were tested with a polynomial regression analysis. Results Afforestation resulted in linear increases of Delta C in the forest floor and 0-10 cm mineral soil with SBA (R-2= 0.67, p< 0.001 and R-2= 0.34, p= 0.003, respectively), but there were no clear relationships between SOC stocks in the soil deeper than 10 cm and SBA. The fine root C stock increased by afforestation across all the soil layers (p< 0.05), and root Delta C had a quadratic curve (the first two mineral soil layers) or linear (the other mineral soil layers) relationship with SBA. About 73 % of the variance of Delta C in the top soil was explained by changes in the forest floor C stock, but changes in plant derived C stocks did not explain the variance of soil Delta C in the deeper layers very well. Conclusions Our study suggest that afforestation increased C sequestration in the forest floor and surface mineral soil, and C stocks in the forest floor and surface mineral soil and roots were strongly controlled by the SBA, which changes with stand development, in the studied semiarid region in northeast China.