Rapid decrease of soil carbon after abandonment of subtropical paddy fields.

Background and aims: Paddy field abandonment has become a major concern in China, particularly among traditional rice-cultivation regions. Abandonment results in the alteration of many processes affecting C sequestration and turnover, but the final effects on C stocks remain unknown. Methods: To examine the effects of paddy abandonment on topsoil organic C (SOC) content and stocks, a long-term experiment was performed in subtropical China, examining an abandoned paddy field with different pre-abandonment fertilization history (from 1991 to 2006 years) and SOC gradients (from 19.2 to 22.0 g C kg). Results: Paddy field abandonment significantly reduced the topsoil SOC content and stock (0-20 cm). Eight years after cultivation ceased, SOC content and C stock had decreased by 9.9-20.9% and 10.2-20.8%, respectively, yielding a mean annual loss rate of 0.30-0.60 g C kg·yr. and 0.50-1.15 t C ha·yr., respectively. Soils with higher initial SOC content were more sensitive to abandonment than soils with low SOC levels. Dissolved organic C (DOC) was more sensitive to abandonment, as evidenced by the faster decrease of DOC than SOC. The rapid reduction of SOC content, combined with a strong decrease in DOC, indicates that post-abandonment C inputs into the soil were far lower than the concurrent SOC mineralization. The SOC content decreases was likely because of the shift from anaerobic to aerobic conditions. This change leads to faster litter decomposition and SOC mineralization, accompanied with decreasing SOC retention or stabilization by soil aggregates, mineral or Fe redox processes. Conclusion: Abandonment of paddy soils leads to switch from a C sink to a C source, resulting in high C losses. The succession of grasses in abandoned fields did not compensate for the losses of soil C stocks. [ABSTRACT FROM AUTHOR]